Analysis Of High Power And Energy All Fiber Laser And The Frequency Doubling Experiments

With the dual advantages in both structure and performance, fiber lasers have attracted depth-study by many researchers. Breakthrough developments in principle and technology have been made in recent years. A mode-locked fiber laser has been paid much attention due to its capacity of generating ultra-short pulses(ps or even fs), with main development directions of high power and high energy. For both aims, fiber lasers are inevitably encountered a problem:when pulses transmit within the fiber (diameter generally less than 10μm), high power and high energy are convergent on a smaller area, which will lead to higher power density, trigger strong nonlinear effects, and result in pulse-breaking at last.Two options are proposed to improve the performance. One option is to increase the mode field area, when pulse power and energy distribute in a larger area, the nonlinear effects are weakened. The other is to develop large normal dispersion lasers. With the help of all normal dispersion, pulses broaden monotonously, thus the peak powers of pulses along with the nonlinearity drop. Due to combination of large-mode-area fibers and all normal dispersion cavity, the current level of single pulse energy has reached micro joules, with average power level nearly kilowatts. The study in this paper is started around an Yb-doped fiber laser with all normal dispersion, and then we move to a large normal dispersion of the erbium-doped fiber laser. A new structural optimization is proposed in order to promote the output energy and decrease the nonlinearity. And finally frequency doubling experiments are carried out using the achieved high energy pulses.The major works of this thesis are as follows:1. Theoretical simulation of high energy pulses achieved by introducing large dispersion is studied base on the pulse domain property propagating in the fiber.2. A structural optimization scheme is proposed in an all-fiber normal dispersion passive mode-locked laser. The optimized parameters include the output position, pumping modes, fiber length after the amplifier and the output coupling ratio. A conclusion that the output coupling ratio and the output energy grow in the same step is drawn. At a higher output coupling ratio of 95%, average power of 460mW is obtained, corresponding to pulse energy of 350nJ.3. The relationship between the output power and cavity length is studied in an all normal dispersion cavity. A coupling ratio of 90% is used to reduce nonlinear in the cavity and increase the out energy. With 320mW pump power, and 2Km long single mode fiber, single pulse energy 1.1μJ is obtained in this ultra-lower repetition rate lasers.4. A research of an erbium-doped fiber (EDF) laser based on nonlinear polarization rotation is carried out, and a similar structural optimization is proposed. In large normal dispersion regime, the high coupling ratio and short fiber after the amplifier are combined used. At a coupling ratio of 95%, average power of 250mW is obtained. By shortening EDF provided normal dispersion, we tune the net dispersion of the cavity to small. Together with a low output ratio of 5%, we get a 3dB bandwidth of 58nm mode-locked wide spectrum covering C+L-band.5. Frequency doubling experiments are constructed. The high energy mode-locked pulses obtained with central wavelength at 1560nm and 1064 are used as fundamental frequency light sources. A PPLN crystal and a waveguide structure PPLN are used as the nonlinear medium. We win the harmonic light of 780nm and 532nm, and an all-fiber frequency doubling scheme is proposed.The innovative work and results in this thesis are as follows:1. We proposed and demonstrated a cavity optimization scheme that may have a commonly application in all large normal dispersion laser. By increasing the output coupling ratio and reducing single-mode fiber length after the amplifier, the nonlinearities intra-cavity decrease and the output energy extra-cavity increase. The highest average power for single-mode laser is achieved.2. Based on the ultra-long normal dispersion passively mode-locked laser, more than 1μJ energy output is achieved.3. We demonstrated a wide-band spectrum based on the dispersion management and output coupling ratio optimization in an EDF lasers. With adjusted net dispersion and a small coupling ratio, we obtain a flat-top spectrum with a 3dB bandwidth of 58nm.4. Based on the resulted high energy pulses, we implement frequency doubling experiments. Frequency doubling lights at wavelength of 780nm and 532nm are obtained, and an all fiber frequency doubling scheme is proposed.