Research progress of high-power and high-energy pulsed single-frequency fiber laser amplifiers
summary
Pulsed single-frequency fiber laser has important applications in the fields of lidar, remote sensing, and coherent optical communication. Due to the narrow linewidth of single-frequency lasers, the power and energy increase of pulsed single-frequency fiber amplifiers are severely limited by the stimulated Brillouin scattering effect. Starting from the stimulated Brillouin scattering effect suppression method, this paper summarizes the research progress of pulsed single-frequency fiber laser amplification technology, systematically sorts out the representative results of high-power and high-energy pulsed single-frequency fiber amplifiers in the near-infrared band, and further discusses the challenges and potential technical paths for the development of high-performance pulsed single-frequency fiber amplifiers.
significance
Pulsed single-frequency fiber lasers can be used in fields such as lidar, remote sensing, and spectroscopy. For example, with rapid developments in the fields of air travel, meteorology, and clean wind energy, enabling long-range, real-time, and high-resolution detection of 3D wind fields is becoming increasingly important. Therefore, the laser source carried on the LIDAR system should exhibit high power/energy, narrow linewidth, and good beam quality. Fiber waveguide-based fiber laser sources are not only of interest for their high-performance laser output, but also meet the requirements of the above applications due to their high compactness and robustness.
In recent years, single-frequency fiber laser technology has made significant progress in terms of laser power, linewidth, noise, and operating wavelength. Although some reviews have been published on single-frequency fiber lasers, no specific work has focused on pulsed single-frequency fiber laser amplifiers. This prompts us to review the progress of pulsed single-frequency fiber amplifiers, considering the application areas, key technologies, and bottlenecks for further development.
progress
Considering the narrow linewidth of single-frequency lasers, the main problem 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 increase laser power and energy. The first is the development of a new type of gain fiber structure. Based on a polarization-maintaining (PM) erbium-doped fiber with a mode domain of up to 1100 μm2 (Figure 1), a pulsed single-frequency laser at 1572 nm has an energy of 541 μJ, compared to a 1480 nm core pumping scheme of 21.1 μm for M. Microstructured fibers provide more space for large-mode area (LMA) single-mode fibers. Stacking 39 erbium-doped cores with dimensions of 24 μm × 32 μm, a multifilament core fiber was developed to enable pulsed single-frequency lasers with energies up to 750 μJ, where M2 is 1.3, because the core numerical aperture (NA) is only 0.022. In addition, tapered gain fibers with progressively increasing core diameters are also used to increase the SBS threshold due to the reduced laser power density. A linearly polarized single-frequency laser with a peak power of 2.2 kW and a beam mass of only 1.08 M2 has been demonstrated using doped PM Yb tapered fibers with core and cladding diameters of 17 μm/170 μm and 49 μm/490 μm at the input and output ports, respectively. Compared to quartz glass, multi-component glass exhibits higher rare-earth ion doping capacity and allows for more precise refractive index control, which facilitates low numerical aperture at large fiber core diameters. The mJ-class single-frequency pulsed laser energy based on rare earth-doped silicates, phosphates, and germanate glass fibers is demonstrated.
Temperature and strain gradients are also used to manipulate the gain spectrum of Stokes light along the fiber, thereby reducing the gain accumulation of the SBS effect. A 500 ns single-frequency laser was demonstrated to have a pulse energy of 540 μJ at 1540 nm by a strain gradient along the Er/Yb co-doped fiber.
Considering that the SBS effect arises from the interaction between the signal light and the phonons, and its lifetime is about 10 ns, it is possible to suppress the SBS effect using laser pulses shorter than 10 ns. In addition to the 12 cm long Er/Yb co-doped phosphate fibers, the 1.5 μm 5 ns single-frequency laser has a peak power of up to 128 kW.
So far, the performance of high-power and high-energy pulsed single-frequency fiber amplifiers in the wavelength range of 1 μm to 2 μm has been significantly improved. In 1 μm, a 3 ns pulsed laser with a repetition rate of 10 MHz achieved an average power of about 913 W. A 2.4 ns single-frequency pulsed laser based on a commercial ytterbium-doped LMA silica fiber achieves a peak power of up to 91 kW. at 1.5 μm waves
Conclusions and prospects
After more than 20 years of development, the performance of pulsed single-frequency fiber amplifiers in terms of laser power, energy, linewidth and beam quality has been greatly improved. Peak power of up to 100 kW and pulse energy in the millijoule range have been demonstrated. For further development, the appropriate balance between laser gain and different nonlinear effects and laser beam quality should be considered. New gain fiber designs, manipulation of pulse characteristics in the time and frequency domains, and combinations of fiber and solid-state laser amplifiers can be further explored to achieve a new milestone in the development of high-performance pulsed single-frequency fiber lasers.