All-optical, silicon based, fiber optic modulator using a near cutoff region

We demonstrate, using molecular beam epitaxy and chemical vapour deposition grown silicon thin films, an all-optical modulator for single-mode guided light at 1.32 μm. A control beam of above band-gap light is used to generate electron–hole pairs via inter-band absorption and the resultant lowering of the effective refractive index brings the waveguide to cutoff thus limiting throughput. Near 100% modulation is obtained with less than 150 pJ energies with subnanosecond initiation and recovery times. The operation is stable and is polarization and wavelength independent in a three port geometry suitable for use as a logic gate. To reduce the total energy required to bring the waveguide to cutoff, the effects of using a small, already near cutoff and leaky region at the interaction region is investigated here. Preliminary measurements on a graded index, leaky mode, chemical vapour deposition sample resulted in a reduction by a factor of five or better in drive energy. By keeping the leaky region short, the losses can be kept to negligible values. Results in time resolved imaging of the guided mode are also presented along with the necessary design considerations for performance compatible with presently available communication solid state laser sources.