An all-optical M-ary amplitude shift keying (ASK) signal demultiplexer is proposed and demonstrated. It allowed us to seamlessly demultiplex a high bit-rate optical M-ary ASK signal into on-off keying (OOK) signals without O/E conversion. It is composed of multilevel thresholding using self-frequency shift and OOK signal generation using optical interconnection. A level identification signal is provided as a result of multilevel thresholding and it is fed to an optical interconnection circuit which can generate corresponding OOK signals. We demonstrate the quadrature ASK signal demultiplexing at 100 Gsymbol/s and its error free operation at 10 Gb/s.
© 2007 Optical Society of America
Recent tremendous growth of optical network requires the large capacity of wavelength division multiplexing (WDM) transmission system . Since, however, the available bandwidth of WDM is limited by the gain region of optical amplifiers, to increase the capacity of WDM transmission system, the improvement of a spectral efficiency is indispensable. For this purpose, the use of multilevel signaling has attracted much attention recently because it enables to carry two or more information in a single symbol . A differential quaternary phase shift keying (DQPSK) using four phase levels becomes essential signaling technique in large capacity and high spectral efficiency transmission systems . In addition, advanced multilevel signaling techniques such as M-ary phase shift keying (PSK), M-ary amplitude shift keying (ASK), and M-ary quadrature amplitude modulation (QAM) have been studied and demonstrated strenuously [2,4,5]. In general, a multilevel signal, whose number of level is M=2N, is generated by multiplexing N on-off keying (OOK) signals. To deal with the individual OOK signal in a high symbol rate multilevel signal over 40 Gsymbol/s, a high-speed multilevel signal demultiplexer is indispensable. An optical one-bit delay line is one promising method for DQPSK demultiplexing because it realizes high-throughput operation with a simple configuration. On the other hand, since the conventional demultiplexing technique for a M-ary ASK signal is realized by an electronic comparator composed of a lot of D Flip Flops (D-FFs) after O/E conversion [6,7], there are some following problems. 1) Since the number of D-FFs depends on that of power levels, the complexity and power consumption of the demultiplexer increase with increasing the number of power level M. 2) The operation speed is limited by a bandwidth of an electronic D-FF. 3) The more power levels in a M-ary ASK signal are required, the severer the setting of the threshold voltages of D-FFs becomes. To overcome these problems, we try to apply an all-optical approach because it enables to completely remove the electrical bandwidth limitation and realize high-speed signal processing with a simple configuration . To apply an all-optical approach to a M-ary ASK signal demultiplexer, we focus on the compatibility between a M-ary ASK signal demultiplexer and an analog-to-digital conversion (ADC). Namely, we regard each symbol in a M-ary ASK signal as an analog signal. Previously, we have proposed a photonic ADC which consists of optical quantization  using self-frequency shift in a fiber and optical coding using optical interconnection based on a binary conversion table . This type of photonic ADC enables to convert an analog sampled signal into a multiple-bit parallel binary code. Therefore, we can promptly adopt our ADC technique to a M-ary ASK signal demultiplexer. In this paper, we propose and demonstrate an all-optical M-ary ASK signal demultiplexer based on a photonic ADC.
A schematic diagram of the proposed all-optical M-ary ASK signal demultiplexer based on a photonic ADC is shown in Fig. 1. It is composed of two parts; optical multilevel thresholding and OOK signal generation. In optical multilevel thresholding, we use the phenomenon of self-frequency shift (SFS) in a high nonlinear fiber (HNLF) and a dispersion device . The M-ary ASK signal is propagated in a HNLF for generation of SFS. Since the amount of SFS increase with increasing a power of an input pulse, the power level of each symbol in a M-ary ASK signal can be discriminated by referring to differences of the center wavelengths. Each symbol in a M-ary ASK signal after SFS is demultiplexed by an appropriate dispersion device such as AWG, and promptly output to one specific port as the level identification signal of each power level. In OOK signal generation, we use an optical interconnection based on a
binary conversion table. Here, the quaternary-to-binary conversion table for quaternary ASK (QASK) signal demultiplexer is shown in table 1 as an example of a binary conversion table. The inputs a0–a3 are quaternary code and the outputs b0–b1 are binary code in Table 1. From Table 1, the outputs b0–b1 are described by eq. (1).
To generate OOK signal based on eq. (1), a set of inputs a0–a3 and outputs b1–b2 are corresponding to the power level in a QASK signal and the demultiplexed OOK signal. From eq. (1), we can see that the demultiplexed OOK signal is formed by combination of the power level of each symbol in a QASK signal. Since the sum operation in eq. (1) can be realized by optical interconnection, OOK signals can be output to corresponding output ports by connecting ports of a dispersion device according to Eq. (1). Because we can easily prepare fiber connection patterns corresponding to various binary conversion tables, it is applicable for demultiplexing of a M-ary ASK signal which has more than four power levels. As a result, an optical M-ary ASK signal can be seamlessly demultiplexed into N OOK signals without O/E conversion.
To demonstrate the proposed all-optical M-ary ASK signal demultiplexing, we executed preliminary experiments of QASK signal demultiplexing. The experimental setup of the all-optical QASK signal demultiplexing is shown in Fig. 2. We used a short pulse irradiated from a fiber laser as a light source. The pulse width, the center wavelength, and the repetition rate were 1.5 ps, 1558 nm, and 50 MHz, respectively. After the pulse compression using an erbium doped fiber amplifier (EDFA) and a single mode fiber (SMF), its pulse width was reduced to 570 fs. We generated four symbol QASK signals with 25 ps and 10 ps intervals using optical couplers, optical attenuators, and optical delay lines. The power level 1, 2, and 3 of the symbols in a QASK signal were set to -6.61 dBm, -1.23 dBm, and +0.60 dBm, respectively. To generate SFS, the QASK signals were propagated in a 10 m HNLF. The QASK signals after SFS were divided and fed to optical band-pass filters (OBPFs) being set for the center wavelength after each power level signal is suffered by SFS. To synchronize the timings and equalize the powers of level identification signals, we adjusted the time delays and powers in advance using optical delay lines and optical attenuators placed at output ports of OBPFs. Since the output ports of OBPFs were connected based on a quaternary-to-binary conversion table, two demultiplexed OOK signals were output to corresponding output ports. We measured the temporal waveforms of demultiplexed OOK signals using an oscilloscope with 53 GHz Photodiode (PD) (Agilent technologies, Inc.). Figure 3 shows the output spectrums of the optical pulses after SFS at corresponding input power levels and the relationship between the input power and the center wavelength. From Fig. 3, the pass wavelengths of OBPFs were set as 1567 nm, 1585 nm, and 1606 nm. The temporal waveforms of input QASK signals at 40 Gsymbol/s and output OOK signals are shown in Figs. 4(a)–4(d). From Fig. 4, the input QASK signals at 40 Gsymbol/s, whose power levels were “3-1-2-3” (Fig. 4(a)) and “3-2-1-3” (Fig. 4(c)), were successfully demultiplexed into two corresponding OOK signals (Fig. 4(b) and Fig. 4(d)). The temporal waveforms of input QASK signals at 100 Gsymbol/s and output OOK signals are shown in Figs 5(a)–5(d). From Fig. 5 the input QASK signals at 100 Gsymbol/s, whose power levels were “3-1-2-3” (Fig. 5(a)) and “3-2-1-3” (Fig. 5(c)), were successfully demultiplexed into two corresponding OOK signals (Fig. 5(b) and Fig. 5(d)). The pulse widths of the demultiplexed OOK signals ranged from 3.6 ps to 4.8 ps. In these experiments, we observed weak noise signals in demultiplexed OOK signal 1. These noise signals are caused by the spill-over spectral component into OBPF 1 and 3 when the power of the symbol in a QASK signal is -1.23 dBm (Fig. 3). In Figs. 4(c) and 4(d), although we equalized the powers of level identification signals in advance, the power of each symbol in OOK signal slightly vary. They are caused by the transient response of a PD because the bandwidth mismatching between a PD and output OOK signals.
To confirm the operation of the all-optical M-ary ASK signal demultiplexer, we executed the bit error rate (BER) measurement of the QASK signal demultiplexing at 10 Gb/s. The experimental setup is almost same as Fig. 2 except for laser source, pseudorandom bit sequence (PRBS) modulation, the method of QASK generation, and pass wavelengths of OBPFs. We used a pulse train at 9.95328 Gb/s irradiated from a mode-lock laser diode (MLLD) as a light source. The pulse width and the center wavelength were 8.0 ps and 1546 nm, respectively. The pulse train was modulated with LiNbO3 modulator (LNM) to form a 215-1 PRBS data by a pulse pattern generator (PPG). The generated data signal was launched into a pulse compressor composed of a EDFA and a dispersion compensation fiber. After the pulse compressor, its pulse width was reduced to 900 fs. To prepare the QASK signals, we changed the power of the data signal using a EDFA and a variable optical attenuator by three steps. The powers of the QASK signals were set as 11.7 dBm, 15.9 dBm, and 17.3 dBm, respectively. The pass wavelengths of OBPFs were set as 1556 nm, 1563 nm, and 1570 nm corresponding to the center wavelength after each power level signal is suffered by SFS. We measured the temporal waveform and BER of each demultiplexed OOK signal. Experimental results of the temporal waveforms of output demultiplexed OOK signals are shown in Figs. 6(a)–6(c). From Fig. 6, the demultiplexed OOK signals can be successfully output to appropriate ports according to the power levels of input QASK signals and we confirm the operation of the proposed all-optical QASK demultiplexer for 10 Gb/s PRBS data signal. Fig. 7 shows the BER measurement result for each demultiplexed OOK signal at each power level. From Fig. 7, the demultiplexed OOK signals show that BER of less than 10-9 and the power penalty of about 2.7 dB can be achieved. This power penalty occurred in the demultiplexed OOK signal 1 when the power of QASK signal is 17.3 dBm. The cause of the power penalty is expected spill-over spectral component into OBPF 1 which is also observed in Fig. 4. The proposed scheme has several limiting factors to apply a larger number of power levels and a higher bit rate system. Since the available number of power level is in inverse proportion to the spectral width after SFS and in proportion to the amount of SFS, we can improve it up to 40 level by using HNLF to generate a large amount of SFS . On the other hand, the upper bit rate is restricted by the pulse width of the output OOK signals and the delay time caused by wavelength dispersion in a fiber. The delay time is defined by the center wavelength after SFS, the dispersion D and the fiber length Z of the HNLF. Figure 8 shows the relationship between the delay time and the center wavelength after SFS by changing D and Z. From Fig. 8, the delay time increase with increasing Z and D. Since we used the HNLF whose D and Z are 0.2 ps/nm/km and 0.01 km in the experiment, the delay time is less than 1 ps. Since the pulse widths of the output OOK signals are under 5 ps, the proposed scheme has the potential for 200 Gsymbol/s system.
We have proposed and demonstrated the all-optical M-ary ASK signal demultiplexer based on a photonic ADC. From experimental results, we can successfully demonstrate the demultiplexing of high bit rate QASK signal at 100 Gsymbol/s. The results of evaluation using 10 Gb/s PRBS data signal showed the error free operation of the proposed all-optical QASK signal demultiplexer with 2.7 dB power penalty. These results indicate that the proposed M-ary ASK signal demultiplexer can be operated for a QASK signal at 100 Gsymbol/s in optical communication systems. Since this subsystem hardly depends on the bit rate, we can upgrade the bit rate over 200 Gsymbol/s. In addition, because the available power level is determined by the amount of SFS, this subsystem is applicable for demultiplexing of M-ary ASK signal which has more than four power levels by optimizing the parameters of HNLF for SFS.
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