Beamed-energy propulsion: optical phase noise in 1064nm fiber amplifiers

Effective transfer of beamed optical energy to a spacecraft requires that the source have a sufficiently small angular divergence to contain the beam within the area of the spacecraft collector. Such a directed energy system could be achieved by coherently combining many small optical sources distributed throughout a large-scale array. The fundamental unit of such a system is a master oscillator power amplifier (MOPA) which consists of a frequency stabilized laser source distributed to amplifier trees in the array. A fundamental challenge in long baseline coherent beam combination is that each source be combined with sub-wavelength accuracy over the entire array. In addition to perturbations due to mechanical and atmospheric disturbances, phase noise introduced by the amplifiers and the seed distribution network must also be accounted for in order to achieve the necessary accuracy. This work investigates the excess phase noise introduced by the amplifier stages and fiber optic links. Locking schemes that could be used to synchronize such an array are presented. The test bed used to interrogate phase noise is based on an all polarization maintaining fiber Mach-Zehnder interferometer (MZI), controlled by an FPGA based digital quadrature detection at a sampling rate of 1 MS/s yielding direct measurement of amplitude and phase with servo control for phase locking. Results for various MOPA and fiber link configurations based on Yb-doped fiber amplifiers operating at 1064 nm and kilometer scale link lengths are presented.