A tunable and narrow-bandwidth Q-switched ytterbium-doped fiber (YDF) laser is investigated in this paper. The non-pumped YDF acts as a saturable absorber and, together with a Sagnac loop mirror, provides a dynamic spectral-filtering grating to achieve a narrow-linewidth Q-switched output. By adjusting an etalon-based tunable fiber filter, a tunable wavelength from 1027 nm to 1033 nm is obtained. When the pump power is 1.75 W, the Q-switched laser pulses with a pulse energy of 10.45 nJ, and a repetition frequency of 11.98 kHz and spectral linewidth of 112 MHz are obtained. This work paves the way for the generation narrow-linewidth Q-switched lasers with tunable wavelengths in conventional ytterbium, erbium, and thulium fiber bands to address critical applications such as coherent detection, biomedicine, and nonlinear frequency conversion.

An analytical model has been formulated to describe the generation of Hermite–Gaussian (HG) modes within Raman lasers through manipulation of the Stokes field’s optical axis deflection angle and its spatial offset relative to the pump field. This model predicts the dependence of HG modes’ oscillation on the angular deviation and off-axis distance between the optical axes of the pump and Stokes fields. Furthermore, an optical system has been engineered specifically for the generation of HG modes in a continuous-wave external cavity diamond Raman laser configuration. Experimental validation demonstrates strong concordance with theoretical predictions.