Applications Of Optical Phase Conjugation To Coherent Communications

Abstract

ABSTRACT We describe an all-optical method to transform a weak incident optical beam with constant amplitude and pulsated phase jumps into an amplified optical beam with amplitude modulation. The proposed method uses either the reflected or the transmitted beams by a phase-conjugate mirror. Our analysis assumes that the operation of optical phase conjugation is due to nearly degenerate four-wave mixing where two monochromatic pump fields are mixed in a Kerr-like material with the incident field, producing a transmitted field and a phase- conjugate reflected field. Numerical results show that the depth of amplitude modulation decreases with the gain of the phase-conjugate mirror, while the duration of the amplitude pulses increases along with that gain. An original configuration with a single output beam is proposed. The equivalence of the device with coherent detection schemes is discussed. 1. INTRODUCTION Phase conjugation of an optical beam is an operation which allows to produce a reflected beam that retraces the optical path of the incident beam-*-. Such an operation can be realized through various nonlinear processes among which stimulated Brillouin scattering2 and degenerate four-wave mixing (DFWM)^^ are most common. The latter process has two main advantages over any stimulated scattering mechanism: the reflected and transmitted fields can be amplified with respect to the incident field and the incident, reflected and transmitted fields have the same frequency. As a result, DFWM has found a variety of applications in optics such as image processing, interferometry and associative memories, while being extensively used for phase conjugation. All these applications are based on the spatial properties of DFWM. The process has also some intrinsic properties in the spectral and temporal domains which can lead to applications^ such as optical gating, temporal convolution or correlation and compensation of pulse spreading effects in optical fibers.In this paper, we describe how the temporal and spectral properties of DFWM can be used for coherent optical communications. Essentially, the paper is concerned with the proposal and the analysis of a purely optical method to transform an incident optical field of constant intensity and with a phase modulation into an amplified output field with amplitude modulation. We use the fact that the operation of optical phase conjugation, when realized through DFWM, can be viewed, in the frequency domain, as tunable filtering with amplifica­ tion; the frequencies close to the pump frequency will experience a larger amplification than the frequencies remote from the pump frequency. Hence, the frequency selective response will reshape the spectral content of the phase-conjugate field with respect to the incident field, thereby changing its temporal structure. Clearly, a sudden phase jump of the incident field will generate frequency sidebands with a smaller amplification than a continuous field, resulting in a depression in the phase-conjugate field amplitude. Similar considera­ tions also hold for the transmitted field. The purpose of the paper is to provide physical insight into the rules to achieve both a significant amplification and an efficient transfer of phase information into amplitude information. So far, conversion of phase modulation to amplitude modulation has been studied^ only in the limit of a weak phase-conjugate reflec­ tivity.The paper is divided as follows. In the next section, we analyze the spectral response of DFWM, for both the reflected (phase-conjugate) and the transmitted fields. In Section 3, we present numerical results showing the temporal responses of the reflected and transmitted fields to incident fields with a discontinuous (rectangular) phase jump; we indicate how the results depend upon the phase-conjugate gain and the duration of the phase jump. A com­ parison is also made with the response to incident pulses of rectangular temporal shape. A discussion of the results is made in Section 4, with practical considerations upon materials and sources required to achieve such devices; we also point out the equivalence of the proposed method with coherent detection schemes.