基于复合腔结构的被动调Q拉曼自锁模激光器

Passively Q-switched Nd∶YVO_(4)-YVO_(4)-Cr^(4+)∶YAG Self-Mode-Locked Raman Laser Based on Composite Cavity

介绍了一种基于复合腔结构的Nd∶YVO_(4)-YVO_(4)-Cr^(4+)∶YAG被动调Q内腔拉曼自锁模激光器。利用复合腔结构中基频光谐振腔与拉曼光谐振腔相互独立的优势,通过实验验证了受激拉曼散射(SRS)自锁模效应的存在;同时通过合理地控制拉曼光谐振腔的腔长,优化了基频光与拉曼光的模式匹配,大幅提高了拉曼调Q锁模激光的输出功率及转换效率,在17.15 W 808 nm二极管(LD)泵浦功率下获得了调制深度为100%、平均功率为1.23 W的1176 nm拉曼调Q锁模激光输出,光-光转换效率为7.17%,相比于直腔结构提升50%以上。锁模脉冲宽度为125.8 ps,脉冲重复频率为942.9 MHz,调Q包络重复频率约为70 kHz,脉宽为4.25 ns。将拉曼腔前腔镜的曲率半径由100 mm增加至150 mm,对应的拉曼腔长由120 mm增加至180 mm,在相同泵浦功率下获得了调制深度为100%、平均功率为1.19 W的1176 nm拉曼调Q锁模激光输出,转换效率为6.94%,脉冲重复频率降低至675.6 MHz。这些结果说明了该复合腔结构具备在保证输出功率及转换效率的同时,对1176 nm锁模激光的重复频率进行主动调控的能力。

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-modelocking 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∶YVO_(4)-YVO_(4)-Cr^(4+)∶YAG passively Q-switched intracavity self-modelocked Raman laser based on a folding coupled cavity is shown in Fig.2.The fundamental resonator consisted of M_(1),M_(2),a Nd∶YVO_(4)crystal,and a Cr^(4+)∶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 150mm,and the output coupler(OC)M_(2)was a flat mirror.The pump source was a fibercoupled LD emitting at 808.2nm 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∶YVO_(4)crystal with a radius of≈200μm near the incident facet of the laser gain medium,and the dimensions of the Nd∶YVO_(4)crystal were 3mm×3mm×20mm.A 4mm×4mm×3mm Cr^(4+)∶YAG crystal with 80%initial transmittance at 1064nm was employed and placed as closely as possible to the OC.An a-cut 4mm×4mm×30mm YVO_(4)crystal was used as the Raman crystal,which was 1°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 110mm.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∶YVO_(4)-YVO_(4)-Cr^(4+)∶YAG passively Q-switched intracavity self-modelocked Raman laser was first studied.When the transmittance of the OC was 5%,a maximum output power of0.81 W was obtained at 1176nm 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≈219.16ps,respectively(Fig.3).After that,a 45°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 100mm and the length of the Raman cavity was 120mm,a maximum power of 1.23 W with 1176nm 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 125.8ps,respectively(Fig.5).The linewidth was 0.2nm,and the beam quality factors M_(2)^(x) and M_(2)^(y) were 1.39and 1.42,respectively(Figs.7and 8).Replacing the ROC of M_(4) with 150mm and increasing the length of the Raman cavity to 180mm,a maximum power of 1.19W at 1176nm 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 675.6MHz(Fig.7).Conclusions A passively Q-switched Nd∶YVO_(4)-YVO_(4)-Cr^(4+)∶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 1176nm 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.