Coherent beam combination is an effective way to develop high-power lasers with high beam quality and high brightness. Coherent combination of six solid-state lasers based on the technique of mutual injection phase locking by using the natural coherent combination property of corner cube is first investigated. The coherent combination with 15.3 J of output energy, 1.7 mrad of divergent angle is obtained, and the combining efficiency is as high as 95.6% at 10 Hz and 85 A. The far-field profile is flattened protuberance.

Mutual injection phase locking coherent combination of solid-state lasers based on corner cube.

Coherent combination of mutual injection phase-locked fiber lasers with a corner cube reflector

A pair of special diffractive optical element (DOE) is used in the combining of six Nd:YAG lasers to obtain flat-top output beam. The function of this pair of DOE in the system has been discussed. The property of output beam distribution has been analyzed and simulated. Both the measured far field and near field intensity distributions match the simulated results. Combining output energy of 15.3J is obtained, and the transfer efficiency of the DOEs is about 81%.

Investigation of combining output a uniform flat-top beam of six solid state lasers by DOE

A corner cube (CC) as a peculiar coherent combination element is first, to the best of our knowledge, theoretically and experimentally proved by the authors. When a CC is used as a total-reflecting mirror in the solid-state laser resonator it can improve the laser far-field energy focalization. Furthermore, the differences between the coherent characteristics of the lasers with a corner cube resonator (CCR) and those with a Fabry-Perot resonator have been investigated, respectively. Theoretical calculation and numerical simulation have proved that the symmetric output beams of the CCR laser are coherent and the adjacent output beams are partially coherent. Based on these special coherent characteristics, a new laser coherent combining configuration, in which a CC was utilized as a total-reflecting mirror, was proposed and experimentally investigated. In our experiments, the measured far-field intensity profiles of coherent combing laser arrays are in good agreement with the numerical simulation. These novel coherent characteristics of a CC may be important for applications in solid-state lasers and laser coherent combining systems, and coherent combination may be one of the development trends and future research directions for CCR lasers.

Coherent characteristics of solid-state lasers with corner cubes

High Coupling Efficiency of the Fiber-Coupled Module Based on Photonic-Band-Crystal Laser Diodes

Coherent laser beam combining is potentially attractive way to increase the combined beam brightness beyond the limits imposed on single‐mode lasers by technological bounds. The active control of every individual laser beam characteristics is more flexible but essentially more complicated in both, necessary equipment and service. Passive phase locking is an attractive alternative, since it does not need external management and leads to strong simplification of the system. A specific feature of fiber amplifiers and lasers is that they possess optical path differences (OPD) of many wavelengths magnitude. Cold‐cavity theory predicts in this case fast decline in efficiency of coherent fiber laser beam combining with number of lasers. Experiments, in contrast, demonstrated in such systems that high degree of phasing takes place for laser arrays of up to 16 lasers. As lasers are strong non‐linear systems, explanation of this discrepancy should rely on a role of non‐linear effects: gain saturation and intensity‐dependent index. Besides, since the gain band width is significantly broader than the distance between spectral lines responding to different longitudinal modes, it is a freedom in adjusting laser wavelength to a value, which corresponds to a best balance between gain and loss of laser radiation. As a first step, we consider a fiber laser array with external global coupling, which means that the same fraction of the combined laser beam is returned into the each element of the array. In this case, every laser in the array is operated as an injection controlled (slave) laser. The specific features of Yb‐doped fiber lasers were taken into account in our model: 1) existence of multiple longitudinal modes; 2) typically low‐Q cavity used in these lasers. This approach allows us to quantify the mechanism of laser wavelength self‐adjustment taking into account the effect of gain saturation. Taking the injection signal intensity within limits of locking range, the output signal was studied as a function of wavelength detuning and small signal gain magnitude. Then the maximal phase locking efficiency is found numerically as a function of coupling strength and of optical pumping intensity at random values of the OPD for laser arrays of variable size. Just the gain saturation effect taken into account in our model leads to comparatively slow reduction of the maximal phase locking efficiency with the laser array size.

N. N. Elkin

Papers

Analysis of Problem of High Power Fiber Laser Combining for Arbitrary Large Optical Phase Differences