Study On Active And Passive Coherent Combining Of Fiber Lasers In All-fiber Configuration

Fiber lasers have been widely applied in industry manufacture, medical equipments and defense for the advantages of high conversion efficiency, high beam quality, compactness and easy operation. However, the output power of single fiber laser is restricted by the nonlinear effects, thermal effect and so on. Coherent combining of fiber laser arrays can greatly improve the output power with beam quality maintained well, which becomes one of hot topics in laser field. Several beam combining techniques, mostly spatial configuration, are researched currently. The all-fiber coherent combining can strength the stability and practicability. In the present paper, the active and passive phasing all-fiber coherent combining of fiber lasers are studied and the main contents are as follows:Firstly, the development of the fiber laser and coherent combining are reviewed and the significance of the all-fiber coherent beam combining is elucidated. The theory of coherent combining using all-fiber configuration is analyzed in the paper. The phasing methods of passive phasing with optical feedback ring cavity and the active phasing in MOPA architecture are introduced and explained, respectively. By comparison of the current evaluation methods, root mean square phase error is chosen to evaluate the precision and stability of phasing techniques while the power in bucket is selected to evaluate the beam quality of combined output laser.All fiber ring-cavity coherent combining scheme is proposed and investigated. In low power level, four channels and eight channels of fiber lasers are passively phased using fiber coupler as the all-fiber combiner. The combining efficiency in four-channel combining is near theory limit, while the combining efficiency in eight-channel combining is 96.7%. The spectra, phase control bandwidth, polarization maintaining ability, far field beam intensity and the wavelength tunability of the scheme are investigated as well. Four channels of pulse laser with microsecond/ nanosecond pulse width are passively coherent combined, respectively, proving the ability of combining pulsed lasers. In higher power level, four channels of W-level fiber lasers are coherently combined using tapered fused bundle combiner, which validates the possibility of achieving high power laser using the all fiber ring-cavity coherent combining scheme.All fiber active phasing in MOPA architecture is also investigated in the paper. In low power level, four channels of fiber lasers are active phased stably and beam quality improves. The impact of the size of the pinhole and tapered fused bundle combiner on the active phasing and combined laser beam quality are studied separately. After improving the output power of each laser, active coherent combining of four 10W-level fiber laser in all-fiber configuration is realized stably.Finally, the power scaling and laser number scaling of the ring-cavity passively phased all fiber combining scheme and actively phased all fiber combining scheme with fiber combiner are investigated and compared.