Research Progress on High Power and High Energy Pulsed Single-Frequency Fiber Laser Amplifiers

Significance Pulsed single- frequency fiber lasers can be applied in fields such as LIDAR, remote sensing, and spectroscopy. For example, with the rapid development of the air travel, meteorology, and clean wind energy sectors, it has become increasingly important to realize long-distance, real-time, and high- resolution detection of 3D wind fields. Therefore, the laser source carried on the LIDAR system should exhibit high power/energy, narrow linewidth, and good beam quality. Based on optical fiber waveguides, fiber laser sources have attracted attention owing to not only their high-performance laser output, but also their high compactness and robustness, which fulfill the requirements of the aforementioned applications. In recent years, significant progress has been made in single- frequency fiber laser techniques in terms of laser power, linewidth, noise, and operation wavelength. Although some reviews on single- frequency fiber lasers have been published, no specific work has focused on pulsed single- frequency fiber laser amplifiers. This motivated us to summarize the progress in pulsed single- frequency fiber amplifiers considering the application areas, critical techniques, and bottlenecks for further development. Progress Considering the narrow linewidth of single- frequency lasers, the main issue in the development of pulsed single- frequency fiber laser amplifiers is the severe stimulated Brillouin scattering (SBS) effect. Different strategies have been developed to suppress the SBS effect in pulsed single- frequency fiber amplifiers to improve the laser power and energy. The first is the development of a novel gain- fiber structure. Based on a polarization- maintaining (PM) Er- doped fiber with mode area up to 1100 mu m2 ( Fig. 1 ), pulsed single- frequency laser at 1572 nm has been demonstrated with an energy of 541 mu J, while good beam quality can be maintained with an M 2 of 1.1 by virtue of 1480 nm core- pumping scheme. Microstructured optical fibers provide more space for achieving a large- mode- area (LMA) single- mode fiber. With 39 Er- doped cores stacked in size of 24 mu mx32 mu m, a multifilament- core fiber was developed to achieve pulsed single- frequency laser with energy up to 750 mu J, in which the M 2 is 1.3 due to the low core numerical aperture (NA) of only 0.022. Moreover, tapered gain fibers with gradually increasing core diameters have also been used to improve the SBS threshold owing to the decreased laser power density. With a PM Yb-doped tapered fiber, whose core and cladding diameters at input and output ports are 17 mu m/170 mu m and 49 mu m/490 mu m, linear- polarized single- frequency laser with peak power of 2.2 kW has been demonstrated while the beam quality M 2 is only 1.08. Compared with silica glass, multicomponent glass exhibits a much higher rare- earth- ion doping capability and can achieve more precise manipulation of the refractive index, which facilitates a low NA under a large fiber core diameter. mJ-level single- frequency pulsed laser energy was demonstrated based on rare- earth- doped silicate, phosphate, and germanate glass fibers. Temperature and strain gradients have also been employed to manipulate the gain spectrum of Stokes light along the fiber, which decreases the gain accumulation of the SBS effect. With the strain gradient along an Er/Yb co- doped fiber, pulse energy of 540 mu J was demonstrated for a 500-ns single- frequency laser at 1540 nm. Considering that the SBS effect originates from the interaction between the signal light and the acoustic phonons, whose lifetime is approximately 10 ns, the SBS effect can be suppressed using a laser pulse shorter than 10 ns. In addition to a 12- cm long Er/Yb co- doped phosphate fiber, a peak power of up to 128 kW was demonstrated for a 5-ns single- frequency laser at 1.5 mu m. Up to now, high- power and high-energy pulsed single- frequency fiber amplifiers have seen significant performance improvement in the wavelength region from 1 mu m to 2 mu m. In 1 mu m, around 913 W average power has been realized for a 3-ns pulsed laser with repetition rate of 10 MHz. A peak power of up to 91 kW was achieved from a 2.4 ns single- frequency pulsed laser based on a commercial Yb-doped LMA silica fiber. At the wavelength region of 1.5 mu m, peak power up to 200 kW for a 0.9 ns laser has been demonstrated with a piece of tapered Er- doped fiber and further power improvement was only constrained by the self- phase modulation effect other than SBS effect. For 2 mu m pulsed single- frequency laser, around 1 mJ has been demonstrated with 41- cm long Tm-doped germanate fiber. Conclusions and Prospects Over 20 years of development, the performance of pulsed single- frequency fiber amplifiers in terms of laser power, energy, linewidth, and beam quality has greatly improved. Peak powers of up to hundred kW, and pulse energies at the millijoule level have been demonstrated. For further development, a proper balance between the laser gain and different nonlinear effects, as well as the laser beam quality, should be considered. New gain fiber designs, manipulation of pulse properties in both the time and frequency domains, and the combination of fiber and solid laser amplifiers can be further explored to achieve new milestones in the development of high-performance pulsed single- frequency fiber lasers.