We demonstrated all-optical label storing and switching for 40 Gb/s optical packets by using all-optical signal processing of multimode interference bistable laser diode optical flip-flops. The stored optical labels in the optical flip-flop controlled an all-optical switch to forward the injected optical packets. The 40 Gb/s optical packets were successfully switched in the all-optical scheme with 11-dB extinction ratio. Error-free operation for the output packets from the switch was also obtained with 1.3-dB power penalty. The presented all-optical packet switching has the advantages of ultrafast switching and transparency of data rate and format, suitable for the future optical networks.
©2006 Optical Society of America
Optical routers for optical packet switching have been expected to play an important role in the future optical networks because of its high bandwidth efficiency, high throughput, low power consumption, and small footprint, as compared to conventional electrical routers. Optical packet switching using tunable all-optical wavelength converters and arrayed waveguide gratings (AWGs) was demonstrated with 4-ns switching time . An X-shaped semiconductor switch was also developed for optical packet switching with high-speed electrical label processors, and the switching time of 1.5 ns was reported . However, in most researches for optical packet switching, the optical label was converted to the electrical signal for label processing, and the optical switches were controlled with electrical drivers. Although the switching time of less than 1 ns was reported , the switching time was still restricted to be more than 1 ns in realistic situations due to the limitation of electrical control. In the future packet switching networks, a faster switching speed will be desired for higher efficiency and lower latency. In order to overcome the limitations of the switching speed, all-optical packet switching with label processing and packet forwarding in the optical domain has been investigated . We have also proposed and demonstrated all-optical switching of optical packets using all-optical label storing and switching by an optical flip-flop .
Figure 1 shows an architecture of the proposed all-optical packet switching node. First an optical label is extracted from an input packet. Then, the optical label is converted to a parallel signal and injected to optical flip-flop memories to store the label information. Finally, the optical flip-flops drive all-optical switches to forward the injected packet. The optical packets are thus switched in the optical domain without any optical-to-electrical conversions. There is no restriction of electrical label processors and switch drivers, and ultrafast switching can be achieved owing to the high-speed operation of both the optical flip-flops and the all-optical switches. Transparency of data rate and format can also be obtained due to transparency of the all-optical switch. In order to realize this system, we have developed multimode interference bistable laser diodes (MMI-BLDs) for the optical flip-flop memories [6, 7], and 10-Gb/s optical packet has been successfully forwarded . In this paper, 40-Gb/s optical packet switching is presented using the MMI-BLD optical flip-flop and the all-optical switch based on a Mach-Zehnder interferometer semiconductor optical amplifier (MZI-SOA) . Owing to transparency of the proposed architecture, 40-Gb/s optical packets were successfully switched with the same configuration for 10-Gb/s packets.
2. Device fabrication
For the optical flip-flop memory, the MMI-BLD with distributed Bragg reflectors (DBRs) was prepared on an InP substrate as shown in Fig. 2(a) . An MMI-SOA, saturable absorbers (SAs), passive DBRs, and passive port waveguides were monolithically integrated using offset quantum well technology . Owing to the DBRs, single mode lasing occurred at the Bragg wavelength of 1554 nm, with side mode suppression ratio (SMSR) of 23 dB. The flip-flop operation was also obtained over 1540 - 1570 nm, with the switching speed of 470 ps or less .
The all-optical switch based on the MZI-SOA was also fabricated on an InP substrate as shown in Fig. 2(b) . Tensile strained bulk SOAs and phase shifters were monolithically integrated with butt-joint growth technology. An optical signal from the MMI-BLD optical flip-flop is injected to one of the SOAs on the MZI’s arms, and injected optical packets are thus switched through cross-phase modulation (XPM) in the SOA. Figure 3 shows the static switching characteristic of the all-optical MZI-SOA switch. The wavelengths of the signal and probe lights were 1549.32 nm and 1552.52 nm, respectively. The switch was configured for inverted operation in this experiments, and the on-off ratio of 11 dB was achieved with +8.0-dBm signal power.
3. Experimental results
All-optical packet switching was demonstrated using the MMI-BLD optical flip-flop and the all-optical MZI switch. The experimental setup is depicted in Fig. 4. Optical packets with 1550-nm labels and 1552.52-nm payloads were created at a 40-Gb/s transmitter. The data format of the payloads was nonreturn-to-zero (NRZ) 27-1 pseudo-random bit sequence (PRBS) at 40 Gb/s. The PRBS length was limited to 27-1 because of the limitation of a 40-Gb/s pulse pattern generator (PPG). The optical labels were injected to a label memory based on the MMI-BLD optical flip-flop, and the label information was stored for the one packet duration. The output of the label memory was injected to the all-optical MZI-SOA switch as a control signal, and the 40-Gb/s payloads were thus switched through XPM effect in the SOA.
To demonstrate packet switching, two alternating packets: Packet1 with a 10-ns label and Packet2 with no label were prepared as shown in Figs. 5(a) and 5(b). The one packet length was set to 160 ns. In this experiment, the guard time for the payload was not inserted, and the continuous PRBS data was used as the payloads due to the limitation of the 40-Gb/s modulator. The optical labels of Packet1 were fed into the MMI-BLD optical flip-flop as set and reset signals, and the label information was stored for 160 ns as shown in Fig. 5(c). These stored labels were fed into the MZI-SOA to switch the injected payloads. The switched 40-Gb/s payloads were shown in Fig. 5(d). As can been seen in Fig. 5(d), only Packet1 was switched because the optical label was attached to only Packet1. In this experiment, the switched Packet1 can not be measured because the MZI-SOA was designed as an all-optical wavelength converter and it had only one output port for Packet2. However the MZI-SOA can work as a cross-bar switch when it has two output ports . The on-off ratio of the all-optical switch was improved from 8 to 11 dB, as compared with the previous report . The switching time of 305 ps was also achieved owing to the improvement of the switching speed of the MMI-BLD optical flip-flop. The switching time could be reduced to less than 100 ps through optimizing the design of the MMI-BLD.
Bit error rate (BER) curves for the all-optical switch was also measured with a 40 Gb/s bit error rate tester (BERT) as shown in Fig. 6(a). We obtained error-free operation with no error floor for the output 40-Gb/s payloads. Power penalty for a BER of 10-9 is shown in Fig. 6(b) at varying optical input powers. As can been in Fig. 6(b), the minimum penalty of 1.3 dB was obtained at the input power of -1 dBm. The sensitivity degradation for lower power was observed because of the degradation of the optical signal-to-noise ratio (OSNR) by the amplified spontaneous emission (ASE) noise from the SOAs. On the other hand, the primary limitation of sensitivity for higher power was gain saturation. It can be seen in the eye diagram of Fig. 6(b) that the output signal was distorted due to pattern effects in the SOAs. The dynamic range of 4 dB was obtained with 1-dB excess power penalty for input power at -1 dBm.
40-Gb/s optical packets were successfully switched by all-optical signal processing of an MMI-BLD optical flip-flop and an all-optical MZI switch. The injected optical labels were stored in the optical flip-flop memory, and the optical output of the label memory controlled the all-optical switch to forward the injected optical packets. The switching time of 305 ps was achieved owing to fast operation of the MMI-BLD, and the on-off ratio of the all-optical switch was improved to 11 dB. The all-optical packet switching scheme presented in this paper can easily be applied to much higher data rate packets because of its transparency of data rate and format, suitable for ultrafast and high efficient optical packet switching networks.
This work was performed under management of the OITDA supported by NEDO. Photolithography masks were fabricated using an EB lithography apparatus of VLSI Design and Education Center (VDEC), University of Tokyo.
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