Study Of Tunable Multiwavelength Rare-earth-doped Silica Fiber Laser Based On Novel All-fiber Filter

Multiwavelength fiber laser has been extensively investigated in high-speed high-capacity fiber communication and fiber sensor systems, due to its lots of advantages, such as multiwavelength output, compact structure, high beam quality, and low insertion oss. Moreover, single-frequency, wavelength-switchable, wavelength-tunable and spacing-tunable multiwavelength fiber laser has greatly potential application in lidar, multi-parameter sensing, microwave photonics and high-power laser. Under the supports of National Nature Science Foundation of China, the thesis is devoted to investigate the tunable multiwavelength fiber laser based on novel all-fiber filters, theoretically and experimentally. The main innovative achievements are listed as follows:1、Three novel fiber filters based on mode-coupling effect are proposed including a polarization-maintaining chirped moire fiber Bragg grating (PM-CMFBG), a tunable twin-core-fiber-coupler (TCF-coupler) filter based on bending-tuning, and a polarization-dependent multimode-singlemode-multimode (PD-MSM) fiber filter. Firstly, according to mode-coupling theory, coupling occured between forward and backward fundamental modes in PM-CMFBG, between two fundamental modes in two cores in TCF-coupler, and between modes in single-mode fiber and multimode fiber in PD-MSM filter are analyzed in detail, as well as spectral characteristics of these three fiber filters. Then, they are fabricated with such specifications: PM-CMFBG has more than two single-polarization narrow-band transmission peaks with the polarization extinction ratio (PER) high than15dB; TCF-coupler filter tuned by bending the TCF has more than27nm continuously wavelength-tuning range; PD-MSM filter has a comb-like transmission spectrum with alternate filtering at two orthogonal polarizations, and the maximum PER of the filter is25dB.2、Two types of switchable dual-wavelength thulium-doped fiber laser are proposed and demonstrated. Due to the enhanced PHB using polarization-maintaining fiber Bragg grating, the dual-wavelength thulium-doped fiber laser at1.94μm can oscillate at room temperature. Wavelength-spacing is0.81nm, and optical signal to noise ratio (OSNR) is higher than60dB. The comparative analyses reveal that the OSNR and stability of the laser output are improved with the increase of pump power due to the PHB in thulium-doped fiber. The maximum output power is378 mW, and the slope efficiency is15.6%. The PER between these dual wavelengths is49.1dB. By adjusting the polarization controller (PC), switchable single-wavelength laser output is achieved, In further research, a PM-CMFBG is used in a thulium-doped fiber laser as a narrow-band filter and a linear polarizer, which contributes to the suppression of the multiple longitudinal modes oscillation. Dual-wavelength lasers at1942.88nm and1943.32nm are achieved at room temperature, which can also be separated by polarization, In a long time observation, the power fluctuation and the wavelength shift within50minutes are less than0.6dB and0.02nm, respectively. The proposed thulium-doped fiber laser can achieve alternate single-wavelength laser output by simply tuning the PC. In the experiment, self-heterodyne method is adopted to verify that the laser output operates in single-frequency state. With a launched pump power of2.8W, the total output power of170mW and the slope efficiency of10%are achieved.3、Two structures of tunable multiwavelength Brillouin/Erbium fiber laser with a dense spacing are proposed and demonstrated by using tunable TCF-based filter. Combining the good stability and broad tuning range of the TCF-coupler filter and the dense spacing of cascaded stimulated Brillouin scattering, tunable multiwavelength lasers are oscillating at room temperature with a spacing of0.088nm, which corresponds to the Brillouin frequency shift.40nm wavelength-tuning range and up to11laser wavelengths are achieved by bending the TCF incorporating with adjusting the Brillouin pump wavelength. Within3minutes, the power fluctuation and wavelength shift of the first-order Stokes line are less than0.56dB and14pm, respectively. In another proposed Brillouin/Erbium fiber laser, a tunable Mach-Zehnder interferometer (MZI) filter based on TCF is introduced. Different from the former proposed laser, this Brillouin/Erbium fiber laser utilizes the Brillouin pump laser self-seeded in the same cavity instead of the external laser injected, which is quite cost-efficient. Up to17wavelengths oscillate simultaneously with a spacing of0.088nm and an OSNR higher than8dB. By adjusting the bending curvature of TCF-MZI, a continuously tuning range of8nm is achieved and the tuning process has a linear relationship with the bending curvature.4、Two types of spacing-tunable multiwavelength erbium-doped fiber laser based on PD-MSM filter are proposed and demonstrated. Due to the enhanced PHB induced by PD-MSM, single-polarization dual-wavelength laser output is achieved at room temperature. The power fluctuation and the power difference between dual wavelengths in50minutes are less than0.7dB and1dB, respectively. By adjusting PC states only, the wavelength spacing between dual lasers can be tuned continuously from0nm to3nm. In another proposed fiber laser, a dense spacing comb filter is formed by a broad-band PD-MSM filter and a segment of polarization-maintaining fiber (PMF), and the intensity-dependent loss induced by the same PD-MSM filter is employed to suppress the gain competition. In this case, up to19wavelengths oscillate simultaneously at room temperature. According to the same principle, the spacing-tunability can be achieved by cascading multiple segments of PMF.5、A spacing-tunable multiwavelength erbium-doped fiber laser based on nonlinear dual-pass MZI filter is proposed and demonstrated. Nonlinear dual-pass MZI filter is used as not only a multi-channel filter and also a gain-competition suppressor. By adjusting the polarization state in the cavity, multiwavelength laser comb with two different spacing,0.2nm and0.4nm can be switched to oscillation. In the situation of0.2nm laser spacing, up to44-wavelength laser combs with the OSNR of14dB is achieved; the wavelength shift and the power fluctuation is less than0.05nm and0.8dB, respectively. In the situation of0.4nm laser spacing, up to25-wavelength laser combs with the OSNR of30dB is achieved; the wavelength shift and the power fluctuation is less than0.05nm and0.4dB, respectively.