| Becouse of high beam quality, compact configuration, fiber output and immunity from thermal lensing, fiber-based high power pulsed amplifiers have being widely used in many fields such as precision machining, material processing, medical treatment, radar and out space communication. This work is financially supported by the National Key Basic Research and Development Programme of China under Grant No.2003CB314906 and the Project of the Key Laboratory Foundation of Solid Laser Technology under Grant No.5148020202JW1502. And in this dissertation, we mainly focous on the research and realization of all-fiber pulsed amplifier, including the principles and experiments of output characteristics of nanosecond pulsed amplifier.The details are described as follows:1. Firstly, the energy level and the velocity rate equation of Yb3+ are analyzed. We get the explicit expressions of the dynamic velocity rate equation by using the density of photons flow and the density of reversed population during the amplifier. And under the dynamic condition, the solution of the dynamic velocity rate equation is obtained and deeply discussed. So the output power and the gain of the amplified pulse can be easily got. Also we further research on the pulse distortion and the gain under different input signal power.2. The single- and dual-stage single mode Yb-doped fiber amplifiers (YDFAs) are deeply researched and realized, respectively. We optimize the fiber length of the single-stage YDFA. During the dual-stage YDFA, because of the fiber length is decided, in order to improve the output power and have a good signal to noise ratio (SNR), we focus on the reasonable match of the pump power levels on the two stages. Finally, At repetition rate of 50kHz, pulse duration of 20ns, and 0.5mW average power of the input signal, the amplified signal with 27.7mW output power and the SNR of 25dB was obtained. The output pulse has good polarization and almost has no pulse distortion. During the experiment, the cascaded Wavelength-division-multiplexed (WDMs) are used in enhancing isolation to the backward ASE and protecting the laser diode effectively.3. All-fiber Watt-level pulsed amplifier with high output power is realized by using the double-cladded Yb-doped fiber(DCYF) for further amplification. In this part, we design two double-cladded Yb-doped fiber amplifiers using two different configurations (including the different pump couplers and DCYFs with different doped concentration). The pump coupler in the first structure is specially designed to protect the laser diodes, in despite of its low coupling efficiency. And under the higher pump level, up to 2.12W average power, 2.12kW peak power is realized. By contrast, the coupler in the second structure has the high coupling efficiency of 98%. So the output pulse with 2.10W average power can be easily obtained under the lower pump power. We have deeply discussed the pulse shape under the influence of the different DCYFs. And also the initial analyze of the side-pulse is given.4. The condition of four-wave mixing (FWM) is analyzed theoretically. FWM process in the multi-wavelength Erbium doped fiber amplifier (MW-EDFA) induced by high-nonlinearity photonic crystal fiber (HNL-PCF) avoids mode competition and ensures stable dual-wavelength oscillation at room temperature. The moiréBragg grating (MBG) is made and used in the dual-wavelength EDFA as the wavelength selector. By adjusting the temperature applied on the MBG, the output lasers can be tuned with the fixed wavelength spacing. Then the tilted fiber Bragg grating (TFBG) is also made and the dual-wavelength EDFA based on TFBG is studied. By adjusting the intracavity polarization state, the proposed laser can operate in stable dual-wavelength or switch between two wavelengths at room temperature. |
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