Photonic switches can be made from a number of 1 x N switches, used as building blocks in part of a larger free-space optical network node. Bruno Fracasso and other researchers at ENST de Bretagne (Brest, France) demonstrated a liquid-crystal-based switch that worked with 10-Gbit/s channels. The researchers believe their device could be made to work in WDM systems with throughputs of up to 1.25 Tbit/s.
Silicon backplane liquid-crystal spatial-light modulators operate with no moving parts and can be configured for a wide range of switching topologies and environmental constraints, but they also have higher losses than competing MEMS micromirrors, and are usually sensitive to polarization. The devices create an electrically controlled hologram, which changes the phase of light reflected from (or passing through) it.
A major advantage of the holographic approach is that the available spatial bandwidth can be used not only to steer or route the optical signals but also for other purposes, such as reducing insertion loss from local channels (due to local fiber misalignment or integration tolerance), making uniform the insertion losses, compensating for wavelength drift, and monitoring device faults. "The last point," say the authors, "becomes crucial when a large number of output channels is considered." This approach also can address one of many output fibers simultaneously, or shift the spectral response of the selected optical output.
The group used a 45° tilt angle smectic C* liquid-crystal halfwave plate in a planar configuration as the holographic steering module (HSM), which makes the device polarization insensitive. The device has a switching time of several hundred microseconds. The liquid crystal device has 512 pixels with a 15-µm pixel pitch.
An array of single-mode fibers at one end of the switch contains both the input and output fibers, which improves alignment (see figure). Light from the input fiber spreads as it travels to a collimating lens, and then the collimated enlarged beam travels on to the HSM. The HSM deflects the beam, depending on how it is programmed. The reflected beam is imaged by the collimating lens back onto the core of an output fiber. The free-space distance traveled by the beam in four focal lengths of the collimating lens.
The researchers demonstrated a 1 x 14 switch with a positive lens that has a 40.5 mm focal length (at 1.55 µm) and a 1 x 16 fiber array. The fibers are irregularly spaced to optimize the bandwidth and minimize crosstalk. Fiber-to-fiber coupling induces 2-dB losses, the liquid-crystal device produces 6 dB of loss.
Losses ranged from 8.5 to 11 dB, presumably because of nonoptimal alignment. Optical bandwidth ranges from 10 nm to more than 50 nm. Polarization-dependent loss was less than 0.6 dB. Crosstalk varies from -21 to -45 dB.
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- B. Fracasso et al., OSA Photonics in Switching Meeting, July 13, Monterey, CA, paper PThB3.