A compact and fast-response wavelength monitor is described that can determine the wavelength of individual laser pulses with a resolution of a few pm. It combines a position-sensitive photo detector with an optical coating that converts the wavelength information of the incident light into a spatial intensity distribution on the photo detector. Differential read-out of the photo detector is used to determine the centroid of this distribution. Wavelength change between individual laser pulses is detected as a shift of the centroid of the spatial light distribution on the detector. The wavelength monitor is demonstrated with results from a wavelength-tunable fiber laser that can produce randomly accessible sequences of laser pulses.
Free-space laser communication has been demonstrated with application potential in many areas such as line-of-sight communications, satellite communications and the last mile solution in a fiber optics networking. Both 0.8 and 1.5 micron wavelengths are currently used in state-of-the-art free space laser communication systems; unfortunately the system performance is imposed by atmospheric turbulence. To reduce the atmospheric effect in free-space laser communication systems, several techniques have been used, such as adaptive optics, aperture averaging and multiple transmitters; however, significant improvement has not been achieved. Theoretically, the seeing effect may be released using a longer wavelength. In this paper, we present a 3.5 micron free-space laser communication system model and its system performance evaluation. A 3.5 micron propagation model based on MODTRAN simulation results in different weather patterns is presented first, and a propagation link budget system model is described after that. The propagation channel performance evaluation results are presented by means of bit error rate versus various propagation distances.