We experimentally studied the coherent beam combining characteristics of fiber laser arrays in all-fiber passive
configurations using polarization maintaining fibers. In addition, we simulated the coherent performance by including
fiber nonlinearity. The beam combining performance is affected by both optical feedback and the laser cavity length
difference. In addition, Kramers-Kronig and n2 induced nonlinearity plays an important role for the coherent phase
locking. We describe the scalability of the coherent array to high power via scaling of laser power and fiber count. We
show coherently combined output powers of 27.4 Watts and 12.2 Watts at 1083 nm in 2-laser and 4-laser arrays.
Passive combining of fiber oscillators may one day lead to a simple power scaling capability that multiplies
the best results obtained by single devices. Along with HRL's unique approach and early experimental
demonstrations, some extensive modeling is underway to sort through the current controversies in this area,
to design for large numbers of fibers, and to create the most stable inphase beam output.
We report a laser link that can correct atmospheric aberrations. We use a fiber collimator array, fed by a master oscillator with multiple fiber amplifiers (MOMA), and accomplish phase adjustment via pump diode current control. Each of seven channels is tagged by a different 1-20 kHz diode current dither. At the receiver, each channel's phase information is extracted from the <50 kHz signal. Our measurements show 5 kHz phase adjustment capability, so even turbulence-induced aberrations, as well as typical atmospheric aberrations (< 200 Hz) can be corrected. Only in >~100 km-range scenarios is the correction bandwidth limited by light's travel time. The low dither frequencies and amplitudes do not interfere with the typically GHz laser communications signal. Importantly, our system reduces transmitter power requirements by correcting small pointing errors and atmospheric-path aberrations. Of course the multiple-fiber amplifier array also enables power scaling. We describe our near- and far-field beam measurements in the laboratory.
Self-organized coherence between fiber lasers has been reported both via all-fiber 2x2 directional coupler trees and in spatially multi-core fibers. We have taken this a major step forward, coupling together a number of independent fiber lasers to obtain a spatially and spectrally coherent far field, with no active length, polarization, or amplitude control. The near field output comes from a spatial array rather than from a single fiber, making this approach scalable to extremely high power.
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