Tunable filter enables packet switching on all-optical network

Tunable filter enables packet switching on all-optical network

By YVONNE CARTS-POWELL

A recently developed fast tunable filter should enable packet switching to be implemented on a new all-optical network, reported Nina Taranenko of Los Alamos National Laboratory (Los Alamos, NM) at the Conference on Lasers and Electro-Optics (cleo `98) in San Francisco, CA, in May. Taranenko and co-authors Richard Thomsen and Andrew Dubois, in the Computer, Information, and Communication (cic) Div. of Los Alamos, described how the fiber Fabry-Perot tunable filter operates at a rate three times faster than commercially available Fabry-Perot filters--fast enough to allow packet switching to be implemented on an experimental all-optical wavelength-division multiplexed network.

The work derives from the Rainbow-2 experimental network developed by ibm and located at Los Alamos. The all-optical network transmits data between 32 nodes, including several supercomputers; each node transmits at 1 Gbit/sec. The network uses a broadcast-and-select design, in which each node has a laser transmitter and a dedicated wavelength. The data is transmitted over the passive-star network to all other nodes. The receiver at each node scans through all the wavelengths and selects signals addressed for that node.

Rainbow-2 operates using a circuit-switched configuration, much like a telephone system, in which a physical circuit is formed between two nodes that are communicating. Each time the wavelength to be received changes, the receiver scans all the wavelengths again. This tuning process takes about 25 msec.

Although this system works well for strings of data several thousand bits long and for limited numbers of users, Taranenko explains, packet switching (such as is used on the Internet) would work better when many users compete for use of the system. In the packet-switched scenario, no physical circuit would be formed; instead, the data would be bundled into short packets, which would be sent separately. The major challenge of combining packet switching with an all-optical network, Taranenko says, "is the lack of an optical switch that would have a fast tuning speed, wide tuning range, low insertion loss, and be highly selective with a narrow passband at the same time."

Specifically, the tuning at the receiver must be performed at least in microseconds, as opposed to milliseconds. The cic-5 group at Los Alamos and Kevin Hsu at Micron Optics (Atlanta, GA) therefore built a piezoelectrically driven fiber Fabry-Perot tunable filter with a special driver that demonstrated switching speeds as fast as 3 microsec. The tunable filter uses a Fabry-Perot cavity with an air-gap of variable length. For a wavelength to be passed by the filter, the cavity length must be a multiple of the wavelength. The filter is tuned by changing the cavity length, accomplished by using a piezoelectric transducer (pzt) to change the length of the air-gap within the cavity.

For the fast filter used in this work, a special pzt was used with lightweight components to reduce the device`s inertia. A special driver for the filter was developed at Los Alamos to provide fast tuning without ringing.

The filter will be incorporated in the upcoming Rainbow-3 network. A media access control (MAC) protocol proposed for the Rainbow-3 network would allow packet switching and also reduce the amount of time spent in tuning the receivers. In this system, unlike the current circuit-switched system, the filter will not have to scan all the channel wavelengths after accepting a signal at one wavelength. The fast filter will allow this protocol to be used because it keeps the tuning time to within 25 microsec--about 20% of the time required to receive a hippi (high-performance parallel interface) burst, which is an acceptable amount of overhead. The MAC protocol will minimize the searching time for the appropriate wavelength: The filter will scan only once through the entire optical region and then tune to all the required channels, one after another, in a few microseconds. "It should provide much higher flexibility and optical throughput for the network," according to Taranenko.

The filter could provide a technical breakthrough for the bottleneck of high-speed packet switching--but it also could present the problem of clock-recovery and bit-synchronization of the packets. Taranenko says, "If the data packets are running at a few gigabits, a gigabit clock recovery scheme is needed for detection." She further explains, "Conventional circuits are inadequate when a fast clock recovery for the short length packets is required because they need thousands of bits to lock on."

Taranenko and researchers from MultiLink Technology Inc. (director of research Vladimir Katsman and president Richard Nottenburg) have developed a system for recovering the clock synchronization packet-by-packet using a surface acoustic wave filter for narrowband filtering. The work is continuing, but a preliminary system that uses a series of filters and amplifiers demonstrated clock recovery. The combined optical and electronic components recover the clock circuit quickly, as fast as 200 to 250 bits. Taranenko says, "This clock recovery technique is very useful for gigabit clock recovery and is realizable using commercially available technology."

"The next step in our development of optical packet switching," says Taranenko, "is to study the trade-off between the ber [bit-error rate] performance and the filter`s locking time." q

Yvonne Carts-Powell writes on photonics issues from Belmont, MA.