Progress on High-Power Supercontinuum Laser Sources at 3-5 μm
Abstract High-power 3~5μm band mid-infrared supercontinuum lasers have important applications in environmental monitoring, biomedical, surveying and metrology, national defense and security. Among the nonlinear media used to generate mid-infrared supercontinuum lasers, optical fibers are easy to achieve large nonlinear coefficients, and have long effective operating distances, flexible structures, and easy integration, which have been widely studied. In recent years, the technical solutions of high-power mid-infrared supercontinuum fiber lasers in the 3-5 μm band have become increasingly abundant, and the performance indicators have gradually improved. The development status of mid-infrared supercontinuum fiber lasers with spectral coverage of 3~5μm is summarized, and its development trend is prospected. Key words laser optics; supercontinuum generation; nonlinear effect; optical fiber; optical fiber amplifier
High-power SC laser sources have a variety of potential applications including active hyperspectral imaging, long-range environmental sensing, homeland security, and so on. The growing trends of MIR-SC lasers are mainly listed as below.
1) Output power is increasing. With the technological development of near-infrared and short-wave infrared fiber lasers and soft glass fiber processing, the output powers of mid-infrared SC fiber lasers based on fluoride fibers have been continuously improved to 30 W. With the development of mid-infrared fiber laser technology and the demand for SC lasers, the output powers of 3-5 μm mid-infrared SC fiber lasers should be further improved.
2) Power conversion efficiency is gradually improved. In the early research, the inefficient spatial coupling and high fiber transmission loss limit the improvement of power conversion efficiency of SC lasers. With the development of low-loss beam coupling technologies such as end-butting and fusion splicing, the successful development of low-loss optical fibers, and the red shift of pump wavelength, the power conversion efficiency of SC lasers has been gradually improved. It can be expected that with the further reduction of transmission loss in mid-infrared fibers and the further red shift of pump wavelength, there is still room for improvement in power conversion efficiency.
3) Optimization of spectral characteristics is continuing. Due to the limitation of the short wavelength or narrow spectrum of the pump laser, the short-wavelength component of the early high-power mid-infrared SC fiber laser has a high proportion of power, or there is a residual spectral peak at the pump wavelength. After the emergence of SC lasers as pumping sources, spectrally flat mid-infrared SC lasers have been widely studied, and the power ratio of long-wavelength spectral components has been significantly improved. With the development of long-wavelength pump source technology, the spectral characteristics (such as spectral shape and long-wavelength power ratio) have become a clear trend.
4) Pump wavelength is gradually moving in the long wavelength direction. From the development of high-power SC fiber lasers, it can be found that with the development of fiber laser technology, the pump wavelength of mid-infrared high-power SC laser gradually moves in the long wavelength direction, from the 1.5 μm band to the 2 μm band, then to the 2-2.5 μm band, and now to 3 μm band. The increase of the pump wavelength is beneficial to reduce the quantum defect, reduce the heat generation, and improve the power conversion efficiency during the conversion from the pump light to the mid-infrared SC laser. It can be predicted that pumping wavelength would be extended to long wavelength, which is an important trend of mid-infrared SC fiber lasers.
It can be expected that in the near future, high-power mid-infrared SC fiber lasers would move from experimental researches to practical applications, and play a unique role in scientific researches, production, and daily life.
