Study On High Power Pulsed Thulium Doped Fiber Laser And Supercontinuum Laser Source

The researches on fiber lasers has attracted more attentions due to its special advantages, such as high photoelectric conversion efficiency, compact configuration, good beam quality, good heat dispersion and all-fiber structure. So far, the researches focus on high power, short pulse, narrow linewidth, mid-infrared, and so on. Because of the development of 2 μm fiber components, the all fiber high power 2 μm thulium doped fiber laser and its application on mid-infrared high nonlinearity have made a great progress. 2 μm fiber lasers located in the eye-safe range and atmospheric transmission window, which can be widely used in biomedical, atmospheric remote sensing, polymer material processing and so on. Besides, the high power thulium doped fiber laser can be achieved easily through the silica fiber. Therefore, the high power pulsed thulium doped fiber laser can also been used in mid-infrared supercontinuum generation, optical parametric oscillator, Raman fluoride fiber laser and THz wave generation.The continuous wave output power of thulium doped fiber laser has reached kilo-watt, picosecond pulsed average output power has achieved hundred-watt level, together with the femtosecond pulse realized by chirped pulse amplification. However, output power of the nanosecond pulse has been limited to tens of watts, and it’s more difficult to achieve all-fiber structure. This dissertation is mainly focused on the high power nanosecond pulse generation from the all-fiber thulium doped fiber laser. Firstly, the key technique to realize all fiber high power laser of mode field adaptor(MFA) has been investigated numerically. Secondly, the nanosecond pulses of thulium doped fiber laser have been achieved by gain-switching, Q-switching and saturable absorber(SA). The high power output has also been realized by master oscillator power amplifier(MOPA) structure. At last, the high peak power 2 μm fiber laser has been used to pump fluoride fiber to generate the mid-infrared supercontinuum. The main contents with novelties are as follows: 1. The key technique of all fiber high power fiber laser of MFA was investigated numerically.The coupling losses versus the thermally expanded core and tapered fiber parameters were theoretically analyzed. The coupling loss between high nonlinearity photonic crystal fiber (PCF) and conventional single mode fiber as well as large mode area double cladding fiber was studied using the graded index fiber and coreless fiber as fiber lens. The experimental results show a good agreement with the numerical analysis. The splicing loss between standard single mode fiber and PCF(mode field adaptor is 3.9 μm@1060 nm)was 0.26 d B. The splicing loss between double cladding fiber(mode field adaptor is 18.7μm@1060 nm) and PCF was as low as 0.50 d B. 2. A pulsed Er/Yb co-doped fiber amplifier(EYDFA) was realized to achieve the 2 μm gain-switched thulium doped fiber laser. The EYDFA was based on a modulated nanosecond 1551 nm seed laser and a three stage MOPA. A novel kind of all-solid photonic bandgap fiber(AS-PBGF) was used in the power amplifier stage to suppress the Yb band parasitic lasing for the power scaling and safety operation. Then, this EYDFA was used to pump the high nonlinearity fibers to generate near-infrared supercontinuum. A flat spectrum with 5 d B bandwidth from 1100 nm to beyond 1700 nm(assuming the pump peak was filtered) in the optical spectrum analyzer detectable range was finally obtained by optimizing the fiber length and pump pulse width. 3. The gain-switching thulium doped fiber laser was achieved by using the pulsed EYDFA.Stable output pulse train with pulse repetition rate(PRR) of quartering(1/4 PRR ofpumping pulse), trisecting(1/3 PRR) and halving(1/3 PRR) were observed in the experiment. The self-mode-locked resembling pulses were also observed. Then the stable pulse was amplified by a three stage MOPA, the output power was scaled to 115 W. Tothe best of our knowledge, this is the highest output power of the all fiber nanosecond pulsed gain-switched thulium doped fiber laser. We also confirmed that the self-mode-locked resembling pulses were not suitable to power amplification due to the nonlinear effects in the amplifers. 4. A hundred-watt-level, repetition rate and pulse width tunable, 1950 nm thulium doped all fiber MOPA was demonstrated based on a fiber-coupled acousto-optic modulator(AOM).The generated pulse width and repetition frequency can be tuned by adjusting working frequencies with the tunable region of 46.3 ns-1.16 μs and 100-2000 k Hz, respectively.The maximum output power can be scaled to 113 W. This is the first demonstration of >110-W, all-fiber, actively Q-switched thulium doped fiebr laser, and it has realized the largest tuning ranges both with the pulsewidth and the PRR.No indication of nonlinear effects was observed in the maximum output power. 5. By utilizing the saturable absorber(SA), a mode locked thulium doped fiber laser was achieved with a well self-starting nanosecond pulse generation. The laser cavity is very simple without any polarization controller and the output pulse shows well time stability.Then, the power of mode-locked fiber was scaled to 69.5 W with the spectrum range of 1950-2500 nm by a two stage MOPA. At last, this MOPA was used to pump ZBLAN fiber to realize the mid-infrared supercontinuum. The output power was 5.23 W, and the spectrum range was 1950-3600 nm.