Stephen J. Brosnan

Coherently coupled high-power fiber arrays

A four-element fiber array has demonstrated 470 watts of coherently phased, linearly polarized light energy in a single far-field spot. Each element consists of a single-mode fiber-amplifier chain. Phase control of each element is achieved with a Lithium-Niobate phase modulator. A master laser provides a linearly polarized, narrow linewidth signal that is split into five channels. Four channels are individually amplified using polarization maintaining fiber power amplifiers. The fifth channel is used as a reference arm. It is frequency shifted and then combined interferometrically with a portion of each channels signal. Detectors sense the heterodyne modulation signal, and an electronics circuit measures the relative phase for each channel. Compensating adjustments are then made to each channels phase modulator. This effort represents the results of a multi-year effort to achieve high power from a single element fiber amplifier and to understand the important issues involved in coherently combining many individual elements to obtain sufficient optical power for directed energy weapons. Northrop Grumman Corporation and the High Energy Laser Joint Technology Office jointly sponsored this work.

Coherently coupled high power fiber arrays

A four-element fiber array has demonstrated 470 watts of coherently phased, linearly polarized light. The results of this experiment as well as comparisons to other fiber array approaches will be presented

8-W coherently phased 4-element fiber array

A four-element fiber array has been constructed to yield 8 watts of coherently phased, linearly polarized light energy in a single far field spot. Each element consists of a 2-W single-mode fiber-amplifier chain. Phase control of each element is achieved with a lithium-niobate phase modulator. A master laser provides a linearly polarized, narrow linewidth signal that is split into five channels. Four channels are individually amplified using polarization maintaining fiber power amplifiers. Frequency broadening of the signal is necessary to avoid stimulated Brillouin scattering. The fifth channel is used as a reference arm. It is frequency shifted and then combined interferometrically with a portion of each channels signal. Detectors sense the heterodyne modulation signal, and an electronics circuit measures the relative phase for each channel. Compensating adjustments are then made to each channels phase modulator. The stability of the optical train is an essential contributor to its success. A state-of-the-art interferometer was built with mountless optics. A lens array was constructed using nano-positioning tolerances, where each lens was individually aligned to its respective fiber to collimate its output and point it at a common far field spot. This system proved to be highly robust and handled any acoustic perturbations.

Beam cleanup and low-distortion amplification in efficient high-gain hydrogen Raman amplifiers

We report the results of a theoretical and experimental investigation of two important properties of Raman converters. The first property is Raman beam cleanup, which refers to the generation of high-quality Stokes beams in a Raman amplifier pumped by laser beams of poor spatial quality. The second is nearly distortion-free amplification of aberrated Stokes beams in a Raman amplifier pumped by good-quality laser beams. A parallel beam geometry was used with collinear pump and Stokes beams. In both cases, excellent energy-conversion efficiencies into the first Stokes order, of the order of 60%, were demonstrated at high amplifier gains of 100 to 1000. The spatial characteristics of the amplified beams were found to be in good agreement with model predictions.