Phase modulation formats for coherent optical receivers with photorefractive beam combiners

Abstract

We discuss the effect of high speed phase modulation impressed on one of two optical beams incident on a photorefractive crystal used in a two-wave mixing configuration in a coherent optical receiver. Phase modulation on the signal beam reduces the average visibility of the fringe system inside the photorefractive material. This in turn reduces the coherent superposition properties of the refractive-index grating formed in the material. In the undepleted pump approximation, a single parameter κ=|Eejφs( t )|, where ϕ s (t) is the phase modulation carried by the signal beam, characterizes the reduction in two-wave mixing coherent superposition properties of the grating with κ = 1 yielding the usual energy transfer equations for forward two-wave mixing. This rules out the use of some phase modulation formats commonly used in microwave communication systems. We propose some alternative phase modulation formats that can be detected with photorefractive beam combiners including the phase modulation equivalent of Q = 4 pulse position modulation and a tri-state phase format that has a unity bandwidth expansion factor.