Experimental demonstration/analysis of fiber-bundle-based receiver performance

In order to track, acquire and maintain a free-space optical link between mobile platforms experiencing misalignment due to movement and atmospheric turbulence requires a different approach than traditional free-space optical transceivers. Recently, a fiber-bundle approach for beam steering at the transmitter was proposed and investigated that allowed tracking ot the receiver without the use of mechanical devices. A complimentary receiver using a collection of fibers behind an array of high power, small diameter lenses was found to allow lateral misalignments of up to 3 cm for a 1.2 cm array and up to 45 degrees of angular misalignment between the transmitter and receiver optical axes. This paper investigates methods for optimizing the receiver design, particularly in terms of maximizing throughput of the optical power to the electronic receiver. Theoretical and experimental analyses are used to examine two significant issues and suggest solutions. Maintaining alignment accuracy between the lens array and the collecting fibers is addressed by using a collimator array, thereby fixing each fiber to one lens, or specially constructed fiber array structures in a dedicated housing. A collimator array is preferable for ease of construction but presents trade-offs with respect to power collected and misalignment tolerance. Losses incurred when combining the signals from the many fiber elements is addressed using couplers, optical combining systems, and electronic summing. The advantages and difficulties of the methods are compared with regard to practical implementation.