We have constructed an all-fiber single-frequency amplifier system and an additional all-fiber amplified spontaneous emission (ASE) source system to simulate the effect of generated backward ASE on stimulated Brillouin scattering (SBS) in master oscillator power amplification (MOPA) system. By injecting the artificial ASE source backward into the Yb-doped MOPA system directly and measuring the evolutions of SBS threshold power with the increasing ASE power level, we can get the changes of SBS process during the amplification. Effects of counterpropagating ASE on SBS threshold decreasing were obvious: When there was no counter-propagating ASE being injected, the backscattered power started to rise very slowly. However, with the added artificial ASE increasing, the backscattered power departed from linear quite early. And the SBS threshold power was reduced to different levels with different power level of additional ASE. As the artificial counter-propagating ASE power increased from 0 to 275.9mW, the SBS threshold power was correspondingly dropped down from 4.75W to 3.35W. The 30% reduction of the SBS threshold can be a siginificant influence in a single-frequency fiber amplifier.
To suppress high order modes and improve the beam quality, an active self-imaging mode filter based on multimode interference and self-imaging effect is proposed in large mode area (LMA) fiber amplifier. With this filter structure, transverse mode competition and individual transverse mode power distributions in strongly pumped fiber amplifiers are theoretically demonstrated. Employing this mode selection technique in 30/400 LMA strongly pumped fiber amplifier, the percentage of the fundamental mode rises from 27.8% (without filter) to 96.3%. By the modal power decomposition, the M<sup>2</sup> parameter of beam quality decrease dramatically from 2.24 to 1.11 (0 relative phase) and from 3.01 to 1.24 (π/2 relative phase). This study provides a new method to achieve single mode in LMA fiber amplifier and this filter would be extended to larger mode area fiber amplifier to improve the beam quality.
In this paper, we demonstrate an ytterbium-doped all-fiber master-oscillator power amplifier (MOPA) system which uses a narrow-linewidth seed source, generating narrow-linewidth and high power continuous-wave output power at 1064nm. Our MOPA configuration system consist of three amplifier stages. We use single-mode Yb-doped fiber as the gain fiber in the first and second pre-amplifier stages, so it can keep good beam quality before entering the main amplifier stage. In order to raise the threshold of nonlinear effects, such as SBS and SRS, and to relieve heat effect, our high power system choose large mode area (LMA) fiber as the gain fiber in the main amplifier stage. For the sake of suppressing high-order modes in LMA fiber, we design novel watering cooling plates of different sizes, and using them in our main amplifier stage. By optimizing its structure, we get very good laser beam pattern on CCD at high power output. The beam quality factor (M<sup>2</sup>) was about 1.4 at 1.31 kW.
We demonstrated a passive phase locking of a seven-element 352 W all-fiber polarization-maintaining amplifiers array using an all-optical feedback loop. Every single channel has four-stage amplifiers and is seeded by a broadband master oscillator for stimulated Brillouin scattering free. The seven laser beams are tiled side by side into a hexagonal laser array with a high space duty ratio of 65% in the near field. When system is in closed-loop, a visibility more than 90% of coherent pattern in the far field is obtained. By using the all-optical feedback loop and more pump power, higher power scaling with high beam quality appears to be achievable in a coherent beam combination system.
In the paper, we reported a single-frequency(SF), single-mode(SM), linear-polarized ytterbium-doped all-fiber master
oscillator power amplifier (MOPA) system consisting of two stage amplifier which could be used in coherent detection.
With a pump power of 17.5W at 976nm, the system emitted up to 12.7 W of single-frequency radiation at a wavelength
of 1064 nm with the polarization-extinction ratio (PER) more than 35dB and signal-to-noise ratio (SNR) more than 40dB.
The final-stage amplifier operated with a high conversion efficiency of 73% and the gain of the whole amplifier achieved
31 dB, it has nearly diffraction-limited beam quality.
Polycrystalline ceramic Nd:YAG laser material enables new possibilities in designing the laser medium
with respect to dopant, size and geometry. In this paper, a 184W continuous-wave Nd<sup>3+</sup>-doped ceramic
Y<sub>3</sub>Al<sub>5</sub>O<sub>12</sub> (Nd : YAG) laser with optical to optical efficiency of about 44.3% has been developed. Laser output
power characteristics as well as the thermal lensing and birefringence properties of the ceramic laser rods were
investigated. The sample used in this experiment was a 3.5mm diameter, 87mm long ceramic YAG rod with
0.6% Nd<sup>3+</sup> concentration. And the end faces of the rod were flat and antireflection-coated at 1064 nm. The
pumping geometry used in this work was a diffusive optical cavity with narrow slits for side-pumping.