An all-optical time-serial label and payload separator generating a synchronization pulse

We demonstrate an all-optical label and payload processor based on nonlinear optical processing with semiconductor optical amplifiers. The processor separates the label and the payload, and generates a synchronization pulse. Introduction All-optical label swapping (AOLS) is a technique that enables the implementation of ultra-fast packet routing and forwarding in IP-over-WDM networks [1]. In an AOLS network, bursts of IP packets are equipped with a label containing its routing information. Fig 1 presents the schematic diagram of the AOLS node under study in the IST-LASAGNE project [2]. The first stage of the node is the payload and label processor block. This block has three output signals, namely the separated label and payload signals, and a single optical pulse. The optical label is forwarded to the label and local address comparison subsystem that generates an optical pulse if an address match is found. This match pulse triggers both the new label generation block and the optical flip-flop. A new label is created that contains the required routing information for the next hop. At the same time, the optical flip-flop emits a continuous wave signal at a certain wavelength, which will be used as a probe signal for the wavelength conversion block. The generated single pulse is necessary to ensure synchronization among the local generated addresses and the label bits at the comparison block. Figure 1 : All-optical label swapper unit. ODL: optical delay line. AOLXG: all-optical logical gate. AWGR: arrayed waveguide router. AOFF: all-optical flip-flop. TWC: tunable wavelength converter. Principle of operation and design The schematic diagram of the label and payload processor is depicted in Fig.2. The packet is composed of a payload signal and its corresponding label. The label data is conveyed in OOK modulation and the payload data is conveyed in DPSK modulation on the same lightwave carrier using a time-serial scheme. The separator block is composed Figure 2. Experimental setup. DFB : distributed feedback laser. PC : polarization controller. IM : intensity modulator. PM : phase modulator. EDFA : erbiumdoped fibre amplifier. PBS : polarization beam splitter. of two SOAs, two polarization beam splitters (PBS), an optical circulator (OC), polarization controllers (PC) and optical band pass filters (BPF). The label and payload separator is based on a nonlinear polarization effect in an SOA [3].The packet is split and injected into SOA1 after a fixed delay line, which ensures that the label part of the burst arrives simultaneously with the payload part of the signal version passing through SOA2. The polarization of this label signal is adjusted by using the PC2 to match the orientation of PBS2. The other part of the split burst signal is amplified by SOA2 and used as a highintensity pump control signal for SOA1. The injected pump signal, aligned to coincide with the label at the SOA1, introduces additional birefringence in SOA1 as compared to the case when no pump signal is present. Therefore, the label data experiences a rotation on its polarization state in SOA1 and leaves the system through output1 of PBS1. Because the payload part does not experience excess of birefringence it leaves the system at output2 of PBS1. The single pulse generator is based on selfpolarization rotation in an SOA [5]. In general, when an optical pulse with sufficient optical power arrives at SOA3, the leading edge of the pulse introduces gain saturation in the SOA. Since the SOA gain saturation is polarization dependent, the TE component of the data pulse experiences different gain saturation than the TM component. Thus, the leading edge of a data pulse introduces a rotation of the polarization state. In this approach, a portion of the recovered payload data is taken, and hence only the leading edge of this signal is transmitted to the aligned PBS output (obtaining the pulse). Experiment and results The experimental setup is depicted in the Fig.2. The packet is composed of a payload signal label and its corresponding label. The label information is conveyed in IM modulation of the lightwave carrier generated by a DFB laser signal at 1555.75 nm. The payload information is conveyed in DPSK modulation on the same lightwave carrier using a time-serial scheme. The label is a 16 bits word (‘AAAA’) at 10 Gbit/s. The payload modulation signal is in the non return-to-zero (NRZ) format based on a 2-1 pseudorandom data sequence at 10 Gbit/s. The output average power after the burst generation was measured to be -15.38 dBm. An erbium-doped fiber amplifier (EDFA) was used to amplify the signal up to 2.2 dBm. The payload had two different lengths, corresponding to the short (25.6 ns) and the long (48.8 ns) payload, achieving a variability factor between the long and the short payload equal to 1.9. The short payload conveyed 256 bits and the long one 488 bits. Those values were chosen arbitrarily as a proof-of-concept. The guard-time was chosen to be 500 ps, although it can be decreased up to one label bit time. Figure 3 shows the traces of the original packet, the separated label and payload and the single pulse generated. The suppression ratio (relation between the suppressed signal and the remaining one) is 23.63dB and 24.08dB for the payload separator, in the case of short and long payload, respectively. Figure 3. Scope traces of the results. The time scale is 20ns/div in all the insets. The label separator has a suppression ratio of 18.84dB and 17.50dB for short and long packets respectively. Both the recovered label and the generated pulse show a clear shape. Figure 4 shows the eye diagrams of the original DPSK signal for short and long payloads. The Q factor of these signals was 9.38dB and 9.57dB, respectively. The extinction ratio (ER) was 16.04dB and 16.42dB, respectively. After the separation process, the obtained Q factor was 9.96dB and 9.23dB, and the ER 16.61dB and 17.41dB, respectively. Although there are slight differences in the values (probably due to the measurement equipment) they all remain above 9dB for the Q factor which means that the signal can be recovered for BER values higher than 10. Therefore, this scheme preserves the integrity of the payload data after all the separation process, avoiding critical degradations of the signal. Figure 4. Eye diagrams of the original DPSK signal and the recovered ones, for short and long packets. The time scale is 50ps/div in all the insets. Although the bitrate of the label can not be increased to more than 20 Gbit/s due to fundamental limitations of the SOA [3], this device is transparent for a DPSK signal and hence, the bitrate of the payload might be increased beyond 40 Gbit/s. Conclusions We have demonstrated experimentally the feasibility of a self-controlled, variable burst length all-optical label and payload processor. The processor includes a label and payload separator and a single pulse generator. The employed payload modulation format, DPSK, reduces the pattern dependence effects in the SOA and achieves robust dispersion transmission along optical fiber links. The proposed scheme has the possibility of integration in a photonic circuit because of to the use of SOA-based devices. AcknowledgmentsThis work was performed within the IST projectLASAGNE, partially funded by the IST Program of theEuropean Commission. References1 R. Geldenhuys et al, OSA JON, 3(2004), 854.2 J. M. 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