| As one of the most important technology to obtain ultrashort pulse,passive modelocking has been widely studied and studied by researchers.The passive mode-locking technology based on nonlinear optical loop mirror(NOLM)or nonlinear amplifying loop mirror(NALM)attracts people's attention again because of its all-fiber structure and the design of full polarization maintaining,which has a great of advantages compared to other mode-locking lasers based on real saturable absorber or nonlinear polarization rotation/envolution(NPR/NPE).It is also known as a figure-of-eight or figure-of-nine cavity,because its laser structure resembles an "8" or "9" character.This article focuses on the study of an all polarization maintaining Yb-doped mode-locking fiber laser.The main contents and research work are as follows:1.Theoretical study on the “figure-of-eight” mode-locking fiber laser.Based on the nonlinear effects in the “figure-of-eight” laser,the saturable absorption curve of the mode-locking mechanism is simulated and analyzed,the role of the saturable absorption curve in the mode-locking process is described,and the pulse output characteristics under different conditions are introduced,including soliton,dissipative soliton resonance and noise-like pulse.2.An experimental study on all polarization maintaining ytterbium-doped fiber laser based on the “figure-of-eight” cavity at the 1030 nm wavelength band was conducted.The experiments based on NOLM and NALM mode-locking were carried out respectively.By changing parameters such as fiber loop length,pump power,and 2×2 coupler output ratio,pulses of different output characteristics were obtained.The narrowest compression pulse width we obtained is 197 fs,the center wavelength is 1034.7 nm,with a 3 dB spectral bandwidth of 16.3 nm,and the repetition frequency is 14.7 MHz,corresponding to the cavity length of 13.6 m.The output power is 85 mW and the single pulse energy is 6 nJ.3.An experimental study was conducted to expand the output wavelength of the “figure-of-eight” cavity fiber laser to a longer wavelength.The pulsed output at 1070 nm and 1080 nm waveband was obtained by means of cascaded pumping or cladding pumping,respectively.In the cascading pump scheme,a dissipative soliton-mode-locked output with a center wavelength of 1073.0 nm and a 3 dB bandwidth of 5.8 nm was obtained at the repetition rate of 7.87 MHz,and the output power is 2.7 mW,the output pulse width is 45.5 ps.A bandpass filter with bandwith of 8 nm was inserted to the cavity,and we obtain the dissipative soliton resonance pulse output with a center wavelength of 1067 nm.the pulse width was increased from 293 ps to 1.28 ns when the pump power was increasing continuously.And then a noise-like pulse with a center wavelength of 1064 nm was also obtained when a bandpass filter with bandwith of 8 nm was inserted to the cavity.In the cladding pump scheme,a noise-like pulse with a center wavelength of 1088.0 nm and a 3 dB spectral bandwidth of 11.6 nm at the repetition frequency of 8.69 MHz was obtained,and the average pulse output power is 210 mW,the single pulse energy is 24.2 nJ.4.By introducing a Faraday rotator and two wave plates,the mode-locking threshold of the eight-word cavity laser was reduced.In this experiment,the dissipative soliton mode-locking output at 1030 nm band was also obtained.When the pump power reached 216 mW,mode locking was self-starting.The center wavelength is 1042.1nm,the 3dB bandwidth of the spectrum is 1.5nm,the output power is 3.7 mW,the repetition frequency is 16.1 MHz,and the pulse width is 20.8 ps.In addition,a “figure-of-eight like” mode-locked fiber laser was realized by means of polarization-maintaining fiber twisted,and a dissipative soliton pulse output of 1060 nm wave band was obtained,which the center wavelength of the pulse is 1067.2 nm,with a 3 dB bandwidth of 1.5 nm at the pulse repetition frequency of 6.3 MHz,and the output power is 23 mW.A noise-like pulse output was also obtained when the pump power continued to increasing. |
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