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A simplified all-optical label swapping scheme is proposed and experimentally demonstrated for all-optical packet switching. This scheme is based on the sub-carrier label multiplexing technique but it significantly reduces the cost and complexity of the label erasure and rewriting process at the intermediate nodes. This is accomplished using a shared interferometric filter for all wavelength channels to separate the payloads and labels as well as using gain-saturated RSOAs for label erasure and rewriting. Label swapping process is realized through re-assigning optical subcarriers to each output wavelength channel and rewriting the new label with RSOAs. The BER-free transmissions experimentally demonstrate the feasibility of the proposed scheme.
©2010 Optical Society of America
1. Introduction
Optical label-switching (OLS) technology is a promising approach to improve the scalability, speed, and throughput of the current Internet networks [1, 2]. All-optical label scheme (AOLS) avoids the complexity of optical-to-electrical and electrical-to-optical conversions of high-speed data payloads and plays an important role in OLS networks. In AOLSs, a label carrying routing information is generated and transmitted together with the packet payload. At each intermediate node, the label is detected to acquire the switching information and a new label is generated and attached to the payload. As a result, the label detection and swapping are important processes for error-free packet forwarding and cascadability of switching nodes [3].
So far, many different labeling schemes have been studied and in general they can be grouped into a few categories: serial labeling technique [3–5], orthogonal modulation scheme [6–9], and subcarrier multiplexed (SCM) labeling scheme [10–12]. In the serial labeling technique, the label is located temporally ahead of packet payload with a guard band in between the two, and this requires a rather complicated label detection method. In the orthogonal modulation scheme, the payload is modulated with some advanced formats, such as phase shift keying (PSK) [6, 7], VSB-CSRZ [8], or dark return-to-zero format [13, 14], instead of a simple amplitude shift keying. This scheme not only makes the switching system more complicated, but also introduces inter modulation crosstalk. In SCM labeling scheme, the label is modulated onto an RF subcarrier and then multiplexed with the packet on the same wavelength. This technique enables an easy extraction of the label information and simplifies the detection architecture. However, the SCM label technique requires a relatively costly optical filter to separate the payload and label for each wavelength channel. Besides, additional tunable laser sources and external modulators are required for generating new SCM labels at each intermediate node [10–12], and this increases the deployment and operating costs.
In this paper, the optical spectral separation of the payload and label at intermediate nodes is achieved by a single interferometric filter that can be shared by all wavelength channels in a single fiber. The ASK label is carried on double-sideband optical subcarriers (which is generated by a Mach-Zehnder modulator (MZM)) but erased, rewritten and amplified by a reflective semiconductor amplifier (RSOA). This eliminates the need for tunable laser diodes and external modulators in the generation of new SCM labels at each intermediate node. Another novel feature of our proposal is that the RSOAs for label erasure are grouped under a common label processing module, and this can be used for the generation of new labels for the egress payloads regardless whether the wavelengths of the switched payloads have been changed or not. Our experimental study demonstrates the feasibility of the proposed label scheme. Results show there is no significant BER performance degradation after label swapping if an appropriate extinction ratio for the NRZ-modulated label is chosen.
2. Proposed SCM label scheme
The proposed SCM labeling scheme is shown in Fig.1. The light from wavelength division multiplexed (WDM) light source is first split into two parts. One part is used for baseband payload modulation and the other part is modulated with a clock signal (f c) to generate the optical subcarriers. A MZM biased at null point is used to suppress the optical carrier. The carrier suppressed optical subcarriers are then modulated with label signals. Both payload and label are simple amplitude shift keying (ASK) signal. After data modulation, the payloads and labels that are carried on the optical carrier and optical subcarriers, respectively, are combined and transmitted to the intermediate node via a fiber link.
At intermediate node, the optical signals are amplified by an Erbium-doped fiber amplifier (EDFA) to compensate for the transmission losses and then they are sent to an interferometric filter (IF), which has a free spectral range of FSR = 2f c. Due to the periodicity of the IF’s filtering characteristics, the payloads and labels for all WDM channels are separated simultaneously [15]. The payloads are then launched into one input port of the optical switch (e.g. input port N in Fig.1) and switched according to their destination address information obtained from their respective labels. After optical switching, the payloads coming from different input ports but heading to the same intermediate or destination node will appear at the same output port of the switch.
At the same time, the labels are sent to a label processing module for label detection and label rewriting. In the label processing module, the labels are demultiplexed with a wavelength demultiplexer (DMUX) and each channel is split into two parts with a power splitter. One part is used for label detection, and the other part is used to seed the RSOA. The RSOA works in the gain saturation region to erase the old label [7]. A new label is simply rewritten on the erased label signal by direct modulating the RSOA.
The label swapping is realized through re-assigning optical subcarriers to each output wavelength channel and rewriting the new label information via the RSOA seeded by assigned optical subcarriers. In label rewriting process, the optical subcarriers (optical label) are disassociated with their corresponding payload and can be reused for any output payload, provided they belongs to the same wavelength cannel. On the other hand, the RSOAs seeded by the optical labels (i.e. optical subcarriers) belonging to the same input fiber link are grouped into one set. Each set can be used to rewrite the new label information for any of the payloads who are distributed to the same output port of the optical switch (e.g. output port n in Fig.1). For any output wavelength channels, one dedicated RSOA and optical subcarriers can be assigned to complete the label rewriting and swapping process. Assuming the optical packet is switched from channel Si,m to channel Uj,n (as shown in Fig.1), RSOAj,n will be programmed to rewrite the label information for this packet. Here, Si,m is the input channel with wavelength λi at output port m of the optical switch, and Uj,n is the output channel with wavelength λj at output port n of the optical switch (i,j ∊[1,K] where K is the number of wavelength channels in a single fiber and m,n∊[1,N] where N is the number of ports in the optical switch). RSOAj,n is the RSOA that belongs to the n-th set and it is seeded by the optical subcarriers located in wavelength channel λj. Thus the set of optical subcarriers that carried the label information for the ingress payloads and launched into the same input switching port can now be reused to carry the label information for a new set of egress payloads appear at the same output switching port. As a result, the optical labels are swapped without requiring additional lasers and external modulators (for subcarrier generation).
Another advantage of this scheme is that the labels need not be switched together with the payload, thereby lowering the bandwidth and crosstalk requirements of the switch fabric. Moreover, the proposed labeling scheme can be used with any type of optical switch fabrics, even if the wavelength of the payload is changed after switching [16].
After the generation of new labels and payload switching, the payload and label are spectrally combined and sent to next node through a common fiber. A piece of short fiber is inserted before payload and label re-combination to compensate for the transmission delay in the optical switch. At the edge node, all payloads and labels are separated again by another IF. Each channel is then demultiplexed for signal detection.
3. Experimental setup
To demonstrate the feasibility of the proposed scheme, an experiment for label erasure and rewriting is depicted in Fig.2.
The light from a distributed feedback laser (DFB) is first split into two parts by a 30:70 fiber coupler. The light from the output port with lower optical power is modulated by an MZM (whose pre-chirping parameter α = -0.7) with a 10- Gb/s pseudo-random bit sequence (PRBS) NRZ-OOK signal with a PRBS length of 223-1 to realize the payload. Light from the other port is modulated with a 20-GHz clock signal to generate the optical subcarriers. The modulator is an MZM which is biased at null point to suppress the optical carrier. The generated optical subcarriers are then modulated with a 1.25-Gb/s PRBS NRZ-OOK signal with a PRBS length of 231-1 to obtain the label. The payload and label are combined and then launched into two separate transmission fibers using a 3-dB coupler to emulate two separate fiber link transmissions (Link1 and Link2). After transmission in 40-km standard single mode fiber (SMF) via Links 1 or 2, the combined payload and label are separated using a delay interferometer (DI). The free spectral range of the DI is 40 GHz. The separated payloads from the respective fiber links are launched into a 2×2 optical switch for signal routing. A small portion of the label signal power (10%) is channeled into an optical receiver for label detection, while the majority of the label signal power (90%) is injected into an RSOA to act as a seeding light. The seeding power and bias current of the RSOA are fixed at -8 dBm, and 75 mA, respectively. The saturation power and noise figure are 3.5 dBm and 8.7 dB, respectively. A 1.25-Gb/s new label signal from a pulse pattern generator (PPG) with amplitude of 2Vpp is directly modulated on the erased the label light via the RSOA (there is no driver amplifier between the PPG and RSOA). The new label is then combined with its corresponding payload in Link 4 via an optical circulator and a 3-dB optical coupler, and then transmitted along another 40-km SMF to the edge node for detection.
To effectively erase the old label and rewrite the new label, some extinction ratio (ER) tradeoffs for the label can be expected. However, since the bit rate of the label is much lower than payload, its power budget will be sufficient in most scenarios. In this experiment, the extinction ratio of the label is first fixed at 3 dB. But the influence of the extinction ratio on the system performance is also studied at the end of next section.
4. Experimental results and discussion
The optical spectra of payload and label before and after combination are shown in Fig. 3(a). Since the payload and label signals are modulated using the same light source, their powers are concentrated within the same WDM channel but on different wavelengths (optical carrier or subcarriers). For the payload, the optical power is mainly concentrated around the optical carrier. But for label, the power is mainly concentrated around the optical subcarriers which have a 20-GHz wavelength separation from the optical carrier. And in this experiment, the power of the suppressed optical carrier is 20 dB lower than that of the subcarriers (label). As a result, these two signals can be combined and transmitted together through the same fiber without significant crosstalk. After separation with the delay interferometer, the payload and label signals are shown in Fig. 3(b). Here, the power of the optical subcarriers of the separated payload signal is 20 dB lower than that of the optical carrier, and the power of the optical carrier of the separated label signal is 11 dB lower than that of the optical subcarriers.
Fig. 4. Eye diagrams of the labels: (a) original, (b), after label erasure, (c) after label rewriting.
After optical separation, the label signal is detected and erased for new label rewriting. The eye diagrams of the label signal before and after erasure are shown in Fig. 4(a) and Fig. 4(b). For the original label signal, the ER is 3 dB and the eye opening is wide and clear after 40-km fiber transmission. However, it is evident that there is no eye opening after label signal has been erased at the output of the RSOA. Once the new label signal is applied to the RSOA, the eye is open once again, as shown in Fig.4(c). Since there maybe more than one intermediate node before the data reaches its destination, the ER of the new label is also maintained at 3 dB for the next label rewriting process.
To better evaluate the system performance, the bit error rate (BER) of the label before and after rewriting are shown in Fig. 5(a). In this paper, the receiver sensitivity is defined as the received optical power when the BER is 10-9. The receiver sensitivity of back-to-back (BTB) case for the original label is -30 dBm. With 40-km SMF transmission, the receiver sensitivity becomes -28.9 dBm. The 1.1-dB power penalty mainly comes from fiber dispersion and crosstalk from payload due to the non-ideal optical separation. Compared with the BER curve of the label transmission without payload, the crosstalk induced power penalty is about 0.45 dB, which is quite tolerable in this system. After label erasing and rewriting, the BTB receiver sensitivity is -28.3 dBm. With another 40-km SMF transmission, the receiver sensitivity of the new label at the edge node can still reach -27.0 dBm. Here, the payload was not combined in the Links 3 and 4 due to the shortage of the delay interferometers. Once the payload is combined, additional power penalty about 0.45 dB could be introduced to the new label because of the crosstalk from the payload.
Fig. 5. (a) Bit error rate (BER) of the label before rewriting: ◇ BTB, + without payload, □ with payload, and after rewriting: ☉ BTB new label, × new label). (b) BER of the payload (◇ BTB, + without label, □ with label).
The BER performance of the payload is shown in Fig. 5(b). Here, the bit rate is 10 Gb/s and the BTB receiver sensitivity is -19.4 dBm. After 40-km transmission, a power penalty of 1.1 dB is introduced due to the fiber dispersion. In this case, the label signal is turned off. Once the label signal is turned on, another 0.3-dB power penalty is induced by the crosstalk from label signal. The eye diagrams before and after transmission are shown as insets in Fig. 5(b).
The extinction ratio of the optical label is an important factor for system BER performance. To enhance the BER performance of original label (i.e. old label), a larger ER is preferable. However, a larger ER will introduce more crosstalk when the new label is rewritten because in such a case, the old label can not be erased significantly. As a result, there should be a trade-off between the BER performances of old and new labels. Figure 6 shows the receiver sensitivity vs. extinction ratio for the label signals before and after rewriting. Here, the receiver sensitivities are measured after 40-km fiber transmission for both old and new labels. The label extinction ratios before and after label rewriting are fixed at a constant value. Experimental results show that the receiver sensitivity is maintained at -27.4 dBm for both old and new labels when the ER is 2.4 dB. In this case, a reasonably good BER performance can be maintained even after several stages of label erasure and rewriting (i.e. passing through several intermediate nodes).
5. Conclusion
A novel all-optical label erasure and rewriting scheme based on optical subcarrier-multiplexed labels has been proposed and experimentally demonstrated. The label erasure and rewriting were simultaneously carried out using an RSOA at each intermediate node. This eliminates the need for costly tunable light sources and optical subcarrier generators. The effect of the extinction ratio of the label signal was studied. Experimental results show the BER performance of the label can be maintained after label rewriting when an appropriate ER is chosen.
Acknowledgements
This work is supported by Singapore’s Agency for Science, Technology, and Research (A*STAR) under SERC project Grant No. 0721010019.
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