The experimental scheme comprised a phase-modulated (to negate Brillouin scatter), 1064-nm seed signal in a fiber integrated geometry that was split into three via a path-length-selectable delay line to enable coherent combination, with each beam then directed through individual phase modulators, polarization controllers, and amplifiers, before passing isolators and into the photonic lantern. The output power of the lantern was around 10 W to enable saturation of a final-stage Yb-doped, 25/400 (core/cladding diameters in µm), 0.065 NA (matching the output characteristics of the lantern fiber), 12-m-long amplifier, enabling kW average power levels. To achieve arbitrary mode excitation and system optimization, the number of input lines to the photonic lantern has to match the number of modes supported by the amplifier. The final-stage Yb amplifier supported the first three modes, i.e., LP01 and LP11 (o and e). A fraction of the output beam was sampled by a pinhole detector, the output of which fed a stochastic parallel gradient controller, dithering the associated polarization, phase, and amplitude of the individual seeds signals. By monitoring and optimizing the on-axis component, the LP01 mode was selected, which was the only supported mode of the system with an on-axis component.
Operation of the adaptive system was directly compared to a conventional fully fiber-integrated scheme where the final-stage amplifier was directly seeded by a conventional 10-W, single-mode, pre-amplified signal. For output powers in the range of 800 W (for pump powers ~1200 W), the output became unstable, with the output beam profile indicating the presence of higher order modes, and the on-axis signal in the time domain exhibited a strong (~50%) periodic modulation around 100 Hz repetition rate, although it should be noted that not all test final-stage amplifier fibers examined displayed the periodic instability, even up to pump power levels in excess of 1500 W. In comparison to the result above, the adaptive scheme fed from the photonic lantern demonstrated an increased efficiency in signal extraction and no periodic instability with output powers up to 1270 W at the maximum pump power of 1560 W. With the adaptive control system turned off, the output became unstable and modal distortion was observed.
The authors have clearly demonstrated the efficacy of their technique, but perhaps the greater impact will be achieved with further power scaling and an associated increase in the core diameter. However, as the authors indicate, this will necessarily have to be associated with a corresponding increase in system complexity in the numbers of adaptive channels and seed fibers in the photonic lantern.
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