Study On The Technology Of High Power All-fiber Lasers And Their Key Components

Constructed with components such as fiber, pump combiners and fiber Bragggratings, high-power all-fiber lasers are of many unique advantages, such as small size,easy cooling, high efficiency, and outstanding beam quality, etc. These superioradvantages have made fiber lasers become very competitive in many applications inmaterial processing, marking, medicine, range finding, free space communication, andsecurity, etc. Recent years, research works on high-power all-fiber lasers are mainlyfocused on increasing pump power, suppressing the optical and thermal damage andnonlinear effects, and also, upgrading output power by use of combining technology. Inan attempt to construct a stable high-power all-fiber laser system with high outputpower and high beam quality, this dissertation present theoretical and experimentalstudy on high-power all-fiber lasers and their key components. The beam combining ofall-fiber laser array is also the main concerned work in this dissertation.Firstly, a MOPA all-fiber laser is experimentally studied and1000W output poweris obtained. In the experiment, tough tasks of splicing all-fiber components, suppressinghigh-order modes and thermal managing of laser system are tackled. The monolithicfiber laser has an optical-to-optical conversion efficiency of up to64%, and the centralwavelength is at1081nm with the spectral FWHM being2nm.Secondly, a novel mutual injection technique is proposed. By using the proposedtechnique, phase-locking and coherent combining of two high power all-fiber lasers arerealized and experimentally demonstrated. Steady interference strips with high visibilityof46%are observed. The coherent combined407W CW output power with apower-combining efficiency of up to98%is obtained.Moreover, partially coherent combining of a2×2all-fiber laser array isexperimentally studied, and up to925W high output power is obtained. The laser arrayconsists of two incoherent laser groups. Each group contains two all-fiber lasers which are phase-locked by using direct mode mutual injection method. The fiber laser array isconstructed by all-fiber components, thereby is monolithic and has stable performance.The efficiency of pump optical power converting to combined output power is up to57%. The beam-quality value BQ of the combined beam is1.95when the fill factor ofthe laser array is0.54.Finally, the key components of high-power all-fiber lasers, including LMA fibers,LMA fiber Bragg gratings, and high-power pump combiners, are studied experimentallyand theoretically. Based on general coupled-mode theory, numerical simulations areperformed to analyze the coupling characteristics of a new LMA gain-guided and indexanti-guided fiber (GG-IAG fiber) and the gain characteristics of the GG-IAG fiber laser.The results show that the coupling direction between the GG-IAG fiber and the generalindex guided fiber (IG fiber) can be changed by changing the real-valuedrefractive-index difference (real part of refractive-index difference between core andcladding, RVRID). To ensure the GG-IAG fiber laser in single-mode operation, the fiberlength and the reflectivity of the output mirror should be valued in their ranges. Thus,with a longer fiber and a smaller reflectivity of the output mirror, the laser obtains moresingle-mode output power.A matrix method is extended to solve the multimode coupling differentialequations, and thereby the spectral characteristics of the chirped Bragg gratings inlarge-mode-area fibers (LMA FBGs) are studied theoretically. Unlike those of Bragggratings in single mode optical fibers (SM FBGs), the reflection spectra of chirpedLMA FBGs contain self-coupling and co-coupling peaks of the existing modes. For thechirped LMA FBGs, the reflectivity decreases and the reflection peaks split. These splitscan be improved on some degree by Gaussian apodization function.By constructing the coupled mode equations, the mode-coupling characteristicsbetween the passive and active cores within a kind of composite structural fiber (CSF)are studied theoretically. And by use of steady-state rate equations and heat conductive equations, the gain characteristics and the temperature distribution of all-fiber laseramplifiers based on CSF are calculated and analyzed numerically. The results show thatthe mode-coupling characteristics depend on the coupled modes and the radius and theseparation of the two cores of the CSF. Compared with the end-pumped fiber laseramplifiers, the amplifiers based on CSF have a slow pump light absorption andconversion, and thus, have low temperature distribution. This kind of newcoupling-pump technology provides a new approach to design higher power fiber lasersand fiber amplifiers.