Research On Novel Double Clad Fibers

Multi-wavelength fiber lasers became more attractive mainly due to the wide application in high speed Dense Wavelength Division Multiplexing (DWDM) systems, fiber sensors, laser LIDAR, spectral analysis, fiber instrumentations and microwave photonics. Especially, lasers with two close wavelength emissions can act as the pump source for THz wave generation if the value of the corresponding frequency difference of the laser is in THz regime. High power pulsed fiber lasers draw much attentions of many research groups because of the advantages such as high efficiency, near diffraction limited beam quality, excellent heat dissipation capability, compactness and free of service. The high power linearly polarized continuous wave (CW) and pulsed fiber lasers working at the wavelength of 1.06μm and 2μm regions were the most important pump source of the mid-infrared lasers. In this thesis, the main work is on the developments of the dual-wavelength high power Yb-doped fiber laser based on a few-mode fiber Bragg grating (FMFBG), all-fiberized double-clad Yb-doped pulsed fiber lasers (including All-fiberized MOPA structured linearly polarized Yb-doped pulsed fiber laser and gain switched Yb-doped pulsed fiber laser) and the Tm-doped double-clad fiber laser working at the wavelength around 2μm.We demonstrated a dual-wavelength high power Yb-doped double-cladding fiber laser based on a FMFBG and the Yb-doped double-clad fiber produced by Nufern. Stable dual-wave length operation was achieved with output power up to 5.67 W under pump power of 10.7 W. The laser wavelengths were centered at 1059.12 run and 1060.80 nm with a full width at half maximum (FWHM) of 37 pm and 43 pm respectively. The signal-to-noise-ratio (SNR) was greater than 50 dB and the beam quality factor (M2) was about 1.9We demonstrated a one-stage MOPA structured all-fiberized linearly polarized high power Yb-doped fiber laser seeded by a fiberized AO Q-switched fiber laser. A maximum average output power of 29.6 W was obtained from the fiber amplifier at a repetition rate of 40 kHz under the pump power of 40 W The laser wavelength was centered at 1060μm with a pulse duration of 30 ns. The slop efficiency was 74.6%. M2 was about 1.36. And the the polarization extinction ratio (PER) was greater than 10 dB.Gain switched Yb-doped fiber lasers were also investigated when pumped by a pulsed Laser Diode (LD) working at wavelengths of 915 nm and 975 nm respectively. Stable pulsed fiber laser output was obtained when pumped by the pulsed LD working at the wavelength of 915 nm. In the experiment, a 4.5 meter double-clad Yb-doped fiber was used as the gain medium. The laser emission wavelength was centered at 1060 nm. The pulse duration was 600 ns-1μs and the pulse energy was 8-20μJ. When a LD working at 975 nm was used as the pump source, and a 1.5 meters double-clad Yb-doped fiber used as gain medium, stable pulsed fiber laser was also obtained. The pulse duration was 250-700 ns and the pulse energy 2-15μJ at the repetition of 30-50 kHz. The pulse duration of 271 ns and the average power of 0.72 W was obtained at the repetition of 50 kHz.The fiber laser was dual-wavelength operated centering at 1060.420 nm and 1059.710 nm.We have also investigated the high power random polarized and linearly polarized Tm-doped fiber lasers using the Fabry-Perot (F-P) cavity structure. The random polarized laser output operating at 2022 nm was achieved with a maximum output power up to 18.4 W when pumped by a LD working at 793 nm. The slope efficiency was 49% and the quantum efficiency was 1.25. The linearly polarized laser output operating at 2030 nm was achieved with a maximum output power up to 21.9 W under the pump power of 63.28 W. The PER was better than 92.7%(11.37 dB).The mechanism for the blue and violet fluorescence observed on the surface of the Tm-doped fiber under high pump power was analyzed.