A research team from the Department of Space and Astronautical Laser Technology and Systems at the Shanghai Institute of Optics and Precision Machinery (SIPM) of the Chinese Academy of Sciences (CAS) has recently reported for the first time a low repetition rate, fully bias-preserving nine-cavity fiber laser in the 915 nm wavelength band. The related research results were published in Optics Express under the title of "Low repetition rate 915 nm figure-9 ultrafast laser with all-fiber structure".
Lasers with wavelengths near 900 nm are favored in the field of optical measurements because of their high responsivity on silicon-based detectors. Low-heavy-frequency fiber lasers at this wavelength can effectively reduce the distance blurring in time-of-flight ranging (TOF), and at the same time have the characteristics of good beam quality, compact structure, and long-term stability, which can be used as an ideal light source for space probing. However, due to the strong dispersion and nonlinear effects of single-mode fibers, it is difficult for the laser to achieve single-pulse operation in a long fiber cavity. Therefore, the repetition frequency of conventional mode-locked fiber lasers is usually limited to tens to hundreds of MHz.
Fig. 1. Experimental setup of 915 nm low-repeat-frequency fully bias-preserving nine-cavity fiber laser
To address the above problems, the researchers proposed a low-repeat-frequency fully bias-preserving nine-cavity fiber laser with a center wavelength of 915 nm. By designing and optimizing the laser structure, a high-energy single-pulse output at low heavy frequency was achieved. The optimized seed was amplified by one stage, and a pulse output with a pulse width of 15.2 ps and a single-pulse energy of 4.7 nJ was obtained at a repetition frequency of 3.1 MHz. The mode-locked performance of the laser has been confirmed by prolonged power and spectral stability tests. This compact and long-term reliable fiber laser is a promising light source for future space laser ranging.
Fig. 2. Long-term stability tests of output power and spectrum and pulse output characteristics in the frequency and time domains.
