Passively Q-switched Nd YVO4-YVO4-Cr4+ : YAG Self-Mode-Locked Raman Laser Based on Composite Cavity

Objective High-performance Q-switched mode-locked lasers can achieve high pulse-repetition-frequency (PRF) ultrashort pulse sequences with nanosecond-pulse envelopes, which are of great importance in applications such as laser remote sensing, adaptive optics, and inertial confinement fusion. Because of the clean-up effect, stimulated Raman scattering (SRS) has been regarded as a potential technical approach to achieve high-performance laser output. In particular, with the discovery of the SRS self-mode-locking phenomenon in recent years, Q-switched self-mode-locked Raman lasers with compact structure, high peak power, and high beam quality have gradually been favored by researchers. However, the mechanism of the SRS self-mode-locking phenomenon is relatively complicated, and there are few theoretical studies at present. Although some experiments have reported the SRS self-mode-locking phenomenon with corresponding explanations, most of them have yet to be verified and improved. In addition, the conversion efficiency of self-mode-locked Raman lasers needs to be further improved. Therefore, to solve the above problems, an elaborate folding coupled cavity design was employed. Taking advantage of the folding coupled cavity, the fundamental and Raman cavities can be adjusted independently. Hence, the SRS self-mode-locking effect can be verified clearly according to the experimental results, and the mode matching between the fundamental and Raman waves can also be optimized by adjusting the length of the Raman cavity to improve the conversion efficiency of SRS. Methods A schematic diagram of the Nd:YVO4-YVO4-Cr4+:YAG passively Q-switched intracavity self-mode-locked Raman laser based on a folding coupled cavity is shown in Fig. 2. The fundamental resonator consisted of M-1, M-2, a Nd:YVO4 crystal, and a Cr4+:YAG crystal. A common L-shaped Raman cavity (designated by mirror path M-2-M-3-M-4) was adopted for the mode matching between the fundamental and Stokes waves. The radius of curvature of M-1 was 150 mm, and the output coupler (OC) M-2 was a flat mirror. The pump source was a fiber-coupled LD emitting at 808.2 nm with a maximum output power of 50 W. A 1:1 multilens coupler was used to focus the pump light into an a-cut 0.3% Nd:YVO4 crystal with a radius of approximate to 200 mu m near the incident facet of the laser gain medium, and the dimensions of the Nd:YVO4 crystal were 3 mmx3 mmx20 mm. A 4 mmx4 mmx3 mm Cr4+:YAG crystal with 80% initial transmittance at 1064 nm was employed and placed as closely as possible to the OC. An a-cut 4 mmx4 mmx30 mm YVO4 crystal was used as the Raman crystal, which was 1 degrees wedged on both facets. All the components were coated according to our requirements. The length of the fundamental cavity composed of M-1 and M-2 was fixed at 110 mm. By adjusting the ROC of M-4 and the length of the L-shaped Raman cavity, optimization of mode matching between the fundamental and Stokes waves can be achieved effectively. Results and Discussions The linear cavity Nd:YVO4-YVO4-Cr4+:YAG passively Q-switched intracavity self-mode-locked Raman laser was first studied. When the transmittance of the OC was 5%, a maximum output power of 0.81 W was obtained at 1176 nm under a pump power of 17.15 W, with an optical-optical efficiency of 4.72% (Fig. 2). The corresponding PRF and pulse width were 885.4 MHz and approximate to 219.16 ps, respectively (Fig. 3). After that, a 45 degrees dichroic mirror M-3 was inserted into the cavity to construct an L-shaped folded Raman cavity with M-4 and M-2 (OC). When the radius of curvature (ROC) of M-4 was 100 mm and the length of the Raman cavity was 120 mm, a maximum power of 1.23 W with 1176 nm Q-switched mode-locked output was obtained under the pump power of 17.15 W, which was an improvement of over 50% compared with the linear cavity [Fig. 4(a)]. The PRF and pulse width of the mode-locked output were 942.9 MHz and approximate to 125.8 ps, respectively (Fig. 5). The linewidth was 0.2 nm, and the beam quality factors M-x(2) and M-y(2) were 1.39 and 1.42, respectively (Figs. 7 and 8). Replacing the ROC of M-4 with 150 mm and increasing the length of the Raman cavity to 180 mm, a maximum power of 1.19 W at 1176 nm Q-switched mode-locked output was obtained at the pump power of 17.15 W, with a conversion efficiency of 6.94% [Fig. 4(b)], and the PRF was reduced to 675.6 MHz (Fig. 7). Conclusions A passively Q-switched Nd:YVO4-YVO4-Cr4+:YAG self-mode-locked Raman laser based on a composite cavity was demonstrated. Taking advantage of the folded-coupled cavity, the length and mirrors of the fundamental and Raman cavities can both be adjusted independently. Hence, the SRS self-mode-locking effect has been clearly obtained in a simple manner according to the experimental results, and the mode matching between the fundamental and Raman waves can be optimized by adjusting the length of the Raman cavity. In this way, the output power and conversion efficiency of the Q-switched mode-locked Raman output can be greatly improved. In addition, the folding coupled cavity structure was proved to control the PRF of the 1176 nm mode-locked output actively by adjusting the length of the Raman cavity together with the ROC of the mirror, without a reduction of output power and efficiency.