A waveguide switch structure with three input and two output ports is designed in SiGe/Si material for the operation around 1.55 μm in wavelength. Strained Si0.96Ge0.04 layer with a thickness of 2.5 μm is used as the waveguide core layer. Single mode ridge waveguide of 10 μm wide and 1µm deep is formed by plasma etching. The switching functiaa on is realized by total internal reflection. Two separate electrodes are used to control the refractive index change in the intersection region through carrier injection. The switch device can also work as splitter, modulator, or add-drop multiplexer, etc. around wavelength of 1.55 μm. An extinction ratio of about 20 dB is achieved for the modulation state from the two side input ports and about 10dB for the central input port.
Improved approach to low-loss and high-uniformity MMI devices is proposed in this paper. The propagation constant error of guided modes in the MMI region is analyzed. An optimized index contrast is selected to decrease the propagation constant errors of those modes near cutoff. 1 X N MMI splitters designed by this means are analyzed by exact mode analysis. Results show that the loss and uniformity of this new structure are notably improved, especially when N is large. The loss of 0.0892 and uniformity of 0.0826 are acquired when N equals 32.