System offers fast optical-label-switched routing of optical packets

Sep 1st, 2001

Optical WDM networks will need to be capable of both switching the wavelengths of optical packets and routing these switched signals quickly. A system that addresses this problem by combining rapidly tunable wavelength conversion with an arrayed waveguide grating router was presented by S. J. Ben Yoo and colleagues at University of California at Davis and NTT Photonics Laboratory (Naka, Ibaraki, Japan).1

Every optical packet includes a header with routing information (like an address on an envelope) and data (the letter). The routing system consists of an optical subcarrier transmitter, an optical header/data separator, a header detector, a forwarding table, a switch controller, a tunable wavelength converter that includes a tunable laser and semiconductor optical amplifier (SOA), a uniform-loss-cyclic frequency arrayed waveguide grating router (AWGR), header rewriters, and receivers (see figure). The researchers demonstrated the optical header extraction and rapid packet routing with optical header swapping.

How it works
The transmitter modulates the original header at 622 Mbit/s on a 14-GHz subcarrier, and combines it with the data packet at 2.5 Gbit/s. The signal then includes double sideband subcarrier header appearing 14 GHz away from the center optical frequency of 192.7 THz (in other words, 1549.3 nm).

The header and data are separated using a fiber Bragg grating as narrow-band filter, which reflects the data payload near the center wavelength but transmits the sidebands containing the header. An optical circulator separated the reflected signal containing data from the input signal.

The 40-bit-long optical label in the header is read and compared to the contents of the forwarding table, which tells the system to which wavelength the packet should be converted. The controller changes the optical label information to include this new wavelength and sends the header off to the appropriate header rewriter. The controller also commands the tunable label to generate this new wavelength.

Meanwhile, the data has been traveling through a 50-m-long fiber that provides a 250-ns optical delay. It emerges from the delay fiber and enters the SOA. The amplitude of the data modulates how much gain the SOA provides to the new wavelength, which effectively encodes the data onto the new wavelength. The AWGR spatially splits the channels, providing one channel per fiber. After the AWGR, appropriate header information for a specific channel is encoded onto the sidebands of the channel. The experiment demonstrated a packet switching time of 600 ps.

For more information contact S. J. Ben Yoo at

Yvonne Carts-Powell


  1. S. J. Ben Yoo et al., Photonics in Switching Meeting, OSA and IEEE LEOS, paper PThD3, Monterey, CA (July 13-15).
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