Planar waveguide devices based on silicon-on-insulator architecture offer the enticing possibility of miniaturization because of the high difference in refractive index between silicon and air (Δn = 2.45) or between silicon and silicon dioxide (Δn = 2.0). This difference translates into a smaller critical angle, which in turn offers the ability to make miniaturized devices with sharper bends without coupling optical power out of the waveguide. Another, perhaps even more enticing possibility of such structures is the fabrication of optical devices integrated with silicon-on-insulator-based CMOS electronics.These exciting possibilities have led a group of researchers at Columbia University in New York to design and fabricate what they believe is the first silicon-on-insulator-based, ultrasmall, low-power, high-speed Mach-Zehnder interferometer thermo-optic switch. The switch, which features alternating layers of silicon and silicon dioxide, takes advantage of the unusually high thermo-optic coefficient (dn/dT = ~1.8 X 10–4/K) of silicon at telecommunications wavelengths (Figure 1). Figure 1. The 0.26 X 0.6-µm silicon core of the strip waveguide is surrounded by silicon dioxide. Because there is a large difference in the refractive index of these two materials, such devices can be fabricated on a small scale with sharp waveguide bends.The switch is based on the principle of a Mach-Zehnder interferometer (Figure 2). Radiation entering from the left is split into two parts at the first Y and recombined at the second Y. If the two arms of the interferometer are of equal length, the two parts combine in phase, and the switch is "on." But if the heat-induced phase change in one arm is π, then the two parts are out of phase when they recombine, and the switch is "off." Figure 2. The Mach-Zehnder switch exhibits a very fast (The input, multimode waveguide is 3 µm wide for efficient coupling with incoming light, but it tapers to a 0.6-µm, single-mode waveguide over a 10-µm distance inside the device. The same design is reversed at the output side of the device.Sinusoidal switchingThe researchers observed sinusoidal switching of the device when they illuminated it with 1.55-µm light and drove the heater with 50 mW of average power. They measured the extinction ratio between "on" and "off" to be greater than 15 dB, and there was a slight polarization dependence to the loss. They believe this polarization dependence resulted from asymmetrical stress from the heater, and that it could be removed with a better heater design.To measure the response time of the switch, the scientists drove it with a 5-kHz, 4-V square wave and fit the resulting data to an exponential fit. They found that 1/e rise time was less than 3.5 µs, which they believe is one of the fastest reported for a thermo-optic switch.The work provides an important proof of the principle of silicon-on-insulator optical switches, but further work will be needed before these devices become practical. In particular, the 32-dB insertion loss precludes immediate application of the device. The researchers believe that sidewall roughness of the waveguide is the dominant loss mechanism and that this roughness is the result of the fabrication process. They are working on techniques to minimize the sidewall roughness and to improve transmission through the device.