Polarization-independent linear detection of optical phase modulation using photo-emf adaptive photodetectors

Abstract

Linear detection of the optical phase modulation in a randomly polarized laser beam is highly important, in particular, for optical fiber sensors and coherent optical communication systems. This type of detection cannot be performed properly by means of conventional techniques, and the problem is that the original optical signal beam has, in general, two orthogonally polarized components with randomly fluctuating phases and relative intensities. We propose a simple original configuration based on utilization of adaptive photodetectors to overcome this problem. The system consists of two independent channels for coherent (with the reference waves) detection of two orthogonally polarized components of the signal beam and by following summation of the output electric signals. The data presented demonstrate applicability of this configuration for polarization-independent coherent photodetection. In general, the adaptive systems in consideration also solve the problem of compensation for the general phase drifts and irregularities of the detected wavefronts. Moreover, if the cut-off frequency of the adaptive detector is high, it can also compensate for the discrepancy between the optical frequency of the signal wave and that of the local oscillator.