Passive and delay-free wavefront correction using photonic crystals in receivers for optical free-space communications

We show that a photonic crystal can be designed as a passive field-of-view expander of optical receivers. In other words, light received at different angle within a given angular range is adjusted to propagate within a smaller angular range when exiting the device and illuminating the actual detector. We plan to employ that function in receivers for optical free-space communications and propose an approach for suppressing beam wander and scintillation of the focal spot on the detector due to the wavefront distortions. Since the alignment of the propagation direction is performed in a passive way, the beam adjustment to stabilize the coupling efficiency of the detector is delay-free. We describe here the idea of this approach in order to show how to utilize effectively the anomalous dispersion characteristics of photonic crystals. In our approach, the photonic crystal behaves like a homogeneous medium with a refractive index less than 1. We discuss the design of photonic crystal structure with such an optical characteristic, and first predict the propagation angle of the beam after passing through the crystal using the dispersion characteristic. Then this prediction is confirmed by electromagnetic analysis using the FDTD (finite-difference time-domain) method. Finally we present a simplified optical setup for the receiver.