The differential 4-quadrature coding scheme for applications in optical communications

The differential 4-quadrature (D4Q) coherent optical system which does not need absolute phase information is proposed. The input information is encod ed in the relative position of the present symbol with respect to the refer ence frame constructed by the previous 3 symbols. The general theory of the system operation including encoding and decoding is presented, This system does not need to track the fluctuation of the states of polarizations whic h is essential for most other polarization modulation systems. As an exampl e, the l-symbol D4Q system is described. And the saddle point approximation is applied to estimate the bit error rate performance. The analytic approx imation agrees with the simulation results very well.