Advanced Solid-State Lasers 2019: focus issue introduction.

This joint issue of Optics Express and Optical Materials Express features 17 state-of-the art articles written by authors who participated in the international conference Advanced Solid-State Lasers held in Vienna, Austria, from September 29 to October 3, 2019. This introduction provides a summary of these articles that cover numerous areas of solid-state lasers from materials research to sources and from design to experimental demonstration.

Suppression of stimulated Brillouin scattering in high power fibers using nonlinear phase demodulation.

We demonstrate a modulation approach that relaxes the limitations imposed by stimulated Brillouin scattering (SBS) on amplification and propagation of narrow-linewidth light in fibers. By imposing synchronous amplitude and phase modulation on an input field, the optical spectrum after high-power fiber transmission is compressed using nonlinear self-phase modulation. This effectively reduces the SBS interaction length and increases the SBS threshold, enabling narrower linewidths. Using this technique, we demonstrate >2 × increase in SBS-limited spectral brightness from a kW-class amplifier. We show that SBS suppression becomes more effective for higher powers and longer fibers.

Focus issue introduction: advanced solid-state lasers.

This joint issue of Optics Express and Optical Materials Express features 28 state-of-the-art articles written by authors who participated in the international "Advanced Solid State Lasers" conference, held in Boston November 4-8, 2018. This review provides a summary of these articles that cover the spectrum of solid state lasers from materials research to sources and from design innovation to applications.

Atmospheric propagation and combining of high power lasers: comment.

Nelson et al. [Appl. Opt.55, 1757 (2016)APOPAI0003-693510.1364/AO.55.001757] recently concluded that coherent beam combining and remote phase locking of high-power lasers are fundamentally limited by the laser source linewidth. These conclusions are incorrect and not relevant to practical high-power coherently combined laser architectures.

Automated co-alignment of coherent fiber laser arrays via active phase-locking.

We demonstrate a novel closed-loop approach for high-precision co-alignment of laser beams in an actively phase-locked, coherently combined fiber laser array. The approach ensures interferometric precision by optically transducing beam-to-beam pointing errors into phase errors on a single detector, which are subsequently nulled by duplication of closed-loop phasing controls. Using this approach, beams from five coherent fiber tips were simultaneously phase-locked and position-locked with sub-micron accuracy. Spatial filtering of the sensed light is shown to extend the control range over multiple beam diameters by recovering spatial coherence despite the lack of far-field beam overlap.

Coherent combining of pulsed fiber amplifiers in the nonlinear chirp regime with intra-pulse phase control.

Two high pulse contrast (> 95 dB) polarization maintaining all-fiber amplifier chains were coherently combined to generate 0.42 mJ, 1 ns 25 kHz pulses with 79% efficiency despite 38 radians of intra-pulse phase distortion. A recursive intra-pulse phase compensation method was utilized to correct for the large nonlinear chirp providing a path for improved coherent waveform control of nanosecond pulse trains.

Group delay locking of coherently combined broadband lasers.

We demonstrate a method for single-detector coherent sensing and automated coalignment of group delays in a coherently combined laser array, enabling robust coherent combining of broadband sources despite initial path mismatches exceeding the laser coherence length. The method is based on Fourier-domain filtering of the coherently combined laser beam to extract error signals, and it is equally applicable to controlling both spatial and temporal misalignments.

Multichannel polarization stabilization for coherently combined fiber laser arrays.

We demonstrate a simplified approach toward active polarization control in coherently combined laser architectures. By leveraging optical phase dithers applied by a phase controller, polarization error signals are generated for an entire laser array from a single beam sample of the combined output, enabling closed-loop polarization locking of non-polarization-maintaining fibers. The concept is shown to be compatible with both hill-climbing and synchronous multidither phase control methods. Simultaneous phase locking and polarization locking was demonstrated for a five-fiber array with >99% phasing efficiency and >20 dB polarization extinction ratio.

Diffractive coherent combining of a 2.5 kW fiber laser array into a 1.9 kW Gaussian beam.

Five 500 W fiber amplifiers were coherently combined using a diffractive optical element combiner, generating a 1.93 kW beam whose M(2)=1.1 beam quality exceeded that of the inputs. Combining efficiency near 90% at low powers degraded to 79% at full power owing to thermal expansion of the fiber tip array.

Two-dimensional diffractive coherent combining of 15 fiber amplifiers into a 600 W beam.

We demonstrate coherent beam combining using a two-dimensionally patterned diffractive optic combining element. Fifteen Yb-doped fiber amplifier beams arranged in a 3×5 array were combined into a single 600 W, M²=1.1 output beam with 68% combining efficiency. Combining losses under thermally stable conditions at 485 W were found to be dominated by spatial mode-mismatch between the free space input beams, in quantitative agreement with calculations using the measured amplitude and phase profiles of the input beams.

Perturbative analysis of coherent combining efficiency with mismatched lasers.

Coherent combining efficiency is examined analytically for large arrays of non-ideal lasers combined using filled aperture elements with nonuniform splitting ratios. Perturbative expressions are developed for efficiency loss from combiner splitting ratios, power imbalance, spatial misalignments, beam profile nonuniformities, pointing and wavefront errors, depolarization, and temporal dephasing of array elements. It is shown that coupling efficiency of arrays is driven by non-common spatial aberrations, and that common-path aberrations have no impact on coherent combining efficiency. We derive expressions for misalignment losses of Gaussian beams, providing tolerancing metrics for co-alignment and uniformity of arrays of single-mode fiber lasers.

Active phase and polarization locking of a 1.4 kW fiber amplifier.

A three-stage Yb-fiber amplifier emitted 1.43 kW of single-mode power when seeded with a 25 GHz linewidth master oscillator (MO). The amplified output was polarization stabilized and phase locked using active heterodyne phase control. A low-power sample of the output beam was coherently combined to a second fiber amplifier with 90% visibility. The measured combining efficiency agreed with estimated decoherence effects from fiber nonlinearity, linewidth, and phase-locking accuracy. This is the highest-power fiber laser that has been coherently locked using any method that allows brightness scaling.

Low-phase-noise, single-frequency, single-mode 608 W thulium fiber amplifier.

A chain of four Tm-doped fibers amplified a single-frequency, 2040 nm diode laser to 608 W with M(2)=1.05+/-0.03, limited by available pump power. Stimulated Brillouin scattering limits were investigated by splicing different lengths of passive fiber to the output of the final amplifier stage. Integrated rms phase noise above 1 kHz was less than lambda/30, suggesting the possibility of further scaling via coherent beam combining. To our knowledge, this is the highest power obtained from any single-frequency, single-mode fiber laser.

Diffractive-optics-based beam combination of a phase-locked fiber laser array.

A diffractive optical element (DOE) is used as a beam combiner for an actively phase-locked array of fiber lasers. Use of a DOE eliminates the far-field sidelobes and the accompanying loss of beam quality typically observed in tiled coherent laser arrays. Using this technique, we demonstrated coherent combination of five fiber lasers with 91% efficiency and M2=1.04. Combination efficiency and phase locking is robust even with large amplitude and phase fluctuations on the input laser array elements. Calculations and power handling measurements suggest that this approach can scale to both high channel counts and high powers.