15 August 2002 Optics Communications 209 (2002) 329 334 www.elsevier.com/locate/optcom All-optical NRZ to RZ format and wavelength converter by dual-wavelength injection locking C.W. Chow, C.S. Wong *, H.K. Tsang 1 Department of Electronic Engineering, The Chinese University of Hong Kong, Shatin, NT, Hong Kong Received 11 March 2002; received in revised form 23 May 2002; accepted 24 June 2002 Abstract We propose and demonstrate nonreturn-to-zero (NRZ) to return-to-zero (RZ) data format and wavelength conversion using dual-wavelength injection locking of a Fabry Perot (FP) laser diode. A negative power penalty of )9.5 db was achieved at 10 9 bit-error-rate. The device operated over a 45 nm wavelength range with over 10 db extinction ratio. We also describe the experimental conditions necessary for negative power penalty of dual-wavelength injection locking. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Injection-locked lasers; Wavelength conversion; Format conversion; WDM communications 1. Introduction * Corresponding author. Tel.: +852-2609-8252; fax: +852-2603-5558. E-mail address: cswong@ee.cuhk.edu.hk (C.S. Wong). 1 Present address: Bookham Technology plc, 90 Milton Park, Abingdon, Oxon OX14 4RY, UK. Future all-optical networks are likely to employ both wavelength division multiplexing (WDM) and optical time division multiplexing (OTDM) [1] and different systems can employ different data formats such as return-to-zero (RZ) or nonreturnto-zero (NRZ) data modulation. Data format conversion will be needed for all-optical crossconnects, which connect networks employing different data formats in terrestrial and undersea optical communication systems. Different approaches for data format conversion include the use of SOA/FBR [2], VCSEL [3], SOA/DFB [4], SOA-loop-mirror [5] and four-wave mixing (FWM) [6]. In WDM networks, a key functionality needed in an all-optical cross-connect is all-optical wavelength conversion. Wavelength conversion using cross-gain modulation (XGM), cross-phase modulation (XPM) [7,8] and FWM [9] has all been extensively reported on. A promising approach for wavelength conversion at low optical power is to use laser diodes. Here we describe an experimental demonstration of simultaneous all-optical wavelength and format converter from NRZ to RZ using dual-wavelength injection locking with commercially available FP laser diode. We also describe the different experimental conditions and techniques needed to achieve a negative power penalty. 0030-4018/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S0030-4018(02)01727-3
330 C.W. Chow et al. / Optics Communications 209 (2002) 329 334 2. Wavelength conversion mechanism The wavelength conversion technique involves coupling a modulated input signal at wavelength, k s, into a FP laser diode that has been injectionlocked by a pump signal at another wavelength k p. If one of the FP laser s longitudinal modes lies at a slightly shorter wavelength than the input wavelength k s, the injected k s signal may reduce the required threshold current for lasing at k s. If the threshold for lasing at k s is reduced to below that for the original lasing wavelength k p, the FP laser diode becomes injection-locked to k s. Since the steady-state carrier density is effectively clamped above threshold, it will operate in this case with a lower carrier density than the original injection locking at k p. The lower carrier density results in a higher refractive index, leading to the required redshift in the longitudinal modes of the FP laser for sustained lasing at k s. The shift in FP modes will also help extinguish the original k p output from the FP laser diode, thus logically inverting wavelength conversion is obtained [10]. 3. Experiment The setup of the experiment is shown in Fig. 1. A FP laser diode, supplied by PHOTRON (model no. PHLD-1220) with a peak wavelength of about 1543 nm and longitudinal mode spacing of 1.68 nm was used for the format and wavelength converter. It was biased at 20 ma. Two DFB laser diodes were used as the single mode laser sources for dual-wavelength injection locking. Firstly DFB laser diode (DFB 1) was dc biased to provide a continuous wave (CW) pump signal k p and a direct modulated DFB laser diode (DFB 2) as the databearing input signal k s. The two light sources were coupled into the FP laser diode via a 3 db fiber coupler and a circulator. The polarization controllers PC1 and PC2 were adjusted to the polarization to injection-lock the FP laser [11]. The filter at the output port of the circulator separated out the wavelength-converted signal. The pump k p was set to initially injection-lock the FP laser diode (the input signal k p was situated within the injection locking range of one of the longitudinal mode of the FP laser and suppressed other modes by over 30 db). The NRZ data-bearing signal k s was then injected to the injection-locked FP laser diode. By fine-tuning the temperature of the thermoelectric cooler of the FP laser diode to shift the wavelength of its longitudinal modes, it was possible to injection-lock the FP laser to the signal wavelength k s in preference over the input signal k p, thus achieving wavelength-conversion. Format conversion can also be achieved. In this case, clock pulses instead of the continuous wave signal for pump k p are employed. The clock (pump) pulses can be generated by gain-switching the DFB laser diode (DFB 1). The clock pulses, with full-width half maximum (FWHM) of 54 2 ps, were synchronized with the data using a variable optical delay line to adjust the relative time delay between input signal k s and pump signal k p. Simultaneous wavelength-conversion and format conversion from NRZ to RZ may thus be achieved. 4. Results and discussion Fig. 1. Experimental setup. PC, polarization controller; SMF, single mode fiber; APD, avalanche photodiode; EDFA, erbiumdoped fiber amplifier and BERT, bit-error-rate tester. Bit-error-rate (BER) measurements were performed by sending a 3.3 Gb/s 2 31 1 pseudo-random bit train at k s, 1550 nm wavelength to the wavelength converter and measuring the converted pump k p, 1547 nm wavelength. The input NRZ signal was generated by a directly modulated DFB laser (DFB 2). The input signal from the directly
C.W. Chow et al. / Optics Communications 209 (2002) 329 334 331 modulated DFB laser had frequency chirp and also had data pattern-dependent amplitude overshooting of about 40% on some pulses because of gain-switching. The wavelength-converted signal was detected by an avalanche photodiode (APD). The bit-rate was limited in this case by the maximum clock-rate of our bit-error-rate tester (BERT) and was not limited by the format and wavelength-conversion mechanism, and will be characterized in the section of impulse response. Fig. 2(a) shows the back-to-back NRZ input signal, Fig. 2(b) shows the wavelength-converted NRZ signal and Fig. 2(c) shows the format and wavelength-converted RZ signal, together with the corresponding eye diagrams. Negative power penalties were observed in the wavelength and format conversion without and with transmission of 50 km single mode fiber (SMF) (total dispersion ¼ 850 ps/nm) as shown in Fig. 2(i) and (ii), respectively. In the dispersive medium (50 km SMF), an increase in error-rate counts was caused by the chirped signal experiencing pulse broadening and hence inter-symbol interference (Fig. 2(ii-a)). In the case of NRZ to NRZ wavelength-conversion, the power penalty improvement ()5.6 db in Fig. 2(ii-b)) in the converted signal was mainly due to Fig. 2. Measured bit-error-rates (i) without transmitted in long optical fiber, (ii) with 50 km optical fiber (D ¼ 17 ps/nm km): (a) backto-back NRZ, (b) converted NRZ and (c) converted RZ with the corresponding eye-diagrams at data rate of 3.3 Gb/s, 2 31 1 pseudorandom signal.
332 C.W. Chow et al. / Optics Communications 209 (2002) 329 334 having less pattern-dependent output and a reduction of frequency chirp [12] because of the wavelength-conversion. We measured the 3 db line-width of the converted signal was reduced from 11.74 to 9.41 GHz. Also, in the case of NRZ to RZ format conversion, a larger negative power penalty ()9.5 db in Fig. 2(ii-c)) was achieved not only due to frequency chirp reduction, but also better timing margin (less inter-symbol interference) and an extinction ratio improvement from the use of pulsed rather than a CW pump. The range of wavelength-conversion was investigated by measuring the extinction ratio that could be obtained for different wavelengths, as shown in Fig. 3. For a constant input power of )5.4 dbm and pump power )1.3 dbm, it was possible to achieve a 10 db or better over a range of about 45 nm. Although the measured results was within the mode comb of FP, the wavelength converter can operate with an arbitrary input wavelength within the gain spectrum of the FP simply by temperature tuning one of the FP modes to within the injection locking range of the input signal. A temperature change of 9.8 K results in a shift of one mode (1.68 nm) of an as-cleaved uncoated (UC UC) FP laser diode, while a change of 16.1 K can shift one mode of an antireflectioncoated high reflection coated (AR-HR) FP laser diode. This large range of wavelength-conversion Fig. 3. Extinction ratio at different converted output wavelengths for a constant input wavelength at 1550 nm, with signal power of )5.4 dbm and pump power )1.3 dbm. (a) (b) (c) (d) Fig. 4. Extinction ratio and bit-error-rate of converted signal for detuning of: (a) input signal wavelength; (b) input signal power; (c) pump wavelength; and (d) pump power.
should be sufficient for most WDM communication applications. C.W. Chow et al. / Optics Communications 209 (2002) 329 334 333 5. Conditions for negative power penalty We have experimental demonstrated different conditions to achieve negative power penalty are shown in Fig. 4. It was observed that there is an inverse relationship between extinction ratio and BER of the converted signal (Fig. 4(a), (b) and (d)). From Fig. 4(a) and (c), for a constant input of signal power of )5.4 dbm and pump power of )1.3 dbm, and by wavelength detuning of input signal wavelength and pump wavelength, it was observed that a low BER or a high extinction ratio was maintained by detuning the wavelength to the higher wavelength side of the FP mode comb, this showed the red-shift of the injection locking. In Fig. 4(b), increasing the input signal power suppressed more the power of the converted output signal, so a higher extinction ratio of converted signal resulted in a larger BER. In Fig. 4(d), increasing in pump power make the BER higher because the APD was saturated by the large injection power (since the pump is where the converted signal come from), and the APD response became too slow at saturation. Another important factor achieving negative power penalty is the dependence on injection ratio, which is defined as the ratio between the externally injected power (P in ) (the sum of the input signal and the pump power) and the output power (P) of the free-running FP laser diode. Fig. 5 shows the optimum injection ratio is around 0.12. 6. Impulse response The dynamic response of the format and wavelength-conversion was studied by testing its impulse response. A 2.5 GHz optical pulses train (Fig. 6(a)) of about 27 ps FWHM pulse width from a gain switched DFB laser was used as the input signal. The logically noninverted wavelength-converted pulses had a FWHM pulse width of 47 ps was obtained (Fig. 6(b)). This response indicates that the format and wavelength Fig. 5. Extinction ratio of converted signal versus injection ratio (P in =P). Fig. 6. (a) Input 27 ps FWHM 2.5 Gb/s optical pulse train at 1550 nm and (b) output wavelength-converted pulse at 1547 nm with 47 ps FWHM. conversion should be capable of working at 10 Gb/s. 7. Conclusion We have experimentally demonstrated for the first time simultaneous wavelength-conversion with a NRZ to RZ format converter using dualwavelength injection locking. A negative power penalty of )5.6 db was achieved in NRZ to NRZ wavelength-conversion and )9.5 db was achieved in NRZ to RZ format and wavelength-conversion measured by a 3.3 Gb/s 2 31 1 pseudo-random bit
334 C.W. Chow et al. / Optics Communications 209 (2002) 329 334 train at BER 10 9 in the transmission of 50 km SMF. The scheme can thus act as a 2R (re-amplify and re-shape) regenerator, which can reduce amplitude fluctuations and frequency chirp. The alloptical wavelength-conversion range is about 45 nm with over 10 db extinction ratio. The data rate is limited by our BER tester, and the expected operation rate is 10 Gb/s. Acknowledgements The work was fully funded by RGC earmarked research grant CUHK 4192/01E. References [1] D. Norte, A.E. Willner, IEEE J. Lightwave Technol. 14 (1996) 1170. [2] P.S. Cho, D. Mahgerefteh, J. Goldhar, Opt. Commun., ECOC 98 1 (1998) 353. [3] H. Kawaguchi, Y. Yamayoshi, K. Tamura, IEEE LEOS 2000 2 (2000) 575. [4] M. Owen, M.F.C. Stephens, R.V. Penty, I.H. White, OFC 2000 3 (2000) 76. [5] H.J. Lee, S.J.B. Yoo, C.S. Park, OFC 2001 1 (2001) MB7-1. [6] A. Reale, P. Lugli, S. Betti, IEEE J. Sel. Top. Quantum Electron. 7 (2001) 703. [7] L. Deming, N.J. Hong, L. Chao, IEEE Photon. Technol. Lett. 12 (2000) 1222. [8] T. Durhuus, B. Mikkelsen, C. Joergensen, S.L. Danielsen, K.E. Stubkjaer, IEEE J. Lightwave Technol. 14 (1996) 942. [9] M.W.K. Mak, H.K. Tsang, K. Chan, IEEE Photon. Technol. Lett. 12 (2000) 525. [10] J. H orer, E. Patzak, IEEE J. Quantum Electron. 33 (1997) 596. [11] L.Y. Chan, H.K. Tsang, S.P. Yam, C. Shu, Integrated Photonics Res. 4 (1998) IWB4-1/389. [12] M. Yoshino, K. Inoue, Electron. Lett. 30 (1994) 1956.