Abstract
A Sagnac interferometer monolithically integrated with parallel-amplifier structure (SIPAS) is described. Filter-free wavelength conversion and full bit-rate conversion from 10-Gb/s random WDM channels to a 40-Gb/s channel were demonstrated.

Introduction
The explosive growth of the internet has pushed an increase in the bit rate of WDM networks and has accelerated the construction of photonic networks, which have the potential to overcome the speed limit of electrical devices through all-optical signal processing.

All-optical wavelength conversion will play an important role in future photonic networks. A wavelength converter based on the cross phase modulation (XPM) in a semiconductor optical amplifier (SOA) is one of most promising devices. Differential phase modulation (DPM), which uses XPM in a differential scheme, can overcome the speed limitation of carrier lifetime in an SOA[1]. Wavelength conversion at over 100 Gbit/s has been reported using DPM[2]. In this scheme, an optical filter is necessary in order to reject the input signal. In the case of wavelength-tunable conversion, the response time of this filter may limit system performance, so simple filter-free operation is desirable [3,4].

With the increase in the bit rate of networks, bit-rate conversion from low-speed (<10-Gb/s) WDM-LANs to high-speed (40-Gb/s) networks is also desirable. All optical bit-rate conversion provides bit-rate-free operation.

In this article, we first describe filter-free wavelength conversion using a newly developed Sagnac interferometer integrated with parallel-amplifier structure (SIPAS)[5,6]. SIPAS has low wavelength dependence, single-signal input without using a delay line and can divide the input and converted signal. We then discuss full bit-rate conversion from 10-Gb/s random WDM channels to a 40-Gb/s channel, including non-return-zero (NRZ) to return-to-zero (RZ) format conversion and reconversion, as an application of SIPAS. Clear eye openings and low power penalties were observed in experimental testing.

Fig.1 Structure of SIPAS

SIPAS is a Sagnac interferometer with parallel-amplifier structure (PAS), which is a Mach-Zehehnder interferometer (MZI) having polarization insensitive SOAs in each arm. It was fabricated using monolithic integration techniques (Fig. 1), and is 4 mm long.

Fig.2 Spectrum of output optical signal: signal: 1552.6 nm CW:1550.0 nm (a) PAS off, SOA1: 212 mA, SOA2: 0 mA (b) PAS on, SOA1: 212 mA, SOA2: 217 mA

The operating principle is similar to that of the SLALOM [7]. An input CW light is divided into clockwise (CLW) and counterclockwise (CCW) traveling lights. Since the PAS is placed asymmetrically in the loop, two lights reach the SOAs at different times, which leads to a different phase modulation between the lights when a signal light is input into the SOAs. After traveling the loop, the lights are superimposed and transmitted to the output port due to differential phase modulation (DPM). We placed the PAS asymmetrically by 0.5mm so that the switching window due to DPM is about 10 ps, which enables high-speed operation over 40 Gbit/s. As the PAS is set in the cross state, the signal light cannot enter the loop, resulting in filter-free wavelength conversion.

Filter-free wavelength conversion
We first examined the filtering effect of the PAS. A CW light and signal light were injected from the CW and signal-input ports respectively.

Figure 2 shows the spectrum from the output port. Figure 2(a) shows the case when a drive current was injected into only SOA1, which corresponds to the conventional SLALOM[7]. Both converted and signal lights were observed. When drive currents were injected into both SOAs, on the other hand, the signal light was suppressed and converted light was increased as shown in Fig.2(b). The suppression ratio was as large as 27 dB, as shown in the figure, which is large enough for filter-free operation.

Fig.3 Eye pattern in filter-free operation

We then performed filter-free wavelength conversion experiments under the conditions shown in Fig. 2(b). The signal light was modulated with a 10-Gb/s RZ signal. Figure 3 shows the eye pattern of the converted signal in filter-free operation. Clear eye openings were observed. A power penalty of 0.9 dB was obtained compared to back-to-back in filter-free operation.

Application: Full bit-rate conversion from 10-Gb/s random WDM channels to a 40-Gb/s channel
Using SIPAS, we demonstrated bit-rate conversion. Figure 4 shows the configuration of the demonstration system. The key features are as follows.

Fig. 4. Full bit-rate conversion

<NRZ/RZ and MUX>

Four 100-GHz spacing WDM channels of 10-Gb/s NRZ format (point a) are launched into an EA modulator and converted simultaneously to RZ formats (point b). A fiber loop arranges the piled up RZ pulse into a serial bit stream. This simple technique for stream forming can also be used in large-scale WDM systems with 8 or 16 channels, for example.

<Bit-rate conversion using SIPAS>

Fig. 5. Eye patterns in bit-rate conversion and RZ/NRZ conversion

For 40-Gb/s bit-rate conversion, the four different wavelengths of the multiplexed bit stream are converted into a single wavelength by using SIPAS (point c). SIPAS has a low wavelength dependency.

<DEMUX and RZ/NRZ>

An EA modulator is used to demultiplex the converted 40-Gb/s stream into 10-Gb/s RZ format (point d). Another DPM device with a gating window of 100 ps enlarges the pulse width, thus completing the 10-Gb/s RZ to NRZ reconversion (point e).

<Experimental result>

Fig. 6. Bit error rate in bit-rate conversion

Figure 5 shows the eye pattern measured at points (a) to (e). Clear eye openings were observed at each point. Figure 6 shows the bit error rate from points (a) to (e). The receiver sensitivity at an error rate of 10^-9 was less than -32 dBm (Fig. 6). Only a small power penalty of less than 0.8 dB was observed.

Conclusion
We have developed a monolithically integrated Sagnac interferometer integrated with parallel-amplifier structure (SIPAS). Filter-free wavelength conversion at a bit rate of 10 Gb/s and full bit-rate conversion from 10-Gb/s random WDM channels to a 40-Gb/s channel were demonstrated. These results show that the SIPAS will be useful in future high-bit-rate photonic networks.

References
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