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Self-controlled All-Optical label and payload separator for variable length bursts in a Time-Serial IM/DPSK Scheme J. J. Vegas Olmos, I. Tafur Monroy, Y. Liu and A. M. J. Koonen COBRA Research Institute, Eindhoven University of Technology, NL-5600 MB Eindhoven, The Netherlands
[email protected]
Abstract: We demonstrate an all-optical label and payload separator. The proposed scheme supports variable length and bitrate burst format and uses IM and DPSK modulation format for the label and payload, respectively. © 2005 Optical Society of America
OCIS codes: (060.4080) Modulation; (060.5060) Phase modulation; (060.4250) Networks; (060.2330) Fiber optics communication
1. Introduction Optical label swapping (OLS) is a promising technique for implementing optical packet switching (OPS) functions over wavelength division multiplexed (WDM) optical networks [1]. Due to the ever increasing line bit rate of data transmission over WDM links, nowadays the bottleneck is moving towards the electronics that processes the signal at the core nodes of the network. All-optical label swapping techniques due to their potential for ultra-fast operation are a promising solution for packet routing speeds matching the line rates; avoiding in this way any processing bottleneck [2]. At the ingress node of an OLS network, packets with a common destination or class of services will be aggregated and equipped with a short fixed optical label. Core nodes process this label and based on its information perform a routing decision. Therefore, the optical label is usually separated from the payload for the corresponding signal processing at the core nodes. Several schemes has been proposed to separate the label (header) and payload information by using all-optical signal processing techniques based on nonlinear polarization switches [3]. Mostly of the proposed approaches use an external control pulse synchronized with the label bits [2], which increases the complexity of the node architecture. Recently, was proposed a scheme [3] without the need of a synchronized external control pulse; however it requires Manchester encoding of the payload data for its proper operation and thus it compromises the payload format. In this paper we propose an all-optical label and payload separator scheme with no need of external synchronized control signal. The proposed scheme uses a combined scheme with an intensity modulation (IM) for the label and differential-phase shift keying (DPSK) for the payload. The separation is performed by using the nonlinear polarization rotation effect in a SOA [4]. The rotation is induced by using the payload signal itself. Thus, the role of the control pulse is performed by the payload signal, exploiting the constant amplitude feature of the DPSK modulation format. In this way the proposed scheme operates asynchronously and self-controlled because the timing for the label separation is contained in the burst format itself. We avoid data pattern effects over the payload produced by the SOA [3], keeping the payload fully untouched during it pass through the node. Moreover, optical transmission by using DPSK modulation format has been demonstrated to be robust against chromatic dispersion, polarization mode dispersion (PMD), and cross-phase modulation (XPM) effects [5], as well as record high capacity and link reach [6]. We show experimental demonstration of the proposed scheme for 10 Gbps DPSK payload and variable label bitrate up to 10 Gbps. Furthermore, we demonstrate variable burst length operation without observed distortion in the eyes diagrams of both label and payload signals. 2. Experimental setup The experimental setup is depicted in the Fig.1. The burst 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 1553.44 nm. The payload information is conveyed in DPSK modulation on the same lightwave carrier using a time-serial scheme. The modulation signal is in the non return-to-zero (NRZ) format based on a 231-1 pseudo-random data sequence. The output average power after the burst generation was measured to be -18 dBm. An erbium-doped fiber amplifier (EDFA) was used to amplify the signal up to 5.1 dBm.
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After the burst is generated, it enters the all-optical label and payload separator block shown in Fig.1. This separator block is composed of two SOAs, two polarization beam splitters (PBS) and one optical circulator (OC). Polarization controllers (PC) and optical band pass filters (BPF) are also necessary for adjusting the operating point of the separator. The first polarization controller (PC1) is used to adjust the incoming signal to the PBS1 so that effective polarization splitting is achieved for the burst. The burst signal, at one of the outputs of the PBS1, is injected into the 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 the PBS2. The signal at the second port of the PBS1 is, is amplified in SOA2 and used as a high-intensity pump control signal for the SOA1. The injected pump signal, aligned to coincide with the label at the SOA1, introduces additional birefringence in the SOA1 as compare to the case when no pump signal is present [4]. Therefore, the label part is polarization rotated in SOA1 and leaves the system through the output of the PBS3. Because the payload part does not experience excess of birefringence it leaves the system by the output of the PBS2.
Fig. 1. Experimental set-up. DFB: distributed feed-back laser; IM: intensity modulator; PM: phase modulator; PC: polarization controller; EDFA: erbium-doped fiber amplifier; BPF: optical band-pass filter; PBS: polarization beam splitter; SOA: semiconductor optical amplifier; OC: optical circulator.
The payload is separated from the label section by a guard-time. The payload can be used as a control signal, supporting variable burst length as far as the control signal does not overlap in SOA1 with the payload section. Therefore, the guard time value can be chosen to guarantee this condition. Supporting variable burst length is an attractive feature for networking in terms of granularity issues [7,8]. 3. Results Two different bitrates for the labels has been investigated. Namely, 5 Gbps and 10 Gbps were used to explore the versatility of the scheme. The used payload length was set to have a duration of 47 ns, i.e. 470 bits at 10 Gbps DPSK. Figure 2 shows the results at the output of the all-optical separator. The extinction ratio of the original signal was 13.5 dB, obtaining similar values at the output for each case. The suppression ratio is 16.9 dB and 15.9 dB for the 5 Gbps and 10 Gbps cases, respectively. Therefore, the label bitrate can vary within the range of the Gbps transmission, obtaining recoverable labels at the output.
Fig. 2. Inset a) and Inset b) show the results of the label separation for a bitrate of 5 Gbps and 10 Gbps, respectively. The first column corresponds to the burst, the second to a detail of the original label of the burst, the third to the separated label and the fourth to a detail of the separated label. The vertical axis has 5 mV/div in all the pictures.
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The payload was then operated at two different lengths, with a fix label at 10 Gbps. The long payload has 470 bits and the short one 140 bits, achieving a variability factor equals to 3.4. The results are shown in the Inset (a) of the Fig. 3. The suppression ratio is 20.1 dB and 21 dB for the long and the short payload, respectively. Inset (b) of the Fig. 3 shows the back-to-back eye diagram of the DPSK signal, which had an extinction ratio (ER) of 14.5 dB, and the eye diagrams of the recovered DPSK signals for the long (14.8 dB of ER) and short (15.3 dB of ER) payload cases. Therefore, the DPSK signal quality is not affected by our system and remains fully untouched in the optical domain.
Fig. 3. Inset a) shows the original burst with a long and a short payload for a 10 Gbps label, and the output of the payload port, where we obtain only the payload. Inset b) shows the eye diagrams of the original DPSK signal and the DPSK signal obtained after the payload separation for two different payload lengths.
4. Conclusion We have experimentally demonstrated the feasibility of a self-routed, variable burst length, variable bitrate alloptical label and payload separator by using non-linear polarization rotation in a SOA. The used modulation format is DPSK which reduces the pattern dependence effects in the SOA and moreover is a promissing candidate for achieve robust ‘dispersion’ transmission along optical fiber links. The proposed scheme has the prospects for integration in a photonic circuit due to the use of SOA-based devices. References [1] [2] [3] [4] [5] [6] [7] [8]
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