Optical control of period doubling in a gain-switched Fabry-Perot laser diode and its application in all-optical clock division

K.K. Chow, C. Shu, Y.M. Yang and H.F. Liu

Abstract: Optical injection is shown to he an effective means to suppress or to enhance period doubling of a gain-switched laser diode. The role that the injection plays depends on whether the laser resonance frequency is larger or smaller than half the modulation frequency. Reversible control of period doubling has heen demonstrated experimentally using a modulated injection source. An injection power as low as 0.03 mW has been used to control the output pulse rcpctition frequency of the slave laser. The response time is measured to be shorter than l00ps. The phenomenon has been applied successfully for use in all-optical clock frequency division. Replacing the electrical modulation with an input clock signal at 19.6 GHz, a frequency-halved output clock with a remarkable low-level phase noise (-87.7 dBc/Hz at 10 kHr offset frequency) has heen obtained. The change in the phase noise level is observed to he smaller than I dB over a frequency detuning range of 400 MHz.

1 Introduction

Optical networks employing ultrafast optical time-division- multiplcxing (OTDM) will be required to accommodate increasing traffic in communications. A reliable and conve- nient approach to generate picosecond optical pulses for high-speed OTDM cornmunication systems is by gain- switching of laser diodes [ I , 21. Such large signal direct modulation of semiconductor lasers is a simple and flexible technique to generate single- or multi-wavelength optical pulses for high-bit-ratc optical fibre communication systems. However, a number of reports showed that directly modulated semiconductor lasers may exhibit various kinds of irregular behaviour such as period doubling, period quadrupling and deterministic chaos [3-71. These kinds of irregular behaviours affect the stability and applications of the Iascrs.

Considering the nonlinear dynamics of a semiconductor laser, the transient behaviour is governed mainly by the population inversion and the optical field inside the laser cavity [3]. Hence, semiconductor lasers are expected to be stable when only a DC current is applied. However, current modulation and/or external light injection may lead to

ID IEE. 2003

IEE P,ocredings online no. 2003021 I Dol: IO.I04~iip-opt:2003021 I Paper tirst received 10th May and in revised form 3rd December 2002 K.K. Chow and C. Shu are with the Depanmenl of Iil~clronic Engineering. The Chinerc University of Hung Kong, Shatin. N T . Hong Kong

Y.M. Yang is with the Photonics Research Laboratory, Australian Photonics Cooperative Research Centre. Dcpanment of Electrical and Electronic Engineering, l h e University of Melbaurnc. Piirkville, Victoria 3010, Australia H.F. Liu i s on l e a w from the University of Melboumc and i s currently with the Photonics Technology Operation, lnicl Cuip.. 300 Cnro Drive, San Jose, CA 95138, USA

I€€ Pmc -Oplo&mon., 1/01, 180, rvo. 3. .Jtmc 2003

instabilities. Investigation of the nonlinear dynamics of semiconductor lasers and their relations with device para- meters is important not only for stability but also for developing new applications of the nonlinear characteris- tics. Previous work revealed that the optical feedback to the laser diode is a critical factor that affects the laser non- linearities [8-IO]. Further investigations show that the combination of both current modulation and cxternal optical injection can add one more dimension in control- ling such nonlinear behaviour [ I I , 121. In this paper, we address a simple and efficient technique to control the laser nonlinearities by adjusting the optical power injected into a directly modulated Fabry-Perot laser diode. As a result, the pulsc repetition frequency can he optically controlled and the prohlcm of instability is solved by optimising the injection power.

With regard to the period doubling of a directly modu- lated laser diode, one application is in all-optical clock frequency division. In an OTDM network, a local clock is always required at each node. For all optical 3R regcnera- tion, the clock rate should be the same as that of the transmitted optical data. In the case of all optical demulti- plexing in the detection process, a base rate clock is required and it should be a subharmonic of the data rate transmitted on the fibre link. Many methods of realising all-optical clock recovery at the transmitted ratc have been demonstrated. I t is also important to have a direct recovery of the base rate clock. One possiblc solution is to employ a frequency division unit in the process. The all-optical approaches are intrinsically simpler in the sense that optical-to-electrical conversion and electrical circuit noise can he eliminated. Some techniques have been suggested to realise all-optical clock frequency division using semi- conductor optical amplifiers [13, 141. Our work provides another effective method to serve this purpose using relatively inexpensive components and a simple approach.

A preliminary experiment is introduced to investigate the control of period doubling by external CW optical

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injection. The effects ofthe power level and the wavelength detuning of the external injection light are studied and discusscd, and the reversible control of period doubling in the configuration is described. A relatively weak CW injection signal is found to be sufficient for switching the pulse repetition rate between 5 and 10 GHz. By exploiting period doubling, all-optical clock division from 19.6 GHz to 9.8 GHr has been demonstrated and the phase noise is relatively low. The result promises potential use in ultrafast control of the repetition frequency for different system applications.

2 Control of period doubling by external light injection

When period doubling occurs, the frequency of the laser output becomes half of the modulation frequency. Under most situations, period doubling should be avoided for the applications. However, the appearance of sub-harmonic components in period doubled output suggests the possi- bility of realising optical clock frequency division. There- fore, from an application point of view, it is important to understand how to control the occurrence of period doubling in directly modulated semiconductor lasers.

This Section focuses on the investigation of the control of period doubling in a directly modulated semiconductor laser with external CW optical injection. We show that period doubling can cither be suppressed or enhanced by appropriate external CW injection. The dependence on the injection wavelength and the injection power is investi- gated systematically to establish an improved understand- ing towards the control of period doubling in modulated semiconductor lasers.

The experimental set-up is shown schematically in Fig. 1. The semiconductor laser is a I .57 pm Fabry-Perot laser diode (FP-LD) with a cavity length of I75 pm and a threshold current of 28.4 mA. The estimated threshold gain is 170cm-l and the carrier life time is 0.4 ns, with a linewidth enhancement factor of 4. The lascr diode is directly modulated by a radio frequency (RF) signal gene- rated from a low phasc noise synthesiser. A 40 dB power amplifier is used to boost the signal power. A tunable laser is employed to provide an external CW optical signal. Aftcr bcing amplified by an erbium-doped fibre amplifier (EDFA), the optical signal is injected into the FP-LD through an optical circulator. An attenuator is used to adjust the optical power level of the injection light and a polarisation controller i s used to control the coupling efficiency of the light into the lasing mode. The output pulses from the circulator are observed using a light-wave test set.

PC EDFA

attenuator

light wave synthesiser

Fig. 1 Exprririrentnl set-up on the conrml qfprriod dovhling b), memo1 light bljecrio~ FP-LD: Fabry-Pcrot laser diode: E D F A erbium-doped fibre amplifier; PC: polansadon conliollcr

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When the laser is biased at 42.9 mA, the resonance frequency is 6.9 GHz. Under this bias condition, the laser exhibits period doubling when it is directly modulated by a large current signal at IOGHz with an RF power level of 18 dBm, corresponding to a modulation index of 1.17. The avcragc output power of the laser is -6dBm. Fig. 2a shows the measured RF spectrum of the modulated laser. The power of the 5 GHz component is 1.5 dB higher than that of the modulation frequency (Fmod) of 10 GHz. By injecting an appropriate CW optical signal, the 5 GHz component can be suppressed. Fig. 20 shows the corres- ponding RF spectrum of the modulated laser with a CW optical injection at a wavclcngtb of 1579.53 nm and an optical power of -5.5 dBm. By comparing Figs. 2a and 2b, i t is noted that the 5 GHz component has been suppressed by 27 dB. Period doubling has been completely suppressed by the optical injection. A pulse train with a repetition rate of IOGHz is obtained in a separated time domain measurement.

To characterise the suppression effect caused by optical injection, we define the suppression ratio as the ratio of the RF intensity at half of the modulation frequency to that of the modulation frequency. The dependences of this ratio on the injection power and the injection wavelength are investigated through measuring the RF spectrum of the output light from the laser with a light-wave test set.

- io -

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Thc impact of injection power is evaluated by measuring the suppression ratio against injected optical power over a range from -25dBm to OdBm for a fixed wavelength detuning. The wavelength detuning is referenced to 1579.53 nm, which is the centre of the chirped spectrum defined at the midway between the 3 dB points. Fig. 3a shows the measured suppression ratio as a function of the injected optical power when the wavelength detuning is zero. The effect of optical injection is negligible when the injected optical power is smaller than -22 dBm. As the optical power is increased from -22 dBm to -7.5 dBm, the suppression ratio is reduced from 0 to -27 dB. For injection power in excess of -7.5 dBm, the suppression ratio remains nearly a constant at -27 dB within a range of 1 dB.

The injection wavelength is also varied across a chirped spectrum to examine the range of the injection wavelength variation when the injection power is fixed. Fig. 36 shows three curves for a corresponding injected optical power level of -3.5 dBm, -5.5 dBm and -9.5 dBm, respectively. The spectral width of the chirped mode is 0.62 nm. The suppression effect decreases as the injection wavelength moves away from the centre to the edge of the chirped spectrum, as shown in Fig. 36. However, the dcpcndence on the injected wavelength becomes less sensitive when the injection power is higher. When the injection power is -9.5 dBm, the maximum suppression is only about 25 dB occurring over a very limited injection wavelength range. When the injection power is increased to -5.5 dBm, larger

-20 -1 5 -1 0 -5 0

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2 17

0 0

0 0 A

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suppressioii (27 dB) is achieved over an injection wave- length range of 0.12 nm near the centre of the chirpcd spectmm. For an injection power level of -3.5 dBm, this range is widened to 0.2211117 around the centre of the chirped spectrum.

The period doubling enhancement experiment is conducted using a similar set-up as shown in Fig. I . The DC bias is set at 35.6 mA for the experiment. Under this bias condition, the resonance frequency is 3.6 GHr. The laser is directly modulated by a current signal at 10GHz with an RF power level of 0 dBm and a modulation index o f0 . 18. A smaller signal is used here to avoid a shift in the resonance frequency caused by the modulation. The aver- age output power of the FP-LD is -8 dBm.

Fig. 4 shows typical experimental results on the enhancement of period doubling as the injected optical power is increased. Without the optical injection, there is no period doubling. The resonance frequency of the laser is -3.6 GHz and the amplitude of the resonance peak is negligible. Fig. 40 shows the measured RF spectrum under this condition.

By injecting a CW optical signal with an injection wavelength of 1577.09nm that is 0.04 nm shorter than the central wavelength of the laser mode, the resonance response of the laser is changed. As the injected optical power increases, the resonance frequency shifts to a higher frequency. Fig. 46 shows the measured RF spectrum when the injected optical power is -14.0dBm. The resonance peak shifts to -4.5 GHz with a much larger amplitude. When the resonance frequency is further shifted towards 5 CHz, which is half the modulation frequency, period doubling enhanccment occurs. When the injection power level is -12.0dBm a very sharp peak appears at 5.0 GHz as shown in Fig. 4c. The intensity of the F,,,,d/2 compo- nent is -7.5 dB larger than that of the F,nod component. As the injection power is further increascd, the period doubled signal is reduced gradually since the resonance frequency is shified to an even higher frequency. Fig. 4d shows the measured RF spectrum when the injection power is -10.0dBm. The resonance frequency is shifted away from 5.0 GHz to 5.6 GHz and period doubling disappears.

The resonance peak shift caused by CW optical injection has been verified clearly in the experiment. When the resonance frequency is shifted to half of the modulation frequency, a very sharp peak appears at FInod/2. As thc injection power is further increased the resonance frequency is also increased. Standard rate equations for an FP-LD with optical injection can be solved to provide information on the photon density, the corresponding phase and the carrier density. Assuming a sinusoidal modu- lation of the injection current, the numerical solutions with different injection power level can be obtained [ I I]. In our experiments, chaotic behaviours are observed under some conditions when the FP-LD is initially in the period- doubled state. When the injection power is increased from -25 dBm to 0 dBm, chaotic behaviours occur when the injection power is low. Under moderate injection power, weak components at period quadrupling and period tripling can be observed. With high injection power, optical injec- tion locking occurs and period doubling suppression can be

-30 I realised, and this result is in good agreement with previous

In summary, optical injection o fCW laser light can serve b both the suppression and the enhancement of period

doubling in a directly modulated FP-LD. The role of the light injection depends on the initial biasing condition and hence the resonance frequency of the FP-LD. At a given bias, the amount of the injected light can also govern the

4.15 4 . 1 0 -0.05 0 0.05 0.10 0.15 work [ I O , 111. wavelength detuning. nm

Fig. 3 Output sidr~mode~su~p,.~ssion-rrrtio a Dependence on optical injection power h Dcpcndcnce on detuning of injection wavelength

IEE Pmc.-Uploeleclmn., Y u l 150, No. 3, June 2003 24 1

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frequency. GHr

C

b

0 5 10 15 20

frequency. GHz

d

Fig. 4 o Without optical injcctian trd With optical injection power of -14, -12, -IOdBm, respectively

Output RF .spectra ofFP-LD under direct n,oduIariorr wirh 0 dBaz RF power 01 10.0 GHi

output repetition frequency of the FP-LD. In the following Section, wc describe the use of a modulated injection source to switch an FP-LD between the normal operation and the period-doubled states. Reversible control of period doubling is demonstrated with a transient response faster than loops.

3 Reversible control of period doubling

In this Section, optical control of period doubling is demonstrated through injection seeding of a directly modu- latcd laser diode. The experimental configuration is shown in Fig. 5. The slave laser is a 1.55 p n FD-LD with a 350 pni cavity length and a 20 mA CW threshold current. A 21 dBm, IOGHz radio frequency (RF) sinusoidal signal is derivcd from a frequency synthesiser and is used to directly modulate the laser diode. The DC bias is 27 mA. The modulation index is 2.63 and the average output power is -5 dBni. The optical source for injection seeding is a wavelength-tunable laser diode. In contrast to the set-up in Fig. I, the tunable output is now fed to an optical modulator before it is directed to the FP-LD. Thc modu- lator is driven by a 1 GHz square-wave that repeatedly switches on and off the light injected into the FP-LD through a circulator. The system output is characterised with an optical spectrum analyser of 0.1 nni resolution and

242

a 32 GHz photodetector together with a digital sampling oscilloscope.

When the FP-LD is operated with the above driving condition, i t exhibits nonlinear behaviour, and period doubling of the output pulses is observed. The resonance frequency for relaxation oscillation of the FP-LD is found to decrease with increasing power of the direct modulation signal. The frequency drift will stop at about half the modulation frequency when the RF power reaches a certaiu level, causing period doubling of the output pulses [9]. It implies that the repetition rate of the output pulses can be

frequency trigger square-wave Synthesiser ge"Wa10,

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Fig. 5 duirbling in a Fohq-Pemt /user diode (FP-LD) PCI, PC2: polarisation controllers

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x 28 mA 0 2 7 m h

0.05 0.10 0.15 0.20 injection power, mW

Fig. 6 Dependences of output sirle-,nodr-s*,~p~',.~~i~l, ratio and repetition rate on opticul injectioii power Doncd-line: 5 GHr pulses: solid-line: mixed pulses; dashcd-iinc: 10 CHr pulses

iOGHr 29 mA ............... 28 mA ...... -. -. .. -. .

._..__..____.___..__.. 5 GHr mixed pulse5 ..................

-. . -. .. -. . -. . -. . .?7 FA. -

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optically controlled. Fig. 6 shows the dependences of the output frequency and the side-mode suppression ratio (SMSR) on the optical injection power with different DC bias of the FP-LD. Without injection, the frequency of the pcriod-doubled signal is 5 GHz. As the injection power increases, the output frequency changes from 5 GHz to a mixture of 5 and 10 GHz. In the case of 27 niA DC bias, the output frequency becomes stable at 10CHz with an injection power above 0.03 mW. I t is worth mentioning that at 0.03 mA injection power level, the SMSR is only 12 dB. To increase the SMSR to over 30 dB, the injection power needs to he increased to -0.17mW. The result indicates that the injection powcr required to suppress period doubling is far weaker than that used for single-mode injection seeding. Therefore, the output repctition rate can be controlled by a relatively low-power light signal. Whcn the bias current is incrcased to 29 mA, the in.jection

time

power needed to stabilise the 10 GHz output is increased to 0.08 inW.

Using an optical modulator with the injection signal, the output response of the FP-LD can be analysed. Fig. 7 shows the optical waveform used for injection, graph (i), roughly approximated by a square-wave turning on and off at I GHz. The corresponding output from the FP-LD is shown in graph (ii). We find that the switching of frequency between 5 and IOGHz can be completed within tens of picoscconds, showing the potential for application in ultra-fast fibreoptic links.

The experimental results i n this Section show the fast and reversible control of period doubling in a gain- switched laser diode. One of the applications of period doubling is in all-optical clock frequency division. To realise an all-optical processing scheme, the electrical modulation on the FP-LD is being replaced by optical modulation. With this modification, optical clock division from 19.6 to 9.8 GHz has been demonstrated successfully and the work is described in the following Section.

4 All-optical clock frequency division

The fact that period doubling can be cnhanced by optical injection suggests that it is possible to realise all-optical clock frequency division by rcplacing the electrical current modulation with optical modulation using an input optical clock signal. The principle is shown in Fig. 8.

Injection seeding on the FP-LD with thc input clock signal causes optical modulation on the carrier and photon densitics. The resonance frequency of the FP-LD is depen- dent on the power of thc input clock. As the input level changes and the resonance frequency is shifted to half of thc input clock frequency, period doubling will occur. As a result, the frequency of the Iascr output power becomes half of the input clock signal. Hence, a frequency halved optical clock signal can be obtained.

Thc expcriniental sct-up that performs all-optical clock frequcncy division at 19.6 CHz is shown schematically in Fig. 9. The semiconductor laser used is a 1.55 pm FP laser with a threshold current of 18.4mA. The laser exhibits period doubling under large signal current modulation. The input optical clock signal is generated by modulating the CW light from a tunable laser at 19.6GHz with an amplitude modulator. After being amplified by an EDFA, the 19.6 GHz optical clock signal is injected into the laser through an optical circulator. A tunable attenuator is used to adjust the input power level. The injected optical signal is removed from the output port of the circulator by an optical filter. The output clock signal is observed in both

input optical Glock at F

laser diode exhibits period doubling

output optical clock at F E

Fig. 8 Schematic diagram o/oll-opricul clnck/rPyuenc.v division using period doubling in (I srmiconductur laser

243

frequency

I attenuator

test set FFP EDFA

Fig. 9 Erperimentol serup on all-opliwl clockfiequency division using period douhling ~ I I a FahT-Peror laser diode (FP-LD) EDFA: erbium-doped fibre amplifier; PCI, PC2: polarisation controllers: FFP: tunable tibrc Fabry-Perot tilter

the time domain and the frequency domain using a sampling oscilloscope and an RF spectrum analyser, respectively.

When the laser is biased at 43.5mA, its resonance frequency is -8.0 GHz. Without the input clock signal; the laser is operating under CW condition with multiple FP modes. Injection locking occurs when the wavelength of the injected optical clock signal is set at one ofthe lasing modes and the laser is modulated at the input clock frequency. As the injection power is increased, both the frequency and the amplitude of the resonance peak increase. When the resonance frequency approaches half the input clock frequency, period doubling occurs. When the average optical power of the input clock signal is -S.OdBm, a very sharp peak at 9.8 GHz is observed on the RF spectrum analyser.

Fig. IOa shows the time domain traces ofthe input clock signal at 19.6 GHz and the output clock signal at 9.8 GHr. I t can be clearly seen that frequency halving has been achieved. Fig. 10b shows the measured RF spectrum of the output clock signal generated from the system. The inten- sity at 9.8 GHz is -28 dB greater than that at 19.6 GHz.

The quality of the generated output clock signal is assessed by measuring its phase noise. For the 9.8GHz output clock signal, the phase noise at an offset frequency of IO kHz is -87.7 dBc/Hz. At the same offset frequency, the phase noise of the 19.6GHz input clock signal is -82.2 dBc/Hz.

The dependence of the phase noise of the output clock signal on the input clock signal parameter is also investi- gated. Fig. I 1 shows the measured phase noise of the output clock signal as a function of the input clock frequency when the power of the injected clock signal and the bias current of the laser were fixed. The results show that a low phase noise optical clock signal can be obtained even when the repetition frequency of the input clock signal is deviated from the designed frequency. If we define the detuning range as the frequency range over which the change in phase noise is < I dB, the detuning range is as high as 400 MHz. This large tolerance to the repetition-frequency fluctuation is an advantage compared with other techniques that used a semiconductor optical amplifier based nonlinear loop mirror to realise all-optical clock frequency division [ 131.

As the operation principle of the clock division is based on period doubling in semiconductor lasers, all-optical clock frequency division over a large input frequency range by the same laser is possible. By setting proper working parameters, we have observed all-optical clock

244

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. .. .. ._. .. .. .. .. . .. .. .. ._. .. .. .. . .. .. ....... .. .. ..........,, ............................................ I I

time

a

I

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b

Fig. 10 Sinusoiilul signals showing input 19.6 GHz clock signal nnd ourput 9.H GHz clock signal of 011-opficul clock division scheme, with measured output RF spectrum RBW=I.OMHz, VBW=IOkHz n Sinusoidal signals (time scale: 100 ps/div) b RF spectrum ( IOdBjdiv)

-88 I 19.1 19.3 19.5 19.7 19.9 20.1

input clock frequency. GHz

Fig. 11 input clock frequency

Dependence of output clock phase noise on detuning of

frequency division from an input optical clock ranging from 8 to 20 GHz. The result shows that all-optical clock division based on period doubling is applicable to a wide range of input clock frequencies.

IEE Pma -O~roeIec!mm. Vol. 150. No. 3, June 2003

5 Conclusion

Period doubling in an FP-LD has been observed under large signal current modulation. Optical injcction is proved to be an efficient means to suppress or to enhance the period-douhled state, depending on the DC bias and hence the resonance frequency of the FP-LD with reference to the modulation frequency. The effects of the injection level and the wavelength detuning of the injection source have been studied. A scheme to reversibly control the period doubling phenomenon by optical injection is also demon- strated. A CW injection level as low as 0.03 mW is sufficient to switch the pulse repetition frequency between 5 and 10GHz. The switching response of the FP-LD is observed to he faster than 100 ps. Period doubling has also been applied successfully for use in all-optical clock frequency division. A 19.6GHz input clock signal has been cmployed experimentally to yield a 9.8 GHz output clock. We have further shown that the frequency-halved clock signal exhibits a low phase noise over a large input frequency detuning range of 400 MHz. By adjusting the bias current of the laser and changing the input clock parameters, the resonance frequency of the laser can be shifted. Frequency halving has been dcmonstrated succcss- fully over an input clock frequency range of 8-20 GHz.

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