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SEMINAR REPORTon

Photonic Micromachined Tunable laser

Submitted by

ARYA. K. R.

In partial fulfillment of the requirements for the award of the degree of

Bachelor of Technology in

ELECTRONICS AND COMMUNICATION ENGINEERING

of

COCHIN UNIVERSITY OF SCIENCE AND TECHNOLOGY

NOVEMBER 2011

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

Toc H INSTITUTE OF SCIENCE & TECHNOLOGY

Arakkunnam P.O, Ernakulam District, KERALA682 313


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Toc H INSTITUTE OF SCIENCE & TECHNOLOGYArakkunnam P.O, Ernakulam District, Kerala 682 313.

DEPARTMENT OF ELECTRONICS & COMMUNICATION ENGINEERING

CERTIFICATEThis is to certify that the seminar entitled Photonic Micromachined Tunable Laser byArya.K.R. of semester VII is a bonafide account of the work done by her under oursupervision during the academic year 2011 - 2012.

Seminar Guide Head of the Department

Head of the Institution


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ACKNOWLEDGEMENT

The satisfaction and euphoria of successful completion of any task would beincomplete without the mention of the people who made it possible through their constant

guidance and encouragement.

First and foremost I would like to express my gratitude to the invisible, the

indomitable God for his blessings without which I would not have been able to complete my

work on time.

I would like to extend my heartiest thanks to the management for extending a

generous hand in providing the best of resources to the college

I would like to express my deep gratitude to Prof (Dr). V. Job Kuruvilla, our

Director, who has been a source of motivation to all the staffs and students of our college. I

am also thankful to Dr.Justus Rabi, our Principal and to Prof (Col.) P. M. Xavier, our

Dean (Student & Academic Affairs)for their co-operation and support.

I would like to extend my heartfelt thanks to Asso. Prof. Deepa Elizabeth George

(H.O.D, ECE) for the inspiration inculcated in us and for the apt guidance.

It would be a grave error if I forget to make a mention of our seminar coordinators

Asst. Prof.Dhanya.S, Asst. Prof.Deepa Mary Varghese and Asst. Prof. Lenin Joseph

(Faculty members, Dept. of ECE) and our class in charges Ms.Sajitha.K.S (Lecturer, ECE)

and Asst. Prof. Emil Thomas (Dept. of ECE) whose constant persistence and support

helped me in the completion of this seminar report.

Last, but not the least, I take this opportunity to thank all the faculty members, Lab

instructors & other staff members of the Department of Electronics and Communication

Engineering. I would like to extend my heartiest thanks also to my parents, family

members, classmates and friends who offered an unflinching moral support for the

completion of this effort.


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ABSTRACT

Over the past decade, tremendous efforts have been devoted to the design and

development of tunable lasers. To provide an access to laser sources with the precise

emission wavelengths required by the system becomes a great challenge to set up any

Wavelength Division Multiplexed (WDM) network. Therefore, MEMS tunable lasers will

support a broadband communication system that will replace current fixed-wavelength or

narrowband semiconductor tunable lasers. This paper covers design, fabrication,

packaging and experiments of photonic Microelectromechanical Systems (MEMS) tunable

laser sources. Two different types of MEMS tunable lasers, which are MEMS coupled-

cavity lasers and dual-wavelength tunable lasers, are demonstrated as examples of

natural synergy of MEMS with photonics. The expansion and penetration of the MEMS

technology to silicon nano-photonics creates on-chip optical systems at an unprecedented

scale of integration. While producing better integration, robustness and compactness,

MEMS improves the functionalities and specifications of laser devices. Additionally,

MEMS photonic tunable lasers are able to deliver their merits of small size, fast tuning

speed, wide tuning range and CMOS compatible integration which broaden their

applications to many fields.


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TABLE OF CONTENTS

SL NO: TITLE PAGE NO:

LIST OF FIGURES

LIST OF SYMBOLS AND ABBREVIATIONS

1 INTRODUCTION 1

1.1 MEMS 2

1.1.1 MEMS MANUFACTURING TECHNOLOGIES 3

1.2 SEMICONDUCTOR TUNABLE LASER 4

1.3 COMPARISON BETWEEN MEMS TUNABLE 4

LASER AND SEMICONDUCTOR TUNABLE

LASER

1.4 SINGLE MODE SUPPRESSION RATIO 5

1.5 WAVELENGTH DIVISION MULTIPLEXING 5

2 MEMS COUPLED CAVITY LASER 6

2.1 CONSTRUCTION 6

2.2 EXPERIMENTAL OBSERVATIONS 8

2.3 FEATURES 11

3 MEMS DUAL WAVELENGTH LASER 13

3.1 CONSTRUCTION 13

3.1.1 DIFFRACTION AND DIFFRACTION GRATING 15

3.2 EXPERIMENTAL OBSERVATIONS 16

3.3 FEATURES 19

4 CONCLUSION 20

4.1 APPLICATION 20

4.2 FUTURE SCOPE 21


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LIST OF FIGURES

SL NO: FIGURE NAME PAGE NO:

2.1 SEM of MEMS coupled-cavity laser 6

2.1.1 Structural design of the MEMS coupled- cavity laser

using a parabolic mirror for optical coupling and cavity

length adjustment

7

2.2.1

2.2.2

2.2.3

Output spectra of the coupled- cavity laser for different

gap selections

Characteristics of wavelength tuning

The measured actuation relationship and coupling

efficiency change

9

10

11

3.1

3.1.1

3.2.1

3.2.2

3.2.3

3.2.4

3.2.5

SEM of the MEMS dual-wavelength laser

Schematic diagram of the tunable MEMS dual-

wavelength laser

Wavelength tuning and the spectral separation versus

the rotation of the mirror 2

Variation of rotation angle with the driving voltage

Superimposed spectra of the tunable wavelength

Spectrum of the dual-wavelength output

Wavelength tuning verses the rotation angle of mirror

13

14

16

17

17

18

19


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LIST OF SYMBOLS AND ABBREVIATIONS

Wavelength

Diffraction Angle

MEMS Microelectromechanical System

SOI Silicon-on-insulator

SMSR Single Mode Suppression Ratio

WDM Wavelength Division Multiplexing

CCL Coupled-Cavity Laser

DRIE Deep-Reactive ion Etching


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Photonic Micromachined Tunable Laser

Department of Electronics and Communication Engineering, TIST1

CHAPTER 1

INTRODUCTION

Tunable diode laser has been extensively applied in a wide range of areas. In

particular, it is the enabler of novel network architectures for wavelength division

multiplexing (WDM) systems, such as passive optical networks, packet switching

networks etc. Such new applications require a wide tuning range and a fast tuning

speed in the nano-second level. Many of the developed tunable lasers only meet one

requirement. In addition, monolithic sources are especially desirable for compactness,

mechanical stabilities and integratability with other systems. For these reasons, MEMS

tunable laser sources have attracted renewed interests since they have the potential to

meet all these requirements.

Microelectromechanical Systems (MEMS) tunable laser sources are natural synergy

of MEMS with photonics. Two different types of MEMS tunable lasers, which are MEMS

coupled-cavity lasers and dual-wavelength tunable lasers, are reported. The expansion

and penetration of the MEMS technology to silicon nano-photonics creates on-chip

optical systems at an unprecedented scale of integration. While producing better

integration, robustness and compactness, MEMS improves the functionalities and

specifications of laser devices. The introduction of MEMS has endowed two special

features to tunable lasers:

One is that MEMS facilitates external cavities at very short (


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platform is compared with traditional systems making use of only discrete component

systems, which are typically bulky, expensive and of lower performance.

1.1 MEMS

Microelectromechanical systems (MEMS) (also written as micro

electromechanical, MicroElectroMechanical or microelectronic and

microelectromechanical systems) is the technology of very small mechanical devices

driven by electricity; it merges at the nano-scale into nanoelectromechanical systems

(NEMS) and nanotechnology. MEMS are also referred to as micromachines (in Japan),

or microsystems technology - MST (in Europe). MEMS are made up of components

between 1 to 100 micrometers in size (i.e. 0.001 to 0.1 mm) and MEMS devices

generally range in size from 20 micrometers (20 millionths of a meter) to a millimeter.

They usually consist of a central unit that processes data, the microprocessor and

several components that interact with the outside such as micro sensors. At these size

scales, the standard constructs of classical physics are not always useful. Because of

the large surface area to volume ratio of MEMS, surface effects such as electrostatics

and wetting dominate volume effects such as inertia or thermal mass.

Silicon, polymers and metals are used for materials for MEMS.

The economies ofscale, ready availability of cheap high-quality materials and ability to incorporate

electronic functionality make silicon attractive for a wide variety of MEMS applications.

Silicon also has significant advantages engendered through its material properties. In

single crystal form, silicon is an almost perfect Hookean material, meaning that when it

is flexed there is virtually no hysteresis and hence almost no energy dissipation.

As well as making for highly repeatable motion, this also makes silicon very

reliable as it suffers very little fatigue and can have service lifetimes in the range of

billions to trillions of cycles without breaking. The basic techniques for producing all

silicon based MEMS devices are deposition of material layers, patterning of these layers

by photolithography and then etching to produce the required shapes.Even though the

electronics industry provides an economy of scale for the silicon industry, crystalline

silicon is still a complex and relatively expensive material to produce.


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Polymers on the other hand can be produced in huge volumes, with a great

variety of material characteristics. MEMS devices can be made from polymers by

processes such as injection molding, embossing or stereo lithography and are

especially well suited to micro fluidic applications such as disposable blood testing

cartridges. MEMS devices can be made from polymers by processes such as injection

molding, embossing or stereolithography and are especially well suited to microfluidic

applications such as disposable blood testing cartridges. Metals can also be used to

create MEMS elements. While metals do not have some of the advantages displayed by

silicon in terms of mechanical properties, when used within their limitations, metals can

exhibit very high degrees of reliability. Metals can be deposited by electroplating,

evaporation, and sputtering processes. Commonly used metals include gold, nickel,

aluminium, copper, chromium, titanium, tungsten, platinum, and silver.

1.1.1 MEMS MANUFACTURING TECHNOLOGIES

The commonly used technologies are Bulk micromachining, Surface

micromachining and High aspect ratio (HAR) silicon micromachining. Bulk

micromachining is the oldest paradigm of silicon based MEMS. The whole thickness of a

silicon wafer is used for building the micro-mechanical structures. Silicon is machined

using various etching processes. Anodic bonding of glass plates or additional silicon

wafers is used for adding features in the third dimension and for hermetic encapsulation.

Bulk micromachining has been essential in enabling high performance pressure sensors

and accelerometers that have changed the shape of the sensor industry in the 80's and

90's.

Surface micromachining uses layers deposited on the surface of a substrate as

the structural materials, rather than using the substrate itself. Surface micromachiningwas created in the late 1980s to render micromachining of silicon more compatible with

planar integrated circuit technology, with the goal of combining MEMS and integrated

circuits on the same silicon wafer. The original surface micromachining concept was

based on thin polycrystalline silicon layers patterned as movable mechanical structures

and released by sacrificial etching of the underlying oxide layer.

A new etching technology, deep reactive-ion etching, has made it possible to

combine good performance typical of bulk micromachining with comb structures and in-
http://en.wikipedia.org/wiki/Pressure_sensorhttp://en.wikipedia.org/wiki/Accelerometerhttp://en.wikipedia.org/wiki/Accelerometerhttp://en.wikipedia.org/wiki/Pressure_sensor


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plane operation typical of surface micromachining. While it is common in surface

micromachining to have structural layer thickness in the range of 2 m, in HAR silicon

micromachining the thickness can be from 10 to 100 m. The materials commonly used

in HAR silicon micromachining are thick polycrystalline silicon, known as epi-poly, and

bonded silicon-on-insulator (SOI) wafers although processes for bulk silicon wafer also

have been created. Bonding a second wafer by glass frit bonding, anodic bonding or

alloy bonding is used to protect the MEMS structures.

1.2 SEMICONDUCTOR TUNABLE LASER

A tunable laser is a laser whose wavelength of operation can be altered in a

controlled manner. While all laser gain media allow small shifts in output wavelength,only a few types of lasers allow continuous tuning over a significant wavelength range.

There are many types and categories of tunable lasers. They exist in the gas, liquid, and

solid state. Among the types of tunable lasers are excimer lasers, CO2 lasers, dye

lasers (liquid and solid state), transition metal solid-state lasers, semiconductorcrystal

and diode lasers, and free electron lasers. Tunable lasers find applications in

spectroscopy, photochemistry, atomic vapor laser isotope separation, and optical

communications.

1.3 COMPARISON BETWEEN MEMS TUNABLE LASER AND

SEMICONDUCTOR TUNABLE LASER

MEMS tunable lasers have many superior characteristics than typical semiconductor

lasers. Some of them are listed below:

They produce better integration, robustness ad compactness. They improve the functionalities and specifications of laser devices.

They are small in size.

They are mechanically stable than traditional semiconductor tunable lasers.

They have fast tuning speed, wide tuning range and CMOS compactable

integration.

They will support a broadband communication system that will replace the

current fixed-wavelength or narrowband semiconductor lasers.
http://en.wikipedia.org/wiki/Active_laser_mediumhttp://en.wikipedia.org/wiki/Excimer_laserhttp://en.wikipedia.org/wiki/CO2_laserhttp://en.wikipedia.org/wiki/CO2_laserhttp://en.wikipedia.org/wiki/CO2_laserhttp://en.wikipedia.org/wiki/Dye_laserhttp://en.wikipedia.org/wiki/Dye_laserhttp://en.wikipedia.org/wiki/Solid-state_laserhttp://en.wikipedia.org/wiki/Crystalhttp://en.wikipedia.org/wiki/Diode_laserhttp://en.wikipedia.org/wiki/Free_electron_laserhttp://en.wikipedia.org/wiki/Spectroscopyhttp://en.wikipedia.org/wiki/Photochemistryhttp://en.wikipedia.org/wiki/Atomic_vapor_laser_isotope_separationhttp://en.wikipedia.org/wiki/Atomic_vapor_laser_isotope_separationhttp://en.wikipedia.org/wiki/Photochemistryhttp://en.wikipedia.org/wiki/Spectroscopyhttp://en.wikipedia.org/wiki/Free_electron_laserhttp://en.wikipedia.org/wiki/Diode_laserhttp://en.wikipedia.org/wiki/Crystalhttp://en.wikipedia.org/wiki/Solid-state_laserhttp://en.wikipedia.org/wiki/Dye_laserhttp://en.wikipedia.org/wiki/Dye_laserhttp://en.wikipedia.org/wiki/Dye_laserhttp://en.wikipedia.org/wiki/CO2_laserhttp://en.wikipedia.org/wiki/Excimer_laserhttp://en.wikipedia.org/wiki/Active_laser_medium


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MEMS technology facilitates the integration of laser chips with other optical

and electronic components onto a single chip.

1.4 SINGLE MODE SUPRESSION RATIO (SMSR)

An expression of the quality of a single mode laser based on the difference in

amplitude between the main spectral mode to the most dominant side mode. It is

preferably high for a good tunable laser source.

1.5 WAVELENGTH DIVISION MUTIPLEXING (WDM)

Wavelength division multiplexing is the process of different channels being encoded

on different wavelengths and being sent through the same fiber. This method is

employed for easy channel provision. This increases the bandwidth even further.


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CHAPTER 2

MEMS COUPLED CAVITY LASER

MEMS coupled-cavity laser (CCL) is one of the new approaches to the MEMS

tunable laser. It is formed by coupling two or more laser cavities, which can be

fabricated by etching a narrow groove inside a cleaved single chip or by employing

cleaved-coupled-cavity structures. A CCL commonly consists of two or more optically

coupled but electrically isolated cavities, which are separated by narrow air gaps. MEMS

technology provides a great reduction in packaging cost and size, with increased

reliability and lower power dissipation. The MEMS part is for the initial phase match

while the electrical injection is for the wavelength tuning. Therefore, the proposed laser

could realize single chip integration while circumventing the speed limit of mechanical

response.

Fig.2.1 Scanning electron micrographs (SEM) of the coupled-cavity laser

2.1 CONSTRUCTION

The design of CCL consists of two Fabry-Perot laser chips, which are optically

coupled by a movable parabolic mirror but electrically independent. CCL employs a

deep-etched parabolic mirror to adjust the gap of the CCL for optimal phase match. In

experiment, such mirror measures an initial coupling efficiency of 70.5% and a low


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variation (


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for DRAM and more recently for creating through wafer via's (TSV)'s in advanced 3D

wafer level packaging technology .

In a recent work, a 3-dimensional (3D) optical coupling system is introduced to

improve the optical coupling in the external cavity. It makes use of an optical fiber as the

rod lens to collimate the light in the vertical plane and a curved mirror in the horizontal

plane. This is the first realization of 3D coupling in single-chip MEMS lasers. With such

3D system, the tunable laser has obtained an efficiency of 47% and a tuning range of

53.2 nm, significantly higher than the typical values of 8% and 10 nm, making it possible

for many advanced applications. Successive work has focused on new designs to

improve the coupling efficiency and the wavelength tuning range.

2.2 EXPERIMENTAL OBSERVATIONS

In the experiment, it is observed that the single chip operates in a multi-

longitudinal-mode regime. After the two chips are assembled onto the MEMS substrate

to form the micromachined CCL, the output spectrum is then characterized. During the

experiment, the lasing current is kept constant at 14.1 mA while the tuning current is

varied. The optimal cavity gap d0 is experimentally determined by moving the parabolic

mirror back and forth while monitoring the output spectrum until a single longitudinal-mode output is obtained. It is seen that as the cavity gap increases the chip shifts its

mode from single to multimode. The tuning current is not varied, only the cavity gap is

increased to shift the mode. Instead of getting a single peak, a series of peaks are

obtained which is actually undesirable.

Such an unsuitable gap only allows the two chips work independently instead of

affecting each other, even though they are optically coupled. It proves that the gap

between the two chips is the dominant factor for the modes matching and wavelength

selection. It is also observed that the SMSR of the single mode output of the

micromachined CCL (~ 24.5 dB) is higher than the peak value (~ 17 dB) of the single

chip as predicted by the analysis. In addition, while the single longitudinal mode is

maintained as SMSR > 19.0 dB, the wide wavelength tuning range can reach 51.3 nm..

When the gap of the external cavity is adjusted, the mode selectivity and output stability

are improved. Furthermore, the speed of the wavelength switching is estimated to reach

the level of nanosecond based on the free carrier plasma effect.


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Fig.2.2.1 Output spectra of the coupled cavity laser for different gap selections.(a) Multimode output at d < d0

(b) single-mode output in the optimal state d = d0(c) multimode again at d > d0

MEMS technology does not only offer high compactness and fine position

adjustment, but also facilitates the integration of laser chips with other optical and

electronic components onto a single-chip, making the micromachined CCLs promising

for many applications. During the fabrication, all the MEMS structures, including the

micromirror, comb-drive actuator, are defined by a single photolithographic step and

then etched into the wafer.

The movable parts are released by dry method. To improve the reflectivity of the

mirror, shadow mask technology is employed to selectively coat an aluminum layer (with

a thickness of 0.4 m) on the reflected mirror surface. After the MEMS fabrication, the

laser chips (with one of facets has antireflection coated, R < 0.2 %), a rod lens and the

detection fiber are integrated. Benefited from the process, the profile of the parabolic

mirror is maintained after the fabrication. To further reduce the optical loss between the

chips, such as the beam divergence in the vertical direction, a rod lens is introduced by

use of a section of the common single mode optical fibre.


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The output spectrum is given in Fig.2.2.2 (a) and wavelength tuning is illustrated

by Fig.2.2.2 (b).

Fig.2.2.2 Characteristics of wavelength tuning

(a) Spectral output at different tuning currents

(b) Wavelength shift corresponding to different current


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The movement of the mirror is generated by an electrostatic comb-drive actuator.

The measured actuation curve is plotted in Fig.2.2.3. No significant movement is

observed when the driving voltage is smaller than 5 volts. Then the displacement is

increased steadily with higher voltage, and it goes to 29.4 m at 30 V. The coupling

efficiency is also plotted in Fig.2.5. The coupling is maintained at the level around 70.5%

over the 29.4 m mirror displacement, with the variation less than 5%.

Fig.2.2.3 The measured actuation relationship and coupling efficiency change

2.3 FEATURES

MEMS CCLs have exhibited the advantages of simplicity, high side mode

suppression and stable single longitudinal- mode output. In a tuning mode, by varying

the current flowing through one of the cavities, its effective refractive index is altered. As

a result, the optical cavity length is changed and the total mode comb is subsequently

modified. Finally, the output wavelength is tuned. Such electrical tuning yields a fast

tuning speed at the nano-second level.

Wide tuning range which can reach up to 51.3nm if SMSR is maintained

greater than 19.0dB.

SMSR of the single mode output of the micromachined CCL (~ 24.5 dB) is

higher than the peak value (~ 17 dB) of the single chip.


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The measured output power is approximately -10dBm.

Microelectromechanical systems (MEMS) technology provides a great reduction

in packaging cost and size, with increased reliability and lower power dissipation.

However, most of the developed MEMS lasers obtain the wavelength tuning by

mechanical movement of the MEMS reflectors (mirrors or gratings), and thus have the

tuning speed limited by the mechanical response speed to about 1 ms.

As the wavelength tuning is obtained by injecting electrical current, the tuning

speed is no longer limited by the mechanical speed of MEMS and thus can achieve a

tuning speed at the nanosecond level.


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CHAPTER 3

MEMS DUAL WAVELENGTH LASER

Dual-wavelength tunable laser is a key device for wide range of applications such

as optical recording, wavelength division multiplexing and optical instruments etc. Dual-

wavelength laser, simultaneously achieves wide tuning range and two-longitudinal-mode

emission. Such single-chip solution may offer a potential to reduce the packaging cost,

while support better performances.

Fig.3.1 Scanning electron micrograph (SEM) of the MEMS dual-wavelength laser

3.1 CONSTRUCTION

The construction of this laser is based on Littman external cavity configuration.

The key point is to use two micro-mirrors to select two different wavelengths at their

independent diffraction modes, and the emission of both wavelengths will be collected

through the 0th order of the grating. Meanwhile, wavelength tuning is achieved by

rotating one of the mirrors. As a result, a tunable spectral separation of dual-wavelength

output is obtained.The external grating cavities are formed by the antireflection (AR)coated facet of the gain chip, a collimating lens, a grating element and two mirrors. The

two mirrors are used to select two individual resonance wavelengths of the cavities.Light from the facet (AR) is collimated before striking a grating element. Since the light


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of different wavelengths are diffracted at different angle, the diffraction grating serves as

an opticalfilter in the cavity.In this Littman configuration, there is a double pass through a reflection grating

via a further external mirror. That is, a beam at certain diffraction angle (associated with

the desired output wavelength) is retro-reflected by a mirror, sending the light back to

the diode for oscillation at that particular wavelength in the cavity. Because there are

two mirrors located at the positions corresponding to different diffraction angles, such

laser configuration supports a simultaneous dual-wavelength output. The grating

diffracts different wavelengths to different directions, and the mirrors (mirror 1 & mirror

2) are used to select two individual wavelengths (1 and 2)in the cavities.

Fig.3.1.1 Schematic diagram of the tunable dual wavelength MEMS laser

(a) Laser configuration (b) rotation of the mirror 2

The 0-th order diffraction of the grating is coupled as the output, which contains

both 1 and 2. Based on the laser cavity resonance and the grating diffraction

conditions the spectral separation (= 2 1) is expressed as:

=p0 [(sin2/m2)-(sin1/m1)]


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Where p0 is the grating period, m1 and m2 are the diffraction orders, and 1 and 2

are the diffraction angles. In our design, 1 and 2 are +1 order and -1 order diffraction

angle, respectively. In order to achieve wavelength tuning, one of the mirrors (mirror 2)

is designed to be rotatable. Thus, the wavelength shifting can be achieved, as

determined by:

d=pcos . d

The spectral separation of the two modes can be tuned to different levels, which

is obtained by rotating the mirror 2 with respect to a fixed pivot (Fig.3.1.1b). It should be

noted that the rotation of the mirror 2 only affects one of the wavelength (2), while the

other wavelength (1) is kept constant.

3.1.1 DIFFRACTION AND DIFFRACTION GRATING

Diffraction refers to various phenomena which occur when a wave encounters an

obstacle. In classical physics, the diffraction phenomenon is described as the apparent

bending of waves around small obstacles and the spreading out of waves past small

openings. Similar effects occur when light waves travel through a medium with a varying

refractive index or a sound wave through one with varying acoustic impedance.

Diffraction occurs with all waves, including sound waves, water waves, and

electromagnetic waves such as visible light, x-rays and radio waves.

In optics, a diffraction grating is an optical component with a periodic structure,

which splits and diffracts light into several beams travelling in different directions. The

directions of these beams depend on the spacing of the grating and the wavelength of

the light so that the grating acts as the dispersive element. The relationship between

the grating spacing and the angles of the incident and diffracted beams of light is known

as the grating equation. According to the HuygensFresnel principle, each point on the

wave front of a propagating wave can be considered to act as a point source, and the

wave front at any subsequent point can be found by adding together the contributions

from each of these individual point sources.

An idealized grating is considered which is made up of a set of long and infinitely

narrow slits of spacing d. When a plane wave of wavelength is incident normally on

the grating, each slit in the grating acts as a point source propagating in all directions.

The light in a particular direction, , is made up of the interfering components from each


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slit. Generally, the phases of the waves from different slits will vary from one another,

and will cancel one another out partially or wholly. However, when the path difference

between the light from adjacent slits is equal to the wavelength, , the waves will all be

in phase. This occurs at angles m which satisfy the relationship dsinm/=|m| where dis

the separation of the slits and m is an integer. Thus, the diffracted light will have maxima

at angles m given by

If a plane wave is incident at an angle i, the grating equation becomes

3.2 EXPERIMENTAL OBSERVATIONS

The wavelength tuning is demonstrated by rotating the micro mirror. With one

wavelength being tuned and the other fixed, the laser output presents a tunable spectral

separation from -28.38 to +24.18 nm. The laser output reaches 2.9 mW with far-field

divergences of 37 and 30 in the vertical and horizontal directions, respectively.

Besides, line broadening is observed with the reduction of the spectral separation.

Fig.3.2.1 Wavelength tuning and the spectral separation versus the rotation of the mirror 2

In the experiment, simultaneous two wavelength emissions are observed in the

output. An output power of 2.9 mW is obtained when the driving current of 30 mA is

applied on the gain chip. Higher output power of up to 10 mW can be achieved with a

higher driving current (e.g., 80 mA), provided a thermoelectric cooler employed to


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stabilize the MEMS laser operating at 25 C. For the rotation of mirror 2 ,a potential is

applied to the comb-drive, and the electrostatic force is produced between the moving

fingers and the fixed ones which generates an electrostatic torque. The lengths of the

individual combs have been optimally adjusted. Fig.3.2.2. shows that the rotation angle

of the mirror is increased with higher driving voltage.

Fig.3.2.2 Variation of rotation angle with the driving voltage

As the voltage goes from 6 V to 20 V, the output wavelength is increased from

1550.32 nm to 1569.36 nm. Single longitudinal mode is well maintained over the whole

range, with a side-mode suppression ratio (SMSR) greater than 30 db.

Fig.3.2.3 Superimposed spectra (2) of the tunable wavelength (with 1 = 1550 nm)


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Fig.3.2.4 Spectrum of the dual-wavelength output

(a) Red-shift of2 (b) blue-shift of2

A simultaneous emission at two different wavelengths is obtained as shown in

Fig.3.2.4. In both spectra, the tunable wavelength ( 2) is achieved by the rotation of

mirror 2. It is noted that the powers of both wavelengths are almost the same, indicating

the similar reflectivity of the two mirrors. The spectral separations between the two

oscillation modes are 6.5 nm and 8.1 nm, respectively. In this case, the fixed

wavelength ( 1) is set at about 1550 nm, stabilized by mirror 1, while a tunable

wavelength ( 2) is provided by mirror 2.

The wavelength tuning is shown in Fig.3.2.5, where 2 has a maximum tuning

range of more than 50 nm, corresponding to the rotation of mirror 2. It also shows that

the tunable wavelength is almost linearly decreased with the increase in the rotation

angle. The simultaneous emission at two different wavelengths is also shown in the

inset.


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Fig. 3.2.5 Wavelength tuning (2) verses the rotation angle of mirror 2 (movable mirror)

3.3 FEATURES

A tunable multiwavelength laser source is of interest for many applications in

science and engineering, including wavelength-division multiplexing (WDM), differential

absorption lidars, two wavelength interferometers, optical data storage, and so on.

The underlying advantage of this design is that semiconductor lasers have a

number of interesting characteristics for dual-wavelength emission, including large

tuning range and high gain.

It is a key device for wide range of applications such as optical recording,

wavelength division multiplexing and optical instruments etc.

Simultaneous two wavelength emissions are obtained.

An output power of 10mW is obtained on applying a driving current of30mA.

Maximum tuning range of more than 50 nm.


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CHAPTER 4

CONCLUSION

Two different MEMS tunable lasers are reported such as MEMS coupled-cavity

lasers, and dual-wavelength lasers. The MEMS tunable lasers achieve great

improvement in high tuning speed, large tuning range and mechanically stability as

compared with the traditional semiconductor tunable lasers. They have the advantages

of high accuracy (~ 0.1m), fast response (~ 1 ms), small volume (~ 1mm3), lightweight

(~ 10 g), single-chip integration, easy IC integration and low cost implementation.

A tunable dual-wavelength laser is made by integration of a semiconductor gain

chip with silicon-micromachined grating and mirrors onto a silicon substrate. The

wavelength tuning is demonstrated by rotating the micromirror. With one wavelength

being tuned and the other fixed, the laser output presents a tunable spectral separation

from 28.38to+24.18nm. The laser output reaches 2.9mW with far-field divergences

of 37 and 30 in the vertical and horizontal directions, respectively. Besides, line

broadening is observed with the reduction of the spectral separation.

A miniature tunable coupled-cavity laser is made by integrating a Fabry-Prot

chip, a gain chip and a deep-etched parabolic mirror using micromachining technology.

The mirror is to actively adjust the gap between chips, enabling the optimal mode

selection. Single-mode operation with a tuning range of 16.55nm and a side-mode-

suppression ratio of >25.1dB is demonstrated. The device overcomes phase

mismatching and instability problems encountered in conventional fixed-gap coupled-

cavity lasers.

4.1 APPLICATIONS

MEMS-based products offer substantial cost and performance advantages

for optical networking applications in the area of switching fabrics, variable

attenuators, tunable lasers, and other devices.

It is a key device for optical recording, wavelength division multiplexing,

optical instruments etc.


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It have potential applications in optical networks, two wavelength

interferometers and differential lidar systems, and so on.

4.2 FUTURE SCOPE

It has been proven that the MEMS tunable laser sources offer significant

advantages and the fusion of MEMS tunability and photonic principles is expected to

open up new opportunities for developing the next generation devices for applications of

telecommunications and biophotonic medical instruments.

The fusion of MEMS tenability and photonic principles is opening up new

chances for developing next-generation devices for not only laser applications but also

cell studies.

The explosion of the Internet has brought about an acute need for broadband

communications, which can only be filled with optical networking. This in turn has

resulted in an unprecedented interest in optical micro-electromechanical systems. Since

the early days of fiber optics, it has been recognized that micro-optics was a fertile

ground for the applications of MEMS.


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REFERENCES

[1] Cai H, Liu B, Zhang X M, Liu A Q, Bourouina T, Zhang Q X. A micromachined tunable

coupled-cavity laser for wide tuning range and high spectral purity, Optics Express,

vol. 16, no. 21, 2008:16670-16679.

[2] Liu A Q. Photonic MEMS Devices- Design, Fabrication and control, Taylor and

Francis, 2008.

[3] Zhang X M, Cai H, Lu C, Chen C K, Liu A Q. Design and experiment of 3-dimensional

microoptical system for MEMS tunable lasers, Proc. MEMS 2006, 22-26 Jan. 2006,

Istanbul, Turkey, pp. 830-883, paper MP45.

[4] Cai H, Liu A Q, Zhang X M, Tamil J, Tang D Y, Wu J, Zhang Q X. Tunable dual-

wavelength laser constructed by silicon micromachining, Appl. Phys Lett. vol. 92,

no.05, 051113, 2008.

[5] Li J, Liu A Q, Zhang X M, Zhong T. Light switching via thermo-optic effect of

micromachined silicon prism, Appl. Phys. Lett. vol. 88, no. 24, 243501, 2006.

[6] Liu A Q, Zhang X M, Murukeshan V M, Lam Y L. A novel device level micromachined

tunable laser using polysilicon 3D mirror, IEEE Photo. Tech. Lett. vol. 13, 2001: 427-

429.


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APPENDIX


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5-7 May, Seville, Spain

EDA Publishing/DTIP 2010 ISBN:

Photonic Micromachined Tunable LasersA. Q. Liu

School of Electrical & Electronic Engineering

50 Nanyang Avenue, Nanyang Technological University, Singapore 639798

Tel: +65 6790-4336; Fax: +65 6793-3318; Email: [email protected]

PRESENTATION: Oral Presentation

Abstract-This paper covers design, fabrication, packagingand experiments of photonic Microelectromechanical

Systems (MEMS) tunable laser sources. Two different types

of MEMS tunable lasers, which are MEMS coupled-cavity

lasers and dual-wavelength tunable lasers, are demonstrated

as examples of natural synergy of MEMS with photonics.

The expansion and penetration of the MEMS technology to

silicon nano-photonics creates on-chip optical systems at an

unprecedented scale of integration. While producing betterintegration, robustness and compactness, MEMS improves

the functionalities and specifications of laser devices.

Additionally, MEMS photonic tunable lasers are able to

deliver their merits of small size, fast tuning speed, wide

tuning range and CMOS compatible integration which

broaden their applications to many fields.Keywords: MEMS, tunable laser, photonics.

I. INTRODUCTION

Over the past decade, tremendous efforts have been

devoted to the design and development of tunable lasers.The applications of tunable lasers in optical networks have

inspired significant research to support the perceived need

for dynamic networks and wavelength reconfigurability in

wavelength division multiplexing systems. One of the key

current trends in optical networking is redirecting a time-

division multiplexed (TDM) trend towards wavelength-division multiplexed (WDM) systems. To provide an access

to laser sources with the precise emission wavelengths

required by the system becomes a great challenge to set up

any WDM network. Therefore, MEMS tunable lasers will

support a broadband communication system that will

replace current fixed-wavelength or narrowband

semiconductor tunable lasers.

MEMS technology experiences rapid development in

optical and photonic devices [1]. It provides tunable lasers

[2-4] with an integration platform solution by fully utilizing

the capability of MEMS tunable lasers of rapidly switchingfrom one light wavelength to another. In so doing, it offers

carriers benefits in networking efficiency through

significant performance of the network flexibility and

seamless scalability.

Inspired by the inherent advantages of MEMS

technology, different types of photonic MEMS tunable laser

sources, such as coupled lasers, injection locked tunablelaser and duel-wavelength tunable laser have been

developed for a series of new generation devices. The

design, fabrication and experiments of the two different

types of MEMS tunable laser sources are presented in this

paper.

II. MEMS TUNABLE LASERS

A. MEMS coupled-cavity lasers

Based on MEMS fabrication and integration technology,

MEMS can provide an approach to fabricating optical

components as well as substrates for integration and

packaging. Recently, MEMS tunable laser research has been

focused on new designs with improved wavelength tunabilityand other optical performances [410].

MEMS coupled-cavity laser (CCL) is one of the new

approaches to the MEMS tunable laser. It is formed by

coupling two or more laser cavities, which can be fabricated

by etching a narrow groove inside a cleaved single chip [11]

or by employing cleaved-coupled-cavity structures [12]. Itconsists of two Fabry-Prot (FP) laser chips, which are

optically coupled by a movable MEMS parabolic mirror

that is electrically independent. It is fabricated using deep-

reactive-ion-etching on a silicon-on-insulator wafer with a

75-um-thick structural layer. To improve the reflectivity of

the mirror, shadow mask technology is employed to

selectively coat an aluminum layer on the reflected mirror

surface. It is challenging to design an actively adjustable

cavity gap that gives a large tuning range as shown in Fig. 1

[13].

Fig. 1. SEM of the MEMS coupled-cavity laser.


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EDA Publishing/DTIP 2010 ISBN:

Fig. 2. Comparison of the output spectra of the micromachined

CCL in different states. (a) single-mode output spectrum ofmicromachined CCL when the cavity gap is optimal; and (b)multi-mode spectrum of the micromachined CCL when the cavity

gap is not optimal (d d0).

In the experiment, it is observed that the single chip operates

in a multi-longitudinal-mode regime. After the two chips are

assembled onto the MEMS substrate to form the

micromachined CCL, the output spectrum is then

characterized (as shown in Fig. 2). During the experiment, the

lasing current is kept constant at 14.1 mA while the tuningcurrent is varied. The optimal cavity gap d0 is experimentally

determined by moving the parabolic mirror back and forth

while monitoring the output spectrum until a single-

longitudinal-mode output is obtained. Fig. 2(a) exemplifies the

single-longitudinal-mode spectrum when the tuning current is

8.6 mA. It has = 1567.7 nm and SMSR = 24.5 dB. When thecavity gap is moved away from its optimal value, the output

spectrum of the micromachined CCL becomes multi-longitudinal mode again as shown in Fig. 2(b). Such an

unsuitable gap only allows the two chips work independently

instead of affecting each other, even though they are optically

coupled. It proves that the gap between the two chips is the

dominant factor for the modes matching and wavelength

selection. It is also observed that the SMSR of the single mode

output of the micromachined CCL (~ 24.5 dB) is higher than

the peak value (~ 17 dB) of the single chip as predicted by theanalysis.

In addition, while the single longitudinal mode ismaintained as SMSR > 19.0 dB, the wide wavelength tuning

range can reach 51.3 nm. The measured output power is

approximately -10 dBm. When the gap of the external cavity

is adjusted, the mode selectivity and output stability are

improved. Furthermore, the speed of the wavelength switching

is estimated to reach the level of nanosecond based on the freecarrier plasma effect. MEMS technology does not only offer

high compactness and fine position adjustment, but also

facilitates the integration of laser chips with other optical and

electronic components onto a single-chip, making themicromachined CCLs promising for many applications.

B. MEMS dual-wavelength lasers

Dual-wavelength tunable laser is a key device for wide

range of applications such as optical recording, wavelength

division multiplexing and optical instruments etc. MEMS

dual-wavelength tunable laser is shown in Fig. 3 [14-15],

which integrates a semiconductor gain chip with silicon-micromachined grating and mirrors onto a silicon chip. The

external cavity is formed by an AR coated facet of the gain

chip, a collimating lens, a grating element and two mirrors.

The grating diffracts different wavelengths to different

directions, and the mirrors (mirror 1 & mirror 2) are used to

select two individual wavelengths (1

and2

) in the cavities.

The 0-th order diffraction of the grating is coupled as the

output, which contains both1

and2

. Based on the laser

cavity resonance and the grating diffraction conditions the

spectral separation (2 1

= ) is expressed as:

2 10

2 1

sin sinp

m m

=

(1)

wherep0 is the grating period, m1 and m2 are the diffraction

orders, and 1 and 2 are the diffraction angles.

Fig. 3. SEM of the MEMS dual-wavelength laser.

In the experiment, simultaneous two wavelength emissions

are observed in the output. An output power of 2.9 mW is

obtained when the driving current of 30 mA is applied on the

gain chip. Higher output power of up to 10 mW can be

achieved with a higher driving current (e.g., 80 mA), provideda thermoelectric cooler employed to stabilize the MEMS laser

operating at 25 C. An example of optical spectrum in

different output states is illustrated in Fig. 4. 1=

(corresponding to mirror 1) is always locked at 1549.04 nm,

while 2 (corresponding to mirror 2) is tuned from 1556.78 nm

(Fig. 4(a)) to 1538.00 nm (Fig. 4(b)). Correspondingly, it

demonstrates the change of of +7.74 and 11.04 nm. All

the wavelength outputs (1 and 2) are in single frequency

modes. Such oscillation spectra of the dual-wavelength laser

are measured at 25 C.


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Wavelength (nm)1520 1540 1560 1580

Power(a.u.)

Power(a.u.)

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

=1549.04 nmRed shift

=7.74 nm

Blue shift

= - 11.04 nm

Wavelength (nm)1520 1540 1560 1580

Power(a.u.)

Power(a.u.)

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

Wavelength (nm)1520 1540 1560 1580

Power(a.u.)

Power(a.u.)

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

Wavelength (nm)1520 1540 1560 1580

Power(a.u.)

Power(a.u.)

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

=1549.04 nmRed shift

=7.74 nm

Blue shift

= - 11.04 nm

Fig. 4. Spectra of the MEMS dual-wavelength laser. 1 is locked at1549.04 nm, and 2 is tuned.

The wavelength tuning ( 2 ) is shown in Fig. 5, where 2

has a maximum tuning range of more than 50 nm,corresponding to the rotation of mirror 2. It also shows that

the tunable wavelength is almost linearly decreased with the

increase in the rotation angle. The simultaneous emission at

two different wavelengths is also shown in the inset.

Fig.5. Wavelength tuning ( 2 ) verses the rotation angle of mirror

2 (movable mirror).

III. CONCLUSIONSTwo different MEMS tunable lasers are reported such as

MEMS coupled-cavity lasers, and dual-wavelength lasers.

The MEMS tunable lasers achieve great improvement in

high tuning speed, large tuning range and mechanically

stability as compared with the traditional semiconductortunable lasers. It has been proven that the MEMS tunable

laser sources offer significant advantages and the fusion of

MEMS tunability and photonic principles is expected to

open up new opportunities for developing the next

generation devices for applications of telecommunications

and biophotonic medical instruments.

ACKNOWLEDGMENT

This work is supported by the Agency for Science,

Technology, and Research (A*STAR) Singapore. The

author sincerely acknowledges the collaborative partners of

Institute of Microelectronics (IME) for their supports.

REFERENCES

[1] Ukita H, Uenishi Y, Tanaka H. A photomicrodynamic system witha mechanical resonator monolithically integrated with laser diodes

on gallium arsenide, Science, vol. 260, 1993: 786-789.

[2] Liu A Q, Zhang X M, Murukeshan V M, Lam Y L. A novel devicelevel micromachined tuanble aser using polysilicon 3D mirror, IEEE

Photo. Tech. Lett. vol. 13, 2001: 427429.

[3] Berger J D, Anthoon D. Tunable MEMS devices for otpical networks,

Opt. Phot. News, vol 14, 2003: 4262.

[4] Zhang X M, Cai H, Lu C, Chen C K, Liu A Q. Design and experimentof 3-dimensional microoptical system for MEMS tunable lasers, Proc.

MEMS 2006, 2226 Jan. 2006, Istanbul, Turkey, pp. 830833, paperMP45.

[5] Liu A. Q. Photonic MEMS Devices Design, Fabrication andcontrol, Taylor & Francis, 2008.

[6] Li J, Liu A Q, Zhang X M, Zhong T. Light switching via thermo-opticeffect of micromachined silicon prism, Appl. Phys. Lett., vol. 88, no.24, 243501, 2006.

[7] Liu A Q, Zhang X M, Tang D Y, Lu C. Tunable laser usingmicromachined grating with continuous wavelength tuning, Appl. Phys.

Lett., vol. 85, no. 17, 2004: 36843686.

[8] Zhang X M, Liu A Q, Tang D Y, Lu C. Discrete wavelength tunablelaser using microelectromechanical systems technology, Appl. Phys.

Lett., vol. 84, no. 3, 2004: 329331.

[9] Cai H, Zhang X M, Yu A B, Zhang Q X, Liu A Q. MEMS tuningmechanism for eliminating mode hopping problem in external-cavity

lasers, Proc.MEMS, 2125 Jan. 2007, Kobe, Japan, pp. 159162.

[10] Liu A Q, Zhang X M, Cai H, Tang D Y, Lu C. Miniaturized injection-locked laser using microelectromechanical systems technology, Appl.Phys. Lett., vol. 87, no. 10, 101101, 2005.

[11] Coldren L A, Miller B I, Iga K, Rentschler J A. Monolithic two-sectionGaInAsP/InP activeoptical-resonator devices formed by reactive-ion-

etching, Appl. Phys. Lett. vol. 38, 1981: 315317.

[12] Tsang W T, Olsson N A, Logan R A. High-speed direct single-frequency modulation with large tuning rate and frequency excursion incleaved-coupled-cavity semiconductor lasers, Appl. Phys. Lett. vol. 42,

1983: 650652.

[13] Cai H, Liu B, Zhang X M, Liu A Q, Bourouina T, Zhang Q X. Amicromachined tunable coupled-cavity laser for wide tuning range andhigh spectral purity, Optics Express, vol. 16, no. 21, 2008:

1667016679.

[14] Coldren L A, Miller B I, Iga K, Rentschler J A. Monolithic two-sectionGaInAsP/InP activeoptical-resonator devices formed by reactive-ion-

etching, Appl. Phys. Lett. vol. 38, 1981: 315317.

[15] Cai H, Liu A Q, Zhang X M, Tamil J, Tang D Y, Wu J, Zhang Q X.Tunable dual-wavelength laser constructed by silicon micromachining,Appl. Phys Lett, vol. 92, no.05, 051113, 2008.

A. Q. Liu received his PhD degree from National University of

Singapore (NUS) in 1994. His MSc degree was from Beijing

University of Posts & Telecommunications in 1988 and BEngdegree was from Xian Jiaotong University in 1982. Currently,he is an Associate Professor at the Division of

Microelectronics, School of Electrical & Electronic

Engineering, Nanyang Technological University (NTU). He

was an Associate Editor of IEEE Sensors Journal 2005 2008,and also Guest Editor of Sensors & Actuators (A Physical) in

2005 and 2006 respectively. He authored a book entitled

Photonic MEMS Devices Design, Fabrication and Control.

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