wherein the intensity of the signal light V* is calculated by the following equation V * = 1 2 ??? ??? m ??? V 2 ??? m - 1 2 ??? m ??? J 2 ??? m - 1 2 ??? ( 2 ??? kA ) + ??? n ??? V 2 ??? n 2 ??? n ??? J 2 ??? n 2 ??? ( 2 ??? kA ) where mmax and nmax are predetermined integers, m is an integer of 1, 2, . . . , mmax, n is an integer of 1, 2, . . . , nmax, the amplitudes of the first signal componen