The effect of the phase deviation on the phase shifter in a 2 × 4 optical 90 hybrid without cross-over is discussed based on a set of formulas derived for the output of this structure. We found that if the two power inputs are equal, the phase deviation of the phase shifter only introduces attenuation in output AC component, and if not equal, it not only introduces the AC component attenuation, but also leads to DC component differentiation, which degrades the optical signal-to-noise ratio of the quadrature output.
Optical signal to noise ratio monitoring using variable phase difference phase portrait with software synchronization