| High power narrow-linewidth fiber lasers have excellent characteristics such as ultra-long coherent length,narrow linewidth and low phase noise,which ensure the laser output with high spectral purity,low-intensity noise,and high beam quality.Its outstanding lasing performance mainly depends on the single longitudinal mode(SLM)fiber laser seed with high stability,and combined with the master oscillator power amplifier(MOPA),a technology that can further boost the output power of the seed laser while ensuring the output quality.The resulting narrow-linewidth lasing can satisfy the requirements for various applications in industry,military and scientific research.The research work of this thesis mainly focuses on two key directions of the MOPA laser system: the fabrication of high stability SLM thulium-doped fiber laser(TDFL)seeds and its key technologies,and the study of the nonlinear effects in the MOPA amplifier stage.The high stability single /multi-wavelength switchable SLM TDFLs are designed and fabricated.The troublesome nonlinear effect,transverse mode instability(TMI)in the current MOPA amplifier stage,is analyzed.The effect of bending-induced mode distortion on TMI is analyzed as well.The main innovative achievements are listed as follows:1.Theoretical analysis of the optical fiber coupler(OC)based compound ring cavity is carried out,and a numerical simulation method for analyzing this kind of compound ring cavity is proposed.The filtering performance of single-coupler single ring cavity and dual coupler single ring cavity structure is simulated and analyzed.Further,a new type of dual-coupler double-ring cavity and tri-coupler double-ring cavity structure is proposed,the output characteristics of them are simulated and the mode selection abilities are confirmed,which lays the foundation for the subsequent SLM selection in TDFL.2.A cascaded 3×3 symmetrical OC based passive tri-ring compound cavity SLM TDFL is proposed.The wavelength selection is realized by a homemade fiber Bragg grating(FBG).The experimental results show that the proposed laser can be lasing stably in the single-wavelength SLM state,the optical signal-to-noise ratio is about70 d B in the 2050 nm band,and the wavelength and power fluctuations within 100 minutes are within 0.02 nm and 0.453 d B,respectively.3.A six-wavelength switchable SLM TDFL based on cascaded sampled fiber Bragg grating(SFBG)and the novel passive double-ring compound cavity is proposed.The switchability of the fiber laser is realized by polarization-dependent loss modulation,and the wavelength tuning is realized by stretching SFBG.The constructed SLM TDFL can realize six switchable single-wavelength outputs in the2050 nm band.For each lasing wavelength,the OSNR is >71 d B,>72 d B,>62d B,>65 d B,>61 d B,>59 d B.The maximum optical power and wavelength fluctuation of the output laser is 0.56 d B and 0.01 nm,respectively.The linewidths of the three measured lasing wavelengths are 7.5 k Hz,8.5 k Hz,and 9.2 k Hz,respectively.Nine switchable dual-wavelength operations with different wavelength intervals and two triple-wavelength operations are also obtained.The obtained single wavelength tuning range is 4.83 nm.4.A five-wavelength switchable SLM TDFL is proposed.A homemade FBG based F-P acts as the narrow bandwidth five-channel filter.Each wavelength in the channel can be filtered out by a tunable FBG.In addition,a specially-designed cascaded triple-ring cavity is embedded to realize mode selection and ensure the SLM operation of each selected wavelength.For the five obtained laser wavelengths in the 1940 nm band,the OSNR exceeds 50 d B,and the wavelength fluctuations are less than 0.01 nm.Each lasing wavelength operates in a stable SLM state with the laser linewidths of 7.69 k Hz,5.38 k Hz,5.13 k Hz,5.16 k Hz,and 7.92 k Hz,respectively.5.A numerical analysis of the influence of the bending-induced mode distortion on the TMI effect in the conventional large mode area(LMA)step-index fibers,which are commonly used in the MOPA amplifier stage,is proposed.The mode field distribution of 20/400 μm LMA fiber and 25/400 μm LMA fiber under the various bending radii as well as the corresponding bending loss are simulated.The range of bending radii of the two fibers is determined according to the bending loss of each mode in fiber,the TMI coupling coefficient of the LP01 to LP11 modes in the given range of bending radii is calculated,and the TMI threshold under the quantum noise seeding and relative intensity noise seeding can be further analyzed.The simulation results show that the bending induced mode field distortion does not have a significant improvement on the TMI threshold. |
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