Fig. 1. (a) Schematic diagram of YCDSF fabrication using the preform with a YAG crystal core and a silica cladding, where the insets are the optical images of the (1) side view and (2) cross-sectional view of the YCDSF; (b) XRD analysis of the YAG crystal rod and the fiber cores.

Fig. 2. (a) Absorption and emission cross sections of the YCDSF; (b) gain cross section of the YCDSF.

Fig. 3. Setup of the DBR SFFL based on a 0.7 cm long YCDSF and the laser measurement system (WDM, wavelength-division multiplexer; ISO, isolator; LR-FBG, low-reflectivity fiber Bragg grating; HR-FBG, high-reflectivity fiber Bragg grating; TC, temperature controller; VOA, variable optical attenuator; OSA, optical spectrum analyzer; PM, power meter; ESA, electric spectrum analyzer; PD, photodetector).

Fig. 4. (a) Calculated effective length of the HR-FBG and LR-FBG with respect to reflectivity; (b) calculated longitudinal mode spacing as a function of YCDSF length.

Fig. 5. Radio-frequency beating spectra of the built fiber laser with a 0.7 cm long YCDSF at different pump powers, measured by a delayed self-heterodyne measurement system.

Fig. 6. (a) Output power of the SFFL as a function of pump power, and the inset shows a magnified view of the graph at a pump power range of 0 to 70 mW; (b) output spectrum of the single-frequency fiber laser under the maximum output power, and the inset is an enlarged view at the wavelengths of 1028–1031 nm.

Fig. 7. (a) Laser stability record within 13 h at 210.5 mW; (b) beam quality of the fiber laser and its two-dimensional beam profile.

Fig. 8. Measured (a) relative intensity noise (RIN), (b) heterodyne signal, and (c) linewidth of the SFFL versus the pump power.