s transmission and cascaded all-optical

40 Gbit=s transmission and cascaded all-optical wavelength conversion over 1 000 000 km J. Leuthold, G. Raybon, Y. Su, R. Essiambre, S. Cabot, J. Jaques and M. Kauer A transmission span of 4
100 km followed by an all-optical wavelength converter is cascaded 2500 times. Thereby transmission of a 40 Gbit=s signal over a total distance of 1 000 000 km is demonstrated.

Introduction: All-optical wavelength converters (AOWC) are envisioned to be part of future high-speed networks. In particular, they might replace optical-to-electrical-to-optical (OEO) wavelength translator units in optical crossconnects [1]. However, to be practical, AOWCs must be able to convert signals that have been degraded during transmission over reasonable distances. Tolerance to timing jitter, noise accumulation and pulse degradation of the incoming signal are therefore required. Moreover, the converted signal has to have a signal quality that allows for subsequent fibre transmission. 3R regenerative (re-amplification, re-shaping and re-timing) capabilities are therefore required of an AOWC. Various 3R regenerative AOWCs for 40 Gbit=s and higher have been demonstrated [2–5]. However, while fibre transmission based on all-fibre regenerators over 1 million km has been demonstrated [6], there exist only few experiments that show operation of all-optical wavelength converters in a cascade and with wavelength conversion ranges of e.g. the whole C-band such as needed in a network with all-optical crossconnects. So far, loop experiments comprising AOWCs have demonstrated transmission distances of 2000 [7] and 4000 km [8] with loop lengths of 100 and 40 km, respectively. In this Letter we report the first 400 km recirculating loop experiment with an all-optical wavelength converter. We cascade 2500 devices, with a total of 1 000 000 km of fibre transmission. The result has been achieved with a 3R regenerative wavelength converter comprising a semiconductor optical amplifier (SOA) based AOWC for performing both wavelength conversion and retiming and a fibre based regenerator for signal reshaping.

output to select components of the same intensity in the SPM spectrum. The 2R stage also provides some small wavelength shift of 1.5 nm. The single-stage 2R regenerator at the output is therefore used to reset the signal wavelength to the loop wavelength. The SOA based delay interferometer (SOA-DI) wavelength converter [5] maps the input signal Pin onto the clock signal Pclk and thereby performs wavelength conversion of the input signal wavelength onto the wavelength of the clock signal. Wavelength conversion can be performed to any wavelength within 30 nm around the SOA gain maximum. Also, bit inversion is performed during the process of wavelength conversion i.e. the zeros are converted to ones and vice versa. A 40 GHz pin photodiode detects the signal and generates an electrical signal, which is filtered using a very high Q (  900) microwave filter and then reamplified before modulating an EA modulator. The EA then carves a clock signal into a CW signal. The time delay between clock and input signal was adjusted with a tunable optical delay line. To further improve the extinction ratio after the wavelength converter we added a saturable absorber (SA) behind the filter at the output of the SOA-DI. This way the quality of the signal improved from a Q2 ¼ 17.4 dB to a Q2 ¼ 18.2 dB. The additional two-stage 2R regenerator at the output of the scheme further improved the quality of the signal to Q2 ¼ 18.5 dB. These additional reshaping devices are not needed if only a few wavelength converters are cascaded. However, to perform thousands of recirculating loops we needed the best signal quality. A simplified loop setup is shown in Fig. 1b. The transmitter provides a 40 Gbit=s, 33% duty cycle, RZ signal at 1552.5 nm that is obtained by multiplexing 10 Gbit=s signals with a PRBS of 231  1 [6]. The receiver consists of a high Q filter based clock recovery and an OTDM demultiplexer. An EA modulator demultiplexes the 40 Gbit=s signals down to 10 Gbit=s for error detection. The transmission span consists of four 100 km spans of TrueWave1 reduced slope (TWRS) nonzero dispersion fibre and EHS-DCF fibre for dispersion compensation. EDFAs and backward pumped Raman amplification are used to compensate the 21 dB span losses. A tunable dispersion compensator adjusts the dispersion before the regenerator. The launch power into the spans was 3 dBm. The wavelength at the output of the AOWC was set to give the initial input wavelength. Input-signal and clock signal powers into the SOA-DI were 6 and 8 dBm.

All-optical wavelength converter and loop: A schematic diagram of the AOWC is shown in Fig. 1a. The AOWC comprises three sections. Two fibre based regenerators for signal reshaping and reamplification and a retiming section encompassing a wavelength conversion scheme. A clock-recovery (CR) is part of the wavelength converter section. This CR depends on narrowband electronics. However, it could potentially be replaced by an all-optical CR such as e.g. shown in [10] and thus completely eliminate the need for power consuming RF electronic circuits and amplifiers.

Fig. 2 BER curves

Fig. 1 Regenerator and loop setups a 3R regenerator setup comprising fibre 2R regenerators for reshaping and SOAbased all-optical wavelength converter for retiming and wavelength conversion b Loop setup

The fibre based 2R regenerator at the input is shown on the left side of Fig. 1a. The two-stage fibre regenerator contains a compression stage and a regenerator stage [6]. The compression stage is for suppressing Brillouin scattering. In the regenerator stage self-phase modulation (SPM) of the nonlinear fibre is exploited to broaden the optical spectrum of the input signal [9]. A 1 nm optical filter is placed at the

Results: Bit error rate (BER) measurements against received power curves are shown in Fig. 2. At an error rate of 109 we find a 2 dB penalty for both the simplified wavelength converter scheme over 1600 km and the enhanced wavelength converter scheme over 1 000 000 km. In the simplified scheme we used the SOA-DI wavelength converter in combination with the two-stage 2R fibre regenerator at the output without any additional SA. The BER curves show that this scheme performs well for up to four cascades or 1600 km of combined transmission and wavelength conversion. However, for cascading thousands of wavelength converters the enhanced scheme of Fig. 1a was required. Fig. 3 shows the measured Q value against distance for the enhanced all-optical wavelength converter of Fig. 1a. Q-values above 15.6 dB (109 BER level) are shown at all distances. BER against decision

ELECTRONICS LETTERS 1st August 2002 Vol. 38 No. 16 , pp 890-892

threshold or v-plots such as used to measure the Q-factor are given for transmission distances at 400, 40 000 and 1 000 000 km in the insets of Fig. 3. The v-plots have nearly identical shapes for transmission at 400 and 40 000 km and up to 400 000 km (not shown), which is a sign that we have hardly any degradation. Yet, a first degradation is seen after 1 000 000 km. We attribute this degradation to polarisation sensitivity issues of the SOA and the SA that occur at the end of the experiment.

Fig. 3 Quality of signal against distance Insets: v-plots showing BER against decision threshold for different distances

Conclusion: We have successfully cascaded four and 2500 wavelength converters separated by 400 km fibre by means of a simplified and an enhanced nonlinear fibre and SOA-based 3R regenerator scheme, respectively. # IEE 2002 Electronics Letters Online No: 20020595 DOI: 10.1049/el:20020595

J. Leuthold, G. Raybon, Y. Su, R. Essiambre, S. Cabot, J. Jaques and M. Kauer (Lucent Technologies, Bell Laboratories, Holmdel, NJ 07733, USA)

References 1

LEUTHOLD, J.,, et al.et al.: ‘All-optical nonblocking terabit=s crossconnect based on low power all-optical wavelength converter and MEMS switch fabric’. Proc. OFC’2001, Anaheim, USA, March 2001, Paper PD 16 2 OTANI, T.,, et al.et al.: ‘40 Gb=s optical 3R regenerator using electroabsorption modulators for optical networks’, J. Lightwave Technol., 2002, 20, (2), pp. 195–200 3 FISCHER, S.,, et al.et al.: ‘Optical 3R regenerator for 40 Gbit=s networks’, Electron. Lett., 1999, 35, (23), pp. 2047–2049 4 UENO, Y.,, et al.et al.: ‘Penalty-free error-free all-optical data pulse regeneration at 84 Gb=s by using a symmetric-Mach–Zehnder-type semiconductor regenerator’, IEEE Photonics Technol. Lett., 2001, 13, (5), pp. 469–471 5 LEUTHOLD, J.,, et al.et al.: ‘Novel 3R regenerator based on semiconductor optical amplifier delayed-interference configuration’, IEEE Photonics Technol. Lett., 2001, 13, (8), pp. 975–977 6 RAYBON, G.,, et al.et al.: ‘40 Gbit=s pseudo-linear transmission over one million kilometers’. Proc. OFC’2002, Anaheim, USA, March 2002, Paper PD FD10 7 THIELE, H.J.,, et al.et al.: ‘Recirculating loop demonstration of 40 Gbit=s all-optical 3R data regeneration using a semiconductor nonlinear interferometer’, Electron. Lett., 1999, 35, (3), pp. 230–231 8 LAVIGNE, B.,, et al.et al.: ‘Cascade of 100 optical 3R regenerators at 40 Gbit=s based on all-active Mach Zehnder interferometers’. ECOC’01, September 2001, Vol. 3, pp. 290–293 9 MAMYSHEV, P.V.: ‘All-optical data regeneration based on self-phase modulation effect’. Proc. ECOC’98, September 1998, Vol. 1, pp. 475–476 10 SARTORIUS, B.: ‘All-optical clock recovery for 3R optical regeneration’. Proc. OFC’2001, Paper MG7

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ELECTRONICS LETTERS 1st August 2002 Vol. 38 No. 16 , pp 890-892