Path Length Matching and Phase control for Coherently Combined Fiber Laser Arrays

Laser Accelerator applications require fiber laser arrays that can be coherently combined to create diffraction limited outputs of both high energy pulses and these pulses at high repetition rates for high average powers. Fiber lasers can provide average power levels to the multi kW level yet are limited in pulse energy due to the onset of fiber nonlinearities. There is recently great interest in coherently combined fiber laser arrays where a single seed source is split many ways and directed into a parallel array of fiber amplifiers and then coherently combined into the far field either in a tiled configuration, a mirror structure or a diffractive optical element. Even lower power Accelerator applications, however, would require hundreds, even thousands of individual fiber lasers. A key element in these fiber laser arrays is the ability to match the path lengths from a number of parallel fiber laser channels. It is essential for efficient coherent combining that the relative path lengths be adjusted on the order of 2cm for gross matching of fiber lengths because the individual fiber amplifiers can be several meters in length, also path length matching to account for temperature changes which can be on the order of several millimeters, and finally path length matching on the order of fractions of a wavelength to account for the phase adjustments necessary for true coherent combination of the fiber laser channels. During Phase I, Optical Engines will, during this phase 1 program, through analysis, simulation, and experimentation will develop the necessary glass processing technologies to demonstrate a single channel low cost path length equalization device that will be under 3mm in diameter, have less than 1dB loss and will be able to equalizes phase and path length from fractions of a wavelength to 20mm with an over 1kHz bandwidth, while costing two orders of magnitude less than current path length matching, optical delay line devices. In addition, OEI will design the means for providing a coarse and sub um fine control in a complete packaged device. Optical Engines proposes to develop an entirely new low cost delay line concept by utilizing OEIĺs world class glass processing technology to micro-miniaturize the fiber collimators from discrete optical lenses to monolithic fiber based optics. Current techniques involve the combination of an electro-optic phase modulator with a path-length ôtrombone tunerö to alter the optical path length. These current path length tuners are large, expensive, and are unstable in alignment due to the precise nature of aligning in 5 dimensions. To address this challenge, OEI has developed specialized glass processing techniques to create spliced fiber collimators such that the 5-dimensional alignment problem can be reduced to a 1-dimension problems which can be handled with a simple capillary. Initial work has shown that these path length matching devices can be extremely small, rugged, and very low cost. We have also believed that these devices, can provide true path length equalization from cm to nm such that one device can be used to provide path length matching for a single channel to take coherent combining of fiber lasers from being a very complicated and costly endeavor to being very inexpensive and straight forward to make coherent combining much more widely employed. The OEI 1D delay line applies (with modifications) to any optical path length matching system that require cm range and sub-micron levels control and is especially useful for coherent combining of fiber lasers and other types of optical signal processing applications (for example, LiDAR and Coherent Digital Holography. This technology could be used, for example, to provide super high resolution airborne traffic mapping.