Symmetric dual polarization diverse digital optical coherent receiver

Digital coherent receivers are an essential building block of optical coherent communication systems. Improved traffic capacity, increased spectral efficiency, and support for advanced modulation formats are amongst their attractive features. The two electrical outputs of a coherent detector may be considered to provide the real and imaginary parts of a complex beat signal representing the value of a sesqui-linear form that combines the analytic signals representing its two optical input ports. An interchange of signal and reference may therefore be compensated by complex conjugation of the beat signal. Conventional polarization diverse coherent receivers use a polarizing beam splitter to provide two orthogonal linearly polarized components and then the polarization of one component is rotated so both components align with two copies of the linearly polarized local oscillator provided by a conventional beam splitter. This arrangement destroys the symmetry of the coherent detector to interchange of signal and reference.