1. Introduction

With recent growth of ultrafast optical networks based on optical time-division-multiplexing (OTDM) technologies, single channel bit-rates have been drastically increased [1]. However, as the bit-rates increase, and the transmitted pulses become shorter, OTDM system is highly sensitive to higher-order dispersion and nonlinear effects in the transmission line. These effects give rise to waveform distortion and timing jitter, and cause degradations of the transmitted signal quality. Although the waveform distortion can be reduced by suppressing nonlinear effects, and compensating for cumulative dispersion in transmission line, the reduction of the timing jitter requires a clock recovery circuit with complicated and expensive components. Therefore at the receiver, it is important to improve the quality of the received signal without using a clock recovery circuit. In such transmission systems, one of the key technologies that obtain sufficient jitter tolerances is to employ a rectangular switching window or waveform [2, 3].

Semiconductor optical amplifier (SOA)-based delayed-interferometric switches are very attractive for OTDM networks employing all-optical signal processing without limits of electrical processing. These switches provide a rectangular-like switching window characteristic that is suitable for most of the switching applications. So far, various functional devices, including demultiplexing, wavelength conversion, and 3R (re-amplification, re-shaping, re-timing) regeneration, have been demonstrated [4]. Recently, all-optical format converters, which convert from return-to-zero (RZ) to nonreturn-to-zero (NRZ) signal format have been reported and demonstrated [5, 6]. Since these converters enable us to generate output pulses with a rectangular-like profile, they act as all-optical waveform converters. These features also enable us to suppress the timing jitter of the data signal through the 3R regeneration [7].

In this paper, as a new application of SOA-based switch, we propose an optically preamplified RZ-receiver scheme for demultiplexed OTDM signals by utilizing all-optical waveform converter, which can control the pulse-width by adjusting optical path delay in the delayed-interferometer [8]. In a conventional receiver, the received signal pulse-widths are broaden by an electrical low-pass filter (E-LPF) to improve the jitter tolerance of the signals. However, in order to minimize the interaction of induced eye opening penalty, optimizing the filter bandwidth is also required. This means that there exists a trade-off between jitter tolerance and inter-symbol interference (ISI). On the other hand, rectangular-like waveform is very effective to improve the jitter tolerance without increasing the ISI. Although receiver performance improvement with rectangular-like waveform converted by dispersion and Kerr nonlinearity in fiber was already reported [9], it requires a long-length normal dispersion fiber over 20 km and a high-power optical amplifier. This paper reports, for the first time, the improved receiver performance by utilizing SOA-based waveform converter with a practical configuration. We have evaluated the receiver characteristics as a function of the pulse-width of the waveform-converted pulse. The performances and advantages of our proposed receiver have been experimentally verified.

2. Configuration and experimental setup

The receiver configurations of the conventional and proposed schemes are depicted in Fig. 1. It was assumed that the demultiplexed OTDM signal propagated into a transmission line, and then was detected at the conventional or proposed receiver as shown in Fig. 1(a). Each of the receivers employs an erbium-doped fiber amplifier (EDFA), an optical bandpass filter (O-BPF), and a photo-diode (PD). In the conventional scheme shown in Fig. 1(b), the optical RZ pulses are converted to electrical pulses, and solely broaden by proper electrical low-pass filtering to reduce the jitter influence. On the other hand, in the proposed scheme in Fig. 1(c), the optical RZ pulses converted by the all-optical waveform converter before the detection. Due to the larger jitter tolerance, the receiver performance is expected to be improved in comparison with the conventional scheme.

 

Fig. 1. (a) Example of OTDM transmission system. Receiver configurations of (b) conventional and (c) proposed schemes

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Figure 2 shows the experimental setup for comparison of the conventional and proposed receiver. As shown in Fig. 2(a), the received signal pulses were generated by a sinusoidally 10 GHz driven electroabsorption modulator (EAM) and subsequent nonlinear pulse compressor based on spectrum-slicing [10]. The nonlinear pulse compressor consists of a 8 km dispersion-shifted fiber (DSF) and optical bandpass filter (O-BPF) with 1.0-nm bandwidth. The output pulse-width was 5.5 ps, which corresponds to a pulse-width of the demultiplexed OTDM signal over 40 Gbit/s transmission system. On the other hand, the uncompressed pulse train with 18.0 ps pulse-width was also employed in order to investigate pulse-width dependence of the received signal. The pulses were modulated by 10 Gbit/s data with a pseudorandom bit sequence (PRBS) of length of 2⁷-1 by using a LiNbO₃ modulator (LNM). The wavelength and output power of the received signal were 1550.12 nm and 4 dBm, respectively. In the conventional scheme in Fig. 2(b), the electrical bandwidth of the E-LPF was 7.5 GHz. As a high-speed photo-detector, uni-traveling-carrier photo-diode (UTC-PD) with 40 GHz-bandwidth and high output power was employed. In order to avoid effects of the bandwidth of the electrical components such as an electrical amplifier at the receiver, the UTC-PD was directly connected to bit error rate tester (BERT). The waveform converter [11] with the proposed receiver scheme in Fig. 2(c) was realized by a terahertz optical asymmetric demultiplexer (TOAD) [12] configuration with a variable delay line. This TOAD is based on a SOA that is asymmetrically placed in the Sagnac loop to neutralize slow semiconductor carrier relaxation time. The employed SOA was a polarization insensitive bulk type with 600 µm long active layer. The measured relaxation time of the SOA was approximately 70 ps. The pulse-width of the converted pulse is primarily determined by the time delay Δt of the delay line. The detailed configuration is described in Ref. [11]. Since the waveform-converted signal is originated from a CW probe light with an arbitrary wavelength, this waveform converter also acts as a wavelength converter.

 

Fig. 2. Experimental setup. (a) Configuration of 10 Gbit/s transmitter (with or without 2R regenerator) and receivers ((b) conventional and (c) proposed schemes).

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3. Results and discussion

Figure 3 shows the BER of the received signals with the conventional (with E-LPF) and proposed receivers. The received signal pulse-widths of (A) 18 ps and (B) 5.5 ps were employed. In the proposed scheme, the time delays Δt of the delay line, which determine the waveform-converted pulse-widths, were set to 10, 20, 30, and 40 ps, respectively. As shown in Fig. 3(A) and (B), the highest receiver sensitivity, corresponding to the pulse-widths of 30 and 20 ps, were obtained respectively. Especially in Fig. 3(B), the receiver sensitivity at Δt=20 ps was 0.7 dB higher than that of the conventional receiver. In delayed-interferometric switch, the rise and fall times of the converted pulse are primarily determined by the pulse-width of the input signal. Thus, the shorter delay setting against the long pulse-width of the received signal degrades optical signal-to-noise ratio (OSNR) of the converted pulse in delayed-interferometric switch [13]. As shown in Fig. 3(A) for example, the error floors exist at the delay settings from 10 to 30 ps, and become larger by shortening the pulse-width of the converted pulse. On the other hand, the longer delay setting (Δt≥30 ps) also degrades the BER performances. It is due to the OSNR degradation of the waveform-converted pulse. If the SOA has a relaxation time shorter than delay setting, the converted waveform is no longer flat-top rectangular-like profile, and the OSNR also degrades. It is also due to the receiver sensitivity reduction. In conventional optically preamplified receiver, the receiver sensitivity is decreased by increasing the duty-ratio of the received pulse [14]. These indicate that optimizing pulse-width setting of the converter is required to achieve the highest receiver sensitivity according to the pulse-width of the received signal. In other words, the characteristics of the proposed receiver can be optimized by adjusting the time delay of the converter to a received signal with an arbitrary pulse-width. Further improvement of the sensitivity will be possible with a shorter pulse-width of the received signal, which is demultiplexed at a higher bit-rate OTDM system.

 

Fig. 3. Measured BER results with the conventional (with E-LPF) and proposed receivers. The received signal pulse-widths were (A) 18 ps and (B) 5.5 ps.

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The BER as a function of the phase detection time for the received signal pulse-width of 5.5 ps is depicted in Fig. 4. In order to estimate the jitter tolerance of the received signal, the BER changes against the relative detection time, which is shifted from an optimal detection time to an arbitrary time, were measured. The time delay Δt for the proposed receiver was set to 20 ps, which performs the highest sensitivity in Fig. 3(B). Although the use of E-LPF enable us to improve the phase-margin at error-free operation (BER≤10^-9), the improvement was very little due to the pulse broadening by the low-pass filtering. On the other hand in the proposed receiver, the phase-margin had a rectangular-like profile and was much wider than that of the conventional receiver. The improvement of the phase margin was approximately 60 %. The inset shows the eye diagram of the waveform-converted pulses. The waveform did not exhibit a rectangular profile. This is because the limits of the bandwidth of our measuring equipment. The pulses were detected with a 30 GHz-bandwidth PD and subsequently displayed on a digital sampling oscilloscope. The enlargement of the phase-margin proves that the proposed waveform converter is effective to improve the jitter tolerance of a receiver.

 

Fig. 4. Measured BER as a function of the relative phase detection time. Inset shows the eye diagram of the waveform-converted pulses before injecting the UTC-PD in Fig. 2 (b).

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4. Conclusion

A new receiver scheme with semiconductor-based waveform converter was presented. We have demonstrated error free operations for various pulse-widths of the waveform-converted signals. By using 10 Gbit/s signal with 5.5 ps received pulse-width, the power-penalty improvement of 0.7 dB and the phase-margin enlargement of 12 ps were simultaneously achieved in comparison with the conventional scheme. These results indicate that the proposed receiver is promising for improving receiver performance.