There are many important applications for phase-only liquid crystal on Silicon-based spatial light modulators (LCOS SLMs). Among the applications, the diffractive beam splitting, beam shaping and beam steering with LCOS SLM are finding more and more use in telecommunication applications (e.g. wavelength selective switch for ROADM, space and mode division multiplexing). However, many effects of LCOS device have to be considered if we want to get high quality output light field. For example, the ideal phase, intensity and polarization distribution in far field are usually deteriorated by the pixelated metal structure and fringing field effects. Thus, the total efficiency is decreased. By using electro-optical and electromagnetic simulation methods, we can properly incorporate the effects that influence the optical performance of LCOS and optimize the design. Furthermore we report the implementation of the high-performance high-resolution LCOS SLM for the telecommunication C- and L-band with the average insertion loss (IL) of less than 0.2 dB, achieved by the reflectivity-enhancement coating on the LCOS backplane. The experimental results on reflectivity, diffraction efficiency, crosstalk and other important parameters are compared with the theoretical predictions.