Objective Single-frequency fiber lasers (SFFLs) are widely used in areas of coherent beam combination, gravitational wave detection, lidar, and nonlinear frequency conversion because of their excellent performance. In particular, SFFLs operating at 976 nm are highly demanded for nonlinear wavelength conversion to generate coherent blue light. SFFLs use either a ring- or linear-cavity configuration. The ring-cavity setup is complicated because many additional components must be inserted to enable a single-frequency output, which unavoidably introduces insertion loss. In addition, the stable single-frequency operation of a ring-cavity fiber laser is susceptible to environmental changes and vibrations, thereby resulting in mode hopping. In comparison, linear-cavity construction, such as the distributed Bragg reflector (DBR) scheme, is more compact, which creates a large longitudinal mode spacing, helping to maintain lasing on a stable single longitudinal and hop-free mode. The cavity length of DBR SFFL is limited to only a few centimeters. Therefore, high-gain fibers are demanded to enable sufficiently high gain. A novel Yb∶YAG crystal-derived fiber (YDSF) that exhibits some unique properties in fiber lasers has been developed. The YDSF was fabricated based on a molten core method (MCM) and shows advantages such as high doping levels and high stimulated Brillouin scattering threshold. In addition, the pure silica cladding of the YDSF makes it highly compatible with commercially available silica fiber devices. All the above mentioned characteristics make the YDSF suitable for high-power single-frequency lasers. Based on these fibers, single-frequency lasers emitting at 1 μm have been demonstrated recently. In 2019, we demonstrated a 110-mW single-frequency YDSF laser at 1064 nm. However, to the best of our knowledge, single-frequency YDSF lasers below 1 μm have never been reported.

Methods A commercially available 10% (atomic number fraction)Yb∶YAG crystal was used to prepare a YDSF. In the experiment, the entire preparation process was divided into two steps to maintain the uniformity of the optical fiber. First, a rod fiber having a diameter of ~1.7 mm was fabricated using a 1.6-mm YAG crystal and pure silica tube (D_inner=2 mm, D_external=10 mm). The drawing temperature was controlled at ~2000 ℃. Second, the YDSF was fabricated based on the rod fiber. A short piece of rod fiber was inserted into a different silica tube with the same specification to constitute a new preform, which was drawn into the fiber at 1940 ℃. Next, the physical and optical properties of the YDSF were measured using some devices and methods, such as an optical microscope, energy dispersive spectrometer, fiber refractometer, and cut-back method. Afterward, a homemade all-fiber amplifier was used to measure the gain coefficient of the YDSF at 976 nm. Then, the laser performance of the YDSF was investigated by optimizing the gain-fiber length and reflectivity of fiber Bragg grating (FBG). In addition, a DBR SFFL based on an 8-mm-long YDSF was built to further verify the performance of the YDSF.

Results and Discussions The mass fraction of SiO₂ and Yb₂O₃ in the core region of the YDSF were measured to be 58.83% and 5.25%, respectively (Fig. 1). As expected, interdiffusion occurred between the Yb∶YAG core and silica cladding during the drawing process. The refractive index profile of the fiber cross section was measured; the numerical aperture (NA) of the core with a diameter of 8.7 μm was 0.5 (Fig. 1), indicating that the YDSF was a multimode fiber. The absorption peaks of the YDSF were located at 915 nm and 976 nm, corresponding to the transitions from the ground state ²F_7/2 to higher states of ²F_5/2 of Yb ³⁺. The peak absorption coefficients were 6 dB/cm and 30 dB/cm for 915 nm and 976 nm, respectively (Fig. 1). For a signal power of 0 dBm and pump power of 181 mW, the net gain coefficient of the YDSF reached 12.6 dB/cm (Fig. 2), which indicated that the YDSF could be used as a gain medium for a 976-nm laser. By optimizing the gain-fiber length and reflectivity of FBG, a maximum output power of 37.2 mW was obtained with a slope efficiency of 24.3% (Fig. 3). In addition, using the 8-mm-long YDSF as the gain medium, a 976-nm DBR SFFL was demonstrated. A maximum output power of 17.8 mW with a signal-to-noise ratio (SNR) of >45 dB was obtained at a launched pump power of 203 mW, and no output power saturation was observed. The corresponding slope efficiency was 15.1% (Fig. 5), which was low because of the mode mismatch. More efforts should be made for reducing the NA and improving Yb ³⁺ doping concentration. The linewidth of the laser was measured to be less than 41 kHz, which was limited by the measurement setup (Fig. 6). The beam quality of the laser output was also measured using a charge-coupled device (Thorlabs, BC106N-VIS); the beam quality factor was measured to be 1.01 and 1.02 in the horizontal and vertical directions, respectively (Fig. 5).

Conclusions A YDSF with 5.25% Yb₂O₃ doping concentration(mass fraction) was fabricated using MCM. The transmission loss of the YDSF with a core diameter of 8.7 μm was measured to be 1.29 dB/m at 1550 nm. The gain coefficient of the YDSF was 12.6 dB/cm at 976 nm with a pump absorption coefficient of 6 dB/m at 915 nm. Using the DBR linear cavity, a 17.8-mW single-frequency laser at 976 nm was achieved with an 8-mm-long YDSF, exhibiting a slope efficiency of 18.5%. To the best of our knowledge, this is the first demonstration of a single-frequency YDSF laser below 1 μm. The SNR was measured to be >45 dB with a linewidth of less than 41 kHz. Results indicate that the YDSF is a promising candidate material for the SFFL operating in the 976-nm wavelength region.