An all-optical logical AND gate at 10 Gbit/s based on cross-gain modulation (XGM) in two cascaded semiconductor optical amplifiers (SOAs) is demonstrated. Single-port-coupled SOAs are employed and specially designed to improve the output extinction ratio as well as the output performance of the logic operation. The output signal power and extinction ratio from the first-stage wavelength converter are critical to achieving all-optical logical AND operation.
©2004 Optical Society of America
All-optical signal processing is expected to have many applications in communications and computation because it can handle large bandwidth signals and large information flows. All-optical logic gates are key functional elements in all-optical signal processing and have received increasing attention in recent years [1–4] for addressing, demultiplexing, regenerating, and switching. The all-optical AND gate is one of the fundamental logic gates because it is able to perform on-the-fly bit-level functions such as address recognition, packet-header modification, and data-integrity verification. Until now, all-optical AND gates reported in the literature [5–10] could be achieved with a semiconductor laser amplifier loop mirror (SLALOM), a semiconductor optical amplifier- (SOA-) based Mach-Zehnder interferometer (SOA-MZI), a SOA-based ultrafast nonlinear interferometer (UNI), cross-polarization modulation, and four-wave mixing (FWM) in SOAs. These schemes have been shown to have some advantages, but they are difficult to control or construct and polarization states or random phase changes are critical for their output performance. Maeda  reported an all-optical triode at 5 GHz based on cross-gain modulation (XGM) in tandem wavelength converters by use of a SOA with bulk material. Based on a similar structure, a simple scheme for an all-optical AND gate is presented in this paper, and a 10-Gbit/s all-optical logical AND gate is experimentally demonstrated for random bit sequences by proper control of the signal power. To improve the output performance, single-port- coupled SOAs [12, 13] with multi-quantum-well (MQW) materials are specially designed for a large output extinction ratio in the first-stage wavelength converter. Identical with XGM wavelength conversion [14, 15], this scheme has the potential advantages of high operation speed, simple implementation, large wavelength span, high power efficiency, and easy of use. In Section 2, the experimental setup and principle of operation are described. In Section 3, experimental results and related discussion are presented. Finally, conclusions are summarized in Section 4.
2. Experimental setup and principle of operation
An experimental setup diagram for the all-optical AND gate with XGM in two cascaded single-port- coupled SOAs is shown in Fig. 1. A cw signal is generated from the tunable laser source, whose output power and wavelength can be variable. A modulated signal at a wavelength of 1542.6 nm generated from the 10-Gbit/s bit-error-rate test (BERT) system is divided into two channels by the 3-dB coupler (OC1) following the erbium-doped fiber amplifier (EDFA1). One output, called channel A, was used as a pump signal channel and was coupled into a single-port-coupled SOA (SOA1) together with the cw probe signal through the 3-dB optical coupler (OC2) and the circulator (C1). Output signals could be extracted from the SOA1 by the circulator, and the modulation information in the pump channel could be converted into the probe channel through XGM in the SOA. The output signals could be amplified by the following EDFA, and the probe channel with converted information is filtered out by the tunable bandpass filter (TBF1). Another output from OC1, called channel B, was followed by a variable optical delay line (TDL), and its output was coupled into the SOA2 together with the converted signals through the 9:1 coupler (OC3) and the circulator (C2). Identically, the output signals were coupled out from the SOA2 by the circulator, and the logical AND operation output could be selected out by the TBF2. The following communication signal analyzer (CSA) and optical spectrum analyzer (OSA) are used to analyze the output signals.
The principle of operation for the all-optical logical AND gate based on cascaded single-port-coupled SOAs is illustrated by Fig. 2. Assuming that the bit stream for the signal generated from the BERT is 1100, the waveform for channel A is shown in Fig. 2(a).
In SOA 1, wavelength conversion based on XGM occurs, and the modulation information would be converted into the probe channel. As shown in Fig. 2(b), the converted output, called channel C, is inverted to the original pump signal, and its bit stream is 0010. By adjustment of the tunable delay line, the time delay between channels A and B could be controlled to be nT, where n is an integer and T is the bit period. Assuming that the bit stream for channel B is 0110, its waveform is shown in Fig. 2(c). It should be noted that the signal power in channel B is controlled to be far smaller than the signal power in channel C. If the signal in channel C is bit 0, the signal in channel B could be amplified directly by the SOA 2; then the output is bit 0 for input bit 0 in channel B and bit 1 for input bit 1. In contrast, if the signal in channel C is bit 1, the carrier density in SOA would be suppressed and the signal in channel B would not be amplified whether the bit is 0 or 1, in which case the output bit is always 0. As a result, the output bit stream is 0100 as shown in Fig. 2(d). Because bits 0 and 1 in channel C correspond to bits 1 and 0 in channel A, respectively, the truth table for this logic operation is as shown in Table 1. From the truth table we can conclude that the all-optical logical AND gate could be achieved with this scheme.
In this scheme, single-port-coupled SOAs are employed to achieve good output performance. The extinction ratio in channel C is critical for all-optical logical AND operation, and the large power difference between the mark and space signal in channel C is helpful for achieving the large gain difference in SOA2. As we know, extinction ratio degradation always exists in XGM wavelength conversion with ordinary SOAs. However, owing to double-pass gain in the single-port-coupled SOA and transmission loss in its rear facet, output extinction ratio performance could be improved in XGM wavelength conversion with single-port-coupled SOAs , and good logical AND output performance could be achieved with single-port-coupled SOAs. Theoretical analysis results  showed that low rear facet reflectivity is helpful for improving extinction ratio. Therefore, the rear facet reflectivity of the SOA is specially designed to be of the order of ~10-2. The SOAs are fabricated with InGaAsP/InP MQW materials and a vertical cavity; the length of its active cavity is 400 μm, and the net gain for -10 dBm@1550-nm input signal is ~13 dB at 150-mA biased current.
3. Results and discussion
Figure 3(a) represents the spectrum of the input signal before SOA1, and Fig. 3(b) represents the spectrum of the input signal before SOA2. The input cw signal wavelength is 1549.5 nm, and the pump signal wavelength is 1542.6 nm. In SOA1, the signal power in the pump channel is -1.4 dBm, and the signal power in the cw probe channel is -11.6 dBm. After wavelength conversion and optical amplification, the signal power in channel C is 2.4 dBm, and the signal power in channel B is -12.4 dBm. By use of the single-port-coupled SOA, the output extinction ratio after the first-stage wavelength converter is larger than 10 dB.
The signal waveforms for different channels are shown in Fig. 4, which are direct screen captures from the CSA. Among these waveforms, two upper waveforms, labeled R2 and R3, are recalled from the temporary memory in the CSA. For clear contrast they have been upshifted from their original locations, and their power scales and zero levels are different from those of the lowest waveform. The upper waveform represents the signal in channel A, whose bit stream is 1100. The second waveform represents the signal in channel B, in which the time delay is controlled to be (n*400+100) ps, and the bit stream is 0110. The lowest waveform represents the output signal from the TBF2 with the wavelength of 1542.6 nm, and the bit stream changes to be 0100, which is precisely the logical AND operation result of the above two waveforms. We may conclude that all-optical logical AND operation was achieved on the basis of XGM in two cascaded single-port-coupled SOAs.
In experimental study, the signal power in channel C is found to be critical to output performance of logical AND operation. If this signal power is not high enough, the signal in channel C would be amplified to some extent, the amplified signal and the logical AND operation result would be superposed together, and the output signal would be an incomplete logical AND operation result. To quantify the logic operation output performance, a parameter R is introduced, and R=P 01/P 11, where P 11 is the output power for bit 1 in channel A and bit 1 in channel B, P 01 is the output power for bit 0 in channel A and bit 1 in channel B. The smaller of the parameter A is, the better output performance would be. As shown in Fig. 5, the parameter R versus the signal power in channel C is presented, in which the signal power in channel B is -12 dBm, and the bit stream in channel A is 1110. In Fig. 5, the output waveform is an incomplete logical AND result, which corresponds to -0.4 dBm input signal power in channel C. It can be shown that the ratio would decrease as the input signal power increases, and then the output performance would be better and better. It should be noted the average output power would decrease as the signal power increased. There is a trade-off between output performance and average output power.
Although all-optical logic AND operation is achieved only at 10 Gbit/s because of experimental conditions, but this scheme still has the ability to achieve higher-speed logic operation. As we know, wavelength conversion based on XGM in SOAs has been demonstrated experimentally at 100 Gbit/s  with 2-mm-long SOA. Operating with the same XGM principle, all-optical logic operation based on this scheme also has the potential to be demonstrated at 100 Gbit/s. Large input signal power and long SOA biased at large current should be exploited in order to shorten the effective carrier lifetime during operation at higher bit rates. In this case, the temperature of the SOAs should be autoregulated to prevent thermal damage. Simultaneously, this scheme has the same advantages as XGM wavelength conversion in SOAs , such as simple implementation, easy of control, large wavelength span, and high power efficiency. The logical AND operation could be polarization-insensitive if polarization-independent SOA is used in this scheme.
An all-optical logical AND gate at 10 Gbit/s was demonstrated by use of cross-gain modulation (XGM) in cascaded single-port-coupled SOAs. Owing to double-pass gain in the single-port-coupled SOA and transmission loss in its rear facet, a high-output extinction ratio could be achieved in wavelength conversion based on single-port-coupled SOAs, and thus good logical AND operation output performance could be obtained. Output performance versus the input signal power was investigated experimentally. Large-input signal power is helpful for achieving improved output performance, and an incomplete logical AND operation result will be obtained when the input signal power is not large enough. Operating with the same principle, the scheme has characteristics identical with those of XGM wavelength conversion in SOAs.
Related research on SOA has been funded by the High Technology Development Project (863-2002AA312160), the National Great Foundation Project (973-G2000036605), theWuhan Great Special Project (2002100513013), and theWuhan Youth Chenguang Project (2003500201602).
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