Investigation On The Technology Of High-power Tunable Picosecond Fiber Laser

High power fiber lasers and amplifiers feature compactness, stable output, excellent beam quality and very good heat dissipation. They are considered as the prevalent tendency in the future by their many advantages. Therefore, the investigation in the high power fiber laser and amplifiers is very meaningful and helpful for the development of the field in laser. In this paper, we demonstrate the generation of tunable high-power picosecond laser based on Yb-doped fiber amplifier and the frequency doubling of fundamental signal output from the laser system. The main research works and innovations of this dissertation are summarized as follows:1. A gain switch semiconductor laser diode which is an attractive technology for compact, robust and reliable picosecond pulse source with non-mode-locked operation is chosen as seed source. The seed spectrum gain is compensated by Yb-doped fiber amplification (YDFA). This is a novel techniques used to reshape and broaden spectrum to our best knowledge. Using this method, 15 nm bandwidth of seed pulse signal is broadened to 20 nm and rectangular-shaped, flat-top spectrum is achieved as well.2. For achieving tunable wavelength output, a self-made tunable filter with signal mode fiber coupling is used to filter the "flat top" broadband spectrum of picosecond laser pulse from the first stage pre-amplifier by spectrum gain compensation. Consequently, 88 ps pulses with 1.8 nm line bandwidth and a central wavelength tuned from 1053 nm to 1073 nm are achieved by adjusting the filter, the continuously tunable spectral bandwidth reached 20 nm. Although the wavelength-tunability is an important feature of a light source, the tunable range depends on the spectral bandwidth of seed source. Especially, the variable spectral line width is also tuned from 0.8 nm to 2 nm by adjusting the line width of the filter.3. We present a novel spectrum compression and pedestal elimination filter based on Sagnac loop. In our experiment, signal with tunable spectrum range of 1053 nm-1073 nm will be amplified to a high power and then frequency doubled. When the signal is pre-amplified, spectrum is broadened and spectrum pedestal is introduced. Obviously, signal which has a broad spectral line bandwidth and a big spectrum pedestal is unfavorable for frequency doubling and will lose a large part of energy. A polarization controller (PC) is introduced in Sagnac loop to control phase shift relation of two counter-propagating incoming lights. By adjusting the PC in loop, input spectrum with line bandwidth varied from 2.5 nm to 3.2 nm is compressed to 1.4 nm-1.7 nm and spectrum pedestal is eliminated. In addition, pulse duration of more than 150ps is suppressed to less than 100 ps.4. We demonstrate the generation of tunable high-power picosecond laser based on Yb-doped fiber amplification of gain switch semiconductor laser diode. A multi-stage single mode Yb-doped fiber preamplifier is combined with two-stage single mode large mode area doubled-clad Yb-doped fiber main amplifier to establish the amplification system, which is seeded by a gain switch laser diode. Finally, this laser system produces the average power of 10.3 W with 1 MHz repetition rate and 150 ps pulse duration within the central wavelength tunable range from 1053 nm to 1073 nm with excellent beam quality.5. The second harmonic generation of near infrared is investigated. 5*5*10 mm BBO crystal and KTP crystal are used as frequency doubling nonlinear crystal. Average power of more than 1 W harmonic wave with 1 MHz repetition and 100 ps pulse duration within the central wavelength tunable range from 527 nm-537 nm is generated.