All-optical pulse data generation in a semiconductor optical amplifier gain controlled by a reshaped optical clock injection

All-optical pulse data generation in a semiconductor optical amplifier gain controlled

by a reshaped optical clock injection

Gong-Ru Lin, Yung-Cheng Chang, and Kun-Chieh Yu

Citation: Applied Physics Letters 88, 191114 (2006); doi: 10.1063/1.2203213

View online: http://dx.doi.org/10.1063/1.2203213

View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/88/19?ver=pdfcov Published by the AIP Publishing

Articles you may be interested in

Ultrafast all-optical switching via coherent modulation of metamaterial absorption Appl. Phys. Lett. 104, 141102 (2014); 10.1063/1.4870635

40 GHz small-signal cross-gain modulation in 1.3 m quantum dot semiconductor optical amplifiers Appl. Phys. Lett. 93, 051110 (2008); 10.1063/1.2969060

Femtosecond switching with semiconductor-optical-amplifier-based Symmetric Mach–Zehnder-type all-optical switch

Appl. Phys. Lett. 78, 3929 (2001); 10.1063/1.1379790

Observation of dark-pulse formation in gain-clamped semiconductor optical amplifiers by cross-gain modulation Appl. Phys. Lett. 75, 3760 (1999); 10.1063/1.125447

The wavelength-dependent transfer function for optically controlled semiconductor lasers J. Appl. Phys. 83, 5056 (1998); 10.1063/1.367322

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 140.113.38.11 On: Thu, 01 May 2014 01:59:23

All-optical pulse data generation in a semiconductor optical amplifier gain

controlled by a reshaped optical clock injection

Gong-Ru Lin,a兲 Yung-Cheng Chang, and Kun-Chieh Yu

Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, 1001 Ta Hsueh Road, Hsinchu, Taiwan 300, Republic of China

共Received 26 October 2005; accepted 28 March 2006; published online 10 May 2006兲

Wavelength-maintained all-optical pulse data pattern transformation based on a modified cross-gain-modulation architecture in a strongly gain-depleted semiconductor optical amplifier 共SOA兲 is investigated. Under a backward dark-optical-comb injection with 70% duty-cycle reshaping from the received data clock at 10 GHz, the incoming optical data stream is transformed into a pulse data stream with duty cycle, rms timing jitter, and conversion gain of 15%, 4 ps, and 3 dB, respectively. The high-pass filtering effect of the gain-saturated SOA greatly improves the extinction ratio of data stream by 8 dB and reduces its bit error rate to 10−12at −18 dBm. © 2006

American Institute of Physics. 关DOI:10.1063/1.2203213兴

All-optical data processing has been comprehensively investigated owing to the implementation of versatile optical logic gates and data format conversion. Previously, numer-ous laser diode or semiconductor optical amplifier 共SOA兲 based ultrafast all-optical data pattern transforming architectures1–9have emerged via the effects of the direct or the cross gain modulation,2,3 interferometer related cross-phase modulation 共XPM兲,4,5 SOA-based wavelength conversion,6 SOA-based nonlinear optical loop mirror 共NOLM兲,7

Fabry-Perot laser diode based dual-wavelength injection locking,8 and under threshold scheme,9 etc. Typi-cally, a wavelength- or format-converted data pattern can be demonstrated at a bit rate of ⬎10 Gbits/s due to the sub-nanosecond carrier lifetime of the optically cross-gain-modulated共XGM兲 SOA, in which the gain recovery process is strongly affected by the injection-power dependent nonlinearity.10 Since the wavelength conversion mechanism relies strictly on the significant gain saturation of the SOA caused by the intense injection, a relatively high power of the input data pattern at original wavelength is thus mandatory. Such an operation is somewhat impractical in a real network with interfacial data pattern transforming requirement since the level control共usually the level amplification兲 of the op-tical data stream is required. Recently, a modified XGM technique has emerged to precisely control the gain of the SOA in the time domain by injecting a dark-optical-comb reshaped optical clock,11which simultaneously enhances the XGM depth and the modulation bandwidth of the SOA. Ob-viously, the SOA can be controlled by a reshaped optical clock to offer extremely high XGM depth and narrow gain window, which should be an alternative to achieve the high bit-rate pulse data pattern transformation. In this letter, we propose for the first time a high-power dark-optical-comb pulse injection architecture to temporally control the gain of the SOA, providing a sufficient XGM depth for a pulse data pattern transformation at low input levels. With the injection of such a reshaped optical clock, the parametric analyses on wave form, extinction ratio, and sensitivity of the SOA trans-formed pulse data pattern are reported. The gain-shifting and high-pass filtering mechanisms of the SOA corresponding to

the enhanced bit-error-rate and signal-to-noise ratio perfor-mances are elucidated.

Figure 1共a兲 schematically depicts the SOA-based pulse data pattern transformer. In experiment, an optical non-return-to-zero 共NRZ兲 formatted pseudorandom random bi-nary sequence共PRBS兲 signal with pattern length of 231− 1 is employed as the input data stream. A radio-frequency syn-thesizer共Agilent, E8457A兲 at 10 GHz simulates the electri-cal clock for the data generator and the pulse data pattern transformer. The output power of the optical NRZ PRBS data stream is attenuated to −1 dBm with its wavelength coinci-dent with the peak wavelength of the SOA gain spectrum. The pulse data pattern transformer is configured by the traveling-wave typed multi-quantum-well SOA共QPhotonics LLC, QSOA-1550兲 and a backward optical injecting source. The small signal gain and typical output power of the single-mode-fiber pigtailed and butterfly packaged SOA are 20 dB and 15 mW, respectively. As shown in Fig. 1共b兲, the

ampli-a兲_{Author to whom correspondence should be addressed; electronic mail:} [email protected]

FIG. 1.共a兲 The schematic diagram of a SOA-based data pattern transformer; 共b兲 ASE spectra of SOA at different biasing currents and the operating principle of a dark-optical-comb pulse-train generator.

APPLIED PHYSICS LETTERS 88, 191114共2006兲

0003-6951/2006/88共19兲/191114/3/$23.00 88, 191114-1 © 2006 American Institute of Physics This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

fied spontaneous emission共ASE兲 of the SOA ranges between 1510 and 1550 nm, which exhibits a 3-dB spectral linewidth of 35 nm and a peak wavelength of 1530 nm under a biased current of 280 mA. The key components of the backward optical injector are a tunable laser, a 10-GHz electrical comb generator, and a Mach-Zehnder intensity modulator 共MZM兲.11

The dark-optical-comb pulse train is obtained by passing the tunable laser output through the MZM operated at nonlinear region共without dc bias兲, as shown in Fig. 2共a兲. The electrical comb generator triggered by the amplified electrical clock 共with power of 26.4 dBm兲 from the afore-mentioned radio-frequency synthesizer is used to drive the MZM for generating dark-optical-comb pulse train. To gain deplete the SOA at highly biased condition, the dark optical comb is then amplified to 18 dBm by using an erbium-doped fiber amplifier共EDFA兲 and backward injected into the SOA via an optical circulator. To perform the bit-error-rate共BER兲 analysis at 10 Gbits/ s, the transformed data pattern is de-tected by a high-gain avalanche photodiode共APD兲 incorpo-rating a gain-controlled amplifier and a clock/data recovery circuit. The pulse data pattern is analyzed by a digitized sam-pling oscilloscope共Agilent, 86100+86109A兲 and a bit-error-rate detector共Agilent, 71612C兲.

In principle, an input data pattern is transformed into a pulsed one in the SOA when the backward injected optical clock turns off, as illustrated in Fig. 2共b兲. The effect of the backward injected wave form on the gain window and the transformed pattern shape in SOA can be simulated by solv-ing the modified rate equations of time-varied carrier density 共Nj兲, input data 共Pi兲, and backward injected 共Pb兲 signals with

average powers of P¯m,j and P¯s,j in jth gain section of the

SOA, as listed below: ⳵Nj共z,T兲 ⳵T = I qV− NJ ␶C

再

冎

where I, Across, and V denote the injection current, cross-section area in active area, and volume of the SOA; q is the electron charge;បwiandបwbdenote the input and backward

injected photon energies; ␶Cdenotes the spontaneous

emis-sion lifetime; and ␣int, gi, and gb are the internal loss and

asymmetric gain coefficients in the SOA. Our simulation re-veals that the pulse width as well as the duty cycle of the data pattern can be narrowed down as the duty cycle and the peak amplitude of the backward injected wave form greatly increase. Shrinking the gain window of the SOA in the time domain is mandatory to optimize the pulse data pattern trans-formation. By contrast, the backward optical injection of the typical pulse train with short pulse width only causes a broader and reshaped gain window with insufficient gain-depletion depth in the SOA, which consequently generates a nontransformed but seriously distorted data pattern,11 as il-lustrated in Fig. 2共b兲. The wave forms of the backward in-jected dark-optical-comb pulse train and the output of the SOA under a cw input are shown in Fig. 3共a兲. Under an electrical comb pulse width of 40 ps at 10 GHz, the duty cycle of the dark optical comb generated from the MZM is up to 80%. The pulsation of the input data pattern results from a serious gain depletion and a greatly narrowed gain window in the SOA backward injected by large duty-cycle signal. For example, a NRZ data stream共10101110兲 encoun-ters the residual gain in the SOA and transforms into a pulse pattern, as shown in Fig. 3共b兲.

On the other hand, the use of the high-level electrical pumping accompanied by a strong optical injection to con-currently suppress lasing and enhance carrier/photon interac-tion in the SOA can significantly result in a transforming speed larger than the carrier recovery rate of the SOA.10The speed limitation for the SOA-based data pattern transformer is determined by the gain recovery time of the SOA. The gain recovery time ␶ in the SOA is described as ␶−1_=␶

nr−1 + aS,10where␶_nris the nonradiative recombination time, a is the stimulated emission rate, and S is the internal

photonden-FIG. 2.共a兲 The operating principle of a dark-optical-comb pulse-train gen-erator;共b兲 the illustration of the data pattern transformation processes in a SOA under the backward dark共left兲 and bright 共right兲 optical-comb injec-tion condiinjec-tions.

FIG. 3. 共a兲 Upper: the dark optical comb injected into SOA; lower: the generated pulse train under a cw input.共b兲 Upper: the input data stream with 10101110 pattern; lower: the pulse data stream transformed by SOA.

191114-2 Lin, Chang, and Yu Appl. Phys. Lett. 88, 191114共2006兲

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 140.113.38.11 On: Thu, 01 May 2014 01:59:23

sity in the SOA. It is elucidated that the gain recovery time of the SOA as well as the rising time of the transformed pulse data pattern can be effectively shortened by greatly increasing the internal photon density of the SOA after strong backward injection. The backward injection of the dark optical comb with extremely large duty cycle thus re-sults in an ultranarrow gain window with enhanced switch-ing response, facilitatswitch-ing the convertible data rate up to 10 Gbits/ s and beyond. A nearly error-free 共BER⬍10−12兲 pulse data stream can be achieved at a received optical power of −19 dBm. Such an excellent performance is correlated with the less fluctuated signal amplitude and the improved signal-to-noise ratio of the transformed pulse data stream, which originates from the gain-saturation induced high-pass filtering effect in the SOA operated at high-gain condition.13 In principle, such a self-gain modulation effect in SOA can further be enhanced by increasing the input power. As ex-pected, the BER of the pulse data pattern significantly de-creases from 10−8 to 10−12as the input data power increases from −6 to − 1 dBm, as shown in Fig. 4共a兲.

Furthermore, such an operation further enhances the ex-tinction ratio共defined as the power ratio of the data level “1” to the data level “0”兲 of the transformed data pattern under a backward injection at longer wavelength. This result is mainly attributed to both the XGM induced by strong injec-tion power and the large red shift in the gain spectrum of the SOA at the data wavelength. The effect of the injected dark-optical-comb wavelength on the extinction ratio of the RZ PRBS is determined and is shown in Fig. 4共b兲. We observed that the extinction ratio of the input data at 1530 nm is greatly improved from 7 to 14.9 dB after the pulse data pat-tern transformation in the SOA with a backward injection at 1550 nm. In fact, the trend of the extinction ratio almost coincides with that of the SOA gain spectrum. The shift in gain spectrum of the SOA is given by␭N=␭0−␬0共N−N0兲,12 where␬0is a constant and␭Nand␭0denote the peak

wave-lengths of the SOA gain spectrum at carrier densities of N 共under backward injection兲 and N0 共transparency兲, respec-tively. It is seen that a blueshift of the SOA gain spectrum occurs when operating the SOA at a high-gain共NⰇN0兲 con-dition. Meanwhile, the intense optical injection strongly de-pletes the carriers as well as gain of the SOA within most of one period. This changes the SOA from gain to loss condi-tion共N⬍N0兲 and also leads to a redshift in the gain spectrum of SOA. It is thus desirable to locate the data wavelength at the gain peak of the unsaturated SOA, as the gain depletion at this wavelength is more pronounced than others. Such a high-gain and strong-injection induced gain-shift effect fur-ther benefits from the advantage of raising extinction ratio in the SOA transformed pulse data pattern. In this case, the gain shape of SOA not only depletes but also shifts to longer wavelength by means of backward dark-optical-comb injec-tion. Experimentally, the enhancement on extinction ratio of the transformed pulse data pattern strictly relies on the back-ward injecting wavelength. Under constant backback-ward inject-ing power, a shorter data wavelength could result in a deeper gain depletion as well as better extinction ratio at the output. In other words, the power required to deplete the gain of SOA strongly depends on the data wavelength.14

In conclusion, a modified cross-gain-modulation archi-tecture is demonstrated by using a backward dark-optical-comb injected SOA for all-optical pulse data pattern trans-formation of the low-power input data stream at an identical wavelength. The transformed pulse data stream exhibits a duty cycle, a rms timing jitter, and a conversion gain of 15%, 4.2 ps, and 3 dB, respectively. The signal-to-noise ratio and the extinction ratio of the input data stream can be greatly improved owing to the high-pass filtering effect of the SOA operating at a saturated gain condition. The input power de-pendent carrier lifetime shortening effect of the SOA further benefits from the increasing on transforming bit rate up to 10 Gbits/ s with a nearly error-free performance.

This work is supported in part by the National Science Council, Taiwan R. O. C. under Grant No. NSC94-2215-E009-040.

1_{C. G. Lee, Y. J. Kim, C. S. Park, H. J. Lee, and C.-S. Park, J. Lightwave} Technol. 23, 834共2005兲.

2_{D. Norte and A. E. Willner, IEEE Photonics Technol. Lett. 8, 712共1996兲.} 3_{D. Norte and A. E. Willner, IEEE Photonics Technol. Lett. 8, 715共1996兲.} 4_{B. Mikkelsen, M. Vaa, H. N. Poulsen, S. L. Danielsen, C. Joergensen, A.} Kloch, P. B. Hansen, K. E. Stubkjaer, K. Wunstel, K. Daub, E. Lach, G. Laube, W. Idler, M. Schilling, and S. Bouchoule, Electron. Lett. 33, 133 共1997兲.

5_{L. Xu, V. Baby, I. Glesk, and P. R. Prucnal, IEEE Photonics Technol. Lett.} 15, 308共2003兲.

6_{P. S. Cho, D. Mahgerefteh, and J. Goldhar, Proceedings of the 24th} Euro-pean Conference on Optical Communication, Madrid, Spain, 20–24 Sep-tember共IEEE, New York, 1998兲, Vol. 1, p. 353.

7_{H. J. Lee, S. J. B. Yoo, and C.-S. Park, Proceedings of the Optical Fiber} Communication Conference and Exhibit, Anaheim, CA, 17–22 March 2001共IEEE, New York, 2001兲, Vol. 1, p. MB7-1.

8_{C. W. Chow, C. S. Wong, and H. K. Tsang, Opt. Commun. 209, 329} 共2002兲.

9_{Y. C. Chang, Y. Lin, J. H. Chen, and G.-R. Lin, Opt. Express 12, 4449} 共2004兲.

10_{J. M. Wiesenfeld, B. Glance, J. S. Perino, and A. H. Gnauck, IEEE} Pho-tonics Technol. Lett. 5, 1300共1993兲.

11_{G.-R. Lin, I. Chiu, and M. Wu, Opt. Express 13, 1008共2005兲.} 12_{I. D. Henning, M. J. Adams, and J. V. Collins, IEEE J. Quantum Electron.}

2, 609共1985兲.

13_{K. Sato and H. Toba, IEEE J. Sel. Top. Quantum Electron. 7, 328共2001兲.} 14_{K. Inoue, T. Mukai, and T. Saitoh, Electron. Lett. 23, 328共1987兲.} FIG. 4.共a兲 The bit error rate measured at different input data powers; 共b兲 the

extinction ratio of the transformed pulse data stream as a function of back-ward injecting wavelength.

191114-3 Lin, Chang, and Yu Appl. Phys. Lett. 88, 191114共2006兲

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 140.113.38.11 On: Thu, 01 May 2014 01:59:23