Study On Coherent Beam Combination Technology Of Fiber Lasers

Fiber lasers have wide potential for industrial and military applications due to their essential characters such as high conversion efficiency, excellent beam quality, convenient heat management and compact configuration. The ultimate output power of single fiber laser is limited due to nonlinear effect, facet fracture and thermal lens. Meanwhile, coherent beam combination (CBC) of multiple fiber laser beams provide an effective approach for achieving high power laser beam with good beam quality. The high energy laser system (HELS) built on coherently combined fiber laser beam has distinct advantages in system cost, heat management and beam control when compared with traditional single-aperture type HELS. Coherent beam combination has been under intensive research in recent years.Coherent beam combination technology of fiber lasers is studied theoretically and experimentally in an all-round way. Five essential issues, i.e., the necessity and feasibility of CBC, the selection of proper experimental scheme and its realization, the method for evaluating the coherently combined beam quality and the propagation properties of coherently combined beam in the atmosphere, are studied and analyzed systematically. The paper begins with reviewing the history of CBC technology, the CBC of gas laser, chemical laser, diode laser, and solid-state laser is reviewed in detail, the recently developed CBC technology of fiber lasers is introduced to demonstrate the fast and amazing development of CBC of fiber lasers. Comparison and analysis on coherent beam combination, incoherent beam combination (IBC) and spectral beam combination (SBC) is presented.The necessity of CBC is studied systematically. Single-frequency radiation is demonstrated to be unnecessary in CBC in master oscillator power amplifier (MOPA) configurations both theoretically and experimentally, which avoids the power limitation due to the nonlinear effect induced by single frequency radiation. In addition, CBC has great advantages in power beaming properties when compared with IBC and SBC.With respect to oppugn proposed by the researchers when faced with the difficulty in CBC of high power fiber laser beams, the feasibility of CBC of high power fiber lasers is analyzed from time, spatial and frequency domains. It is shown that former analyzing the feasibility in only frequency domain is not comprehensive; there is requirement for single laser beam in time, spatial and frequency domains. The phase noise measurement results of high power fiber lasers are concluded and analyzed, and the influence of power fluctuation and wavefront distortion on the result of CBC is theoretically analyzed and experimentally studied. The feasibility of CBC of high power fiber lasers is predicted with respect to the performance of high power fiber laser and beam cleanup technologies.Quantitative analysis is performed on the CBC technologies in being, the feasibility and practicability in scaling to high-power and large-number is analyzed from the respects of combining efficiency, beam quality and scalability. It is revealed that the CBC using stochastic parallel gradient descent (SPGD) algorithm has the potential in scaling to a large number of lasers while adding no complexity to the whole system. In addition, CBC using SPGD algorithm is also the key technologies in APPLE, which is a new system configuration of high energy laser system presented by the US military. In this way, CBC using SPGD algorithm deserves to be studied in detail.Theoretical configuration of CBC using SPGD algorithm is built based on numerical computing. The feasibility of CBC using SPGD algorithm is validated by static simulation, and the parameter optimization is performed by numerical computing. The control bandwidth and its dependence on laser numbers are analyzed by dynamical simulation. Cross-grouped SPGD algorithm and adopting relative cost function are proposed to improve the system bandwidth and tolerance on laser power instability. Numerical calculation results show that the bandwidth for CBC of 16 and 36 lasers can be improved by a factor of 2.6 and 3.4 when Cross-grouped SPGD algorithm is employed, and the tolerance can be improved by a factor of 4~5 times under rapid laser power fluctuations when adopting relative cost function.Experimental system of CBC using SPGD algorithm is built in the lab. Six ten-watt-level fiber amplifiers are coherently combined with an output power of 73.5W is achieved, which is the highest record for CBC based on SPGD algorithm in both laser numbers and output power. CBC of up to sixteen fiber channels and CBC of fiber amplifier in intensive phase noise environment are carried out to validate the feasibility of scaling to large-number and high-power level. The combining efficiency for CBC of 16 lasers is 75% and the phase control error for the phase noise with the character frequency of 3.1 kHz is restricted to be withinλ/25. CBC of multi-wavelength and wide linewidth fiber amplifiers are carried out to demonstrate that single-frequency radiation is not required in CBC in MOPA configuration. The appropriate method for evaluating the beam quality of coherently combined beam is studied. Beam propagation factor (BPF) is proposed as a proper beam quality factor, the physical principle and the measuring technique in practical engineering are extensively analyzed. The influence of aperture truncation, beam jittering, duty ratio of the laser array, phase error, tilt error, intensity error and out-of-work, polarization error, coherence of single laser beam on BPF is studied in detail. Handy scaling laws are obtained by numerical computing, which present good reference for designing and analyzing CBC system.With respect to the practical properties of single laser and beam projection system, influence of beam orders, correlation length, projecting configuration on the beam quality in the receiving plane is studied in detail. The propagation model of conformal-truncated-partially-coherent-flattened laser array is presented, and the analytical propagation equations are deduced, which present good reference for evaluating the performance of CBC system.