The Study On High-power All-fiber Mid-infrared Supercontinuum Generation

Supercontinuum(SC) sources in the mid-infrared(mid-IR) wavelength region of 2-5 μm have variety of attractive applications such as in basic science, environmental monitoring, biomedical, and security. There is intense effort focused on the development of all-fiber mid-IR SC sources due to their compact configuration without free-space components, excellent beam quality, and easy to maintain. As the development of Tm-doped fiber lasers, 2 μm pulsed fiber laser is proved to be an attractive pump choice for efficient mid-IR SC generation. However, mid-IR SC generation based on 2 μm pulsed fiber lasers is still in its infancy, and reports on watt-level SC sources are still sparse. This paper carried out a comprehensive theoretical and experimental research on the generation of high-power all-fiber mid-IR SC in ZBLAN(Zr F4-Ba F2-La F3-Al F3-Na F) fluoride fibers. At last, high-power all-fiber mid-IR SC generation in a ZBLAN fiber pumped by a 2 μm master oscillator power amplifier(MOPA) system is demonstrated. This paper mainly contains the following several aspects:1. We present a brief historical review of SC generation, introduce the applications of mid-IR SC sources, discuss the material choice for mid-IR SC generation, and review recently development of mid-IR SC generations. At present only ZBLAN fluoride fibers are considered to be the most stable and technologically mature for high-power mid-IR SC generation.2. Numerical research of mid-IR SC generation is carried out. Both the time-domain and frequency-domain models of generalized nonlinear Schr?dinger equation(GNLSE) and their solving methods are introduced. The characteristics such as dispersion, loss, Raman response function and nonlinear coefficient of the ZBLAN fiber are researched, and how to select the parameters of ZBLAN fiber is discussed. Mid-IR SC generations in the ZBLAN fiber with different pumping wavelengths and pulse widths are compared. The numerical results show that the power distribution in the longer wavelengths of the mid-IR SC can be improved by pumping a ZBLAN fiber with 2 μm pulses instead of 1.55 μm pulses, and there will be a better extension result for the longer pulse width under the same peak power and pumping wavelength.3. The generations of 2 μm pulsed lasers are researched. A semiconductor saturable absorber mirror(SESAM) passively Q-switched fiber laser near 2 μm is achieved, pulse width from 1 μs to 362 ns under repetition rate of 20 to 80 k Hz is obtained. A SESAM passively mode-locked fiber laser at 1960 nm with pulse width in picoseconds range is demonstrated. A pulsed fiber laser at 1960 ns with pulse repetition rate of 100 MHz and pulse width of 850 ps is achieved by composite cavity mode-locked technique. A novel 2 μm pulsed fiber laser based on a SC source is studied for the first time, and a pulsed laser at 1957.8 ns is obtained based this method.4. High power 2-2.5 μm region SC source is accomplished. A SC with 62.1 W average power is generated in the last stage tm-doped fiber amplifier(TDFA) of the 2 μm MOPA system, the output spectrum extends from 1.9 to 2.7 μm and the 10 d B spectral flatness is maintained in the range of 1945-2579 nm if the signal peak around 1960 nm is neglected. Both the 62.1 W output power and 39.8% optical efficiency as respect to the pump power of the last stage fiber amplifier are higher than those in previous work. The 62.1 W output power is the highest average power of SC generation in this spectral range.5. Mid-IR SC generation in a ZBLAN fiber pumped by a 1550 nm nanosecond MOPA system is demonstrated, the SC has an output power of 304 m W and spectra extending from 1.5 to 3μm. The performance of the output SC is improved by subsequent Tm-Ho codoped fiber amplifier(THDFA) and TDFA after the Er-Yb codoped fiber amplifier(EDFA). A mid-IR SC extending from 1.9 to 4.3 μm with 185 m W average output power is obtained. This is the first time of experimental study of mid-IR SC generation with spectral extending beyond 3 μm in our country.6. Mid-IR SC generations in single mode ZBLAN fibers with 1 W, 7.11 W and 13 W output power are demonstrated by 2 μm MOPA system pumping. The 13 W average output power is the highest output power for mid-IR SC generation in soft-glass fibers. The output spectrum of the 13 W mid-IR SC extends from ～1.9 to 4.3 μm, the overall optical conversion efficiency from the pump light of the last stage fiber amplifier to the total SC output is 20%, and the SC power for wavelengths longer than 2.5 μm is 6.85 W with a power ratio of 52.69% with respect to the total SC power.