A rectangular array of Y-junction electro-optical switches has been proposed as a prototype of reliable, high-speed cross-point switches for fiber-optic-based photonic communication systems. The proposed array would feature a hybrid digital/interference design. The proposed array would be an integrated-optics device, fabricated in the silicon-on-insulator material system; it would be compatible with silicon-based electronic integrated circuits and with up-to-date V-groove packaging.
Some previously developed electro-optical switches are said to be of the interference (more precisely, two-mode interference/single-mode propagation) type, for which the switching characteristic is sinusoidal. The performances of interference-type switches are sensitive to (and can thus be degraded by) imperfections of fabrication and variations of polarization, wavelength, operating voltage, and temperature. Hence, it is difficult to make a high-performance large interference-type switching array.
Some other previously developed electro-optical switches are said to be of the digital type. One subtype includes InP- and GaAs-based symmetric-Y-junction waveguide switches. In the case of an InP switch of this subtype, there are two electrodes on each of the two arms of the Y. When no current is made to flow through the electrodes, the switch behaves as a 3-dB power splitter. When sufficient electrical current is injected via the electrodes on one waveguide arm, the index of refraction of that arm decreases by enough that light no longer enters that arm and instead goes entirely into the other arm. Because each waveguide arm is designed to support propagation in only one mode, the light transmission versus the applied current exhibits a step-function characteristic; that is why a switch of this type is denoted a digital optical switch. When the current is turned on, there is an optical loss because (1) the reflection wall is tilted because of spreading of the injected current and (2) imperfections of the tips of the Y give rise to radiation and scattering.
The proposed array would contain ridge-waveguide structures. The waveguide ridges would be made of silicon, and the upper and lower cladding (optical-confinement) layers would be made of air and silicon dioxide, respectively. Very low optical-propagation loss has been achieved in such a structure; this is attributable to low optical loss in the air and SiO2 layers. The silicon wave-guiding layer in the proposed array could be designed with low doping concentration for low optical absorption.
The principle of operation of the switches in the proposed array would differ from the principles of operation of previously developed electro-optical switches. A switch in this array would exhibit two states - denoted "through" and "switching." These states are defined in the figure. The through state would involve multimode interference (instead of single-mode propagation). The principle of operation and the design to implement it would significantly reduce (in comparison with an InP switch as described above) the optical loss in through state, while retaining the step-function switching characteristic essential for effectively digital switching.
The throughput-waveguide portion of each switch would be curved; the precise shape would be chosen to minimize the throughput loss. The electrodes for each switch would be formed in a lateral acceptor-doped/intrinsic/donor-doped [positive/intrinsic/negative (PIN)] configuration that would suppress spreading of the injected current into the branch waveguide; as a result, the optical loss that would otherwise be associated with current spreading in the switching state could be minimized.
This work was done by Chi Wu and Simiak Forouhar of Caltech for NASA's Jet Propulsion Laboratory.
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