Adaptive photodetection using wavemixing and real-time holography with application to laser-based ultrasound, remote sensing, and laser communication

Summary form only given. A fundamental requirement of coherent detection systems for optimal performance is the necessity for wavefront-matching of a received signal to that of a local oscillator. In light of the potential applications of lasers to remote sensing for manufacturing process control using laser-based ultrasound and the renewed interest in free-space optical communication networks, the optics community has been motivated to examine how one can improve the performance of coherent receivers in the face of highly aberrated signal beams. In general, the antenna theorem states that a huge penalty in signal-to-noise is experienced (upwards of 30 dB) when dealing with signals that contain many spatial modes. It is therefore incumbent that one consider real-time methods to provide for beam clean-up or wavefront-matching of dynamically varying phase-fronts to a local oscillator. In this tutorial, we discuss and compare a variety of nonlinear optical techniques to achieve these goals. Examples include phase-conjugate mirrors, double-pumped phase-conjugate mirrors, two-wave mixers, and nonsteady-state photo-induced EMF sensors. The latter has the novel property of combining real-time holographic wavefront-matching with photoconductive detection, all in a single nonlinear optical element. The elegance of all these devices is that they proceed via "all-optical" techniques and do not require computationally intensive processing.