| Compared to common double-cladding linear cavity high power fiber laser, MOPA (Master Oscillator Power Amplifier) fiber laser system has many advantages such as flexible structure, high efficiency, small bulk, light weight, high beam quality, et al. With the development of fiber lasers, both high power and high beam quality are required in many application areas such as high-precision sensing, medical treatment, industry processing, free space laser communication and military weapons. In the future, the developing trend of MOPA fiber laser system will be all-fiber, high power and high beam quality, and the research on all-fiber MOPA laser system will be focused on two directions. The one is the selection, design and characterization of gain fibers used in the amplifier stage; the other one is study on the key techniques of high quality seed fiber laser fabrication. In this thesis, the main work includes the characterization of residual stress (RS) and refractive index (RI) in gain fibers used in the amplifier stage and study on the key techniques of high quality seed fiber laser fabrication. The innovations of the thesis are as follows:1. Using a state-of-the-art concurrent three-dimensional RS and RI characterization technique, for the first time, the RS and RI in large-mode-area single-mode erbium-doped fibers (LMA-SM-EDFs) were characterized concurrently. The effects of fiber manufacturing, cleaving and arc fusion splicing for the LMA-SM-EDF properties were measured and studied carefully. The research results will contribute to the design, manufacturing and applications of large-mode-area single-mode gain fibers, and be useful to the fabrication and improvement of the high power all-fiber lasers and amplifiers.2. For the first time, the arc fusion splicing effects in large-mode-area single-mode ytterbium-doped fibers (LMA-SM-YDFs) were characterized carefully. The obtained results proved that the splicing-induced RS and RI perturbations, which were often ignored before, could significantly affect the fiber coupling and mode propagation properties of the LMA-SM-YDFs. The effects were also investigated by using the simulation results based on the experimental data. This research indicated that the mode field diameter of the LMA-SM-YDF could be changed by as much as39.6%by the arc fusion splice, and the core/cladding RI difference could be changed by as much as75.8%by the release of the RS and frozen-in viscoelasticity and the diffusion of the heavy concentration doped ytterbium ions.3. Based on the theoretical and experimental study on fiber Bragg grating (FBG) and Fabry-Perot (F-P) filter, a single-frequency and single-polarization erbium-doped fiber laser was proposed. The fiber laser could well operate in stable single-frequency state. By simply stretching the FBQ the switchable operation between four wavelengths were achieved. During5h measurement, the. fluctuations of wavelength and output power were less than0.01nm and0.04dB respectively. The degree of polarization (DOP) was close to100%and the optical signal to noise ratio (OSNR) was better than60dB. The characteristics of the proposed fiber laser will be improved if the F-P filter used is replaced by a chriped Moire FBG F-P filter.4. Through the theoretical analysis on single-longitudinal-mode selection using compound cavity structure, three single-wavelength fiber lasers based on different cavity structures, which are cascaded asymmetric three-cavity structure, parallel asymmetric four-cavity structure and hybrid compound cavity structure, were proposed. The former two structures gave two simple and low-cost methods to achieve single-frequency fiber lasers with good beam quality and high stability. The hybrid compound cavity structure was composed of a long active linear-cavity and a short passive subring cavity; Using the umpumped-EDF-based saturable absorber, meanwhile, the fiber laser achieved single-frequency operation with very high beam quality, whose many output parameters had been better than the similar fiber lasers reported. During1h measurement, the wavelength and power fluctuations were less than0.7pm and0.07dB respectively. The laser linewidth was as narrow as650Hz, the OSNR was better than67dB, the DOP was close to100%and the polarization extinction ratio (PER) was as high as30.57dB. By using special FBG recoating technique, the fiber laser achieved a wavelength-tunable range of as wide as8.1nm.5. A switchable and wavelength-spacing-tunable dual-wavelength single-freque-ncy fiber laser based on a hybrid compound three-cavity structure was proposed. In dual-wavelength operation, the wavelength-spacing could be tuned form0.430nm to4.487nm and the fiber laser could be used to fabricate the microwave signal generator; in single-wavelength operation, the fiber laser could not only work at the fixed wavelength of1544.108nm (λ1), but also work at the wavelength with a tuning range from1544.538nm to1548.595nm (A2); the linewidths of two wavelengths were as norrow as760Hz and810Hz respectively, and the PER for any operation mode was better than30dB. Additionally, the fiber laser possessed high wavelength and power stabilities and high OSNR.6. A switchable and wavelength-spacing-tunable dual-wavelength single-freque-ncy fiber laser based on a novel ring compound cavity structure was proposed. The compound cavity has excellent mode selection capability. In dual-wavelength operation, the wavelength-spacing could be tuned form0to4.83nm and the laser output could be used to generate the microwave signal with a tuning range from0to606GHz; in single-wavelength operation, the fiber laser could work at the fixed wavelength of1543.65nm (λ1) or oscillate with a wavelength-tuning range from1543.65nm to1548.48nm (λ2); the linewidths of two wavelengths were550Hz and600Hz respectively, and the PER for each wavelength was higher than21dB. |
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