CMOS Laser Diode Drivers for Supercontinuum Generation

University of Calgary

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Graduate Studies The Vault: Electronic Theses and Dissertations

2016

CMOS Laser Diode Drivers for Supercontinuum

Generation

He, Yuting

He, Y. (2016). CMOS Laser Diode Drivers for Supercontinuum Generation (Unpublished master's

thesis). University of Calgary, Calgary, AB. doi:10.11575/PRISM/25339

http://hdl.handle.net/11023/3030

master thesis

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UNIVERSITY OF CALGARY

CMOS Laser Diode Drivers for Supercontinuum Generation

by

Yuting He

A THESIS

SUBMITTED TO THE FACULTY OF GRADUATE STUDIES

IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE

DEGREE OF MASTER OF SCIENCE

GRADUATE PROGRAM IN ELECTRICAL ENGINEERING

CALGARY, ALBERTA

MAY, 2016

© Yuting He 2016

ii

Abstract

There have been intense research efforts on developing compact and low-cost supercontinuum

generation (SCG) systems, which have various application areas including telecommunications,

spectroscopy, and optical coherence tomography. This research employs complementary metal–

oxide–semiconductor (CMOS) technology to design and implement two integrated laser diode

drivers for reducing the size and cost of SCG systems. A continuous-wave CMOS driver with a

maximum output current of 600 mA is developed for driving a laser diode in an erbium-doped

fiber amplifier (EDFA). A picosecond pulsed CMOS driver is designed and applied for gain-

switching a laser diode to produce optical pulses with a pulse width of 200 ps and a repetition rate

of 5.6 MHz. The gain-switched laser diode output is amplified by an EDFA and then launched into

a highly nonlinear fiber for SCG. The generated supercontinuum has an average power of 62 mW

and a spectral bandwidth of 806 nm.

iii

Acknowledgements

Firstly, I would like thank my supervisor Dr. Orly Yadid-Pecht for her invaluable guidance,

support and encouragement throughout my research work.

Secondly, I thank all the I2Sense lab students, post-doctoral fellows, my friends throughout

the ECE department and department staff for their help and support. Many thanks to Dr. Kartikeya

Murari, Dr. Yuhua Li, Michael Himmelfarb, Nikhil Vastarey, Pauling Cummings, Dr. J.P.E.

Hadden, Prasoon Ambalathankandy, Matthew Jackson, Dr. Arthur Spivak, Linhui Yu, Donuwan

Navaratne, Zhixing Zhao, Christopher Simon, Kathryn Simon and Shem Chenoo for their helpful

discussion and support regarding this thesis work.

Thirdly, I would like to thank my committee members Dr. Paul Barclay and Dr. Leonid

Belostotski for taking their time to serve in my committee.

Last but not least, I would like to thank the CMC Microsystems for the access to the design

tools, workshops and fabrication services through MOSIS. Special thanks to Dr. Shahriar

Mirabbasi from UBC for organizing the CMOS Electronics for Photonics training course and for

helpful discussions regarding the pulsed CMOS driver design during the course.

iv

Dedication

To my parents for their unconditional love and support!

v

Table of Contents

Abstract ............................................................................................................................... ii Acknowledgements ............................................................................................................ iii Dedication .......................................................................................................................... iv Table of Contents .................................................................................................................v List of Tables .................................................................................................................... vii List of Figures .................................................................................................................. viii List of Abbreviations and Symbols......................................................................................x

Chapter 1: Introduction ........................................................................................................1 1.1 Background and Review ............................................................................................1 1.2 Motivation and Objectives .........................................................................................4 1.3 Thesis Scope and Contributions ................................................................................6 1.4 Thesis Outlines ..........................................................................................................7

Chapter 2: High Current Continuous-Wave CMOS Laser Diode Driver ............................8 2.1 Introduction and Objectives .......................................................................................8 2.2 Characteristics of Laser Diodes .................................................................................9 2.3 Continuous-Wave CMOS Driver Circuit Design Methodology ..............................11

2.3.1 Current Reference Circuit ................................................................................12 2.3.2 Start-up Circuit ................................................................................................14 2.3.3 Current Source Circuit .....................................................................................14

2.4 Circuit Design and Simulations ...............................................................................15 2.5 Measurements ..........................................................................................................19

2.5.1 Measurement Setup .........................................................................................19 2.5.2 Measurements of the Output Current ..............................................................21 2.5.3 Measurements of the Optical Output Power ...................................................22

2.6 Summary ..................................................................................................................25

Chapter 3: Picosecond Pulsed CMOS Laser Diode Driver ...............................................26 3.1 Introduction and Objectives .....................................................................................26 3.2 Gain-switched Laser Diodes ....................................................................................26 3.3 Pulsed CMOS Driver Circuit Design Methodology ................................................28

3.3.1 Voltage-Controlled Ring Oscillator ................................................................29 3.3.2 Voltage-Controlled Delay Line .......................................................................31 3.3.3 Exclusive-OR Circuit ......................................................................................33 3.3.4 Current Source Circuit .....................................................................................34

3.4 Circuit Design and Simulations ...............................................................................35 3.5 Measurements and Analysis ....................................................................................38

3.5.1 Measurement Setup .........................................................................................38 3.5.2 Measured Results .............................................................................................39 3.5.3 Measurements of Tunable Output ...................................................................41 3.5.4 Analysis ...........................................................................................................43

3.6 Summary ..................................................................................................................44

vi

Chapter 4: Supercontinuum Generation in a Highly Nonlinear Fiber Using CMOS Laser Diode Drivers.......................................................................................................................45

4.1 Introduction and Objectives .....................................................................................45 4.2 Erbium-Doped Fiber Amplifier ...............................................................................46 4.3 Highly Nonlinear Fiber ............................................................................................49 4.4 Nonlinear Optical Effects ........................................................................................50

4.4.1 Chromatic Dispersion ......................................................................................50 4.4.2 Stimulated Raman Scattering ..........................................................................51 4.4.3 Four-Wave Mixing ..........................................................................................51 4.4.4 Modulation Instability .....................................................................................51

4.5 Supercontinuum Generation ....................................................................................52 4.5.1 Experimental Setup .........................................................................................52 4.5.2 Measured Results and Discussions ..................................................................54 4.5.3 System Performance Comparison ...................................................................57

4.6 Summary ..................................................................................................................60

Chapter 5: Conclusions and Future Work ..........................................................................62 5.1 Conclusions ..............................................................................................................62 5.2 Future Work .............................................................................................................63

References ..........................................................................................................................65

vii

List of Tables

Table 2.1: Design parameters of transistors in the proposed CMOS circuit ................................ 16

Table 2.2: Performance summary of the designed CW pumped laser diode ................................ 25

Table 3.1: Performance summary of the designed pulsed laser diode .......................................... 44

Table 4.1: Optical properties of the HNLF ................................................................................... 49

Table 4.2: Characteristics of the amplified laser diode pulses ...................................................... 53

Table 4.3: Performance summary of the seed laser module ......................................................... 57

Table 4.4: Comparison of the designed SCG system and the reference SCG system .................. 61

viii

List of Figures

Figure 1.1: Simplified SCG process in the spectral domain ........................................................... 1

Figure 2.1: Block diagram of a basic EDFA configuration ............................................................ 8

Figure 2.2: Typical output light versus injection current (L-I) curve of laser diodes ................... 10

Figure 2.3: Typical forward voltage versus injection current (V-I) curve of laser diodes ........... 10

Figure 2.4: Block diagram of the CW laser diode driver circuit design methodology ................. 11

Figure 2.5: Circuit schematic of the proposed CMOS laser diode driver ..................................... 11

Figure 2.6: Simulation results of transistors’ operation regin versus the resistance ..................... 17

Figure 2.7: Comparison of modelling current output and simulation current output at the resistance of the off-chip resistor range from 1100 Ω to 1950 Ω. ........................................ 18

Figure 2.8: Die micrograph of the fabricated CW laser diode driver ........................................... 19

Figure 2.9: Package of a complete driver circuit with two identical CMOS dies; Note that the designed driver circuit is only one small part of the whole CMOS die, there are unrelated circuits shared on the same die. ............................................................................................ 20

Figure 2.10: (a) Host PCB with a driver package and four identical potentiometers which controls three current output ports; (b)Laser diode package mounted on a PCB ................. 20

Figure 2.11: Comparison of the output current between the measurement result, simulation result and mathematical modelling result; Note that the resistance at x-axis represents the resistance of each one potentiometer on the PCB in the measurement. ............................... 21

Figure 2.12: Measured data of optical output power with respect to the injection current, and the fitting curve based on measured data .............................................................................. 22

Figure 2.13: Relation curve of the optical output power of the laser diode and the resistance of potentiometers ....................................................................................................................... 23

Figure 2.14: Stability test of the optical output power over a 400 minute period. ....................... 24

Figure 3.1: Evolution of the photon and carrier density during a gain-switching cycle [37] ....... 27

Figure 3.2: Block diagram of the proposed pulsed laser driver circuit design ............................. 28

Figure 3.3: Design methodology for generating pulse waves ....................................................... 29

Figure 3.4: Circuit design schematic of the VCRO ...................................................................... 30

Figure 3.5: Schematic of the VCDL circuit .................................................................................. 31

ix

Figure 3.6: Timing sequence diagram of the VCDL operation .................................................... 32

Figure 3.7: Design schematic of the XOR circuit ......................................................................... 33

Figure 3.8: Negative power supply operation of the laser diode .................................................. 34

Figure 3.9: Post-layout transient simulation of the proposed CMOS laser driver: (a) the output current waveform; (b) a sample current pulse. ..................................................................... 36

Figure 3.10: Output plots when tuning (a) repetition rate and (b) pulse width in simulations ..... 37

Figure 3.11: (a) Die micrograph and (b) PCB layout of the pulsed laser source .......................... 38

Figure 3.12: (a) Laser pulse waveform and (b) one Gaussian fitted laser pulse ........................... 39

Figure 3.13: Optical spectrum of the pulsed laser diode output ................................................... 40

Figure 3.14: Measured results of tuning the optical output pulses’ repetition rate (a) by adjusting control voltage Vvar and (b) by adjusting control voltage Vctr ............................... 42

Figure 3.15: Measured results of tuning the optical output pulses’ pulse width (a) by adjusting control voltage Vvar2 and (b) by adjusting control voltage Vb .............................................. 42

Figure 4.1: Supercontinuum generation system design block diagram ........................................ 45

Figure 4.2: EDFA amplification mechanism based on an energy-level diagram of erbium ions . 46

Figure 4.3: Block diagram of the EDFA module setup ................................................................ 47

Figure 4.4: Spectrum of the output light from the EDFA monitor output .................................... 48

Figure 4.5: Experimental setup diagram of the SCG system ........................................................ 53

Figure 4.6: Spectrum of supercontinuum output .......................................................................... 54

Figure 4.7: Supercontinuum output evolution at different pump power level .............................. 55

Figure 4.8: Temporal profile of the seed laser pulse, the EDFA monitor pulse and the supercontinuum pulse ........................................................................................................... 56

Figure 4.9: Output optical pulses from the commercial seed laser module: the left figure shows the pulse waveform; the right figure shows a single pulse shape. ........................................ 58

Figure 4.10: (a) Spectrum of optical pulses output from the seed laser module; (b) spectrum of the amplified pulses output from the monitor of EDFA ....................................................... 58

Figure 4.11: SCG results from two different systems .................................................................. 60

x

List of Abbreviations and Symbols

Abbreviation Definition ASE Amplified Spontaneous Emission CBL Current-Balanced Logic CDS Cadence Design System CFP Ceramic Flat Package CMOS Complementary Metal-Oxide-Semiconductor CW Continuous-Wave DFB Distributed Feedback DWDM Dense Wavelength-division Multiplexing EDA Electronic Design Automation EDFA Erbium-doped Fiber Amplifier FC Fiber Connector FS Fusion Splicing FWHM Full Width at Half Maximum FWM Four-Wave Mixing HNLF Highly Nonlinear Fiber MI Modulation Instability NMOS N-channel Metal-Oxide-Semiconductor OCT Optical Coherence Tomography OSA Optical Spectrum Analyzer PCB Printed Circuit Board PCF Photonic Crystal Fiber PDK Process Design Kit PMOS P-channel Metal-Oxide-Semiconductor RMS Root Mean Square SCG Supercontinuum Generation SMF Single-Mode Fibers SRS Stimulated Raman Scattering TEC Thermo-Electric Cooler VCDL Voltage-Controlled Delay Line VCRO Voltage-Controlled Ring Oscillator XOR Exclusive–OR ZDW Zero Dispersion Wavelength

xi

Symbol Definition Ith Threshold injection current of laser diodes ROUT Off-chip resistor IREF Reference current IO Output current 𝜇𝜇𝑛𝑛𝐶𝐶𝑜𝑜𝑜𝑜 Transconductance parameter of CMOS transistors Mi Transistor number i L Length of CMOS transistors Vth Threshold voltage Vgs Gate source voltage of CMOS transistors K An integer KI An integer L Optical output power I Injection current N Number of delay stages 𝑓𝑓𝑜𝑜𝑜𝑜𝑜𝑜 Oscillation frequency td Delay time of each delay stage CL Load capacitance Vosc Oscillation voltage amplitude Ictrl Control current Vvar Varactor control voltage Vctr Transistors’ current control voltage Vb Control voltage Rin Input impedance Vf Forward voltage If Forward current 𝑡𝑡𝐹𝐹𝐹𝐹𝐹𝐹𝑀𝑀 Duration time of FWHM a1, b1, c1 Coefficients of the Gaussian pulse Ppeak Peak power Pavg Average power fRR Repetition rate Ep Pulse Energy ω Frequency γ Nonlinear coefficient

1

Chapter 1: Introduction

1.1 Background and Review

A supercontinuum is a special type of light with high intensity, broad spectral bandwidth

and a high degree of spatial coherence. It is considered to be the combination of a lamp with

broadband spectrum and a laser with high brightness. This kind of light does not exist in nature. It

only occurs when high intensity laser light interacts with nonlinear media. The nonlinear process

that broadens the spectrum of the laser light to produce the supercontinuum is called

supercontinuum generation (SCG). A simplified SCG process in the spectral domain is illustrated

in Figure 1.1.

λ λ0 λ λ0 λ1 λ2

Nonlinear Process

Laser Light Spectrum Supercontinuum Spectrum

Figure 1.1: Simplified SCG process in the spectral domain

SCG was first observed by Alfano and Shapiro back in 1970s [1]. They focused powerful

picosecond pulses into a bulk BK7 glass to generate a white light supercontinuum covering a

spectral range of 400 nm to 700 nm. Since then, SCG has been observed when intense picosecond

or femtosecond laser light is incident on various nonlinear materials which can be solid, liquid and

gaseous [2], [3]. SCG in optical fibers was first observed in 1976 by Lin and Stolen [4]. They used

2

nanosecond laser pulses at kilowatt peak power levels to pump the conventional silica fiber in

order to generate a supercontinuum with spectral bandwidth of 180 nm and spectrum centered at

530 nm. Many subsequent research efforts have been made for SCG in standard single-mode

optical fibers [5], [6]. As the nonlinearity of standard fibers is relatively low, it requires pump

lasers with high peak power levels to generate significant spectral broadening in standard fibers.

However, laser light with a high peak power can damage the silica fibers, which limits the power

level and consequently the spectrum bandwidth of the supercontinuum.

The advent of photonic crystal fiber (PCF) greatly improved SCG technology. PCFs are

produced by the cladding of an optical fiber incorporated with photonic crystals that are dielectric

periodic structures on the scale of a wavelength of light. The PCF has the advantage of high

nonlinearity as well as designable dispersion, which enhance nonlinear effects and obtain much

broader spectra than the standard fibers [7]. The first octave spanning SCG was accomplished with

a PCF pumped by nanojoule pulses from a Ti:Sapphire laser in the year 2000 [8].

The successful SCG designs with PCFs have inspired similar work using optical tapered

fibers [9] or highly nonlinear fibers (HNLFs) [10]. Both the optical tapered fibers and HNLFs

present comparable enhanced nonlinearity, which makes them good alternative to PCFs. Optical

tapered fibers are produced by gently stretching optical fibers to a thin core diameter. Optical

tapered fibers have designable dispersion, a controllable tapering process and a reduced fiber

diameter, all of which enhance the SCG performance. SCG in a tapered fiber with a diameter of 2

µm and a length of 90 mm was reported to have a broad spectrum output of more than two octaves

(370 -1545 nm) [9].

HNLFs are produced by fabricating optical fibers with a narrow core and high material

doping level, which reduces the effective core area and thus enhances the nonlinearity. Compared

3

to the PCF and tapered fibers, HNLFs are the easiest to fabricate but exhibit the lowest

nonlinearity. As HNLFs are single-mode optical fibers, they can be coupled with standard single-

mode fibers (SMF) at a low coupling loss, which is an advantage of HNLFs based all fiber SCG

systems. SCG in a 200-meter-long HNLF pumped by a femtosecond fiber laser with 110 fs pulses

at 1550 nm was reported to have a spectrum spanning from 1100 nm to 2100 nm [10].

SCG in optical fibers has found numerous novel applications in the field of

telecommunication [11], optical frequency metrology [12], [13], optical coherence tomography

(OCT) [14] and spectroscopy [15]. For example, in the field of telecommunications, SCG systems

can be used in dense wavelength-division multiplexing (DWDM) systems. One can use optical

filters to slice the supercontinuum spectrum, so that thousands of single wavelengths of laser light

can be obtained and applied to a number of transmission channels. This approach can realize a

high transmission rate with only one light source.

4

1.2 Motivation and Objectives

Although SCG systems have great potential to be applied in these cutting-edge areas, most

commercial SCG systems nowadays are just used as a laboratory tool. Current commercial SCG

systems are still limited in performance by the availability of suitable wavelength ranges and

limited by size, cost, and power. There is high demand for the development of compact, low-cost

and reliable SCG systems that are practical and accessible for use in various application areas [16].

As the supercontinuum is generated when laser light interacts with a nonlinear medium, SCG

systems in general contain two components, which are a laser light source and a nonlinear medium.

Numerous experimental results using different lasers and nonlinear optical media for SCG have

been reported. PCFs, optical tapered fibers, HNLFs, dispersion-shifted fibers [17], and even silicon

waveguides [18] have proven to be successful nonlinear optical media. Mode-locked lasers [19],

Q-switched lasers [20], gain-switched lasers [21] and continuous-wave (CW) pump lasers [22]

with various wavelengths have been used as pump lasers in SCG. Compared to fiber media, pump

lasers often take up most of the space and cost, and cause the most reliability and maintenance

problems in SCG systems. Thus, a compact, low-cost, power-efficient and reliable pump laser

light source would be a turnkey solution for the next generation of SCG systems. The objective of

this thesis is to develop such a laser light source for use in SCG systems.

Among all types of pump lasers mentioned above, CW pump lasers are the most compact

and low-cost. However, CW pump lasers require the highest average power to obtain the same

broad spectral supercontinuum as pulsed lasers. Methods of pulsing lasers are mode-locking, Q-

switching and gain-switching. Ultrashort pulses can be produced by mode-locked lasers using

nonlinear polarization rotation [23], active Q-switched lasers using fiber Bragg gratings [24],

passive Q-switched lasers using standard small-mode-area saturable absorber fibers [25], or gain-

5

switched lasers using ultrashort current pulses. As gain-switched lasers do not require additional

optical components, they are the most compact and cost-effective among all pulsed lasers.

A laser diode is considered to be a good candidate for a compact and reliable pump laser. It

is a simple and cost-effective method to obtain tens-of-picosecond short pulses by gain-switching

widely available telecommunication-grade laser diodes. The drawback of gain-switched laser

diodes is the limitation of the maximum output power. To get a high pulse power, the output of

laser diodes can be amplified by compact fiber amplifiers, which are widely used in optical

communication systems. The combination of telecommunication-grade low-power gain-switched

laser diodes and fiber amplifiers offers a compact, reliable and low-cost approach to generate high-

power short pulses. This approach has been utilized for octave-spanning SCG in the report [17].

SCG systems using this approach have been implemented in the DWDM application [26].

The SCG method employed in this research uses amplified gain-switched laser diode pulses

to pump an HNLF. Based on this method, a laser diode is gain-switched by short current pulses

from a pulsed electronic driver. A fiber amplifier is utilized to amplify the power of optical pulses

produced by the gain-switched laser diode. In the fiber amplifier, a pump laser diode is driven by

a CW electronic driver. To make these electronic drivers compact and low-cost, complementary

metal-oxide-semiconductor (CMOS) technology is utilized to design and fabricate electronic

driver circuits. CMOS technology has the advantages including low power, low fabrication cost,

and high integration. Using CMOS technology for the design of laser diode drivers in

telecommunication is a subject of intense research [28], [29], however, there has not been much

implementation of CMOS laser diode driver circuits in the development of compact and low-cost

SCG systems. This research work aims to design and apply CMOS laser diode driver circuits for

developing such SCG systems.

6

1.3 Thesis Scope and Contributions

The scope of this thesis is to implement CMOS technology in the design of the laser diode

driver circuits for the purpose of developing compact and economical SCG systems. The method

of SCG employed in this thesis is applying amplified picosecond gain-switched laser diode pulses

to efficiently induce nonlinear effects in an HNLF.

Based on the scope, the main contributions of this thesis are:

1. Design and implementation of a CMOS laser diode driver circuit with a tunable CW

high current output. A CW pump laser with a maximum output power of 350 mW is

built using the CMOS driver circuit.

2. Design and implementation of a CMOS laser diode driver circuit with picosecond

current pulses output. A picosecond pulsed distributed feedback (DFB) laser diode is

built using the CMOS driver circuit. To the best of the author’s knowledge, this is the

first such design reported in the open literature.

3. Establishment and measurements of a SCG system using the designed picosecond

pulsed DFB laser diode in combination with an erbium-doped fiber amplifier (EDFA)

and an HNLF.

7

1.4 Thesis Outlines

The rest of this thesis is organized as follows.

Chapter 2 presents the design of a high current CW CMOS laser diode driver. It initially

introduces the design objectives and characteristics of laser diodes. Then, a circuit design

methodology is proposed and analyzed. Circuit simulations in Cadence Design System (CDS) are

presented with detailed simulation parameters. Experimental measurements of the CMOS driver

circuit output as well as the laser diode optical output are conducted. A summary is given to

conclude performance of the design.

Chapter 3 demonstrates the design of a picosecond pulsed CMOS laser diode driver. The

gain switching technique that is implemented to generate picosecond pulses is explained. A circuit

design methodology using CMOS analog logic circuits is demonstrated. Circuit simulations based

on the proposed methodology show the output performance. The CMOS driver circuit is packaged

and connected with a DFB laser diode experimentally. Measured results of the laser output are

reported and analyzed. A performance summary table is given at the end of this chapter.

Chapter 4 establishes a SCG system, which contains the designed picosecond pulsed DFB

laser diode, an EDFA and an HNLF. Characteristics of the EDFA and the HNLF used in the system

are explained. A brief introduction of nonlinear optical effects in the context of a SCG process is

described. The system setup is depicted and measured results are presented. For comparison,

measurements of a reference SCG system utilizing a commercial seed laser module are given. A

comparison between the designed SCG system and a reference SCG system is made in the

summary.

Chapter 5 discusses the results obtained and suggests future work.

8

Chapter 2: High Current Continuous-Wave CMOS Laser Diode Driver

2.1 Introduction and Objectives

Continuous-wave (CW) current sources provide power for a large group of laser diodes.

Many researchers and engineers develop stable CW laser diode drivers with printed circuit boards

(PCBs) and off-the-shelf components. However, there has been an increasing interest in using

commercial CMOS technology to design CW laser diode drivers, for reasons of a compact circuit

size, a low fabrication price and integrable packages.

CW laser diode drivers are required to drive pump laser diodes in erbium-doped fiber

amplifiers (EDFAs), which have proven to be effective fiber amplifiers for supercontinuum

generation (SCG) [17], [29]. A block diagram of a basic EDFA configuration is shown in

Figure 2.1. The gain medium is an erbium-doped fiber. The EDFA amplifies the incident light

using the simulated emission in the erbium-doped fiber, which is continuously pumped by high

energy light from a pump laser diode. The pump laser diode demands a CW laser diode driver to

provide electrical energy.

Figure 2.1: Block diagram of a basic EDFA configuration

975nm Pump Laser Diode

CW LaserDiode Driver

Erbium DopedFiber

Incident Light~1550nm

Amplified Light~1550nm

EDFA

9

Pump laser diodes used in the EDFA usually need to have high power output. A 975 nm

pump laser diode (Bookham LC95B74ET) is selected in this design. It has a maximum output

power of 350 mW in the CW mode, which corresponds to 600 mA injection current based on the

datasheet. Thus the CW laser diode driver requires to be able to output 600 mA current to the laser

diode. Apart from this requirement, a tunable output feature needs to be taken into consideration

for this driver design, as it can add to the flexibility of the laser output. With a tunable output

power from the pump laser, the fiber amplifier system is able to amplify the incident light to

different power levels.

The objective of the research described in this chapter is to implement CMOS technology in

the design of a high current CW laser diode driver circuit with tunable output feature.

2.2 Characteristics of Laser Diodes

In order to design drivers for laser diode, it is critical to understand characteristics of laser

diodes. Laser diodes, also known as injection lasers or diode lasers, are semiconductor lasers in

which the light is generated by injecting an electrical current. In other words, a laser diode emits

light in response to an injection current. The optical power of the emitted light is a function of the

injection current, which is commonly referred as the light versus current (L-I) curve of a laser

diode. Figure 2.2 shows the L-I curve of a typical laser diode. If the injection current is below the

threshold current Ith, the emitted optical power Pout is very small and is generated due to

spontaneous emission in the p-n junction. As the injection current is increased above the threshold

current, the laser diode starts lasing due to the stimulated emission. The efficiency of a laser diode

can be derived from the curve, which is defined as the slope efficiency. The slope efficiency is

denoted as ∆P/∆I (mW/mA) as shown in the figure. It indicates the incremental output power gain

over the increase of injection current.

10

Injection Current (mA)

Opt

ical

Out

put P

ower

(mW

)

ΔP

ΔI

Ith

Figure 2.2: Typical output light versus injection current (L-I) curve of laser diodes

The forward voltage is an important electrical characteristic of a laser diode. It represents

the voltage drop across the laser diode. The relation between the forward voltage and the injection

current of a typical laser diode is shown as the voltage versus current (V-I) curve in Figure 2.3.

This V-I characteristic is similar to the analogous characteristic of other types of semiconductor

diodes. Typical laser diodes exhibit a forward voltage in the range of 1.5 V to 3 V, which requires

CMOS driver circuits to have high voltage supplies and large transistor breakdown voltages [30].

Injection Current (mA)

Forw

ard

Volta

ge (V

)

1

2

Figure 2.3: Typical forward voltage versus injection current (V-I) curve of laser diodes

11

2.3 Continuous-Wave CMOS Driver Circuit Design Methodology

A block diagram of a CW CMOS laser diode driver circuit design methodology is shown in

Figure 2.4. The driver circuit is a current source. There are three sub-circuits including a start-up

circuit, a current reference circuit and a current source circuit. The start-up circuit ensures that the

current reference circuit is turned on. The current source circuit amplifies the reference current

generated from the current reference circuit. Based on this methodology, a CMOS laser diode

driver circuit is proposed and shown in Figure 2.5.

Figure 2.4: Block diagram of the CW laser diode driver circuit design methodology

M6

VDD

M5

M7

M3 M4 M0

M1 M2

RO UT

Laser Diode

IREF IO

Start-up CurrentReference

CurrentSource

Figure 2.5: Circuit schematic of the proposed CMOS laser diode driver

Start-up Circuit Current Reference CircuitTurn On

Current Source CircuitAmplified

12

2.3.1 Current Reference Circuit

This proposed current reference circuit shown in the center of Figure 2.5 is based on the

structure of beta-multiplier voltage reference [31]. Transistors M1-M4 are self-biased to operate in

the saturation region. The p-channel metal-oxide-semiconductor (PMOS) current mirror M3 and

M4 force the same current through each leg of the circuit. An off-chip resistor ROUT has been placed

between the source of M2 and ground. The size of M2 is made larger than that of Ml so that the

difference in the gate to source voltage of Ml and M2 is dropped across ROUT. Through adjusting

the resistance of the off-chip resistor, the reference current will be adjusted accordingly.

As the PMOS M3 current mirrors the PMOS M4, the current I1 passing through M3 and M1

is the same as the reference current IREF passing through M4 and M2, which is expressed as

The gate source voltage of M1, Vgs1 equals the sum of the gate source voltage of M2, Vgs2, and the

voltage across the resistor ROUT, which is shown as

𝑉𝑉𝑔𝑔𝑜𝑜1 = 𝑉𝑉𝑔𝑔𝑜𝑜2 + 𝐼𝐼𝑅𝑅𝑅𝑅𝐹𝐹𝑅𝑅𝑂𝑂𝑂𝑂𝑂𝑂. (2.2)

As all transistors are operated in the saturation region, the current I1 and IREF can be described as

(neglecting the channel-length modulation)

𝐼𝐼1 = 𝜇𝜇𝑛𝑛𝐶𝐶𝑜𝑜𝑜𝑜𝐹𝐹12𝐿𝐿1

(𝑉𝑉𝑔𝑔𝑜𝑜1 − 𝑉𝑉𝑡𝑡ℎ1)2

𝐼𝐼𝑅𝑅𝑅𝑅𝐹𝐹 = 𝜇𝜇𝑛𝑛𝐶𝐶𝑜𝑜𝑜𝑜𝐹𝐹22𝐿𝐿2

(𝑉𝑉𝑔𝑔𝑜𝑜2 − 𝑉𝑉𝑡𝑡ℎ2)2, (2.3)

where 𝜇𝜇𝑛𝑛𝐶𝐶𝑜𝑜𝑜𝑜 is the transconductance parameter of CMOS transistors, W1 and L1 are the width and

length of the transistor M1, Vth1 is M1’s threshold voltage, W2 and L2 are the width and length of

the transistor M2 and Vth2 is M2’s threshold voltage.

Based on Equation (2.3), Vgs1 and Vgs2 can be rewritten as

𝐼𝐼1 = 𝐼𝐼𝑅𝑅𝑅𝑅𝐹𝐹. (2.1)

13

𝑉𝑉𝑔𝑔𝑜𝑜1 = 2𝐼𝐼1𝛽𝛽1

+𝑉𝑉𝑡𝑡ℎ1

𝑉𝑉𝑔𝑔𝑜𝑜2 = 2𝐼𝐼𝑅𝑅𝑅𝑅𝑅𝑅𝛽𝛽2

+𝑉𝑉𝑡𝑡ℎ2,

where β1 = µnCoxW1L1

,β2 = µnCoxW2L2

.

(2.4)

Neglecting the body effect from M2, for the same CMOS technology, the two threshold voltages

Vth1 and Vth2 should be equal

𝑉𝑉𝑡𝑡ℎ1 = 𝑉𝑉𝑡𝑡ℎ2. (2.5)

By substituting Equation (2.1) (2.4) (2.5), Equation (2.2) can be rewritten

2𝐼𝐼𝑅𝑅𝑅𝑅𝑅𝑅

𝛽𝛽1= 2𝐼𝐼𝑅𝑅𝑅𝑅𝑅𝑅

𝛽𝛽2+ 𝐼𝐼𝑅𝑅𝑅𝑅𝐹𝐹𝑅𝑅𝑂𝑂𝑂𝑂𝑂𝑂. (2.6)

The reference current can be derived by solving Equation (2.6)

𝐼𝐼𝑅𝑅𝑅𝑅𝐹𝐹 = (1 − 1√𝐾𝐾

)2 2𝛽𝛽1𝑅𝑅𝑂𝑂𝑂𝑂𝑂𝑂2

where 𝐾𝐾 = (𝐹𝐹1𝐿𝐿1

)/(𝐹𝐹2𝐿𝐿2

). (2.7)

Equation (2.7) shows that the reference current is only dependent on the resistance of the off-chip

resistor and the device parameters of transistors. The device parameters of CMOS transistors are

fixed after fabrication of the design. The reference current can only be tuned by adjusting the

resistance of the off-chip resistor.

14

2.3.2 Start-up Circuit

When solving Equation (2.6), there is another scenario except the answer indicated in

Equation (2.7). That is the zero reference current scenario. In this scenario, the circuit is completely

off even after the power supply is on. This is possible because all transistors are self-biased.

To eliminate the zero reference current scenario, a start-up circuit shown in the left part of

Figure 2.5 is introduced into the design. This start-up circuit is able to initiate the current reference

circuit from a dead (zero current) operating point to its normal operating point [32]. When the

current reference circuit is at a dead operating point, the start-up circuit sets the drain voltage of

the transistor M1, and initiates M1 to draw current. Once the start-up transistor M7 provide a current

path between the supply voltage and ground, the transistors M1, M2, M3, and M4 operate normally

and the reference current will reach the desired amount. After the current reference circuit is turned

on, the gate source voltage of M7 drops to below the threshold voltage and no current flows through

M7. Thus, the start-up circuit has no impact on the value of the reference current.

2.3.3 Current Source Circuit

The current source circuit is shown in the right part of Figure 2.5. It is a single PMOS with

a bias voltage from the reference current circuit. This current source configuration extends the

output voltage to accommodate the required forward voltage of the laser diode.

The gate bias voltage of transistor M0 is connected to the gate bias voltage of transistor M4.

This mirrors the current along M0 and M4. Since M4 operates under the saturation region, M0 does

in the same mode. Thus, the output current Io is

𝐼𝐼𝑂𝑂 = 𝜇𝜇𝑛𝑛𝐶𝐶𝑜𝑜𝑜𝑜𝐹𝐹02𝐿𝐿0

(𝑉𝑉𝑔𝑔𝑜𝑜0 − 𝑉𝑉𝑡𝑡ℎ0)2. (2.8)

15

Since the gate-source voltage Vgs0 and the threshold voltage Vth0 of the transistor M0 are the

same as the gate-source voltage and the threshold voltage of the transistor M4, the amplification

ratio KI of the output current over the reference current can be expressed as

𝐾𝐾𝐼𝐼 = 𝐼𝐼𝑂𝑂𝐼𝐼𝑅𝑅𝑅𝑅𝑅𝑅

= (𝐹𝐹0𝐿𝐿0

)/(𝐹𝐹1𝐿𝐿1

). (2.9)

By substituting IREF with the result from Equation (2.7), the output current can be expressed as

𝐼𝐼𝑂𝑂 = 𝐾𝐾𝐼𝐼(1 − 1√𝐾𝐾

)2 2𝛽𝛽1𝑅𝑅𝑂𝑂𝑂𝑂𝑂𝑂2

. (2.10)

Equation (2.10) shows that the output current has the same output characteristics as the

reference current. The tunable output current is achieved by adjusting the resistance of the off-chip

resistor. Equation (2.10) also shows that the output current is independent of the supply voltage. It

is desirable for the current source to be insensitive of the supply voltage, as most power supplies

fluctuate. By utilizing the self-biasing technique, power supply sensitivity can be greatly reduced.

Self-biasing technique means the biasing voltages especially gate biasing voltages are not

connected to the supply power directly. All transistors in this proposed design are self-biased.

Thus, the output is insensitive to the power supply voltage.

2.4 Circuit Design and Simulations

CMOS circuits are often designed with electronic design automation (EDA) software.

Cadence Design System (CDS) is a ubiquitous commercial tool for CMOS circuit schematic

design, simulation, layout design and verification. To start a CMOS circuit design with CDS, an

industry-standard CMOS process with a foundry-certified process design kit (PDK) needs to be

determined. As the drain voltage of M0 requires a minimum 2 V output voltage to accommodate a

2 V forward voltage of the laser diode, a 0.35 µm CMOS process with a high transistor breakdown

voltage of 3.6 V is selected for designing the laser diode driver circuit.

16

To have a maximum output power of 350 mW from the pump laser, the laser diode driver

circuit needs to inject 600 mA CW current to the pump laser, according to the L-I curve of the

pump laser. This required output current is quite high for the CMOS technology due to the metal

electromigration issue. For each CMOS process, the amount of current carried on a metal wire or

bus is limited. A metal wire or bus carrying too much current causes a change in the metal

dimensions, spots of higher resistance and eventually failure [31]. This is termed as the metal

electromigration effect. The current density limit for the 0.35 µm CMOS process is 1.6 mA/µm.

To avoid the electromigration effect, the output current is distributed to 12 identical metal wires

on chip. Each metal wire has a width of 33 µm. This setup guarantees that the driver circuit can

deliver 600 mA current without experiencing the electromigration effect. These 12 metal wires are

connected together as a bus wire on a PCB, which allows a much higher current density.

The proposed CMOS circuit was designed and simulated with CDS. In order to create a large

tuning range of the output current, the size of the transistor M2 is made four times larger than the

transistor M1. As increasing the channel length can reduce the body effect, transistors of the

reference current circuit are set to a fairly large 3 µm channel length. Detailed design parameters

of transistors are listed in the Table 2.1.

Table 2.1: Design parameters of transistors in the proposed CMOS circuit

Transistor Parameter Value Transistor Parameter Value

M1 L=3 µm, W=10 µm M5 L=0.5 µm, W=20 µm

M2 L=3 µm, W=40 µm M6 L=5 µm, W=20 µm

M3 L=3 µm, W=60 µm M7 L=0.5 µm, W=20 µm

M4 L=3 µm, W=60 µm M0 L=1 µm, W=100 µm *216

17

For the transistor M0, the total width is 21.6 mm. A multi-finger structure is implemented in

the design. The multi-finger structure allows multiple identical transistors with short width

connected in parallel to replace a single transistor with long width. This technique reduces the gate

resistance and the circuit’s physical size. The supply voltage for this driver circuit is 3.3 V and the

laser diode is modeled as a –2 V voltage source in simulations based on the datasheet.

The model of the output current in Equation (2.10) is based on the fact that all transistors

M1, M2, M3, M4 and M0 are in the saturation region. With the help of simulations in the CDS, the

relation between the resistance and transistors’ operation region is shown in Figure 2.6. Transistors

M1 and M4 can only operate in the saturation region. Reg0, Reg2 and Reg3 represent the operation

region of transistor M0, M2 and M3, respectively. In this simulation, Y = 1 means the transistor is

in the triode region and Y = 2 means the transistor is in the saturation region.

Figure 2.6: Simulation results of transistors’ operation regin versus the resistance

1000 1200 1400 1600 1800 20000-Cutoff

1-Triode

2-Saturation

3-Subthreshold

4-Breakdown

Rout (Ω)

Y (O

pera

tion

Reg

ions

)

Reg0Reg2Reg3

18

The plotted result shows that all transistors are operating in saturation region (Y=2) when

resistance is higher than 1100 Ω. Under this condition, the relation between the output current and

the resistance as illustrated in Equation (2.10) is valid. By substituting design parameters K=4,

KI=1080 and β1=7.1667·10-4, the obtained output current is

𝐼𝐼𝑂𝑂 = 1.88371∙105 𝑅𝑅𝑂𝑂𝑂𝑂𝑂𝑂2

. (2.11)

This expression of output current is obtained based on mathematically modelling behaviors

of transistors. To verify the mathematical model, a comparison between the modelling output and

the CDS simulation output is made in Figure 2.7. The comparison shows that the modelling current

output agrees with the simulation current output. In the range from 1300 Ω to 1400 Ω, the

modelling one has an accurate fit to the simulation one.

Figure 2.7: Comparison of modelling current output and simulation current output at the resistance of the off-chip resistor range from 1100 Ω to 1950 Ω.

1100 1200 1300 1400 1500 1600 1700 1800 1900 2000150

200

250

300

350

400

450

500

550

600

650

Resistance of the off-chip resistor (Ω)

The

outp

ut C

urre

nt (m

A)

Simulation OutputModeling Output

19

2.5 Measurements

2.5.1 Measurement Setup

Due to the size restriction of the fabrication process, the design has to be divided into two

identical CMOS dies. Each CMOS die provides half of the total current output, therefore each one

has six identical current outputs. The die micrograph is shown in Figure 2.8. As the complete driver

circuit is connecting two of these dies in parallel, two dies are placed into one package. A complete

driver with the package is shown as Figure 2.9. The package is mounted on a custom-designed

PCB, as shown in Figure 2.10 (a). Four identical off-chip potentiometers are also connected on the

PCB for tuning the output, as each potentiometer controls three current output ports. This driver

circuit with the host PCB can output the driving current to the laser diode with a mount PCB,

which is shown in Figure 2.10 (b).

When the CMOS driver circuit provides the driving current to the laser diode, the laser diode

outputs laser light. The output current from the CMOS driver circuit is measured by an ammeter

placed between the driver circuit and the laser diode. The power of the output light is detected by

an optical power meter (Newport 1916-R). The supply voltage VDD to the driver circuit is 3.3 V.

GND Io1 Io2 Io3 Io4 Io5 Io6

VDD

VDD

VDD VDDRout1 Rout2

W =

2 m

m

Figure 2.8: Die micrograph of the fabricated CW laser diode driver

20

Figure 2.9: Package of a complete driver circuit with two identical CMOS dies; Note that the designed driver circuit is only one small part of the whole CMOS die, there are unrelated circuits shared on the same die.

Figure 2.10: (a) Host PCB with a driver package and four identical potentiometers which controls three current output ports; (b)Laser diode package mounted on a PCB

21

2.5.2 Measurements of the Output Current

By tuning the four off-chip potentiometers with the same pace from 1100 Ω to 1950 Ω, a

wide range of the output current from 200 mA to 600 mA is realized. This result is compared to

the results from the simulation and the modelling. The comparison result is shown in Figure 2.11.

Results indicate that the measurement agrees to the modelling better than the simulation. From a

resistance of 1250 Ω to 1450 Ω, results from three different approaches have good agreement.

Figure 2.11: Comparison of the output current between the measurement result, simulation result and mathematical modelling result; Note that the resistance at x-axis represents the resistance of each one potentiometer on the PCB in the measurement.

1100 1200 1300 1400 1500 1600 1700 1800 1900 2000150

200

250

300

350

400

450

500

550

600

650

Resistance of the off-chip resistor (Ω)

The

outp

ut C

urre

nt (m

A)

SimulationModellingMeasurement

22

2.5.3 Measurements of the Optical Output Power

The optical output power from the laser diode is directly dependent on the amount of

injection current provided by the driver circuit. The measured relation between the optical output

power of the laser and the injection current is shown in Figure 2.12. Measured data shows that the

optical output power from the laser diode can be tuned from 100 mW to 350 mW. Based on

measured data, a linear fitting curve is also plotted in Figure 2.12. The fitting plot indicates that

the threshold current of this laser diode is 40.1 mA. There is a linear relation between the optical

output power L and the injection current I, which can be expressed as

𝐿𝐿 = 0.6433 · 𝐼𝐼 − 25.81. (2.12)

Figure 2.12: Measured data of optical output power with respect to the injection current, and the fitting curve based on measured data

0 100 200 300 400 500 600-50

0

50

100

150

200

250

300

350

400

The injection current (mA)

The

optic

al o

utpu

t pow

er o

f the

lase

r dio

de (m

W)

Measured DataFitting Curve

23

As the injection current is the output current of the laser diode driver circuit and the

resistance of the off-chip potentiometers determines the output current, the optical output power

of the laser diode is controlled by the resistance. The relation between the optical output power

and the resistance is shown in Figure 2.13. Since the current output is in a hyperbolic relation with

a square of the resistance according to Equation (2.11) and the optical output power is in a linear

relation with the injection current according to Equation (2.12), the fitting curve between the

optical output power and the resistance is a hyperbolic function of the resistance’s square. A

desired optical output power level can be obtained by setting the corresponding resistance of

potentiometers, based on the experssion of fitting curve

𝐿𝐿 = 4.6·108

𝑅𝑅𝑂𝑂𝑂𝑂𝑂𝑂2− 14.6. (2.13)

Figure 2.13: Relation curve of the optical output power of the laser diode and the resistance of potentiometers

1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 30000

50

100

150

200

250

300

350

400

The resistance of a potentiometer (Ω)

The

optic

al o

utpu

t pow

er o

f the

lase

r dio

de (m

W)

Measured dataFitting curve

24

As the output current from the driver circuit is relatively high, its thermal stablity needs to

be investigated. During the measurement, a fan with 0.8 W power placed between the driver PCB

and the laser PCB is turned on. The thermo-electric cooler (TEC) inside the laser package is also

turned on. With this setup and an output power of 309 mW, the stability test is carried out for 400

minutes. The test result shown in Figure 2.14 indicates that the optical output power has a 2 mW

(0.65% of the average output power) fluctuation over the 400 minute test. The fluctuation can be

caused by combinated effects of the thermal noise in the CMOS circuit, the laser diode and the

power meter. There is no trend of consistent power increase or decrease. This stable power output

performace makes the laser suitable for various applications including laser pumping and material

processing.

Figure 2.14: Stability test of the optical output power over a 400 minute period.

0 50 100 150 200 250 300 350 400305

306

307

308

309

310

311

312

313

314

315

Test time (minutes)

The

optic

al o

utpu

t pow

er (m

W)

25

2.6 Summary

A CMOS laser diode driver circuit with off-chip potentiometers is designed for driving the

pump laser diode. The driver circuit has compact size and a tunable output up to 600 mA. Using

this driver circuit, the pump laser is able to provide a stable output optical power up to 350 mW.

A detailed performance summary of this pump laser diode is given in Table 2.2. The laser driven

by the designed CMOS circuit is suitable to be used as a pump laser in erbium-doped fiber

amplifier systems. The advantages lie in its compact size, low cost and potential integration with

other CMOS-compatible platforms.

Table 2.2: Performance summary of the designed CW pumped laser diode

Parameter Value

Laser Diode Optical Output Power up to 350 mW

Driver Current Output up to 600 mA

Supply Voltage 3.3 V

Centre Wavelength 975 nm

Technology 0.35 µm CMOS process

CMOS Die Size 4 mm · 4 mm

26

Chapter 3: Picosecond Pulsed CMOS Laser Diode Driver


Picosecond pulsed laser source have application areas such as optical communications,

biomedical imaging [33] and supercontinuum generation (SCG). Gain-switching laser diodes with

short injection current pulses offers a compact, cost effective and power-efficient approach to

generate picosecond optical pulses [34]. Gain-switched distributed feedback (DFB) laser diodes

have been used as seed lasers for master oscillator power amplifiers in SCG [35]. DFB laser diodes

are a type of narrow spectral width laser diodes, which are widely used in optical communication

systems as transmitters. The combination of low-power gain-switched laser diodes and fiber

amplifier systems creates an attractive technological approach to the development of low-cost,

robust, and compact high-power short-pulse optical sources [36].

The objectives of the research described in this chapter are to design a picosecond pulsed

CMOS laser diode driver for gain-switching a DFB laser diode and to establish a compact

picosecond pulsed laser source as the seed laser of a SCG system.

3.2 Gain-switched Laser Diodes

Short optical pulses with pulse widths in the picosecond range can be conveniently generated

by directly driving laser diodes with large amplitude and fast speed current pulses. This technique

is known as gain-switching laser diodes, as the optical gain of laser diodes is modulated by

switching the driving current. A typical gain-switching cycle is shown in Figure 3.1.

27

Lasing threshold

Switching current pulse

Electron density Photon density

t

Figure 3.1: Evolution of the photon and carrier density during a gain-switching cycle [37]

The laser diode is biased below the lasing threshold. When the switching current pulse is

first injected into the laser diode, there is no stimulated emission due to the low initial value of the

photon density. The electron density increases rapidly with a constant injected current. Once the

electron density is above the lasing threshold, the stimulated emission occurs and the laser diode

emits light. With a significant electron compulsion by the stimulated emission, the electron density

decreases to the level below the lasing threshold in a short period. Thus, a short laser pulse is

generated during a gain-switching cycle. As there is no laser light emission at the beginning or the

end of the current pulse, the laser pulse has a shorter pulse width than the current pulse.

Gain-switched laser diodes are attractive lasers for generating picosecond laser pulses

because they are simple, compact and stable [37]–[39]. Compared to gain-switched laser diodes,

picosecond Q-switched lasers and picosecond mode-locked lasers are more expensive, more

complex and less robust. Moreover, the pulse width and pulse repetition rate of optical pulses

generated by gain-switched laser diodes can be easily tuned in a wide range. As the output pulse

width and repetition rate are controlled by the electronic driver instead of the laser resonator setup,

it is simple and compact to achieve a tunable output feature with gain-switched laser diodes.

28

3.3 Pulsed CMOS Driver Circuit Design Methodology

CMOS analog logic circuits are implemented to design the electronic driver for gain-

switching laser diodes at picosecond pulse levels. A block diagram of the proposed CMOS laser

driver circuit is shown in Figure 3.2. The design methodology for generating pulse waveforms is

shown in Figure 3.3. This design implements a logic-based pulse generation method with CMOS

technology. There are four sub-circuits: a voltage-controlled ring oscillator (VCRO), a voltage-

controlled delay line (VCDL), an exclusive–OR (XOR) circuit, and a current source circuit. The

VCRO generates a periodic square wave signal, which determines the repetition rate of the output

pulses. The VCDL sets the delay of the square wave signal at a certain time period, which

determines the pulse width. Electrical pulses are created by the XOR circuit when the delayed

signal is XOR-ed with the original square wave signal. A CMOS current source at the output stage

converts voltage output pulses to current output pulses. The peak current of the output pulses

depends on the size of the CMOS transistor and the supply voltage.

VDD

VCRO Buffer

VCDL

XOR Buffer

NMOS

Laser Diode

VSS_33

Figure 3.2: Block diagram of the proposed pulsed laser driver circuit design

29

Square wave

Delay square wave

XOR

Pulse wave

Figure 3.3: Design methodology for generating pulse waves

3.3.1 Voltage-Controlled Ring Oscillator

The two main categories of CMOS voltage-controlled oscillators are the ring oscillator and

the LC oscillator. Compared to the CMOS LC oscillator, the CMOS ring oscillator has the

advantages of small design area, wide tuning range and ease of integration, which are preferred in

this design.

A ring oscillator requires the connection of an odd number of inverters and feedback from

the output of the last inverter to the input of the first inverter [40]. The proposed VCRO circuit, as

shown in Figure 3.4, implements the varactor-tuned technique and current-starved inverters. A

CMOS varactor is a reverse-biased diode whose capacitance is controlled by the applied reverse

voltage. The varactor-tuned technique means tuning varactors to control the oscillation frequency.

Current-starved inverters are standard inverters with an additional transistor, which is used to

control the current charging the load capacitor. Using current-starved inverters can also increase

the tuning range of the oscillation frequency.

30

C1

M3

M2

M1

Vvar

VoutVin

Vctr

VDD

VSS

Figure 3.4: Circuit design schematic of the VCRO

The oscillation frequency of the ring oscillators is given by

𝑓𝑓𝑜𝑜𝑜𝑜𝑜𝑜 = 12𝑁𝑁𝑡𝑡𝑑𝑑

(3.1)

where N is the number of delay stages and td is the delay time of each stage. The delay time of

each stage is given by [41]

𝑡𝑡𝑑𝑑 = 𝑉𝑉𝑜𝑜𝑜𝑜𝑜𝑜𝐶𝐶𝐿𝐿𝐼𝐼𝑜𝑜𝑐𝑐𝑐𝑐

(3.2)

where Vosc is the voltage amplitude of oscillation signals, CL is the load capacitance of each delay

stage, and Ictrl is the control current. By subtracting Equation (3.2) into Equation (3.3), the

oscillation frequency can be rewritten as

𝑓𝑓𝑜𝑜𝑜𝑜𝑜𝑜 = 𝐼𝐼𝑜𝑜𝑐𝑐𝑐𝑐2𝑁𝑁𝑉𝑉𝑜𝑜𝑜𝑜𝑜𝑜𝐶𝐶𝐿𝐿

. (3.3)

In the proposed design the VCRO has five delay cells in the loop. Each cell consists of a

basic cascaded inverter pair with a varactor for controlling the load capacitance and an additional

n-channel metal-oxide-semiconductor (NMOS) transistor for controlling current passing through

inverters. The delay of each inverter is controlled by a current control voltage Vctr and a varactor

31

control voltage Vvar. Introducing these two voltage control elements in the design enables a wide

tuning range of the oscillation frequency.

3.3.2 Voltage-Controlled Delay Line

Transmission gate based delay lines have high power efficiency and full swing output.

However these designs suffer from the tuning range because the delay changes quadratically with

the number of cascaded transmission gates. Delay lines based on current-balanced logic (CBL)

feature low switching noise and high output voltage swing [42].

The proposed VCDL schematic based on CBL is shown in Figure 3.5. The control voltage

Vb sets the same amount of charging current through transistors M1 and M2, thus the current

balancing is ensured. The delay at the rising edge and the delay at the falling edge are identical

since transistors at first stage (M1, M2) are matched with transistors at the second stage (M3, M4)

and the load capacitance at node Vd2 is the same as at node Vdout.

M4M2

M1

Vdin

VDD

M3

Vvar2

Vdout

Vb

C1 C2

Vd2

VSS

Figure 3.5: Schematic of the VCDL circuit

32

The timing diagram of the VCDL is shown in Figure 3.6. When the rising edge of Vdin is

applied, the node Vd2 is first pulled down by discharging through transistor M2. Then, PMOS M3

at the second stage is turned on and pulls up the node Vdout by charging the varactor C2 and the

load capacitance. When a falling edge transition of Vdin is applied, the node Vd2 is first pulled up

by charging the varactor C1 and the load capacitance. Then NMOS M4 at the second stage is turned

on and pulls down the node Vdout.

Rising and falling edge delays are controlled by the current control voltage Vb and varactors’

control voltage Vvar2. This VCDL has wide delay tuning range with respect to control voltages.

t

Vdin

Vd2

Vdout

VDD

VCC

Figure 3.6: Timing sequence diagram of the VCDL operation

33

3.3.3 Exclusive-OR Circuit

The design of an XOR circuit is shown in the Figure 3.7. This conventional but effective

circuit can operate with a full output voltage swing [43], which is critical to have a high peak

current output. The upper part of this circuit is a complementary pull-up PMOS network while the

lower part consists of pull-down NMOS networks. Only when two inputs V1 and V2 differ, the

output voltage Vxor will be pulled up to VDD.

V1

VDD

V2

VDD

VDD

V2

V1

V2

V1

Vxor

V1

V1

V2

V2

V2

V1

VSS VSS

Figure 3.7: Design schematic of the XOR circuit

34

3.3.4 Current Source Circuit

NMOS-based current source circuits have been approved as good candidates for being

implemented in laser driver circuits [44]. This design applies a single NMOS based current source

at the output stage. Using only one NMOS at this stage maximizes the drive current swing and

accommodates the required forward voltage.

The current source circuit is connected with the cathode of a DFB laser diode (FUJITSU

FLD5F6CX-J). The anode of the DFB laser diode is connected to the laser package metal case. As

the package metal case is usually grounded for low-noise output performance, the anode of this

DFB laser diode is earth-grounded. The positive terminal of the power supply, which is connected

to VDD in the Figure 3.2, is at zero volts with respect to ground. Accordingly, a negative voltage

from the negative terminal is applied to VSS_33. This configuration shown in Figure 3.8 is often

called negative power supply operation of laser diodes.

I

VDD

VSS_33

Figure 3.8: Negative power supply operation of the laser diode

35

3.4 Circuit Design and Simulations

In this design, the 0.13 µm CMOS process with thick oxide transistors is selected with

concerns about the performance, cost, and design requirements. It is a low-cost mature process

with advanced features and high performance. Thick oxide transistors that allow a maximum of

3.6 V voltage supply are utilized in the design. Using these special transistors at the output stage

can increase maximum supply voltage, so that the output voltage can be increased to meet the

required forward voltage of the laser diode. Once the process is specified, a circuit schematic can

be created based on the methodology. Device parameters in the design are usually determined

based on design requirements and process specifications.

To conduct simulations of the CMOS driver circuit in Cadence Design System (CDS), a

circuit model of the DFB laser diode is established as a load circuit. According to the information

from the DFB laser diode datasheet, the voltage versus current characteristic is linear and the input

impedance Rin is matched to 25 Ω. So the relation between the forward voltage Vf and the forward

current If is: Vf = If · Rin + Vth. Based on the test data of the forward voltage (1.6 V) and forward

current (30 mA), the threshold voltage is 0.85 V. Therefore, a simple equivalent circuit model of

this DFB laser diode is derived as a 0.85 V reverse voltage in series with 25 Ω matched impedance

with rise time and fall time of 100 ps at 2.5 Gb/s modulation rate.

In order to withstand the high voltage swing for the laser (above 1.6 V), the 3.3V I/O

transistor with thick oxide (nfet33) is utilized as the NMOS current source. This nfet33 transistor

has a width to length ration of 800. All other transistors are thick oxide transistors (dgnfet/ dgpfet)

with a breakdown of 2.7 V. This ensures a desired high current output. All VDDs in the schematics

are connected to the ground. The negative supply voltage is -3.3 V for the VSS_33 of the NMOS

current source and -2.5 V for the VSS in all other blocks. Control voltages (Vctr, Vvar, Vb, Vvar2)

36

are tuned to obtain pulse train outputs with picosecond pulse widths at repetition rates of several

megahertz.

The critical characteristic of the driver circuit performance is the output current to the load

circuit, because the laser output power is dependent on the injection current. In simulations, the

output current from the proposed driver circuit is plotted. As this driver circuit is designed for the

seed laser in SCG, driving current pulses with a high peak current, a short pulse width and a low

duty cycle are aims of this design. The best simulation performance waveform indicates that the

pulse full width at half maximum (FWHM), the peak current and the repetition rate of drive current

pulses are 200 ps, 80 mA, and 5.8 MHz respectively. The output current waveform from the post-

layout transient simulation is shown in Figure 3.9 (a) and a sample current pulse is shown in

Figure 3.9 (b).

Figure 3.9: Post-layout transient simulation of the proposed CMOS laser driver: (a) the output current waveform; (b) a sample current pulse.

0 200 400 600 800 1000-10

0

10

20

30

40

50

60

70

80

90

Time (ns) (a)

Out

put C

urre

nt (m

A)

36.1 36.2 36.3 36.4 36.5-10

0

10

20

30

40

50

60

70

80

90

Time (ns) (b)

Out

put C

urre

nt (m

A)

37

The repetition rate and the pulse FWHM of output current pulses can be tuned by varying

the control voltages. The repetition rate is determined by the oscillator frequency of the VCRO.

Tuning the repetition rate has a negligible impact on the pulse width. Simulation results of tuning

the repetition rate are shown in Figure 3.10 (a). It is tuned from 5.8 MHz to 45.9 MHz by changing

the control voltages Vctr and Vvar. Tuning the pulse width is accomplished by changing control

voltages Vb and Vvar2 in the VCDL. The plot of simulation results is shown in Figure 3.10 (b). The

output with 200 ps pulse width and 5.8 MHz repetition rate is achieved when Vctr, Vvar, Vb and

Vvar2 are -1.5 V, -2.5 V, -2.5 V and -2.1 V respectively.

Figure 3.10: Output plots when tuning (a) repetition rate and (b) pulse width in simulations

-2.5 -2 -1.5 -1 -0.5 05

10

15

20

25

30

35

40

45

50

Control Voltage Vvar (V)

Rep

etiti

on R

ate

(MH

z)

Vctr=0 V

Vctr=-1.5 V

-2.5 -2 -1.5 -1200

400

600

800

1000

1200

1400

Control Voltage Vb (V)

Pul

ses

FWH

M (p

s)

Vvar2=-2.1 V

38

3.5 Measurements and Analysis

3.5.1 Measurement Setup

This laser driver circuit is fabricated with the 0.13 µm CMOS process. A die micrograph

shown in Figure 3.11(a) has a 0.3 mm2 CMOS chip area. This CMOS die is packaged in a ceramic

flat package (CFP) and interconnected with a 4 GHz DFB laser diode on a PCB as shown in

Figure 3.11(b). The size of this laser source PCB is 28 cm2. Since the gain-switching frequency is

in the RF frequency range, the connection between the driver output and the laser diode cathode

is designed as short as possible (less than 5 mm) to reduce the transmission line effect. Four

potentiometers are used on PCB to set control voltages Vctr, Vvar, Vb and Vvar2.

The VDD is connected to the earth ground. VSS is set as a -2.5 V power supply. In the

measurement, when VSS_33 is set to -3.3 V, the output power is quite low. In order to get the

desired power, VSS_33 is set to -3.65 V for all measurements.

W=

0.3

mm

(a) (b)

L=1 mm

W=

39 m

m

L= 69 mm

CMOSVb

Vctr Vvar VSS Vvar2 VSS_33

Iout

VDD

DFB

Vb

VctrVvar

Vvar2

VSS_33VDD

VCROVCDL

XOR NMOS

Laser Driver

VSS

Figure 3.11: (a) Die micrograph and (b) PCB layout of the pulsed laser source

39

3.5.2 Measured Results

A temporal waveform of this output light is captured by a 5 GHz bandwidth photodiode

(Thorlabs DET08CFC) and displayed on an oscilloscope with 8 GHz bandwidth and 25 GS/s

sample rate (Tektronix DSA70804B). The control voltages Vctr, Vvar, Vb and Vvar2 are set to be

-1.5 V, -2.5 V, -2.5 V and -2.1 V respectively. A part of this periodic waveform is shown as Figure

3.12(a). The repetition rate of this waveform is 5.6 MHz with a standard deviation of 2.8 kHz at a

count of 215 pulses. Figure 3.12 (b) shows a measured laser pulse with Gaussian fitting. The

Gaussian fitting model 𝑓𝑓𝑔𝑔𝑔𝑔𝑔𝑔 and the FWHM of a Gaussian pulse 𝑡𝑡𝐹𝐹𝐹𝐹𝐹𝐹𝑀𝑀 are expressed as

𝑓𝑓𝑔𝑔𝑔𝑔𝑔𝑔(𝑜𝑜) = 𝑎𝑎1𝑒𝑒

−(𝑜𝑜−𝑏𝑏1𝑜𝑜1)2

𝑡𝑡𝐹𝐹𝐹𝐹𝐹𝐹𝑀𝑀 = 2√2 𝐶𝐶1 , (3.4)

where amplitude a1, mean b1 and variance c1 are coefficients of the Gaussian fitting model. The

calculated average FWHM is 200 ps with a standard deviation of 25 ps at a count of 215 pulses.

Figure 3.12: (a) Laser pulse waveform and (b) one Gaussian fitted laser pulse

1.8 1.9 2 2.1 2.2 2.3 2.4-0.1

-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

Time(µs)

Mea

sure

d V

olta

ge(V

)

1988.8 1989 1989.2 1989.4 1989.6 1989.8-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

Time(ns)

Mea

sure

d V

olta

ge(V

)

Measured PointsGaussian Fitting

(a) (b)

40

The laser output spectrum is recorded by an optical spectrum analyzer (OSA, YOKOGAWA

AQ6375), which is shown in Figure 3.13. The output average power is 7.0 µW. The peak power

and the pulse energy of Gaussian pulses can be calculated by [45]

where Ppeak is the peak power, Pavg is the average power, tFWHM is the FWHM of the pulse, fRR is

the repetition rate, and Ep is the pulse energy. Based on measured results, the pulse peak power is

5.9 mW and the pulse energy is 1.25 pJ. The center wavelength of its output light is 1548.1 nm.

The spectral width is 0.24 nm at 20 dB root mean square (RMS) level.

Figure 3.13: Optical spectrum of the pulsed laser diode output

1540 1542 1544 1546 1548 1550 1552 1554 1556 1558 1560-70

-65

-60

-55

-50

-45

-40

-35

-30

-25

Wavelength (nm)

Spe

ctra

l int

ensi

ty (d

Bm

)

𝑃𝑃𝑝𝑝𝑝𝑝𝑔𝑔𝑝𝑝 = 2𝑙𝑙𝑛𝑛2

𝜋𝜋𝑃𝑃𝑎𝑎𝑎𝑎𝑎𝑎

𝑡𝑡𝑅𝑅𝐹𝐹𝐹𝐹𝐹𝐹𝑓𝑓𝑅𝑅𝑅𝑅

𝐸𝐸𝑝𝑝 = 𝑃𝑃𝑎𝑎𝑎𝑎𝑎𝑎𝑓𝑓𝑅𝑅𝑅𝑅

, (3.5)

41

3.5.3 Measurements of Tunable Output

Simulation results show that the repetition rate and the pulse FWHM of the driver circuit’s

output current pulses can be tuned by adjusting control voltages. In measurement, the control

voltages Vctr, Vvar, Vb and Vvar2 can be adjusted by adjusting the potentiometers on the PCB. The

repetition rate of optical output pulses is related with the control voltages Vctr and Vvar, while the

pulse width is related with control voltages Vb and Vvar2. Figure 3.14 shows measured results of

tuning the optical output pulses’ repetition rate by adjusting the control voltage Vvar and Vctr. When

the Vvar is adjusted from -2.5 V to -0.5 V, the output pulses’ repetition rate is increased from

5.6 MHz to 10.4 MHz, as shown in Figure 3.14 (a). This measured result agrees with the simulation

result. When the Vctr is adjusted from -1.5 V to 0.9 V, the repetition rate is increased from 5.6 MHz

to 12.6 MHz, as shown in Figure 3.14 (b). However, when the Vctr is above -0.9 V, the amplitude

of the optical pulses starts decreasing. The maximum repetition rate with a stable output

performance is 13.2 MHz, when the Vctr is -0.9 V and Vvar is -2.1 V. Thus, the tuning range of the

repetition rate is from 5.6 MHz to 13.2 MHz.

Figure 3.15 shows measured results of tuning the pulse width of optical output pulses by

adjusting control voltages Vb and Vvar2. The minimum pulse width occurs when Vvar2 equals 2.1 V

and Vb equals 2.5 V. The tuning range of the pulse width is not wide in measurement. When the

pulse width is increased to above 350 ps, ultrashort pulse trains appear next to the main pulse, and

the output characteristics become unclear.

42

Figure 3.14: Measured results of tuning the optical output pulses’ repetition rate (a) by adjusting control voltage Vvar and (b) by adjusting control voltage Vctr

Figure 3.15: Measured results of tuning the optical output pulses’ pulse width (a) by adjusting control voltage Vvar2 and (b) by adjusting control voltage Vb

-2.5 -2 -1.5 -1 -0.55.5

6

6.5

7

7.5

8

8.5

9

9.5

10

10.5

Control Voltage Vvar (V)

Rep

etitio

n R

ate

(MH

z)

-1.6 -1.4 -1.2 -1 -0.85

6

7

8

9

10

11

12

13

Control Voltage Vctr (V)R

epet

ition

Rat

e (M

Hz)

(a) (b)

-2.5 -2 -1.5180

200

220

240

260

280

300

320

Control Voltage Vvar2 (V)

Pul

se W

idth

FW

HM

(ps)

-2.6 -2.4 -2.2 -2 -1.8200

205

210

215

220

225

230

235

Control Voltage Vb (V)

Pul

se W

idth

FW

HM

(ps)

(a) (b)

43

3.5.4 Analysis

There are certain differences between measurement and simulation in this design. First of

all, the supply voltage (VSS_33) in the measurement setup is higher than in the simulation.

Reasons for this difference could be that the laser diode equivalent circuit model is not accurate

enough or circuit connections in the measurement suffer the power transmission/reflection loss.

Although the supply voltage (3.65 V) is higher than the thick oxide transistor’s breakdown voltage

(3.6 V), it does not cause breakdown in the transistor due to the laser diode load circuit that is

connected with the transistor in series. The laser diode load circuit acts like a voltage dividing

circuit and also limits the in-rush current to the transistor when being powered on.

Secondly, the measured laser pulse width is very close to the simulation current pulse width.

Theoretically, the measured laser pulse width should be smaller than the simulated current pulse

width due to the gain-switching mechanism. However, as the parasitic capacitance from the die

package, the PCB and the laser diode causes a delay on the pulse rise time and fall time, the

switching current pulse width gets larger than the simulated one. A combination of these two

effects makes the measured pulse width close to the simulation current pulse width.

44

3.6 Summary

This work presents a compact picosecond pulsed laser source by gain-switching a DFB laser

diode with a CMOS integrated circuit. The CMOS circuit has a 0.3 mm2 die area. A commercial

DFB laser diode is assembled with the CMOS driver circuit and measurements on the laser light

output have been conducted. A performance summary of this laser source is given in Table 3.1.

This compact picosecond pulsed laser source can be used in SCG as the seed laser source.

Table 3.1: Performance summary of the designed pulsed laser diode

Parameter Value Parameter Value

Pulse Width 200 ps (Minimum) Pulse Energy 1.25 pJ

Repetition Rate 5.6 MHz (Minimum) Duty cycle 1:893

Average Power 7.0 µW Center Wavelength 1548.1 nm

Peak Power 5.9 mW Size (L · W) 7 cm · 4 cm

45

Chapter 4: Supercontinuum Generation in a Highly Nonlinear Fiber Using CMOS Laser Diode Drivers


In general, s supercontinuum generation (SCG) system consist of two parts: a laser and a

nonlinear medium. The method of SCG in a highly nonlinear fiber (HNLF) employed in this

research is shown in Figure 4.1. It consists of a gain-switched laser diode, a fiber amplifier and an

HNLF. The combination of a low power pulsed laser diode and a fiber amplifier is considered as

a simple, efficient and cost-effective solution to establish a high power pulsed laser for SCG.

CMOS drivers designed in Chapters 2 and 3 can be used to reduce the system complexity,

size and cost. The picosecond pulsed distributed feedback (DFB) laser diode using the CMOS

driver designed in Chapter 3 can be applied as a seed pulsed laser in SCG. As the peak power of

optical pulses from the laser diode is not high enough to efficiently induce the nonlinear effects in

the HNLF, an erbium-doped fiber amplifier (EDFA) is employed to boost the peak power. The

pump laser diode with the high current continuous-wave (CW) CMOS driver described in

Chapter 2 can be utilized to pump an erbium-doped fiber in the EDFA. The amplified optical

pulses will be launched into the HNLF for SCG.

The objectives of the research described in this chapter are to design a SCG system using

CMOS laser diode drivers and to compare the performance with a reference SCG system using a

commercially-available nanosecond seed laser module.

Figure 4.1: Supercontinuum generation system design block diagram

HNLF

Gain-switched Laser Diode Fiber Amplifier

Supercontinuum Output

46

4.2 Erbium-Doped Fiber Amplifier

EDFAs are widely used in the optical fiber communications systems, as they can efficiently

amplify light in the 1.55 µm wavelength region (the conventional band), which has the lowest fiber

attenuation. EDFAs have been employed in the design of SCG for pulse power amplification [17].

The erbium-doped fiber is the core technology of EDFAs. The mechanism of light

amplification in EDFAs is illustrated based on an energy-level diagram of erbium ions, as shown

in Figure 4.2 [46]. The 980 nm pump laser light excites the erbium ions to a short-lifetime excited

state 4I11/2 where they make a fast and non-radiative decay to a long-lifetime metastable state 4I13/2.

From there erbium ions undergo spontaneous emission and stimulated emission to the lower-

energy 4I15/2 state. The input light with a wavelength of 1550 nm is amplified by the stimulated

emission, while noise light with wavelengths spanning range from 1520 nm to 1570 nm is

amplified by spontaneous emission.

4I15/2 : low energy state

Fast Decay

980 nm Pump

~1550 nm Input

4I13/2 : metastable state

4I11/2 : short-lifetime state

~1550 nm Amplified output

Stimulated emission1520 nm – 1570 nmspontaneous emission

Figure 4.2: EDFA amplification mechanism based on an energy-level diagram of erbium ions

47

The EDFA employed in this SCG system is a commercial module from Opeak

(EDFA131175). A block diagram of the EDFA module setup is shown in Figure 4.3. The core is

a single mode erbium-doped optical fiber. The fiber is pumped by light from two laser diodes

through couplers. The pumping light is input in two opposite directions such that one travels in the

forward direction and the other one travels in the backward direction with respect to the input. This

bidirectional configuration has a better noise figure, compared to the unidirectional configuration

[47]. The optical isolators are incorporated at the input and the output of this EDFA to prevent the

amplification of back reflected light [48]. The pump light from laser diodes has a wavelength

around 980 nm. Laser drivers are used to control the laser diode output.

Figure 4.3: Block diagram of the EDFA module setup

Apart from the EDFA, the commercial module also contains photodiodes and an optical

splitter for monitoring the optical power. The monitor output from the optical splitter is calibrated

as 1:10000 power ratio to the EDFA output. By measuring the monitor output, the characteristics

of the EDFA output can be studied without direct measurement. Using this approach, one can

avoid damage on equipment caused by the high peak power output from the EDFA.

Pump Laser Diode

CW Laser Driver

Erbium DopedFiber

Input Output

Pump Laser Diode

CW Laser Driver

IsolatorCouplerIsolator Coupler

Photodiode

Splitter

Monitor

Photodiode

48

The pulsed laser driven by the CMOS circuit designed in Chapter 3 is connected to the EDFA

through fiber connectors. After the input laser light is amplified, the monitor output light is

measured by an optical spectrum analyzer (OSA, Yokogawa AQ6375). The output spectrum is

shown in Figure 4.4. The average power is measured to be 6.8 µW. Since this power has a 1:10000

ratio to the EDFA output power, the amplified output power from the EDFA is expected to be

68 mW. The measured average power of the input light is 7.0 µW shown in Figure 3.12, so the

amplification gain of this EDFA is 40 dB, which matches the maximum gain listed on the device

specification sheet. The spectral width gets broadened from 1 nm to 40 nm at the -60 dBm level.

This is expected due to the amplified spontaneous emission (ASE) noise [46].

Figure 4.4: Spectrum of the output light from the EDFA monitor output

1520 1530 1540 1550 1560 1570 1580-70

-65

-60

-55

-50

-45

-40

-35

-30

-25

Wavelength (nm)

Spe

ctra

l int

ensi

ty (d

Bm

)

49

4.3 Highly Nonlinear Fiber

The HNLF has a small core and high refractive index difference between the core and the

cladding. It combines a high nonlinear coefficient with small group velocity dispersion around the

desired pumping wavelength. When high peak power laser pulses propagate through this fiber,

many nonlinear effects occur and lead to the broadening of the spectrum of the laser pulse [49].

The HNLF is a suitable nonlinear medium for fiber-based SCG. Compared to standard

dispersion-shifted fibers, the HNLF exhibits both a higher nonlinearity (5 times higher) and a lower

dispersion slope that are main factors for broadband and flat SCG [50]. Besides, the HNLF has an

advantage of a low coupling loss (typical 0.1 dB) with standard single-mode fibers (SMF) and ease

of processing, compared to popular photonic crystal fibers and tapered fibers. The detailed optical

properties of the HNLF (OFS Standard HNLF) used in this SCG system are shown in Table 4.1.

Table 4.1: Optical properties of the HNLF

Parameter Value

Dispersion @ 1550 nm -0.01 ps/(nm·km)

Dispersion Slope @ 1550 nm 0.018 ps/(nm2·km)

Zero Dispersion Wavelength 1549.5 nm

Nonlinear coefficient 11.4 (W·km)-1

Fiber Attenuation @ 1550 nm 0.72 dB/km

Total Insertion Loss @ 1550 nm 0.31 dB

Splice Loss to Standard SMF Pigtail (Typical) 0.1 dB

Effective Area (Typical) 11.6 µm²

50

4.4 Nonlinear Optical Effects

SCG in HNLFs involves the interplay between nonlinear and linear effects that occur during

the propagation of picosecond optical pulses in the fiber [7]. The major linear effect that plays a

crucial role in influencing the interactions between various nonlinear effects is chromatic

dispersion. The dominant nonlinear effects behind the process of the SCG in HNLFs with

picosecond laser pulses are expected to bet stimulated Raman scattering (SRS), four-wave mixing

(FWM) and modulation instability (MI) [51]. The following sections briefly introduce these effects

in the context of SCG.

4.4.1 Chromatic Dispersion

Chromatic dispersion is the effect that causes propagating light pulses to spread in time

domain. Chromatic dispersion arises from the frequency dependence of the refractive index of the

fiber. In general, when optical light is propagating in the fiber, the response of the fiber depends

on the optical frequency of the light. This characteristic is crucial to the performance of ultra-short

pulses with a wide wavelength range propagating in fibers. Ultra-short pulses consist of a broad

spectrum of frequencies that all experience a different refractive index in a fiber. After a certain

propagation distance, ultra-short pulses get broadened in the time domain. In other words, the

temporal pulse width of ultra-short pulses is increased. The peak power of the pulse is reduced and

thus the nonlinearity is reduced. As both the pulse peak power and the nonlinearity are important

factors for SCG, it is necessary to reduce the impact of chromatic dispersion. In practical, HNLFs

with a small dispersion slope are preferred and the wavelength of ultrashort pulses launched into

HNLFs is usually chosen to be close to the zero dispersion wavelength (ZDW) of HNLFs.

51

4.4.2 Stimulated Raman Scattering

Raman scattering is an interaction between photons of optical pulses and the silica molecules

of fibers. During the interaction, photons transfer a small fraction of energy to molecules as a result

of molecular vibrational motion and low-frequency photons are scattered. These new waves

formed by generated low-frequency photons are called Stoke waves [52]. Spontaneous Raman

scattering is a weak effect. Highly efficient Raman scattering can occur under the influence of a

high intensity pump laser beam, as the Raman gain of the pump laser beam amplifies the low-

frequency photons. This effect is called SRS. The overall nonlinear consequence of SRS is that

the spectrum of optical pulses extends to long wavelength side.

4.4.3 Four-Wave Mixing

FWM is also created by the nonlinear response of the fiber medium. However, the fiber

medium only plays a passive role in FWM, which is different with SRS. When three optical waves

with three different frequencies ω1, ω2 and ω3 interact nonlinearly inside the fiber medium, each

wave sets up a refractive index modulation of the material. If any two waves or three waves are

propagating together with phase matching, the new wave will be generated with a new frequency

given by ω4 = ω1 + ω2 + ω3. The net total energy and momentum should be conserved during the

creation of new frequencies.

4.4.4 Modulation Instability

MI includes the nonlinear and dispersive effects causing a nonlinear system to exhibit an

instability, which results in the breakup of the optical pulse envelope into a series of ultra-short

pulses and the generation of new wavelength side bands [52]. When a pump wave propagates in

a fiber and experiences anomalous dispersion, the nonlinear system including the pump wave and

the nonlinear medium becomes unstable. The main requirement is the anomalous dispersion,

52

which occurs when the wavelength of the pump wave is greater than the zero dispersion

wavelength of the nonlinear medium.

The MI effect produces a symmetric gain spectrum around the center wavelength of the

pump wave. When noise within the gain spectrum is amplified, two sidebands are formed and are

symmetric about the center wavelength. The frequency shift of the first-order MI gain spectral

peak is given by Δω = (2·γ·Pp/|β2|)1/2 [52], where γ is the nonlinear coefficient, Pp is the peak power

of the pump and β2 is the group velocity dispersion parameter of the fiber at the pump wavelength.

The wavelength shift on the spectrum can be further calculated based on the frequency shift

equation. By comparing the calculated wavelength shift to the measured spectrum shape, one can

identify whether the MI is one of the nonlinear effects, which contributes to the SCG.

In the context of SCG, FWM/MI processes dominate the broadening mechanisms in the case

of input pulse width within picosecond-nanosecond range [7]. Efficient spectral broadening

requires the pumping wavelength to be at anomalous dispersion region and close to the fiber ZDW.

The anomalous dispersion induces the MI effect to occur.

4.5 Supercontinuum Generation

4.5.1 Experimental Setup

An experimental setup diagram of the SCG system is shown in Figure 4.5. The system is

composed of a CMOS driver circuit, a DFB laser diode, an EDFA, a 1-meter-long SMF, and a 10-

meter-long HNLF. Fiber connectors (FC) are utilized to connect the DFB laser diode with the

EDFA. Fusion splicing (FS) of optical fibers is also used to make fiber connections in the setup.

The 1-meter-long SMF is placed between the EDFA and the HNLF in order to reduce the fiber

coupling loss. The DFB laser diode is gain-switched by the CMOS driver. The laser output is

optical pulses with a pulse width of 200 ps, a repetition rate of 5.6 MHz, a pulse energy of 1.25 pJ

53

and a peak power of 5.9 mW. This output is amplified by the EDFA, in which an erbium-doped

fiber is pumped by two 980 nm laser diodes at the total 550 mA maximum injection current. The

EDFA monitor output spectrum is shown in Figure 4.4 and its output pulses have a 68 mW average

power. Optical characteristics of the amplified DFB laser diode pulses are listed in Table 4.2.

Table 4.2: Characteristics of the amplified laser diode pulses

The EDFA output is connected to a SMF, which is a bridge between the erbium-doped fiber

and the HNLF. The SCG occurs once the high energy laser light is launched into the HNLF. The

supercontinuum output is analyzed by the OSA with a spectral measurement wavelength range

from 1200 nm to 2400 nm, an InGaAs photodiode and an oscilloscope. The OSA is utilized to

measure the output spectrum and the output average power. The photodiode and the oscilloscope

are employed to record the pulse characteristics in the time domain.

Figure 4.5: Experimental setup diagram of the SCG system

10 mHNLF

DFB Laser Diode

CMOS Driver

EDFA

1 mSMF

FC FCFS FS

Monitor FCOutput

FS


Pulse Width 200 ps Pulse Energy 12.1 nJ

Repetition Rate 5.6 MHz Peak Power 57.3 W

Average Power 68 mW Center Wavelength 1548.1 nm

54

4.5.2 Measured Results and Discussions

Figure 4.6 shows the optical spectrum of the SCG system output recorded by the OSA with

0.5 nm scan resolution and 20000 sampling points. As the spectrum noise floor of the OSA is

around -66 dBm, the broadband spectrum of supercontinuum covers the whole spectral range of

the OSA when measured from the noise level. The spectral bandwidth of the supercontinuum light

is 806 nm, spanning from 1298 nm to 2104 nm at the -40 dB level. It has a flat spectrum of 10 dB

across the wavelength range from 1298 nm to 1517 nm and 1576 nm to 2104 nm. The spectral

peak around 1548 nm is due to the seed laser pumping. The average power of this supercontinuum

light is 62 mW, achieving a 91.2% power conversion efficiency from the injection pump power to

the recorded supercontinuum power. The power loss is due to fiber connection loss and fiber

attenuation.

Figure 4.6: Spectrum of supercontinuum output

1200 1400 1600 1800 2000 2200 2400-60

-50

-40

-30

-20

-10

0

10

20

Wavelength (nm)

Spe

ctra

l int

ensi

ty (d

Bm

)

10 dB

BW=806 nm

55

For picosecond pulses, and when pumping close to the zero dispersion wavelength region of

the HNLF, the SCG is initiated by the MI and the FWM [51], [7], [21]. The MI is observed in the

spectrum with the generation of modulation sidebands located around the pump wavelength. To

confirm that the supercontinuum is initiated by the MI, the spectrum broadening as a function of

the pump power is measured as shown in Figure 4.7. Spectral sidebands around the pump

wavelength are observed during the early evolution of the supercontinuum at low pump power.

When increasing the pump power, the peak power is correspondingly increased so that the

supercontinuum spectrum expands to both sides around the pump wavelength. The red shift is

caused by the SRS of solitons, which are created by the MI. The blue shift is due to a FWM

nonlinear process between solitons and dispersive waves that are also created by the MI [53]. The

red shift side expands faster than the blue shift side.

Figure 4.7: Supercontinuum output evolution at different pump power level

1200 1400 1600 1800 2000 2200 2400-80

-70

-60

-50

-40

-30

-20

-10

0

10

20

Wavelength (nm)

Spe

ctra

l int

ensi

ty (d

Bm

)

4.8 mW Supercontinuum11.6 mW Supercontinuum18.2 mW Supercontinuum31.0 mW Supercontinuum42.9 mW Supercontinuum53.9 mW Supercontinuum62.3 mW Supercontinuum

Pump Peak1st order Sideband

1st orderSideband

2nd orderSideband

2nd orderSideband

56

The attenuated (~40 dB attenuation) supercontinuum pulse is detected by a 5 GHz InGaAs

photodiode and recorded by an 8 GHz oscilloscope. The repetition rate of supercontinuum pulses

remains the same (5.6 MHz) as the pump pulses. A temporal pulse profile of the supercontinuum

pulses is shown in Figure 4.8 with a seed laser pulse and an EDFA monitor pulse. The pulse profile

corresponds to the full bandwidth of the supercontinuum spectrum incident on the photodiode.

Compared to the seed laser pulse and the EDFA monitor pulse, the pulse width of the

supercontinuum pulse is increased around 10 ps due to the dispersion in the HNLF. In general, the

wavelength shifting does not increase the input pulse duration substantially. In other words, the

supercontinuum generation process does not affect the temporal pulse shape.

Figure 4.8: Temporal profile of the seed laser pulse, the EDFA monitor pulse and the supercontinuum pulse

0 200 400 600 800 1000 1200 1400

0

0.2

0.4

0.6

0.8

1

Time (ps)

Nor

mal

ized

Vol

tage

(V)

Seed Laser PulseEDFA Monitor PulseSupercontinuum Pulse

57

4.5.3 System Performance Comparison

For comparison, a reference SCG system using a commercially available nanosecond seed

laser module produced by Opeak (DFB131101) is tested for SCG. The setup of the reference SCG

system is the same as with Figure 4.5, except that the reference SCG system uses a seed laser

module to replace the DFB laser diode and the CMOS driver circuit. The seed laser module

contains a 1550.8 nm DFB laser diode and an electronic control circuit. The module is able to

output optical pulses with 10 ns pulse width (shortest available) and 10 kHz repetition rate under

a 5 V power supply. The temporal waveform characteristics are shown in Figure 4.9. The spectrum

characteristics of the output pulses are shown in Figure 4.10 (a). The average output power of this

seed laser module is 1.1 µW. The performance is summarized in Table 4.3.

Table 4.3: Performance summary of the seed laser module


Pulse Width 10 ns Pulse Energy 110 pJ

Repetition Rate 10 kHz Duty Cycle 1:10000

Average Power 1.1 µW Center Wavelength 1550.8 nm

Peak Power 10.3 mW Size (L · W) 14 cm · 9 cm

58

Figure 4.9: Output optical pulses from the commercial seed laser module: the left figure shows the pulse waveform; the right figure shows a single pulse shape.

Figure 4.10: (a) Spectrum of optical pulses output from the seed laser module; (b) spectrum of the amplified pulses output from the monitor of EDFA

-100 0 100 200-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

Time (µs)

Volta

ge (V

)

99.99 100 100.01 100.02 100.03-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

Time (µs)

Volta

ge (V

)

1550 1550.5 1551 1551.5-70

-65

-60

-55

-50

-45

-40

-35

-30

Wavelength (nm)

Spec

tral in

tens

ity (d

Bm)

1520 1540 1560 1580-70

-65

-60

-55

-50

-45

-40

-35

-30

Wavelength (nm)

Spec

tral in

tens

ity (d

Bm)

59

Compared to the designed DFB laser diode with CMOS driver circuit, this seed laser module

has a lower repetition rate and much higher pulse energy. The designed DFB laser diode has the

advantage of much shorter pulse width over the commercial seed laser.

The seed laser pulses are amplified by the EDFA with the maximum pump power. The

spectrum of the monitor output from the EDFA is shown in Figure 4.10(b). As the average output

power of the monitor port is 5.9 µW, the pump pulses launched into the HNLF have an expected

average power of 59 mW, a pulse energy of 5.9 µJ, and a peak power of 554 W. Although these

pump pulses have almost the same average power as the designed DFB laser diode after

amplification, the peak power of this laser module is 9.7 times higher than the designed one. The

difference of the duty cycle between these two designs results in the peak power difference.

The spectrum at the HNLF output is measured by the OSA, shown as a red trace in

Figure 4.11. The average power of this supercontinuum is 51 mW, achieving 86.4% power

conversion efficiency. The spectrum bandwidth is 892 nm at –40 dB level, spanning from

1258 nm to 2150 nm. A 5 dB spectrum flatness covers almost the full bandwidth except the pump

wavelength region. The nonlinear mechanism leading to the SCG is similar with the picosecond

pulses pumping regime. The wavelength shifting is also initiated by the MI and enhanced by the

SRS and the FWM effects.

Compared to the reference SCG system, the SCG system using the designed DFB laser diode

has a similar supercontinuum spectrum but a high peak at the pump wavelength. The high peak

reflects that wavelength shifting is not efficient enough due to a low peak power of pump pulses.

60

Figure 4.11: SCG results from two different systems

4.6 Summary

A SCG system based on an HNLF pumped by amplified diode-laser pulses is demonstrated.

The designed DFB laser diode with the CMOS driver circuit is employed to provide diode-laser

pulses. The SCG achieves a flat supercontinuum spectrum with 806 nm bandwidth (-40 dB level)

at a low peak power and a 91.2% power conversion efficiency at 62 mW output power. For

comparison, the designed DFB laser diode is replaced by a commercially available seed laser

module in the SCG system. A comparison table between the designed SCG system and the

reference SCG system is shown in Table 4.4.

1200 1400 1600 1800 2000 2200 2400-60

-50

-40

-30

-20

-10

0

10

20

Wavelength (nm)

Spe

ctra

l int

ensi

ty (d

Bm

)

Supercontinuum with proposed DFB LaserSupercontinuum with seed laser module

51 mW 62 mW

61

This comparison shows that the designed DFB laser diode is a good candidate for the seed

laser in picosecond level SCG systems. Additionally, the designed DFB laser diode is attractive

due to reduced complexity, compact size and low cost compared to commercial seed laser modules.

Table 4.4: Comparison of the designed SCG system and the reference SCG system

Supercontinuum Parameter The Designed SCG System The Reference SCG System

Average Power 62 mW 51 mW

Spectrum Bandwidth (-40 dB) 806 nm 892 nm

Spectral Density 0.08 mW/nm 0.05 mW/nm

Pulse Duration ~200 ps ~ 10 ns

Repetition Rate 5.6 MHz 10 kHz

Pulse Energy 11.1 nJ 5.1 µJ

62

Chapter 5: Conclusions and Future Work

5.1 Conclusions

This thesis provides experimental evidence to support that CMOS laser diode drivers can be

used in supercontinuum generation (SCG) systems. SCG systems with CMOS laser diode drivers

have the advantage of compact size and low cost compared to the traditional fiber lasers and the

mode-locked lasers based SCG systems. In this thesis, two different design of CMOS laser diode

drivers are presented.

In Chapter 2, a high-current continuous-wave (CW) CMOS laser diode driver was designed

for driving pump lasers in SCG systems. Based on the design requirements, a circuit design

methodology was proposed. Circuit simulations were conducted in Cadence Design System (CDS)

to predict the driver’s performance. Experimental measurements on both the driver’s current

output and the pump laser’s optical output confirmed that the driver could provide up to 600 mA,

which corresponded to an optical output power up to 350 mW from a 975 nm laser diode. The

driver features tunable output, long-term stable operation and reduced sensitivity to power supply

voltage variations.

In Chapter 3, a picosecond pulsed laser diode driver was designed for gain-switching a

distributed feedback (DFB) laser diode. The driver design implemented CMOS analog logic

circuits to generate picosecond level current pulses. Simulations showed that the driver circuit can

provide current pulses with a pulse width of 200 ps, a repetition rate of 5.8 MHz, and a peak current

of 80 mA. Experimental measurements of the DFB laser diode output showed that the output

optical pulses had a pulse width of 200 ps at FWHM, a peak power of 5.9 mW and a repetition

rate of 5.6 MHz. The average output power of the pulsed DFB laser diode is 7.0 µW and the pulse

energy is 1.25 pJ. The driver design also features a tunable repetition rate output and a tunable

63

pulse width output. The size of the designed package, including the laser and the driver together,

is 7 cm length and 4 cm width. This compact picosecond pulsed laser diode is a suitable seed laser

for SCG systems.

Chapter 4 demonstrated an HNLF based SCG system with the designed picosecond DFB

laser diode. The supercontinuum was generated by using amplified picosecond optical pulses to

induce nonlinear effects in the HNLF. The system consists of the designed picosecond DFB laser

diode, a commercial EDFA module and the HNLF. The supercontinuum output of the designed

system has an average power of 62 mW, a spectral bandwidth of 806 nm, a pulse width of 210 ps

and a repetition rate of 5.6 MHz. For comparison, a reference SCG system using a nanosecond

pulsed commercial seed laser module was tested for supercontinuum generation. With the

commercial seed laser module, the supercontinuum output has a spectral bandwidth of 892 nm and

an average output power of 51 mW. Compared to this commercial module, the designed DFB laser

diode has the advantages of compacter size, lower cost, higher output power and shorter pulse

width. The designed SCG system has potential applications in OCT, spectroscopy and DWDM.

5.2 Future Work

Future work will be focusing on improving performance of the CMOS driver circuits and

developing a compact package for the SCG system.

In the design of the CW CMOS laser diode driver circuit, the CMOS chip size, the

potentiometer setup and the PCB assembly can be further optimized. The CMOS chip size can be

reduced by fabricating a new chip with all circuit parts integrated together. The number of

potentiometers on the PCB board can be reduced to one if the CMOS circuit is fully integrated.

The driver circuit and the laser diode can be assembled into one host PCB to make them a more

compact optoelectronic device.

64

The output of the designed picosecond pulsed DFB laser diode is sensitive to the noise from

the power supply connectors and the surroundings. A new PCB with an improved circuit grounding

and noise filtering setup can be designed to improve the circuit stability performance.

The designed SCG system uses a commercial EDFA module, which meets the requirement

for the purpose of pulse amplification. However, the module is not cost-effective and is difficult

to be integrated with other components in the system. It is necessary to develop a custom-made

EDFA with the designed pump laser. A custom-made EDFA will reduce the cost and size and also

increase the system flexibility.

The designed SCG system has not been fully packaged as a device. The largest discrete

component in this system is the 10-meter-long HNLF, which has a circular shape with an 8 cm

diameter. The device package will be built around the HNLF. The expected full package dimension

will be 10 cm length, 10 cm width and 5 cm height with a power supply input port and a

supercontinuum output port.

Silicon photonics technology has been an intense research interest recently, as it allows

optical devices to be made using standard semiconductor fabrication. This technology provides a

new platform for integration between the optical devices and CMOS circuits. It has already been

approved that the CMOS-compatible silicon wire waveguides can be used as a nonlinear medium

for SCG [18]. This report shows the opportunity for designed CMOS laser diode driver circuits.

In the future, these CMOS circuits can be integrated with silicon waveguides on the same platform

as lab-on-a-chip devices. This will be a revolutionary improvement on the design of compact and

low-cost SCG systems.

65

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CMOS Laser Diode Drivers for Supercontinuum Generation

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