Research On The Diffraction Characteristics Of LCOS Dynamic Phase Grating

In the agile optical network(AON) based on dense wavelength division multiplexing(DWDM) technology, reconfigurable optical add/drop multiplexing(ROADM) nodes can switch optical signal in wavelength granularity. The new generation of ROADM technology based on the wavelength selective switch(WSS) can realize the best wavelength switching flexibility. Liquid crystal on silicon(LCOS) is one of the main approaches for WSS module. This paper researches the diffraction characteristics of dynamic phase grating based on LCOS. The main work is as follow:The applications of WSS technologies and the development of LCOS technologies are reviewed. Based on application of LCOS in WSS module, a simple 2f optical model is built. In the model, the optical fiber collimator array employed as the input/output ports and the LCOS chip are placed on the front and rear focal planes of the Fourier lens, respectively. The model of LCOS dynamic phase grating is also built, which is equivalent as a composite grating composed of a blazed grating and an echelon grating. The phase modulation of each pixel in the LCOS chip is independently controlled by voltage, while electric field interference between the adjacent pixels exists. The influence of fringing-field effect is considered in the grating model, which generates a parasitic grating in the phase modulation return area.Based on above grating model, cosine function is employed to fit the transition region between the phase modulation of adjacent pixels, and the diffraction characteristics of LCOS phase grating are numerically simulated. Considering the application in WSS module, the parasitic grating resulting from the fringing field effect greatly affects the insertion loss and crosstalk of WSS. With the increment of the beam deflection angle diffracted by the LCOS phase grating, the fringing field effect is more remarkable, and a wider phase modulation return area should be set for simulation to match the experimental results.Based on above simulation results, the parameters of the experimental system are designed and an experimental platform is constructed to verify the simulation. The period of the LCOS phase grating is set according to a required diffraction angle. The input optical fiber collimator is placed at an off-axial position on the rear focal plane of the Fourier lens, and another output fiber collimator is employed to receive the optical signal at different positions on the rear focal plane. The curve relating the coupling efficient and the receiving position is obtained. The coupling efficient corresponding to the target port position is the insertion loss, and those corresponding to the other port positions are crosstalk. The measured crosstalk at different diffraction angle meets the simulation results generally. The research work could be reference for the design of a LCOS-based WSS module.