Optical phase modulators for MHz and GHz modulation in silicon-on-insulator (SOI)

This paper reports the simulation of the direct current (dc), transient, and optical characteristics of low-loss single-mode optical phase modulators based on silicon-on-insulator (SOI) material. The devices operate by injecting free carriers to change the refractive index in the guiding region and have been modeled using the two-dimensional (2-D) device simulation package SILVACO and the optical simulator BeamPROP to determine their electrical and optical performance, respectively. These simulators have been employed to optimize the overlap between the injected free carriers in the intrinsic region and the propagating optical mode. Attention has been paid to both the steady state and transient properties of the device. In order to produce quantitative results, a particular p-i-n device geometry has been employed in the study, but the trends in the results are sufficiently general to be of help in the design of many modulator geometries. The specific example devices used are designed to support a single optical guided mode and are of approximately 1 mm in cross-sectional dimensions. The modeling results predict that the transient performance of the device is affected significantly by the contact width and the rib doping depth. Results presented encompass Gaussian and constant doping profiles in the n⁺ regions. The doping profile of the contacts has a tremendous effect on both the dc and transient performances. Phase modulators with drive currents as low as 0.5 mA and transient rise times of 0.3 ns and fall times of 0.12 ns are predicted. Following from these results, a realistic doping profile is proposed that surpasses the electrical results of the Gaussian and most of the constant doping profiles. The improvements in electrical device characteristics are at the expense of a slightly increased optical absorption loss. An alternative switching technique is also presented that could further improve the device speed.