40 Gb/s all-optical NRZ to RZ format conversion using single SOA assisted by optical bandpass filter

Transcription

1 4 Gb/s all-optical NRZ to RZ format conversion using single SOA assisted by optical bandpass filter Jianji Dong, Xinliang Zhang*, Jing Xu, and Dexiu Huang Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan, China, 4374 Songnian Fu, and P. Shum Network Technology Research Centre, Nanyang Technological University, Singapore, Abstract: We propose a novel all-optical format conversion from nonreturn-to-zero (NRZ) to return-to-zero (RZ) based on single semiconductor optical amplifier (SOA) and a detuning optical bandpass filter (OBF). By adopting the ultrafast SOA model associated with intraband mechanism, the 4Gb/s NRZ-to-RZ format conversion is successfully demonstrated with numerical simulation. We investigate the influence of the filter detuning, extinction ratio, conversion efficiency, output pulsewidth, and filter bandwidth. These simulation results confirm the feasibility of our scheme. The proposed scheme is robust and potential for applications in future optical networks. 7 Optical Society of American OCIS codes: (6.45) Optic communication; (5.598) Semiconductor optical amplifier; (.456) Optical data processing. References and Links. D. Norte and A. E. Willner, Multistage all-optical WDM-to-TDM-to-WDM and TDM-to-WDM-to-TDM dataformat conversion and reconversion through 8 km of fiber and three EDFA s, IEEE Photon. Technol. Lett. 7, (995).. C. W. Chow, C. S. Wong, and H. K. Tsang, All-optical NRZ to RZ format and wavelength converter by dualwavelength injection locking, Opt. Commun. 9, (). 3. Y. C. Chang, Y. H. Lin, J. Chen, and G. R. Lin, All-optical NRZ-to-PRZ format transformer with an injectionlocked Fabry-Perot laser diode at unlasing condition, Opt. Express., (4). 4. C. M. Huang, K. C. Yu, Y. C. Chang, and G. R. Lin, Gbit/s all-optical NRZ-to-RZ data format in a darkoptical-comb injected semiconductor optical amplifier, OFC 6, JThB33 (6). 5. A. Reale, P. Lugli, and S. Betti, Format conversion of optical data using four-wave mixing in semiconductor optical amplifiers, IEEE J. Sel. Top. Quantum Electron. 7, (). 6. L. Xu, B. C. Wang, V. Baby, I. Glesk, and P. R. Prucnal, All-optical data format conversion between RZ and NRZ based on a Mach-Zehnder interferometric wavelength converter, IEEE Photon. Technol. Lett. 5, 38-3 (3). 7. W. Li, M. Chen, Y. Dong, and S. Xie, All-optical format conversion from NRZ to CSRZ and between RZ and CSRZ using SOA-based fiber loop mirror, IEEE Photon. Technol. Lett. 6, 3-5 (4). 8. C. G. Lee, Y. J. Kim, C. S. Park, H. J. Lee, and C. S. Park, Experimental demonstration of -gb/s data format conversions between NRZ and RZ using SOA-loop-mirror, J. Lightwave Technol. 3, (5). 9. C. H. Kwok and C. Lin, Polarization-insensitive all-optical NRZ-to-RZ format conversion by spectral filtering of a cross phase modulation broadened signal spectrum, IEEE J. Sel. Tops. Quantum Electron., (6).. J. Yu, G. K. Chang, J. Barry, and Y. Su, 4 Gbit/s signal format conversion from NRZ to RZ using a Mach- Zehnder delay interferometer, Opt. Commun. 48, 49-4 (5).. M. L. Nielsen, B. Lavigne, and B. Dagens, Polarity-preserving SOA-based wavelength conversion at 4 Gbit/s using bandpass filtering, Electron. Lett. 39, (3).. S. Fu, J. Dong, P. Shum, L. Zhang, X. Zhang, and D. Huang, Experimental observations of inverted and noninverted wavelength conversion based on transient cross phase modulation of SOA, Opt. Express. 4, (6). # $5. USD Received 7 November 6; revised 3 February 7; accepted 5 February 7 (C) 7 OSA 9 March 7 / Vol. 5, No. 6 / OPTICS EXPRESS 97

2 3. M. L. Nielsen, J. Mørk, R. Suzuki, J. Sakaguchi, and Y. Ueno, Experimental and theoretical investigation of the impact of ultra-fast carrier dynamics on high-speed SOA-based all-optical switches, Opt. Express. 4, (6). 4. A. Mecozzi and J. Mørk, Saturation induced by picosecond pulses in semiconductor optical amplifiers, J. Opt. Soc. Am. B 4, (997). 5. J. -T. Hsieh, P. -M. Gong, S. -L. Lee, and J. Wu, Improved dynamic characteristics on four-wave mixing wavelength conversion in light-holding SOAs, IEEE J. Sel. Top. Quantum Electron., (4).. Introduction Future all-optical networks are likely to be a hybrid of wavelength division multiplexing (WDM) and optical time division multiplexing (OTDM) networks by adopting the advantages of both technologies. Return-to-zero (RZ) format is widely used in OTDM systems due to its tolerance to fiber nonlinearities in spite of the dispersion-induced effect, while the nonreturnto-zero (NRZ) format is preferred in WDM networks for its high spectral efficiency and timing-jitter tolerance. Therefore, format conversion between NRZ and RZ is desirable in nodes and links of high speed OTDM and WDM networks []. So far, a variety of all-optical format conversions from NRZ to RZ have been demonstrated, such as using an injection-locked Fabry-Perot laser diode [, 3], using nonlinearity of semiconductor optical amplifier (SOA) [4-8], and using nonlinearity of optical fiber [9, ]. The reported injection locking technologies were with low bit rate due to the directly modulated source. The use of SOA as the nonlinear medium has received considerable attention in terms of small footprint, high nonlinearity, and optical integration. For example, cross gain modulation (XGM) is attractive for its simple implementation. Ref. [4] combined XGM effect of SOA and a dark optical comb to realize NRZ-to-RZ format conversion. Four-wave mixing (FWM) scheme is effective due to its transparency to all optical modulation format as well as high bit rate capabilities [5]. Some SOA-based interferometric schemes are also proposed, such as Mach-Zehnder interferometer [6] and nonlinear loop mirror [7, 8]. The nonlinearity of optical fiber is attractive in format conversion due to its ultrafast nonlinear response []. However, a long interaction length is required in order to obtain sufficient nonlinear effects. Ref. [9] presented the NRZ to RZ format conversion by filtering the broadened spectrum induced by cross phase modulation (XPM), which is polarization insensitive by employing a polarization diversified loop and compact by using photonic crystal fiber and highly nonlinear dispersion shifted fiber with short length. In this paper, we propose a novel scheme for NRZ to RZ format conversion using single SOA and optical bandpass filter (OBF). The SOA, acting as nonlinear element, causes the broadened spectrum of input NRZ signal due to XPM effect, and the OBF is used to extract the special spectrum from the broadened spectrum. Our scheme is robust in terms of simple structure and high bit rate operation. The format conversion will be polarization insensitive if the polarization insensitive SOA is employed. This paper is organized as follows. In section, the principle of the NRZ to RZ format conversion is presented and the theoretical approach is put forward. In section 3, we put emphasis on the performance optimization of our configuration and investigate the influence of the filter detuning, extinction ratio (ER), conversion efficiency (CE), output pulsewidth, and filter bandwidth. Finally, conclusions are given in section 4.. Theory and principle The schematic illustration for our NRZ to RZ format conversion is shown in Fig. (a). The format converter consists of the SOA as nonlinear medium and the OBF as spectral filtering. The incoming 4Gb/s NRZ signal with wavelength λ s is combined with a synchronized 4GHz clock pulse train as the control signal, and then launched into the SOA. The pulsewidth of the control signal is picosecond-scale, typically ps~ps, to achieve sufficient XPM-induced spectral broadening to the NRZ signal. The subsequent OBF then extract the sideband spectrum with its central wavelength λs +Δ λdet, where Δ λdet is the filter detuning from the NRZ wavelength. As we know, the converted signal will keep in phase to the input # $5. USD Received 7 November 6; revised 3 February 7; accepted 5 February 7 (C) 7 OSA 9 March 7 / Vol. 5, No. 6 / OPTICS EXPRESS 98

3 control signal if the filter detuning is properly chosen. The principle is similar to the noninverted wavelength conversion (WC) based on the configuration of the SOA and OBF [- 3]. Simply speaking, the NRZ signal will generate a transient frequency shift within the control signal duration, and the OBF with proper detuning will only transmit frequencyshifted component caused by XPM. So the non-inverted WC is realized with the capacity of ultrafast response. Figure (b) shows the operation principle for format conversion. The input control signal will drive the phase of NRZ signal to change periodically. If and only if the NRZ data is mark, the converted signal will output at bit due to the non-inverted WC, otherwise, the converted signal will output bit since the input NRZ data is space. In this way, the format conversion from NRZ to RZ can be obtained. It should be noted that, the converted signal has the frequency shift to the original NRZ signal. λ con λ s λs + Δλ det Fig.. the operation principle of the NRZ to RZ format conversion based on the SOA and OBF. In order to explore the intrinsic mechanism of our proposed scheme, a potent SOA model should be developed to predict the SOA operation. When the SOA is operated with pulses shorter than a few picoseconds, the intraband effects, such as spectrum hole burning (SHB) and carrier heating (CH), become important. The model of Mecozzi and Mørk is a simple and powerful method for calculating the amplification of picosecond pulses in a SOA [4]. Therefore we adopt their model associated with the intraband mechanism. Then the material gain can be expressed as g = gl /( + ε Ptot ), where P tot is the input total power coupled into the SOA. ε is the nonlinear gain suppression factor associated with SHB and CH effect, where ε = εch + εshb. gl is the linear material gain coefficient, which is described with a polynomial formula with empirically determined constants [5]. 3 gl( N, λ) = a( N N) a( λ λn) + a3( λ λn) () where a, a, and a 3 are gain constants and N is the carrier density at transparency for the peak wavelength λ N, which is assumed to shift linearly with the carrier density, i.e., λn = λ a4( N N), and λ is the peak wavelength at transparency, which is 56nm. We consider the nonuniform distributions of carrier density in the SOA active region and use the subsection model, then the traveling wave equations of signals can be expressed as dpji, = ( Γg j, i αin) Pj, i, j = scon, () dz where P is the optical power in the SOA, j represents either the NRZ signal or the clock control signal. i represents the i-th subsection of the SOA. Γ denotes the mode confinement factor. αin is internal loss. The carrier density in the SOA active region, which can be described as Eq. (3) in the i-th subsection dni () t ηi 3 Γ P j, i = ( cn i + cn i + cn 3 i ) g j, i (3) dt ewdl dw j= s, con hν j where N is the carrier density, h Planck s constant, ν optical frequency, I denotes injection current, η is current injection efficiency, the product of wdl is the volume of the # $5. USD Received 7 November 6; revised 3 February 7; accepted 5 February 7 (C) 7 OSA 9 March 7 / Vol. 5, No. 6 / OPTICS EXPRESS 99

4 active region, e is the electron charge. c is the unimolecular recombination constants caused by trapping sites, c is the bimolecular spontaneous radiative recombination coefficient, and c3 is the Auger recombination coefficient. The third term on the right-hand side represents carrier consumption induced by the stimulated emission for the input signal power. To model the total phase variation of the signal traveling through a section, we use the Kramers-Kronig relations, namely so-called α -factor. Because the SHB gain contribution is symmetric with respect to the wavelength, the contribution of SHB is considered to be around zero [4]. Taking into account the phase changes induced by the carrier density modulation and the CH, the phase shift of converted signal Φ NL can be calculated as [3]: L Φ NL = ( ) N gl CH CH gptot dz α Γ + α ε Γ (4) where α N is SOA linewidth enhancement factor, α CH is α -factor relative to the effect of CH. From the solutions of Eqs. (), (3), and (4), the converted signal power Psout () t, the carrier density, and the temporal phase Φ NL () t is calculated. Then the optical field after SOA can be described as Esout ( t) = Psout ( t) exp[ iφ NL ( t)] (5) The frequency response function of the OBF is assumed to have Gauss shape. That is ω Δω H ( ω) = exp[ ln ( ) ] (6) B Then the optical field at the output port of OBF is described as Eout ( t) = F { H( ω) F[ Esout ( t)]} (7) Finally the output power after OBF is expressed as Pout ( t) = Eout ( t) (8) From Eq. (5) to Eq. (7), Δω is the OBF frequency detuning from the probe carrier. B is 3dB- bandwidth of the OBF. F[] and F [] are the Fourier transform and inverse Fourier transform, respectively. In our scheme, we assume the SOA is polarization independent, and the simulation parameters are listed in Table. # $5. USD Received 7 November 6; revised 3 February 7; accepted 5 February 7 (C) 7 OSA 9 March 7 / Vol. 5, No. 6 / OPTICS EXPRESS 9

5 Parameter Table. Parameter List Value SOA length L -3 m SOA width w -6 m SOA depth d. -6 m Material gain constant a 3 - m Material gain constant a m -3 Material gain constant a m -4 Material gain constant a m 4 Carrier density at transparency N.9 4 m -3 Wavelength at transparency λ 56-9 m Nonradiative recombination constant c.5 8 s - Bimolecular recombination constant c m 3 s - Auger recombination constant c 3-4 m 6 s - Internal Loss α in 3 m - Linewidth enhancement factorα N 6 CH-induced parameter α CH CH gain suppression factor ε CH W - SHB gain suppression factor ε SHB W - Injected current I.A Current injection efficiency η.75 Optical confinement factor Γ.3 Bandwidth of the filter B 5GHz Extinction ratio of input NRZ signal db 3. Results and discussion According to the previous SOA model, we simulate the format conversion from NRZ to RZ, as shown in Fig.. The input control signal is 4GHz clock pulse train at 55nm with.5ps pulsewidth and dbm peak power, whose waveform and spectrum are shown in Fig. (a) and (a), respectively. The frequency spacing between two neighbor lines is measured to be.3nm from the spectrum. The original NRZ signal is synchronized to the control signal and modulated at 4Gb/s to form 7 - pseudo random binary sequence (PRBS) signal, whose peak power and wavelength are dbm and 554nm, respectively. The temporal waveform and spectrum are shown in Figs. (b) and (b). Unless stated otherwise, the simulations are performed assuming the parameters listed in Table. After the NRZ signal and the control signal are simultaneously lunched into the SOA, the NRZ spectrum will be broadened due to XPM induced by the control signal. When the detuning Δλdet is -.8nm, the waveform and spectrum of the converted RZ signal are shown in (c) and (c). Similarly, when the detuning is.8nm, we can also obverse the NRZ to RZ format conversion, shown in (d) and (d). The filtered spectrum shapes in (c) and (d) correspond to the spectrum of the RZ pulses. The insets of Figs. (b), (c) and (d) are the eye patterns of the corresponding # $5. USD Received 7 November 6; revised 3 February 7; accepted 5 February 7 (C) 7 OSA 9 March 7 / Vol. 5, No. 6 / OPTICS EXPRESS 9

6 waveforms. Figure reveals the NRZ to RZ format conversion can be obtained whether the filter detuning is positive (red shift) or negative (blue shift). The theory of the WC based on SOA and a detuning OBF manifests that both inverted and non-inverted WCs can be obtained by different filter detuning, however the inverted WC appears at small detuning and non-inverted one appears at relatively large detuning [, 3]. Therefore, the filter detuning will play an important role to optimize the format conversion scheme. Figure 3 shows the output ER and CE vary as a function of the filter detuning. When the absolute value of the detuning increases from.64nm to.6nm, the ER keeps increasing while the CE decreases. So there is a tradeoff between high ER and low CE. When the detuning is near zero, the polarity will turn to be inverted due to the dominant XGM and the format conversion will be terminated. Even in the margin of polarity-inverted region, the ER is rather low because of the XGM-induced crosstalk. Therefore, the detuning should be optimized at a large value (typically.6nm). In order to improve the CE, we may employ an amplifier and another filter to follow the format converter. The amplifier is used to boost the optical power and the filter can suppress the noise Fig.. Simulation results of the NRZ to RZ format conversion. (a) and (b) are the temporal waveforms of input control signal and original NRZ signal, respectively. (c) and (d) are the converted RZ waveforms when the detuning is -.8nm and.8, respectively. (a)- (d) are the corresponding spectrum Fig. 3. The extinction ratio and conversion efficiency vary as a function of the filter detuning. Fig. 4. The nonlinear patterning varies as a function of the control pulsewidth. From the converted RZ waveforms in Fig., we can find the pattern effect appears. The pattern effect is named as nonlinear patterning (NLP), which results from carrier density saturation [3]. The NLP is defined as the maximum power over minimum power of bit. Figure 4 shows the NLP varies with respect to the control pulsewidth when the detuning is.6nm. Since the NLP increases as the control pulsewidth increases, we should reduce the NLP by injecting the control signal with ultrashort pulse train. The reason lies in that the # $5. USD Received 7 November 6; revised 3 February 7; accepted 5 February 7 (C) 7 OSA 9 March 7 / Vol. 5, No. 6 / OPTICS EXPRESS 9

7 ultrashort pulse injection would result in large nonlinear phase shifts of NRZ signal in SOA, and the followed filter converts the phase modulation to intensity modulation. The ultrashort pulses are required to induce large phase modulation and deep carrier density modulation. Hence the NLP could be reduced. It can be seen from Fig., the converted RZ pulsewidth is much wider than that of the control signal. To further test the quality of the converted RZ signal, we have investigated the pulsewidth variation with respect to the filter bandwidth when the detuning is -.6nm and.6nm, as shown in Fig. 5. Both curves suggest that the converted pulsewidth is in inverse proportion to the filter bandwidth. When the detuning is rather large, such as ±.6nm, the filter 3dB-bandwidth is approximately equal to the spectrum bandwidth of converted signal since the envelope of broadened NRZ spectrum is almost horizontal near ±.6nm. Therefore, the curves in Fig. 5 represent the duration-bandwidth product ( Δ Δ t ν ) of the converted signal. The average Δ Δ t ν is estimated at.45, which is very close to the transform limited case for unchirped Gauss pulse with Δ Δ t ν =.44. So our proposed conversion scheme has good performance. Although the frequency chirp is generated when the NRZ signal propagates in SOA, the frequency chirp can be eliminated after filtering. The output signal performance can be improved by reshaping the output spectrum. It should be noted that, our format conversion scheme could realize the variable duty cycle if a bandwidth-tunable filter is employed. 5 detuning -.6nm detuning +.6nm 8 6 detuning +.6nm detuning -.6nm Fig. 5. The converted pulsewidth varies as a function of the filter bandwidth. Fig. 6. The extinction ratio variation with input signal wavelength. We further investigate the tunability of our conversion scheme. Figure 6 shows the output ER of converted RZ signal variation with the input NRZ signal wavelength when the detuning is ±.6nm. In the simulation, the input ER of NRZ signal is db and the control signal pulsewidth is.5ps. When the signal wavelength varies from 53nm to 56nm, the output ER fluctuates from 4dB to 8dB, which is degraded to some extent compared with the original db. The fluctuation of output ER results from the wavelength-dependent gain coefficient. It is worth noting that there is a crosstalk region near 55nm, the control signal wavelength. From Fig. (a), we can see the control signal has a broad spectrum, which will induce the crosstalk to the converted signal if the filter central wavelength is close to 55nm. Therefore, the input NRZ wavelength should be far from the control signal wavelength to avoid the crosstalk. 4. Conclusion In this paper, we proposed a novel all-optical format conversion from NRZ to RZ based on single SOA and a detuning OBF. The principle lies in the spectral filtering of XPM-induced broadened spectrum of the NRZ signal. Our proposed scheme has three advantages, () simple implementation, () high speed operation, and (3) polarization insensitivity if the polarization insensitive SOA is employed. The simulation results show that the conversion scheme can be obtained whether the filter detuning is positive or negative, but there is a tradeoff between the high output ER and low CE. To reduce the pattern effect, the pulsewidth # $5. USD Received 7 November 6; revised 3 February 7; accepted 5 February 7 (C) 7 OSA 9 March 7 / Vol. 5, No. 6 / OPTICS EXPRESS 93

8 of the control signal should be as narrow as possible. The duration-bandwidth product of converted RZ signal is close to the transform limited case for unchirped Gauss pulse, which suggests that our proposed conversion scheme has good performance and the frequency chirp can be eliminated after filtering. The input NRZ wavelength should be far from the control signal wavelength in case of spectrum crosstalk. Our scheme has potential to be applicable in future high bit rate optical networks. Acknowledgment This work was partially supported by National Natural Science Foundation of China (Grant No. 647), the Science Fund for Distinguished Young Scholars of Hubei Province (Grant No. 6ABB7), and the Program for New Century Excellent Talents in Ministry of Education of China (Grant No. NCET-4-75). # $5. USD Received 7 November 6; revised 3 February 7; accepted 5 February 7 (C) 7 OSA 9 March 7 / Vol. 5, No. 6 / OPTICS EXPRESS 94