Demonstration of 8 Error-free Cascades of 2R NRZ SOA-MZI Wavelength Converter

D. Apostolopoulos1, D. Klonidis2, P. Zakynthinos1, K. Vyrsokinos1, N. Pleros3, I. Tomkos2

and H. Avramopoulos1

(1) National Technical University of Athens – Department of Electrical Engineering and Computer Science

9 Iroon Polytechniou Street, Zografou 15773 –Athens, Greece Tel. +30-10-7722057, Fax +30-10-7722077, Email: apostold@mail.ntua.gr

(2) Networks and Optical Communications group - AIT - Athens Information Technology center 19002, Athens, Greece (3) Computer Science Department, Aristotle University of Thessaloniki, Greece

Abstract: We show 8 error-free cascades of a 2R SOA-MZI, 10 Gb/s NRZ wavelength converter. It uses a novel push-pull arrangement with an additional CW signal to differentially bias the MZI SOAs and reduce jitter accumulation. ©2008 Optical Society of America OCIS codes: (060.2330) Fiber optics communications; (230.1150) All-optical devices; (250.5980) Semiconductor Optical Amplifiers;

1. Introduction Wavelength conversion is a central method in effecting transparency in all-optical networks. Wavelength conversion (WC) is also a main technique for effecting switching in all-optical signal processing circuits and semiconductor optical amplifier, Mach-Zehnder Interferometers (SOA-MZIs) are key elements to achieve these. Depending on whether the wavelength conversion is performed on an external, continuous wave (CW) or a clock signal, SOA-MZIs possess either 2R or 3R regenerative properties [1]. Clearly a wavelength converter that uses an external CW signal is significantly simpler to implement, as it does not require a high speed short pulse laser or a clock recovery unit with their associated drivers. Unfortunately this type of simpler wavelength converter with its 2R regenerative properties has also been shown to be capable of a very limited number of repeated cascades, significantly restricting its use as a regenerator in a high speed fiber transmission line [2].

Given the significance of wavelength conversion and the comparative simplicity of 2R, SOA-MZI wavelength converters, effort is warranted in extending their capability to perform successive cascades. In this communication we examine the cascade potential of a novel scheme for 2R wavelength conversion with SOA-MZIs. The scheme uses the well-known bidirectional push-pull data injection scheme [3], but it employs an additional external CW signal on one of the SOAs of the MZI to differentially adjust it with respect to the second SOA. In this way, it assists in optimizing and fixing gain and phase conditions in both arms of the interferometer, while further suppressing jitter effects. We investigate the performance of the scheme in a fiber loop experiment. We show that whilst the standard scheme with a single control for 2R wavelength conversion with SOA-MZI can only support 2 error-free cascades and the push-pull scheme can support 4, the scheme proposed here can extend error-free operation to 8 cascades.

2. Concept and Experimental Setup

Figures 1(a), (b) and (c) show respectively 2R wavelength converter regenerators following the standard single control scheme, the bidirectional push-pull control scheme and the differentially biased scheme proposed in this letter. The latter follows the layout of the bidirectional data injection scheme but employs an additional external CW signal which is used to differentially adjust and to fix the gain/gain recovery the SOAs. In this way the external CW signal helps in reducing the conversion of the amplitude jitter of the incoming signal to timing jitter in the outgoing signal as usually happens in regular, single control SOA-MZI 2R wavelength converter. The cascading performance of the three schemes was investigated in the loop experiment of Figure 1(d).

The loop input signal was generated from a 1556 nm continuous wavelength (CW) laser diode modulated in a Ti:LiNbO3 electro-optic modulator driven with a 10 Gb/s electrical NRZ signal, producing in this way a 231-1

PRBS sequence. The recirculating loop consists of two spans of 75 km SMF fibers each followed by an erbium-doped fiber amplifier (EDFA) and the appropriate dispersion compensating fiber (DCF) (-1360 ps/nm and -1190 ps/nm respectively) to compensate for power loss and chromatic dispersion. The input signal power levels in the two SMF spans and DCFs are 3.2 dBm and -4 dBm. Two SOA-MZI wavelength converters [4] were used in each loop transit, one to convert the incoming signal from 1556 nm to 1551 nm after transmission in the first fiber span and the second to convert back from 1551 nm to the original wavelength 1556 nm. The SOA-MZIs were hybrid integrated devices and were operated with 300 mA current and required 1.7 dBm of CW signal at their input port. The powers of the control signals were 4 dBm for the standard scheme, 2 and -1.5 dBm for the push and pull signals of the bidirectional scheme and 2 and -0.5 for the push and the pull of the differentially biased configuration. Finally, the power of the additional CW (1560 nm) signal in the differentially biased push-pull scheme was 4 dBm.

3. Results and Discussion

Figure 2 illustrates the change in the eye-diagrams after successive transits of the signal through the loop for the three wavelength conversion schemes, noting that each transit through the loop corresponds to a pair of consecutive wavelength conversions. Figure 2(a) depicts the output of the loop for the standard, single control scheme. An almost closed eye is observed after the second transit through the loop, after only 4 wavelength conversions due to the rapid accumulation of amplitude and timing jitter. The performance of the bidirectional push-pull control scheme is shown in figure 2(b) and displays a significant improvement as the counter-propagating control suppresses the accumulation of jitter in amplitude and time. Two transits through the loop are error-free, corresponding to 4 successive wavelength conversions. Finally figure 3(c) shows the performance of the new, differentially biased, bidirectional, push-pull control scheme. It indicates that 4 loop transits are possible

Figure 2: Eye diagrams at the loop output for (a) standard WC scheme with one control signal, (b) bidirectional push-pull scheme (c) differentially biased bidirectional push-pull scheme. Each loop employs two cascaded wavelength converters in sequence.

Figure 1: Principle of operation of (a) standard WC scheme with one control signal, (b) bidirectional push-pull WC scheme and (c)

differentially biased bidirectional push-pull scheme with additional CW signal. (d) Experimental loop setup.

corresponding to 8 successive wavelength conversions with error-free signal at the end. Figures 3(a), 3(b), 3(c) show bit error rate measurements against received power for the single control, the bidirectional push-pull and the differential biased scheme respectively. Error free operation for our scheme is verified even after 8 MZI cascades compared to the 2 and 4 successful cascades for the single and bidirectional schemes. Figure 3 (d) shows the degradation in BERs for the three schemes against the number of cascades through the wavelength converters. A BER of 10-8 was obtained after 10 cascades using the differential biased bidirectional scheme, while an error floor at 10-4 was measured after 4 and 6 cascades for the standard and the bidirectional operation respectively.

Figure 3:Bit-Error-Rate measurements

4. Conclusion

We have examined the cascaded performance of three schemes for 2R wavelength conversion. The novel scheme outlined here that involves bidirectional push-pull control and an additional CW signal for SOA gain and gain recovery management, is shown to be capable of 8 successive, error free wavelength conversions and outperforms the others.

References [1] Olivier Leclerc et al., “Optical Regeneration at 40 Gb/s and Beyond”, IEE/OSA J. Light. Tech., 20, 2779-2790 (2003) [2] M. Funabashi et al, "Cascadability of optical 3R regeneration for NRZ format investigated in recirculation loop transmission over field fibers", IEEE Photon. Technol. Lett., 18, 2081-2083 (2006) [3] M. Hattori, K.Nishimura, R. Inohara, and M. Usami, "Bidirectional Data Injection Operation of Hybrid Integrated SOA–MZI All-Optical Wavelength Converter", IEE/OSA J. Light. Tech., 25, 512-519 (2007) [4] G. Maxwell et al., “Very low coupling loss, hybrid-integrated all optical regenerator with passive assembly,” presented at the Eur. Conf. Optical Commun., Copenhagen, Denmark, 2002, Paper PD3.5

Acknowledgments

The work described in this paper was carried out with the support of EURO-FOS and BONE projects, Networks of Excellence funded by the European Commission through the 7th ICT-Framework Programme and ICT-DICONET.