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.
We report 0.95 kW average output power from a single cw-diode pumped Yb:YAG power oscillator. The 3-mm diameter solid-state laser rod is side pumped by three sets of cw diode arrays each of which has an electrical-to-optical efficiency of up to 50%. Our phase-conjugate master oscillator, power-amp architecture will incorporate this pump cavity as one of the power amplifiers for multi-kW average power, good beam quality laser applications.
Phase conjugation offers a practical, realistic approach for scaling solid-state lasers to high energies and high peak powers with a minimum increase in complexity. The present approach involves coherently combining the outputs of multiple parallel amplifiers in a single phase-conjugate oscillator-amplifier configuration. The use of phase conjugation can eliminate phase distortions that would otherwise result from individual amplifiers having optical lengths that differ from one another by many optical wavelengths. The laser output energy can be scaled well beyond the limits imposed by traditional volume constraints of crystalline media. Hence, instead of selecting a laser medium based solely on the available sizes, a laser system designer can base the medium selection on tradeoffs among many other important material parameters. Since an increase in output energy also requires an increase in the energy incident on the PCM, the energy scalability of PCMs based on SBS has been actively investigated over the past decade. These investigations have shown that, while practical single-cell PCMs offer somewhat limited energy scaling potential, a series combination of two Brillouin cells can accommodate energies of several tens of joules. We summarize our effort in developing such a dual-cell PCM that has achieved excellent performance at energies approaching 5 J and is scalable to even higher energies.