Analytical Study on Propagation Dynamics of Optical Beam in Parity-Time Symmetric Optical Couplers*Supported by the National Natural Science Foundation of China under Grant No. 11465008, the Hunan Provincial Natural Science Foundation under Grant Nos. 2015JJ4020 and 2015JJ2114, the Scientific Research Fund of Hunan Provincial Education Department under Grant No. 14A118. (1st April 2015)