| Fiber laser has become one of the most widely studied laser because of the advantages of simple and compact structure, good beam quality, higher optical to optical conversion efficiency and so on. Single frequency nanosecond pulse fiber laser have been widely applied in coherent synthesis, LIDAR, laser ranging and so forth since the peculiarity of extremely narrow spectral width. Green laser can be obtained from 1 μ m laser by frequency conversion. High power single frequency green pulse laser can be used as the source of LIDAR, long distance and high precision can be provided in the measurement of water temperature, flow rate and so on by it. So it has very important application value to research high energy single frequency nanosecond pulse laser.In this thesis, theoretical and e xperimental researches about the intensity modulation of single frequency laser and single frequency nanosecond pulse laser amplifier have been carried on. The main content can be outlined as follows:1、The research background and significance of single frequency fiber laser amplifier is briefly described. The researches progress at home and abroad of single frequency all fiber laser amplifier are introduced, as well as the practical applications.2、The energy level structure of ytterbium ion is described, then the rate equation and the boundary condition of single frequency ytterbium ion doped fiber laser are given in theory. Several kinds of nonlinear effects which are easily produced when the laser transfers in the fiber is introduced. Emphatically, the nonlinear effects of stimulated Raman scattering(SRS) and stimulated brillouin scattering(SBS) are described. The dynamic process of SBS in fiber is deduced in theory. The reason why SBS is the most easily appear in narrow- linewidth nanosecond pulse fiber laser amplifier is analyzed. Some methods of restraining SBS effect are proposed, and the feasibility of restraining SBS effect by several methods interaction is discussed.3、The basic principle of obtaining nanosecond pulse laser by the means of Q-switch and modulation are introduced. The fundamental of how to convert the CW laser into pulse laser which the pulse width and repetition rate can be controlled by the interaction of arbitrary waveform generator(AWG) and electro-optic modulator(EOM) is expounded. By adjusting the AWG and EOM related parameters, the output pulse laser which is similar to the signal waveform is obtained successfully. It will be a good pulse laser seed for the single frequency nanosecond pulse fiber laser amplifier.4、The way how to deduced the SBS effect in the fiber laser amplifier is described. The SBS threshold is calculated in theory, in the experiment of main power amplifier stage, the means of large mode double cladding fiber and water-cooling are used to improve the SBS threshold. We have successfully demonstrated a single frequency nanosecond pulse all- fiber laser amplification system based on MOPA configuration. The average output power of 23.24 W and 23.12 W is generated at the frequency of 10 k Hz and 5 kHz, respectively. The single pulse energy is 2.3 mJ and 4.6 mJ, corresponding to the peak power of 230 kW and 460 kW when the pulse width is 10 ns. Extra-cavity frequency conversion is made in a preliminary experiment. |
PDF Full Text Request