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Research on All-Fiber Nanosecond Pulsed Yb-Doped Fiber Amplifier

Author: LiuFengNian
Tutor: YuanShuZhong;LiuYanGe
School: Nankai University
Course: Optics
Keywords: Pulse fiber amplifiers Cascade Cladding-pumped Pulse distortion Signal to Noise Ratio Self-pulsation Composite cavity Polarization locking Coherent combining efficiency
CLC: TN248
Type: PhD thesis
Year: 2008
Downloads: 282
Quote: 1
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Abstract


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.51438020205JW1502.With high beam quality, compact configuration, fiber output and immunity from thermal lensing, fiber-based high power pulsed amplifiers attract great interest applications such as precision machining, material processing, medical treatment, lidar and outer space communications. In this dissertation, we mainly focus on the research and realization of all-fiber Watt-level pulsed amplifier, including the principles and experiments of output characteristics of nanosecond pulsed amplifier.The details are described as follows:1. Through the dynamic velocity rate equation, we got the explicit expressions of the density of photons flow and the density of reversed population along transmission direction of the signal. The influence to the output energy, gain of output power, gain of energy and pulse distortion by the intensity of the input signal, the length of active fiber and the initial density of reversed population were analyzed and discussed with solutions of the dynamic velocity rate equation.2. The cascaded wavelength-division-multiplexed (WDMs) with 70% coupled efficiency were used in cascaded single-mode amplifiers, which enhanced isolation to the backward ASE and protected the laser diode from destroyed during long period. Based on the optimization of active-fiber length, we pumped the active fiber in subsection. The pumping in subsection increased the isolation and suppressed the ASE, which improved the amplified output power greatly. To insure high signal to noise ratio, we further optimized the pump level of two laser diode. At repetition rate of 50kHz, pulse duration of 20ns, and 0.5mW average power of input signal, the amplified signal with 26mW output power and signal to noise ratio of 25dB was obtained. There was almost no pulse distortion with 84.5% polarization degree. 3. Using double-cladded Yb-doped fiber with different doped concentration, we studied the influence to the output power and pulse shape by the initial reversed population. At the same pump level, using high-concentration Yb-doped fiber would improve the output power greatly, but the pulse shape was distorted. Spectral broadening due to self-phase modulation. In the method with high coupled efficiency, there need to avoid the backward signal destroying the laser diode. Around the pump threshold, output power of the signal was equal to input power with instable pulse shape. Almost 1.05W average power was obtained after the cladding-pump amplifier.4. Through the cladding-pump amplifier chains, up to 2W average power, 2kW peak power are realized. We further studied the influence to pulse shape by the faint feedback in amplifier on condition of strong input signal. The buildup of interpulse amplified spontaneous emission(ASE) lead to parasitic lasing or destructive self-pulsations, which decreased the output power and signal to nosie ratio. To avoid parasitic lasing and self-pulsations, one or two acousto-optic gates may be used to control the ASE buildup. Suppression of ASE injection between other stages can be ensured by using a bandpass filter centered at signal wavelength. All these method could provide clean, ASE-pedestal-free pulses at the output of the complete system.5. Characteristics of self-organization in compound cavity laser with one fiber bragg grating(FBG) to wavelength selection have been investigated and discussed. The coupled modes emitted from the FBG(4.5% reflectivity) and an coherent efficiency of 94.3% was obtained. The characteristics of leak power was good agreement with numerical simulation. A polarizer and the polarization controller were employed in composite cavity. Based on the combination of two elements controlling the polarization state, coherent polarization locking was realized. Choosing FBGs with different center wavelength to wavelength selection in Michelson compound cavity denoted that the selected wavelength locating the range from 1559nm to 1561nm would increase the coherent efficiency and slope efficiency.

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