Wavelength agile FSO receiver

The design of a free-space optical (FSO) transceiver suitable for the demands of establishing reliable communication between mobile nodes such as UAVs requires the solution of difficult problems related to pointing, acquisition and tracking. To accommodate the inevitably larger misalignment conditions encountered in mobile applications, the receiver must possess a large field of view (for angular misalignments), be tolerant of translations away from the optical axis of the receiver antenna, and maintain a large collecting area to realize a workable power budget for the overall link. The transmitter must be able to deliver the best quality beam, in terms of peak power and power distribution, in the direction of the receiver as possible. Ideally, the ability to tolerate misalignments due to mobility at the receiver and proper choice of parameters at the transmitter allows the system to also tolerate the effects of atmospheric turbulence and weather on the transmitted beam, which normally can cause signal fade or complete signal loss even in well-designed FSO systems between stationary platforms. One approach to turbulence mitigation is to use a wavelength diversity scheme. The optimum wavelength for transmission varies as such factors as turbulence strength, absorption, and weather effects such as rain and fog vary. An FSO system for mobile communication that is wavelength agile will perform better than one that uses a single wavelength. We have constructed a FSO transmitter and receiver based on optical fiber bundles and adapted the transmitter to be capable of wavelength diverse transmission. In this paper, we present an experimental investigation of the performance of the system as a function of transmission misalignment, turbulence, and weather for a wavelength diversity scheme, which consists of switching between multiple transmission wavelengths, for reducing the impact of turbulence. Three wavelengths, 850nm, 1310nm, and 1550nm, are emitted by one or more tran- mitting fibers, and the effects of turbulence and misalignment experimentally evaluated in an indoor environment. A system is designed to detect changes in transmission and switch the transmitter to the appropriate wavelength. The receiver retained the link for a reduced range of misalignment at all wavelengths without adjustments, indicating that adjustment of the receiver immediately after a wavelength changes was not necessarily required.