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In optical packet switching (OPS) and optical code division multiple access (OCDMA) systems, label generation and processing are key technologies. Recently, several label processors have been proposed and demonstrated. However, in order to recognize N different labels, N separate devices are required. Here, we propose and experimentally demonstrate a large-scale, multiple optical code (OC)-label generation and processing technology based on multi-port, a fully tunable optical spectrum synthesizer (OSS) and a multi-wavelength electro-optic frequency comb generator. The OSS can generate 80 different OC-labels simultaneously and can perform 80-parallel matched filtering. We also demonstrated its application to OCDMA.
©2007 Optical Society of America
1. Introduction
The explosive increase in internet protocol traffic calls for advanced photonic networks enabling ultra high-speed and large capacity communications. To this end, it is desirable to develop flexible optical signal processing and transfer technologies with enhanced data speeds and all-optical transmission capability. Additionally, label processing technology needs to be examined as one possible solution to the bottleneck problem in conventional electronic routers.
In optical packet switching (OPS) and optical code division multiple access (OCDMA) systems, label generation and processing are key technologies [1–3]. In conventional optical code (OC)-label processors, in order to recognize N different OC-labels, N separate devices are required. As the number of addresses increases, however, the number of devices needed for label processing also grows. As one means to overcome this problem, an angle-multiplexed spectral hologram serving as a multiple correlator for input labels based on the principle of time-space conversion has been demonstrated [2]. In addition, 16-label generation and processing using an arrayed waveguide grating (AWG) configuration has been proposed and demonstrated [3]. However, a combination of tunability of the processing device and multiple processing capability has not yet been realized.
In this paper, we propose a large-scale, multiple OC-label generation and processing technology based on a multi-port, fully tunable OSS and a multi-wavelength electro-optic frequency comb generator, and we describe its experimental demonstration. We demonstrated 40-parallel matched filtering because of the constraint of the number of devices, although the OSS can perform 80-parallel matched filtering in principle. We also demonstrated its application to OCDMA.
2. Multi-port optical spectrum synthesizer
4.1 Optical en/decoding in spectral domains
We used optical spectrum processing for ultra high-speed signal processing of the pulse trains. The pulse repetition rate corresponds to the reciprocal of the frequency spacing of Fourier components. In optical spectrum processing, the spectral pattern is controlled by changing the amplitude and phase of each Fourier component of the spectrum independently to generate a new spectral pattern following the added amplitude and phase pattern as a filtering function. A pulse train with a different waveform from the input pulse train can be obtained by inversely Fourier-transforming the controlled spectral pattern.
4.2 Optical en/decoding using multi-port OSS
Although an AWG-based spectral domain pulse shaper has been proposed [4], it is not tunable. Therefore, we previously proposed a single-port, fully tunable OSS [5]. The configuration of our proposed, fully tunable, multi-port OSS for multiple optical code label processing is shown in Fig. 1 [6]. The multi-port OSS consists of a 20×20-port cyclic AWG, variable optical attenuators (VOAs), variable optical phase shifters (VOPSs), a mirror, and circulators. All components are fully integrated by planar lightwave circuit (PLC) technology, except the circulators. This multi-port OSS can control the amplitude and the phase of each spectral component. The channel spacing of the AWG is 10 GHz.
3. Multi-wavelength frequency comb generator
Recently, 19 × 10-GHz electro-optic ultra-flat frequency comb generation using a Mach-Zehnder modulator (MZM) has been demonstrated [7, 8]. In this method, the spectrum of the generated frequency comb can even be flattened using a set of conventional LiNbO3 MZMs because the spectral ripples of the comb signal generated in each arm cancel each other out. The multi-wavelength electro-optic frequency comb generator consists of four laser diodes (LDs), a WDM MUX, a polarization controller (PC), a LiNbO3 dual-drive MZM, radio-frequency (RF) amplifiers, and a signal generator (SG), as shown in Fig. 2.
Light from the LDs was introduced into the modulator (MZM) through the PC to maximize the modulation efficiency. The MZM was dual-driven with two sinusoidal signals with different amplitudes (RF-a, RF-b), formed by generating an RF sinusoidal signal at a frequency of 10 GHz from the SG, splitting it into two with a hybrid coupler, and amplifying the resulting signals with microwave boosters, which were then fed into each modulation electrode of the modulator. The intensity of RF-a was attenuated slightly due to loss in the feeder line when connected with the electrode. When the light from multiple LDs was fed into the frequency comb generator, multi-wavelength optical combs were able to be simultaneously generated. In this paper, frequency combs with four different central wavelengths were generated. The central wavelengths of the four LDs were 1556.96 nm (λ1), 1554.18 nm (λ2), 1552.34 nm (λ3), and 1550.69 nm (λ4). These wavelengths were tuned to the central wavelength of each diffraction order of the AWG. Figure 3 shows the simultaneously generated frequency combs with four different central wavelengths, demonstrating that the combination of a multi-port OSS and a multi-wavelength frequency comb generator could realize large-scale OC-label processing.
4. Multiple OC-label generation and processing
Figure 4 shows the experimental set-up used for multiple OC-label generation and processing. It consisted of a frequency comb generator, an erbium-doped fiber amplifier, a band-pass filter, optical circulators to import and export optical signals, polarization controllers, an optical spectrum analyzer, a sampling oscilloscope, polarizers, and a multi-port OSS. To generate 80 different OC-labels at the multi-port OSS outputs, pulses from the comb generator were fed into one input port of the multi-port OSS. We used a prime code pattern, an orthogonal code used for signal encoding and decoding. We suppressed each spectral component by VOAs in the 20 channels of the multi-port OSS in accordance with the repeated on/off pattern of the prime code with prime number p=3. The 80 OC-labels were generated by ports 1 to 20 of the multi-port OSS. We assigned ports 1 to 10 for encoding and ports 11 to 20 for decoding instead of using two separate OSSs for encoding and decoding.
Figure 5 shows encoded waveforms and spectra for each central wavelength (λ1, λ2, λ3, λ4) in ports 1–20, demonstrating that 80 different OC-labels were generated simultaneously using the OSS and the multi-wavelength frequency comb generator.
The OC-labels generated by ports 1–10 were decoded by ports 11–20. Figure 6 shows matched decoded waveforms and spectra at ports 11–20. Figure 7 shows matched and unmatched decoded waveforms and spectra at ports 11–20. Only matched signals were extracted, whereas unmatched signals were well suppressed. The multi-port OSS performed 40-parallel matched filtering.
6. OCDMA experiment
In our OCDMA system, multiplexed data were decoded by matched filtering in the spectral domain, and then separated. We were able to also apply the OC-label processing technique to en/decoding operations. We also demonstrated OCDMA by multiplexing signals from two ports (ports 9 and 10) of the multi-port OSS [9]. A pulse train having a 1 GHz repetition rate was generated from a frequency comb having a 10 GHz repetition rate by the LN-IM. We used a prime code pattern of prime number p=3. The results of the OCDMA experiment are shown in Fig. 8. Figures 8(a)–8(b) represent spectra encoded at ports 9 and 10. A multiplexed spectrum formed by combining the spectra of ports 9 and 10 is shown Fig. 8(c). Figures 8(d)–8(e) represent spectra decoded at ports 19 and 20. We succeeded in obtaining waveforms of matched decoded spectra in the time domain [Fig. 8(f)], and unmatched decoded waveforms at port 18 were well suppressed [Fig. 8(g)]. These results show that our proposed system using the multi-port OSS and the multi-wavelength electro-optic frequency comb generator was effective when applied to OCDMA.
7. Conclusions
We have proposed and developed a new multi-port OSS constructed of VOAs and VOPSs. The system generated 80 different OC-labels simultaneously using the multi-port OSS and a multi-wavelength electro-optic frequency comb generator. The multi-port OSS performed 40-parallel matched filtering. The multi-port OSS effectively acted as a large number of spectral-domain matched-filtering devices. We also demonstrated its application to OCDMA.
Our proposed multi-port OSS can also be used as an optical processor in optical packet switching and pulse reshaping. Such optical processors will be key technologies for future photonic networks.
References and links
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