exter and direct modulator.pdf

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Direct & External Modulation

Direct and External Modulation of Light

Christophe Peucheret

DTU Fotonik

Department of Photonics Engineering

Technical University of Denmark

[email protected]

Abstract

This note is an introduction to the laboratory exercise on direct and external modulationin the course 34129 Experimental Course in Optical Communication. The concepts of direct

and external modulation are introduced and general requirements on the properties of thegenerated signals, such as extinction ratio and frequency chirp, are briefly discussed. Theprinciples and main features of external modulation mechanisms in electro-absorption andelectro-optic modulators, as well as direct current modulation of semiconductor laser diodesare outlined. Finally the assignments of the exercise itself are presented. The focus is on thestatic and dynamic characterisation of a directly modulated laser operating at 10 Gbit/s,as well as on comparison with external chirp-free modulation after propagation through alength of standard single-mode fibre.

Contents

1 Introduction 2

2 Direct versus external Modulation 2

2.1 Modulation concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22.2 Modulation technique requirements . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.2.1 Speed of operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.2.2 Extinction ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.2.3 Frequency chirping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3 External modulation 6

3.1 Electro-absorption modulators . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63.2 Electro-optic modulators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

4 Direct current modulation of semiconductor lasers 8

4.1 Turn-on delay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94.2 Extinction ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94.3 Bandwidth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104.4 Relaxation oscillations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104.5 Frequency chirping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

5 Description of the exercise 12

5.1 Static characterisation of a DFB laser . . . . . . . . . . . . . . . . . . . . . . . . 14

5.2 Dynamic characterisation of a DFB laser . . . . . . . . . . . . . . . . . . . . . . . 145.3 Transmission over standard single mode fibre . . . . . . . . . . . . . . . . . . . . 15

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34129 Experimental Course in Optical Communication

1 Introduction

The vast amount of data we are using and transmitting daily, either for plain telephony or whenusing the Internet, is generated and processed in the electrical domain. However, now that we

are reaching the end of this experimental course, you should have hopefully been made awareof the benefit (and the beauty) of using light to transport this data. For the time being, therealm of optical communication is long haul and high capacity submarine links (from about1000 to a couple of 10000’s km), as well as terrestrial trunk networks (up to a few 1000’s km;usually less in Denmark). Optical communication is also used for metropolitan area networks(MANs; a few 10’s to a few 100’s km) and is currently rapidly moving towards the end users inthe access network (e.g. fibre-to-the-home, FTTH).

In most cases, one of the benefits of optical communication is to allow the transport of rela-tively large capacities expressed in terms of bits per second for the digital optical communicationsystems we will be considering here.1 Typical bit rates employed today are 2.5-10 Gbit/s perchannel (meaning per wavelength in wavelength division multiplexing -WDM- systems where

the capacity can be increased beyond the channel bit rate by simultaneously transmitting alarge number of channels at different wavelengths in a single optical fibre) and the next genera-tion optical transport systems running at 40 Gbit/s per channel are ready, at least technically,to be deployed. So far, this order of magnitude for the bit rate is well above the needs of individual users and applications. Consequently, the data originating from a large number of users is multiplexed in time in the electrical domain before being transmitted at a high bitrate over optical fibres. A key functionality of an optical system is therefore the modulationoperation, which consists in “converting” the high bit-rate electrical data signal into the opticaldomain. Ideal modulation is therefore equivalent to performing a frequency translation fromthe baseband to an optical carrier frequency, of the order of 193 THz (i.e. 193×1012 Hz) forthe usual 1550 nm transmission window. Until now, most optical communication systems make

use of intensity modulation of the lightwave (i.e. its intensity or power is varied accordingto the data to be transmitted) since it allows to use a very simple detection process under theform of a photodiode whose generated photocurrent is proportional to the incoming opticalpower. In this exercise, we will therefore exclusively focus on intensity modulation, althoughother types of optical modulation, such as phase and frequency modulation, are also possibleand intensively investigated.

The goal of this exercise is to briefly introduce two different strategies used for the opticalintensity modulation operation, and to have a more in-depth look at the benefits and limitationof one of those methods, where the optical power emitted by a semiconductor laser is variedaccording to the data that the user wants to transmit.

2 Direct versus external Modulation

In this section, we present the concepts of direct and external modulation and discuss generalrequirements for the optical modulated signal.

2.1 Modulation concepts

As discussed above, the operation of modulation consists in transferring the data to be trans-mitted from the electrical to the optical domain. Two strategies, illustrated in Fig. 1, can beused to perform this operation. In the first option, called direct modulation , light is emitted

1

Optical fibres are also used for the transport of analogue signals, such as for instance in hybrid fibre-coaxialsystems used for the distribution of cable television (CATV).

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externalmodulator

power

time

current

time

voltage

time

current

time

power

time

power

time

DM laser

CW laser

Figure 1 Illustration of the concepts of direct (top) and external (bottom) modulation. In the directmodulation scheme, the driving current to a directly modulated (DM) semiconductor laser is variedaccording to the data to be transmitted. In the external modulation scheme, the laser that is subjectedto a constant bias current emits a continuous wave (CW) while an external modulator switches theoptical power on or off according to the data stream.

from a semiconductor laser only when a “mark” is transmitted. Ideally, no light should beemitted when a “space” is transmitted2. However, we shall see later that this is not alwaysthe case for practical implementations. In the second option, called external modulation , acontinuous wave (CW) laser is used to emit light whose power is constant with time. A secondcomponent, known as modulator , is then used as a switch to let the light pass whenever thedata corresponds to a “mark” and to block it whenever the signal is a “space”. This switchcan be implemented in several ways and we will briefly describe two physical effects that arecustomarily used to perform external optical modulation at high bit rates. Note that here, thekey point is that the physical process that should permit the switch to toggle between its twostates (“open” and “closed”) should be fast enough to allow proper operation at the desired bitrate.

2.2 Modulation technique requirements2.2.1 Speed of operation

As briefly introduced above, the physical processes that are exploited to perform the modulationoperation should be fast enough to allow proper operation at the desired bit rate. At 10 Gbit/s,the bit slot duration is 100 ps and we will expect the transmitter, whether a directly modulatedlaser or a continuous wave laser followed by an external modulator, to be able to switch betweenthe “mark” and “space” states within a fraction of this duration.

2In this note, we define the “mark” and “space” states as the high and low power levels, respectively, of an optical signal whose intensity has been modulated using binary modulation. These states correspond to thetransmission of “0” and “1” bits, respectively, provided no logical inversion of the signal has been performed. In

case the “space” state corresponds to the absence of optical power, the modulation format is known as on-off keying (OOK). In the more general case, it is described as amplitude-shift keying (ASK).

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2.2.2 Extinction ratio

The extinction ratio of the optical signal is defined as

ER =

P 1

P 0 , (1)

where P 1 and P 0 are the power levels corresponding to the “marks” and “spaces”, respectively.You have seen in a previous exercise that it is important to achieve a good extinction ratiofor the optical signal, i.e. to achieve a large separation between the power of the “marks” and“spaces”, and ensure that as little power as possible is present in the signal when a “space” isbeing transmitted. The effect of a poor extinction ratio will otherwise manifest itself under theform of power penalty at the receiver (i.e. an increased required optical power at the receiverin order to achieve a given bit-error-rate, typically 1.0×10−9, compared to the case of an idealsignal with infinite extinction ratio) that will reduce the power budget of the system. Underthe assumptions of a receiver limited by thermal noise, this penalty can be shown to be equal

toδ ER (dB) = 10 log10

ER + 1

ER− 1

. (2)

As an illustration, the power penalty calculated according to (2) is represented as a function of the extinction ratio in Fig. 2. In practice, values of the extinction ratio of the order of 15 dBwill be considered nearly ideal.

0 2 4 6 8 10 12 14 16 18 20

0

1

2

3

4

5

6

7

8

9

10

p o w e r p e n a l t y ( d B )

extinction ratio (dB)

0.0 0.5 1.0 1.5 2.0

0

1

2

3

4

5

normalised distance z/L D

b r o a d e n i n g f a c t o r

T / T

0

C β 2 > 0

C β 2 = 0

C β 2 < 0

Figure 2 Left: power penalty as a function of extinction ratio. Right: broadening of a Gaussian pulseas a function of distance due to the effects of linear chirp and group velocity dispersion. The propagationdistance is normalised to the so-called dispersion length defined as LD = T

20

/|β 2|.

2.2.3 Frequency chirping

The electric field of a lightwave whose carrier angular frequency is ω0 can be expressed as

E (t) = ℜ A (t) eiω0t , (3)where ℜ denotes the real part, and where A (t) is known as the complex envelope of thesignal. As the name suggests, the envelope is a complex number that can be further written interms of its modulus |A| and phase φ according to

A (t) = |A (t)| eiφ(t) =

P (t)eiφ(t), (4)

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where P (t) = |A (t)|2 is the power of the signal. The operation of intensity modulation thereforeconsists in varying P (t) according to the modulating electrical signal. However, the desiredintensity modulation of the lightwave is often accompanied by a modulation of its phase φinduced by the physical process used to realise the intensity modulation. Consequently, not

only the power P (t) becomes a function of time, but also the phase φ (t), which is very oftenan undesired feature. Observing eq. (3) and (4), we can define an instantaneous frequency of the optical signal according to

ω (t) = ω0 + ∂ φ

∂t. (5)

Consequently, a time varying phase is equivalent to a change in the signal instantaneous fre-quency. This frequency modulation is usually referred to as frequency chirping . The amountof frequency chirp depends on the physical mechanism used to achieve light modulation, as wellas on the design and operating conditions of the modulator.

Since the group velocity through an optical fibre is frequency dependent, an effect knownas group velocity dispersion , the different frequency components of the spectrum of a pulse

injected at the fibre input will travel at different speeds, hence leading to pulse broadening. Incase an intensity modulated signal is transmitted, the information pulses will spread out of theirallocated time slots, leading to inter-symbol interference, which will in turn introduce errorsat the decision circuit (where the received signal is compared to a given threshold to decidewhether a “mark” or “space” has been transmitted) and lead to a degraded bit-error-rate.Intuitively, the effect of frequency chirping will be to broaden the spectrum of the modulatedsignal. As the effect of dispersion worsens with increasing signal spectral width3, frequencychirping will, in general, result in reduced tolerance to group velocity dispersion.

We now make abstraction of the mechanism responsible for light modulation, and considerthe propagation of a single chirped pulse in an optical fibre. For simplicity, we assume a linearlychirped Gaussian pulse4 and consider its dispersion induced broadening when propagating along

a fibre whose dispersion is described by the β 2 parameter5

. In this case, the complex envelopecan be expressed as

A (t) = A0 exp

−1 − iC

2

t

T 0

2, (6)

where T 0 can be related to the full-width half-maximum of the signal T FWHM according toT FWHM = 2

√ ln 2 T 0. The instantaneous frequency of the signal is therefore

ω (t) = ω0 + C

T 20t, (7)

which varies linearly with time, hence the “linearly chirped” designation used to describe such

a pulse. One of the nice features of our choice of a linearly-chirped Gaussian pulse is that it3This is the reason why the effect of dispersion becomes more critical at high bit rates. If the system is limited

by dispersion, the maximum tolerable transmission distance for 1 dB power penalty is about 60 km over standardsingle-mode fibre for 10 Gbit/s non return-to-zero (NRZ) modulation. Since the effect of dispersion scales as thesquare of the bit rate, this distance will be reduced to about 4 km when the bit rate is increased to 40 Gbit/s.Hopefully some techniques exist that can compensate for the effects of dispersion and allow for long distancetransmission at high bit rates.

4It is important to emphasise that this is an assumption for the present calculation. Usually, the frequencychirp generated in transmitters is not linear. However the present discussion based on a linear chirp assumptionwill prove useful to qualitatively discuss the interaction of transmitter chirp and group velocity dispersion.

5β 2 is the second derivative with respect to angular frequency ω of the propagation constant β . Therefore β 2characterises the second-order dispersion and can be related to the usual fibre dispersion parameter D , usuallyexpressed in units of ps/(nm·km), through D = − 2πc

λ2 β 2, where λ is the wavelength and c is the velocity of light

in vacuum. For the commonly used standard single-mode fibres, D ≈ 17 ps/(nm·km) at 1550 nm, and thereforeβ 2 is negative.

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enables to calculate analytically the pulse shape after propagation over a distance z throughan optical fibre, and hence to deduce the pulse width. The results are shown in Fig. 2 where itcan be seen that three different propagation regimes can be identified depending on the valueof the product of the chirp parameter C and the dispersion parameter β 2. If C β 2 > 0, the pulse

will broaden faster than in the case of an unchirped signal for which Cβ 2 = 0. On the otherhand, if Cβ 2 E g. Such a shift of the absorption edge of a semiconductorunder the influence of an external voltage is represented in Fig. 3. By properly selecting thesignal wavelength so that it experiences a significant change in absorption when the voltageis applied, it thus becomes possible to achieve optical modulation controlled by an electricalsignal. A typical absorption versus applied voltage transfer function for an electro-absorption

modulator is also shown in Fig. 3.Since the absorption and refractive index of a semiconductor material are related by Kramers-Kronig relations of the type

∆n (ω) = c

π

+∞0

∆α (ω′)

ω′2 − ω2 dω′, (8)

where ∆n is the change in refractive index induced by the change in absorption ∆α, and c isthe velocity of light in vacuum, shifting the absorption edge to achieve optical modulation willalso induce a change in the refractive index of the material, hence in the phase or instantaneousfrequency of the signal being modulated. Consequently, some amount of frequency chirping will

be introduced by electro-absorption modulators. However, the generated frequency chirp willusually be smaller than when direct current modulation of a semiconductor laser is used.

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wavelength

a b s o r p t i o n

V = 0

V ≠ 0

voltage (V)10 2 3 4 5

5

10

15

20

25

30

f i b r e - t o - f i b r e

l o s s ( d B )

Figure 3 Left: absorption of a semiconductor as a function of wavelength with and without an externalapplied electric field (adapted from [2]). Right: typical loss versus applied voltage curves for an electro-absorption modulator (adapted from [1]).

0 1 2 3 40.0

0.5

1.0

a)

b)

t

t

Figure 4 Principle of operation of a Mach-Zehnder modulator.

3.2 Electro-optic modulators

The refractive index of some materials can be modified by applying an external electric field tothem through the linear electro-optic effect . Since the phase shift experienced by a lightwave

of wavelength λ propagating through a length L of a medium with refractive index n is

φ = 2π

λ nL, (9)

a straightforward application is the realisation of phase modulators made from an electro-opticwaveguide subjected to a time dependent electric field. The applied voltage will modulate therefractive index of the waveguide material, hence the phase shift experienced by a lightwavepropagating along the waveguide. However, legacy optical communication systems typically relyon intensity modulation of light. This can be achieved by transforming phase modulation induced by the electro-optic effect to intensity modulation using an interferometric

structure.

In order to illustrate the principle, we consider the simple interferometric structure repre-sented in Fig. 4. It is based on a Mach-Zehnder interferometer including one electro-optic mate-

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Figure 5 Ilustration of the concept of direct current modulation of a semiconductor laser. The laserdriving current is varied according to the modulating signal, resulting in modulation of the emittedpower. The modulated optical signal is represented for two different values of the bias current, resultingin improved dynamic behaviour, but poorer extinction ratio when the bias current increases.

4.1 Turn-on delay

One could in principle achieve an infinite extinction ratio by letting the laser driving currentbelow threshold in order to generate “spaces” and increasing it to an above-the-threshold valuewhenever “marks” need to be modulated. Under these conditions, the laser switches from a statewhere no lasing occurs, to a state where population inversion is achieved, resulting in lasingoperation. However, population inversion is achieved by injecting carriers into the structureand it takes some time for the carrier density to reach its threshold value when lasing begins.Consequently lasing will be delayed from the time the driving current is increased by a time tdknown as turn-on delay. The turn-on delay can be approximated by

td = τ c ln

I 1 − I 0I 1 − I th

, (12)

where I 1 and I 0 are the driving currents corresponding to “marks” and “spaces”, respectively,and τ c is the carrier lifetime, of the order of a few nanoseconds. Such a delay might not becompatible with high speed operation, for instance at 10 Gbit/s, where the bit slot is only100 ps long.

4.2 Extinction ratio

We have seen above that high speed operation of a directly modulated laser diode will requireto operate above threshold, even when a “space” is generated, in order to avoid the turn-ondelay. In this case, lasing occurs all the time, but the laser output power will take one of the

two values, P 1 or P 0 = 0, for “marks” and “spaces”, respectively, depending on the value of thebias current. Consequently, an infinite extinction ratio will not be achieved.

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4.3 Bandwidth

Furthermore, the modulation bandwidth of a directly modulated laser can be shown to increasewith the bias current according to

∆f 3dB ≈

3GN (I b − I th)4π2e

1/2, (13)

where GN is related to the dependence of the rate of stimulated emission on the carrier number,I b and I th are the bias and threshold current, respectively, and e is the elementary charge. Highmodulation bandwidths suitable for high speed operation will therefore be obtained for largebias currents. Note however that the modulation bandwidth as stated above is defined undersmall signal modulation conditions (i.e. the electrical modulating signal is taken as a sinusoidalsignal whose peak-to-peak current is small compared to I b − I th). However it can still providevaluable qualitative information under large signal modulation such as when the semiconductorlaser is used for digital communication.

4.4 Relaxation oscillations

When the laser is subjected to a transient in its bias current, for instance during a transitionfrom a “mark” to a “space” or vice versa, its output power will exhibit damped oscillationsknown as “relaxation oscillations”. Both the oscillation frequency and damping rate dependon the laser output power, hence on the value of the driving current. The physical origin of those oscillations is the interplay between the injected carriers and emitted photons. It can beshown that the relaxation oscillation frequency and the damping rate increase with increasingcurrent. One will therefore pay attention to those oscillations when the laser is biased close tothreshold, as illustrated in Fig. 6.

4.5 Frequency chirping

When the laser is directly modulated, a change in the bias current will lead to a change in thecarrier density, which in turn will lead to a change of the refractive index of the semiconductormaterial. Since the lasing wavelength is determined from the feedback condition in the lasercavity, which itself depends on the refractive index6, the instantaneous frequency of the emittedsignal will be a time varying signal. Consequently, directly modulated lasers are inherentlychirped, an effect that has prevented their use for the generation and transmission of high bitrate signals. The variations of the instantaneous frequency at the output of a directly modulated

laser can be shown to be equal to

∆ν (t) = α

4π

d

dt [ln P e (t)] + κP e (t)

, (14)

where α is known as the linewidth enhancement factor and κ is the adiabatic chirp coefficient . It can be seen that the chirp consists of two terms. The first term, namedtransient chirp only exists when the emitted power varies with time, for instance duringtransients of the applied current I (t) and induced relaxation oscillations, while the secondterm, named adiabatic chirp , is responsible for the different emission frequencies observedunder steady state when a “mark” or “space” is transmitted.

6

For instance the resonance condition in a distributed feedback laser can be expressed as Λ = mλ/(2n), whereΛ is the period of the refractive index corrugation, n is the mode index, λ is the wavelength and m is an integer.

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4.6 Summary

The points above are illustrated in Fig. 6 and 7 where the operation of a single mode semicon-ductor laser has been simulated numerically for bias currents7 of 10 and 30 mA, respectively,

and peak-to-peak current of 20 mA. For this laser, the threshold is equal to about 5 mA. Thelaser is operated at a bit rate of 2.5 Gbit/s.

0 1 2 3 4 5 6

0

2

4

6

8

10

12

14

16

p o w e r ( m W )

time (ns)

-200 -150 -100 -50 0 50 100 150 200

-140

-120

-100

-80

-60

-40

-20

0

p o w e r ( d B m )

frequency detuning (GHz)

0 1 2 3 4 5 6

-15

-10

-5

0

5

10

15

f r e q u e n c y c h i r p ( G H z )

time (ns)

Figure 6 Simulated waveform, frequency chirp and spectrum at the output of a directly modulatedlaser for a bias current of 10 mA. The laser threshold is ∼ 5 mA and the bit rate is 2.5 Gbit/s.

In the case of 10 mA bias, strong relaxation oscillations are visible for both “marks” and“spaces”. By comparing those oscillations, it can be checked that both the relaxation oscillationfrequency and the damping rate increase with increasing laser driving current. The relaxationoscillations constitute a fast variation of the laser output power, hence the laser chirp is dom-inated by the transient component. Only during long sequences of consecutive “marks” doesthe output power stabilise to its steady state value. This dominant transient chirp behaviourresults in significant broadening of the signal spectrum.

On the other hand, when the bias current is increased to 30 mA, the highly damped relax-ation oscillations result in well defined “mark” and “space” levels, hence to a signal waveformthat does not present as much distortion as in the 10 mA bias case. However, the power levelcorresponding to the “spaces” is higher than in the previous case and the extinction ratio isdecreased. Apart from during transients of the modulating signal, a strong adiabatic chirp com-ponent is visible, leading to distinct frequencies when “spaces” and “marks” are transmitted.The dominant adiabatic chirp can also be observed in the spectrum, which is narrower than inthe previous case and exhibits characteristic peaks at the two frequencies corresponding to the“marks” and “spaces”.

7Defined here as the driving current corresponding to the “spaces”.

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0 1 2 3 4 5 6

0

2

4

6

8

10

12

14

16

p o w e r ( m

W )

time (ns)

-200 -150 -100 -50 0 50 100 150 200

-140

-120

-100

-80

-60

-40

-20

0

p o w e r ( d

B m )

frequency detuning (GHz)

0 1 2 3 4 5 6

-15

-10

-5

0

5

10

15

f r e q u e n c

y c h i r p ( G H z )

time (ns)

Figure 7 Simulated waveform, frequency chirp and spectrum at the output of the same directly mod-ulated laser as the one used to generate the results of Fig. 6 , but for an increased bias current value of 30 mA.

5 Description of the exercise

Figure 8 Picture of the packaging of the type of 10 Gbit/s directly modulated laser used in theexperiment. Source: NEL.

In this exercise, we study the static and dynamic characteristics of a state-of-the-art com-mercially available distributed feedback (DFB) directly modulated laser (DML) designed for10 Gbit/s operation. The device is manufactured by NEL (NTT Electronics Corporation) inJapan and its packaging is represented in Fig. 8. The modulation input (coaxial connector)is clearly visible on one side of the packaging, while the other pins are for the bias current,temperature control (temperature sensor and thermo-electric cooler) and monitoring.

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Important:

• Do not apply any modulation to the DML before having adjusted the bias to a sufficientlylarge value. This might otherwise result in reverse biasing the laser diode junction, whichmight irremediably destroy the laser. When you switch the laser off, turn off the modu-lation first, then decrease the bias current.

• When you deliver some radio-frequency (RF) power, for instance the laser modulatingsignal, always ensure that this is done to a matched impedance (typically 50 Ω) device.Consequently, always switch on the bias to an electrical amplifier before you actually turnthe input signal on. Turn on the path of an RF signal from load to source. Switch it off from source to load.

• Never disconnect an optical connector without having ensured first that the light has beenswitched off. Failure to comply might result in damage to the connector and potentialeye hazard.

• Handle the optical connectors with care. Carefully align the connectors to the adapterbefore making a connection. Clean the connector and check it through the inspectionmicroscope before making any connection.

• In case of doubt, remember there is nothing wrong about asking!

Based on some of the recommendations above, always follow the following laser turn-on procedure:

1. First of all, ensure that you know the value of the peak-to-peak current you will apply to

the laser. Check the modulating signal peak-to-peak voltage on an oscilloscope and usethe conversion table provided to assess the corresponding peak-to-peak current. This willenable you to ensure the laser bias current is sufficiently high so that there is no risk todamage the laser by reverse biasing the laser diode.

2. Make sure the temperature controller output is turned on.

3. Increase the bias current to a sufficiently large value, depending on your modulating signalpeak-to-peak current.

4. Switch on the laser driver amplifier.

5. Turn the data output of the pattern generator on.

In order to switch the laser off, take the steps above in reverse order. When turning on thelaser, a safe approach is to increase the bias current above the desired value (but not too high:never exceed 100 mA), switch on the modulation, then decrease the bias current. Note that thisprocedure is safe for the model of laser diode and modulating signal generation method used inthis experiment (where the bias current will correspond to the value of the driving current forthe “marks”). However a different procedure might be required for different laser models andexperimental set-ups.

The experimental configuration is represented in Fig. 9. The actual set-up might actually

slightly differ from the one represented in the figure. It is your responsibility to draw the actualset-up used for your investigations in your laboratory book.

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oscilloscope

opticalspectrumanalyser

10 Gbit/spattern

generator

t°controller

currentsource

data

trigger

ch1

3 dB

ch2

DML

electricalamplifier

SMFEDFA

Figure 9 Experimental set-up.

5.1 Static characterisation of a DFB laser

In this first part of the exercise, the optical power versus current characteristic of the semicon-ductor laser is measured. This static characterisation (i.e. in the absence of modulation) willbe used later to determine the working point (bias) and peak-to-peak modulation current toapply to the laser for 10 Gbit/s modulation.

Assignment:

1. Connect directly the output of the DML to the optical spectrum analyser.

2. Gradually increase the laser bias current from 0 to about 80 mA. Once the threshold hasbeen reached, record the signal power and its centre wavelength on the optical spectrumanalyser. Use a resolution bandwidth of 0.1 nm.

3. Plot the power versus bias current and wavelength versus bias current characteristics of the laser.

4. Extract the value of the laser threshold.

5. Based on the measured static characteristic, decide on the laser biasing point and peak-to-peak current that you will apply for 10 Gbit/s modulation of the laser. Comment yourchoice.

5.2 Dynamic characterisation of a DFB laser

We now apply a 10 Gbit/s NRZ signal to the laser. The signal is a pseudo-random binarysequence (PRBS) generated from a bit pattern generator that is amplified using a 12.5 Gbit/sdriver amplifier. At the output of the amplifier, the signal is split in a 6 dB power splitter so

that it is possible to monitor it on a high-speed sampling oscilloscope. In this exercise, thepeak-to-peak modulating current has been predefined to ensure safe operation of the laser.

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Assignment:

1. Bias the laser diode to a sufficiently large value (e.g. 60 mA)

2. Turn on the modulating signal. Please refer to the turn on procedure described above.

3. Monitor the electrical driving signal. Record the peak-to-peak voltage and calculate thecorresponding peak-to-peak current.

4. Vary the bias current and examine the signal waveform or eye diagram, as well as thesignal spectrum. Compare the eyes and spectra for different values of the bias current.

5. Record typical eye diagrams and spectra depending on the operating conditions (biascurrent) of the laser.

6. Provide an interpretation for the differences observed in the waveforms and spectra under

small and large bias.

5.3 Transmission over standard single mode fibre

The signal generated in the previous step is now transmitted through a length of 35 km standardsingle-mode fibre (SMF). The signal quality after transmission will be assessed by observingthe eye diagrams. In particular, the influence of the bias current will be investigated, and thesignal behaviour will be related to the knowledge you gained from the theoretical section andthe dynamic characterisation of the laser. In a final step, the directly modulated laser will bereplaced by a transmitter consisting of a continuous wave laser and a chirp-free electro-opticMach-Zehnder modulator, and the signal quality after transmission will be compared.

Assignment:

1. Based on the dynamic characterisation of the laser, select a bias current you believe issuitable for the generation of 10 Gbit/s signals.

2. Switch the laser off according to the turn-off procedure described above, and connect35 km SMF at the output of the laser. An erbium doped fibre amplifier (EDFA) will beconnected to the output of the fibre spool in order to compensate for the fibre loss thatis of the order of 0.2 dB/km.

3. Observe the signal waveform and eye diagram after propagation through 35 km SMF.

Comment.

4. Generate a 10 Gbit/s signal using the external lithium-niobate Mach-Zehnder modulatorprovided. Observe the waveform or eye diagram at the output of the modulator.

5. Transmit the externally modulated 10 Gbit/s signal through the 35 km SMF spool andmonitor the eye diagram at the fibre output. Compare your observations with the onesyou made in point 3 above.

6. If time allows, we will also show that some special type of optical fibres, known as disper-sion compensating fibres (DCFs) can be used to compensate for the dispersion accumu-lated over transmission in SMF. We will add a suitable piece of DCF after the SMF in

the direct modulation case and will observe the waveform after propagation through theSMF+DCF.

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References

[1] T. Ido, S. Tanaka, M. Suzuki, M. Koizumi, H. Sano, and H. Inoue, “Ultra-high-speedmultiple-quantum-well electro-absorption optical modulators with integrated waveguides,”

Journal of Lightwave Technology , vol. 14, no. 9, pp. 2026–2034, Sep. 1996.[2] A. Ramdane, F. Devaux, N. Souli, D. Delprat, and A. Ougazzaden, “Monolithic integration

of multiple-quantum-well lasers and modulators for high-speed transmission,” IEEE Journal of Selected Topics in Quantum Electronics , vol. 2, no. 2, pp. 326–335, Jun. 1996.

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exter and direct modulator.pdf

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