In this thesis, we discuss various coupling conditions between optical waveguide devices. We simulate the coupling loss of single mode fiber to multi-mode fiber, analyze optical waveguide based on SOI coupling to single mode fiber, furthermore, consider coupling effect between optical waveguide based on SOI and multi-mode fiber. The introduction of 2x2 Multi-mode Interference (MMI) SOI devices and measurement of photo luminance spectrum of the 2x2 MMI devices are reported. Discussion of coupling loss of different width and slab height of 2x2 MMI devices are also presented. Finally, we minimize the size of 2x2 MMI devices to nano-scale and discuss the coupling loss between nano-scale optical waveguide components and optical film.