Average power of 1.1 kW from spectrally combined, fiber-amplified, nanosecond-pulsed sources

High-power, high-beam-quality spectral beam combination of six narrow-linewidth fiber amplifiers with two transmission diffraction gratings

We demonstrate 10 kW spectral beam combination of six narrow-linewidth fiber amplifiers by two transmission gratings with a combining efficiency about 90%. The wavelengths of incident beams range from 1056 to 1088 nm. Each fiber amplifier delivers 2 kW output power laser with 0.25 nm root-mean-square (RMS) linewidth. A dual-grating beam-combining system is constructed to combine the six beams into a 10 kW level high-power beam and control the beam quality factor ${{\rm M}^2} \lt {2}$M²<2 by dispersion compensation. To the best of our knowledge, this is the highest output power combined by transmission gratings, which verifies the feasibility of transmission grating under high-power density laser radiation and provides an alternative approach for high-power beam combining.

High-efficiency polarization-independent wideband multilayer dielectric reflective bullet-alike cross-section fused-silica beam combining grating

A high-efficiency polarization-independent wideband multilayer dielectric reflective bullet-alike cross-section (combination of trapezoidal-rectangular grating profile) fused-silica beam combining grating (BCG) used in the -1st order for spectral beam combining (SBC) is designed and fabricated. Exact grating profile parameters are optimized by using the rigorous coupled-wave analysis and simulated annealing algorithm. As a comparison, traditional pure trapezoidal and pure rectangular gratings are also designed. Numerical results show that such a bullet-alike cross-section BCG exhibits wide bandwidth with the lowest maximum electric field enhancement in the grating material, which is greatly beneficial for the promotion of the power scaling level of the grating-based SBC system. A two-step dry-etching procedure is developed to fabricate such a grating. The averaged diffraction efficiency of greater than 91% was experimentally demonstrated.

Stimulated Brillouin scattering suppression of thulium-doped fiber amplifier with fiber superfluorescent seed source

We investigate the stimulated Brillouin scattering (SBS) effect in high-power thulium-doped fiber amplifier seeded with a narrow-linewidth fiber superfluorescent source or a conventional narrow-linewidth fiber laser. No random backward SBS pulses are observed when using a narrow-linewidth fiber superfluorescent source as the seed. The corresponding average power and peak power reach 153 W and 3.4 kW, respectively, only limited by the available pump power. This gives the average power and peak power extraction from the thulium-doped fiber amplifier with 17 fold enhancement, in comparison with the situation using the conventional narrow-linewidth fiber laser with similar central wavelength and spectral linewidth as the seed. This work indicates that using low coherent fiber superfluorescent sources is a good solution for power scaling in narrow-linewidth fiber amplifier system in order to overcome the limitation of SBS effect.

Coupling efficiency model for spectral beam combining of high-power fiber lasers calculated from spectrum

We utilize the spectral broadening of Yb-doped fiber lasers in the spectral beam combining scheme to develop an analytical model of the coupling efficiency, which forms a critical factor in evaluating the practicality of the beam combination system. The simulation results predict a trend similar to the measured ones. Via increasing the number of simulating lasers, the model can be extended to calculate the combining efficiency of the resulting multiple-beam-combination system and estimate the optimal output power and combining efficiency. Moreover, the analytical model is suitable to investigate key parameters of Yb-doped lasers and filters, which is beneficial in enhancing the combining efficiency.

10.8 kW spectral beam combination of eight all-fiber superfluorescent sources and their dispersion compensation

We report an 8-element spectral beam combination of Yb-doped all fiber superfluorescent sources around 1070 nm wavelength. Each source consists of a 60 mW front-end and a 1.5 kW three-stage fiber amplifier chain. The eight output beamlets are spectrally combined using a home-made polarization-independent multilayer dielectric reflective diffraction grating. 10.8 kW output power is achieved with an efficiency of 94%. Besides, both theoretical and experimental studies of dual grating dispersion compensation scheme have been performed, which is proved to be a prospective way for high brightness spectral beam combination.

Optical properties and Faraday effect of ceramic terbium gallium garnet for a room temperature Faraday rotator

The optical properties, Faraday effect and Verdet constant of ceramic terbium gallium garnet (TGG) have been measured at 1064 nm, and were found to be similar to those of single crystal TGG at room temperature. Observed optical characteristics, laser induced bulk-damage threshold and optical scattering properties of ceramic TGG were compared with those of single crystal TGG. Ceramic TGG is a promising Faraday material for high-average-power YAG lasers, Yb fiber lasers and high-peak power glass lasers for inertial fusion energy drivers.

Coherent addition of fiber-amplified ultrashort laser pulses

We report on a novel approach of performance scaling of ultra-fast lasers by means of coherent combination. Pulses from a single mode-locked laser are distributed to a number of spatially separated fiber amplifiers and coherently combined after amplification. Splitting and combination is achieved by polarization cubes, i.e. the approach bases on polarization combining. A Hänsch-Couillaud detector measures the polarization state at the output. The error signal (deviation from linear polarization) is used to stabilize the synchronization of different channels. In a proof-of-principle experiment the combination of two femtosecond fiber-based CPA systems is presented. A combining efficiency as high as 97% has been achieved. The technique offers a unique scaling potential and can be applied to all ultrafast amplification schemes independent of the architecture of the gain medium.

100 W all fiber picosecond MOPA laser

A high power picosecond laser is constructed in an all fiber master oscillator power amplifier (MOPA) configuration. The seed source is an ytterbium-doped single mode fiber laser passively mode-locked by a semiconductor saturable absorber mirror (SESAM). It produces 20 mW average power with 13 ps pulse width and 59.8 MHz repetition rate. A direct amplification of this seed source encounters obvious nonlinear effects hence serious spectral broadening at only ten watt power level. To avoid these nonlinear effects, we octupled the repetition rate to about 478 MHz though a self-made all fiber device before amplification. The ultimate output laser exhibits an average power of 96 W, a pulse width of 16 ps, a beam quality M2 of less than 1.5, and an optical conversion efficiency of 61.5%.

Spectral beam combination of fiber amplified ns-pulses by means of interference filters

In this paper we introduce a simple scheme to spectrally combine four single beams using three low-cost dielectric interference filters as combining elements. 25 ns pulses from four independent and actively Q-switched fiber seed-sources are amplified in a single stage fiber-amplifier. Temporally and spatially combined 208 W of average power and 6.3 mJ of pulse energy are obtained at two different repetition frequencies. A detailed analysis of beam quality as well as the thermal behavior of the combining elements is carried out and reveals mutual dependency.

High-power nanosecond ytterbium-doped fiber laser passively synchronized with a femtosecond Ti:sapphire laser

We report on laser synchronization between a high-power nanosecond fiber laser and an ultrashort Ti:sapphire laser by cross-absorption modulation and power amplification. The nanosecond pulses were generated by a long-cavity ytterbium-doped fiber laser that was passively synchronized to a 70 fs Ti:sapphire laser with a remarkable cavity mismatch tolerance of about 8 cm as a result of resonance-enhanced cross-absorption modulation in additional rare-earth-doped fiber. By using a two-stage Yb-doped fiber preamplifier and a two-stage double-clad fiber power amplifier in cascade, the synchronized nanosecond pulses were amplified to 131 W of average power, corresponding to 0.55 mJ of single-pulse energy at a repetition rate of 240 KHz, with a timing jitter of 13 ps.

Tunable flat-top nanosecond fiber laser oscillator and 280 W average power nanosecond Yb-doped fiber amplifier

We demonstrate a nanosecond fiber laser with tunable central wavelengths and narrow spectral bandwidths suitable for high-power amplification. Spectrally narrowband flat-top nanosecond pulses were generated at the fundamental repetition rate of 1.9 MHz in an Yb-doped fiber laser, which could be tuned in central wavelength from 1033 to 1053 nm by changing the nonlinear polarization rotation in the fiber laser cavity. In particular, such flat-top nanosecond pulses could be tuned around 1030 nm to match the gain bandwidth of ytterbium-doped double-clad fibers or 1053 nm to match the maximum gain in Nd-doped phosphate glass. The pulse duration could be changed from 1 to 15 ns by varying the pump power or laser polarization evolution in the cavity. By using an ytterbium-doped single-mode fiber preamplifier and a two-stage large-mode-area Yb-doped double-clad-fiber power amplifier, 280-W average power with pulse duration of 3 ns was obtained at 1034 nm.