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Optoelectronics & Communications

Ultrafast all-optical signal processing and packet switching

Light-based solutions to wavelength conversion and flip-flop could sidestep the need for control electronics in routing data.
16 May 2007, SPIE Newsroom. DOI: 10.1117/2.1200704.0592
Optical networks have become an important part of the global telecommunication infrastructure due to the merits of optical fiber, the only medium capable of moving data at multiple gigabits per second (Gb/s) commercially. In these networks, signals are transmitted through optical fiber and switched by cross-connects. Driven by the Internet, the demand for bandwidth is constantly increasing. Transmission technologies have been able keep up with it thanks to dense wavelength division multiplexing. This technology allows more than a hundred wavelengths to be simultaneously launched into a single optical fiber.

But the electronic packet routers in the cross-connects (see Figure 1) face challenges in terms of power consumption, footprint, costs, and switching architectures. All-optical switching is emerging as a promising technology because it could help overcome the challenges of its electronic counterpart.1 Gradually, more switching functions will be implemented in the optical domain by using photonic integrated circuits. For this reason, all-optical signal-processing technology is essential for future optical packet-switching nodes. Some progress in this field has been reported.2–5

Figure 1. Schematic diagram of an optical cross-connect in an optical network.

Figure 2. Basic configuration of a 1×2 all-optical packet switch.

Figure 2 presents a conceptual view of our all-optical packet switch, which can be realized by using entirely light-based signal processing. The switch consists of three functional blocks: an all-optical header processor, optical flip-flop memory, and a wavelength converter.

The optical packet consists of a header section and a payload section. The routing information is contained in the header. The header processor correlates the header pattern into an optical pulse. This pulse triggers the flip-flop to generate continuous wave (CW) light with a specific wavelength. Different header information produces a different wavelength that can be tailored as desired by feeding it into the wavelength converter. Afterwards, a demultiplexer is used to spatially route the information into a particular port, depending on the wavelength of the packet. This basic structure has been extended4 with more sophisticated functions, such as clock recovery, header erasing, and inserting, included for all-optical packet switching in the optical network nodes.

Figure 3. (a) The experimental setup for all-optical routing. CW: Continuous wave. SOA: Semiconductor optical amplifier. PC: Polarization controller. BPF: Optical bandpass filter. PBS: Polarizing beam splitter. PMF: Polarization-maintaining fiber. (b) Optical pulses for set and reset operation and the output from the flip-flop that shows the switching between the two states. (c) Mask layout of integrated optical flip-flop.

Figure 4. Information about signal and error results. (a) Bit-error-rate values of 80Gb/s wavelength conversion. (b) Eye diagram of 160Gb/s converted signal. (c) Input and switched data packets.

The experimental setup is illustrated in Figure 3(a). The system consists of two parts: an integrated optical flip-flop and an all-optical wavelength converter. The 80Gb/s transmitter is constructed by eight times multiplexing 10Gb/s 1.9ps optical pulses. The 80Gb/s data packets (1545.35nm) have a duration of 35ns, separated by 15ns of guard time. The optical flip-flop is an integrated InP/InGaAsP two-state coupled laser device,6 fabricated on an InP/InGaAsP wafer. The device has dimensions of 2.4×0.8mm, as shown in Figure 3(c).

The flip-flop has two possible states. In state 1, laser 1 is lasing, suppressing laser 2, so it outputs CW light at wavelength λ1 ( 1555.28nm). Conversely, in state 2, laser 2 is lasing, and the flip-flop outputs CW light at wavelength λ2 (1561.36nm). The state can be interchanged by injecting set or reset external optical pulses. The contrast ratio between the states is over 35dB. Moreover, the lasers operate in single mode. In Figure 3(b), the flip-flop dynamic operation is demonstrated by injecting external pulses with a duration of 3ns every 50ns in the dominant laser. It is clea from the lower traces in Figure 3(b) that the flip-flop regularly switches its state, and remains in its new state even after the set or reset pulse has vanished.

The second part of the setup is a device that converts the wavelength of the data packet from the wavelength that outputs the flip-flop. The dashed box in Figure 3(a) shows the wavelength converter, which is constructed by using commercially available fiber-pigtailed components. This configuration allows error-free noninverted wavelength conversion at 160Gb/s.7 The output signal is analyzed either on a conventional oscilloscope or on a 700GHz optical sampling oscilloscope. Bit-error-rate (BER) measurements are carried out after demultiplexing the 80Gb/s signal to 10Gb/s.

Figure 4(a) shows the BER values when the 80Gb/s return-to-zero pseudo-random binary sequence data stream is fed into the wavelength converter. All eight 10Gb/s tributaries are presented. The average sensitivity penalty of wavelength conversion at a BER of 10−9 is less than 2dB. Figure 4(b) shows eye diagrams measured at 160Gb/s at the wavelength converter output. The open eye indicates that the system also operates error free at 160Gb/s. We demonstrate routing of 80Gb/s data packets by operating the flip-flop dynamically. The input packets are routed to different ports when the flip-flop toggles between two states.

In summary, we have demonstrated all-optical wavelength routing of 80Gb/s data packets without using any control electronics. This system can be employed in all-optical packet switching.

This work was funded by IST-LASAGNE (FP6-507509), STW project EET649, the NRC photonics grant, and the NWO/STW Vidi program.

Yong Liu
COBRA Research Institute,
Eindhoven University of Technology
Eindhoven, The Netherlands

Yong Liu received his master's degree from the University of Electronic Science and Technology in China in April 1994, and his PhD degree from Eindhoven University of Technology in October 2004. In 2003, he was awarded an IEEE Lasers and Electro-Optics Society graduate student fellowship. Currently he is a postdoctoral researcher; he will be leaving to take a professorship at the University of Electronic Science and Technology in China. He has coauthored more than 90 journal and conference papers. In addition, he served as a session chair at SPIE APOC 2006, and has given invited talks at SPIE Optics and Photonics 2005 and SPIE APOC 2006.