Mixing inhomogeneous, partially coherent optical fields.
A phase-quadrature (p-q) description of a random optical field is suggested as an attractive alternative to the usual logamplitude-phase representation for the purpose of investigating the behavior of the optical heterodyne. Using the p-q representation, an expression is developed for the average signal-to-noise ratio at the heterodyne's output when both the signal and local oscillator (LO) fields are inhomogeneous and partially coherent. An examination of the expression for the signal-to-noise ratio indicates the general nature of the previously observed saturation effect. A coherence suppression phenomenon is also apparent. It is shown that the heterodyne's field of view may be drastically broadened (relative to that of a system which processes ideal plane waves) as a result of weak spatial coherence in either the signal or LO field. Recently published experimental data are examined in light of the results obtained i this paper, and good agreement between theory and experiment is observed.
Effects of coherence and polarization changes on the heterodyne detection of stochastic beams propagating in free space
We evaluate the heterodyne efficiency of a heterodyne (coherent) detection system for partially polarized, partially coherent, quasi-monochromatic beams. The beam-coherence-polarization (BCP) matrix is used for the description of the statistical ensemble of this class of stochastic beams. We investigate the dependence of the efficiency of the detection process on the beams parameters as the beams propagate in free space. We discuss how the optimization of the detection system can be performed for received beams with different coherence and polarization properties by adjusting the corresponding properties of the local oscillator beam. The dependence of the mixing efficiency on the size of the receiving aperture is emphasized. We derive an analytical expression for the heterodyne efficiency in the case when both the received beam and the local oscillator beam belong to a broad class of so-called electromagnetic Gaussian Schell-model beams. Our analysis is illustrated by numerical examples.
Optical communications in the earth's atmosphere
The main reasons for the present interest in the potentialities of the laser for communication system applications lie in the coherent nature of the radiation and the short wavelength. The property of coherence, as considered in this article, is the ability of an electromagnetic wave to interfere with itself. The phenomenon is characterized by the ability to form a highly collimated beam, which leads to the ability to achieve communications over great distances. However, the perturbing effects of the earth's atmosphere on coherent optical radiation raise a serious obstacle to the use of lasers in earthbound communication systems. Space and interplanetary communications applications appear more promising as atmospheric effects are eliminated.