Spectrum control of solid-state lasers, including wavelength tuning and linewidth control, is crucial for obtaining a specific wavelength and narrow linewidth laser. Narrow-linewidth solid-state lasers have the advantages of high spectral purity, long coherence length, high peak spectral density, and various wavelength options , leading to important application prospects in coherent optical communication, optical precision measurements, laser radar, and other fields. Therefore, investigating wavelength tuning and linewidth narrowing technology in solid-state lasers is essential. To realize the output of a narrow-linewidth laser in the ultraviolet (UV) band, an alexandrite continuous (CW) wave laser with a tunable wavelength and narrow linewidth, pumped by a 638-nm laser diode (LD), is reported in this study.

To obtain a narrow-bandwidth-tunable solid-state laser output, selecting a dispersive element that can perform both narrowband filtering and wavelength tuning is first required. Birefringent filters (BRFs) are widely used as optical elements in solid-state lasers. First, we simulate the control effect of BRFs inserted into the resonator at the Brewster and 0° angles on the spectrum and obtained the transmission curves under different conditions using MATLAB, which enables selection of the best scheme for the experiment (Figs. 2 and 3). The feasibility of intracavity frequency doubling is then analyzed. Next, a V-shaped folding cavity alexandrite laser is constructed, where the related parameters are obtained using reZonator software and according to specific experimental requirements (Fig. 5). The overall structure of the laser is shown in Fig. 4. The pump source used in the experiment is a coherent red light LD with a multi-mode fiber-coupled output, which can provide a laser output with a center wavelength of 638 nm and maximum power of 40 W. The pump light is passed through the collimating system as well as through the half-wave plate HWP₁ and polarized beam-splitting prism (PBS) to obtain horizontally polarized light with a maximum power of approximately 30 W. The polarized light is then adjusted through half-wave plate HWP₂ to make it parallel to the b axis of the alexandrite crystal to achieve the highest polarized light absorption efficiency. Finally, the light is converged in the emerald crystal through lens F₁ at a focal length of 50 mm. The alexandrite crystal is 4 mm×4 mm×15 mm in size, and the mass fraction of the Cr³⁺ ions is 0.2%. To obtain a frequency-doubled UV laser, LiB₃O₅ (LBO, θ_LBO=90°, φ_LBO=37.6°) and β-BaB₂O₄ (BBO, θ_BBO=31.3°, φ_BBO=0°) are selected as type-I phase-matched nonlinear optical crystals for experiments. A tunable alexandrite CW laser with a narrow linewidth and tunable wavelength is obtained by adjusting the BRF and nonlinear crystals.

After a BRF is inserted into the cavity, a narrow-linewidth UV laser with a tuning width of approximately 15 nm is obtained using either of the nonlinear crystals. When LBO is used as a frequency-doubling crystal, the tuned wavelength range is 371.60?386.76 nm (Fig. 6). At this central wavelength, the peak power is 1.53 W (Fig. 8) at 379.49 nm with a linewidth of 7.6 pm (Fig. 9). When BBO is used as a frequency-doubling crystal, the tuned wavelength range is 370.95?386.88 nm (Fig. 10). At this central wavelength, the peak power is 1.98 W (Fig. 8) at 381.29 nm with a linewidth of 8.5 pm (Fig. 11). The conversion efficiencies of the horizontally polarized pump light to the frequency-doubled UV laser are 5.1% and 6.6%, respectively. The linewidth of the UV laser can be compressed to less than 10 pm within the entire tuning range. The output laser linewidth is significantly narrowed, which is important for the generation and corresponding application of tunable all-solid-state UV lasers with narrow linewidths. A self-designed water-cooled temperature control device is used in the experiment, and the temperature control effect is ideal; therefore, no saturation or decrease occurs in the output power due to the thermal focal length effect of the laser crystal. Thus, a 638-nm LD with a higher output power can be used as the pump source in subsequent experiments to obtain a high-power UV CW laser output and thereby meet the requirements of laser light sources in optical precision machining, coherent optical communication, and other fields.

This study conducts a theoretical design and experimental verification to realize an intracavity frequency-doubled UV CW alexandrite laser with a narrow linewidth and tunable wavelength. Based on the V-cavity alexandrite laser, a UV CW laser with a tuning width of approximately 15 nm and spectral linewidth of less than 10 pm is obtained using only a single BRF with a thickness of 2 mm. The results provide a foundation for obtaining the output of solid-state lasers with narrow linewidths and tunable wavelengths within a certain range of the UV band.