The reference value is a value of the nonlinear phase shift which occurs in thefirst case (Type 1) under the condition that an input level is -2 dBm and the pump light is not supplied.Referring to calculation results (solid lines in FIG. 10A) when an influence of a Reyleigh's cross talk is taken into consideration, it is clear that the highest (the most preferable) optical SNR is obtained in the c