A silica-based 16 x 16 strictly nonblocking matrix thermo-optic switch with a bandwidth that covers the entire EDFA gain region offers an average insertion loss of 6.6 dB, extinction ratio of 53 dB for the worst-case scenario, and consumed 17 W without bias power. Takashi Goh and others at NTT Photonics Laboratories (Ibaraki-ken, Japan) recently reported optimizing their previous work to yield this larger-scale switch.1 The switch has 16 input ports, 16 output ports and a matrix of 512 switching units between the two, such that any input signal can be directed to any output port. Each switching unit is a set of two asymmetric Mach-Zehnder interferometers (MZIs) with an optical-path length of a half wavelength, thermo-optic phase shifters, and an intersection (see figure). By manipulating the phase with the heaters, the signal can be shifted from one input port to either output port.
The work is an extension of an earlier 8 x 8 switch to 16 channels. However, the design increased insertion loss because of incremental increases in the waveguide-propagation loss and fiber-coupling loss with each channel. The electrical power consumption also increased—because of the increased number of switch points, the required bias power when the switch was in the "off" state also increased.
The group tackled the problems by decreasing the difference in refractive index between the waveguide and the cladding to minimize insertion loss. They designed the switch to obtain a high extinction ratio and short circuit length, and used a phase-trimming technique to eliminate the waveguide phase error, which minimizes the power consumption.The size of the switch grew to use a 6-in.-diameter wafer—a lower refractive-index difference results in less loss, but increases the minimum radius of curvature.
Even when a switch is "off" it requires a power to the heater to maintain the state because of small errors in the optical path in each MZI. To fix the phase errors and thus minimize the bias powers, the researchers used heaters to apply a one-time high-voltage heating (to about 450°C) method that produces a permanent change in the index of refraction of the waveguide. By carefully selecting heating power and time, the bias power could be adjusted to zero.
- T. Goh et al., J. Lightwave Tech. 19(3), 371 (March 2001).
Yvonne Carts-Powell is a science and technology writer based in Belmont, MA.