Planar Silicon Optical Waveguide Light Modulators

The results of an experimental investigation of a new type of optical waveguide based on planar technology in which the liglht guiding and modulation are achieved by exploiting free carrier effects in silicon are presented. Light is guided between the n+ substrate and two p+ regions, which also serve as carrier injectors for controling absorption. Light confinement of single mode devices is good, giving spot sizes of 9 ¿m FWHM. Insertion loss measurements indicate that the absorption losses for these waveguides are extremely low, less 1 dB/cm. Estimates of the switching speed indicate that values in the nanosecond region should be possible, however, the measured values are high, 20 microseconds, due to the large area of the injector junctions, 1× 10^¿2 cm², and the limitations imposed by the detection circuit. The modulating properties of these devices are impressive, measurements indicating that modulation depth can approach 100 % (¿ 30 dB).