连续波拉曼激光器突破现有波长范围

1928年,交相辉映的蓝色天空和大海激发了拉曼对散射光的实验观察,进而拉曼效应被发现,并获得了1930年诺贝尔物理学奖。

1962年受激拉曼散射借助脉冲红宝石激光器被成功观测,从此激光拉曼散射成为众多领域在分子原子尺度上的重要研究工具。利用受激拉曼散射效应,可以对现有激光波长进行变换,获得更丰富的激光波长输出,以满足在生物医药、通讯、工业、军事等领域的广泛应用需求。

相比脉冲激光器中基频光具有较高峰值功率,比较容易达到拉曼转换阈值,连续波拉曼转换更加难以实现。而且,由于激光器在连续运转方式时工作的热效应更为严重,限制了激光器性能的提升,因此要实现高效稳定的连续波拉曼激光输出,需要仔细优化设计激光器结构,并进行良好的热管理。

扬州大学樊莉副教授领导的研究小组采用波长锁定窄线宽的879 nm激光二极管(LD)半导体激光器共振泵浦键合Nd:YVO4激光晶体,在采用共振泵浦技术和键合晶体改善晶体热效应的同时,利用LD的发射波长与晶体吸收峰的精确匹配提高了泵浦光的吸收率,进一步提高了腔内产生的基频激光功率。

连续波全固态拉曼激光器的结构图

为了进一步改善热效应和避免晶体损伤,半导体激光器输出的泵浦光经过不同放大倍率的耦合器放大入射到激光晶体Nd:YVO4上,产生1064 nm的基频激光,并将高增益的BaWO4拉曼晶体直接放入基频激光腔内,利用腔内高的基频激光功率密度来达到受激拉曼转换阈值。最终利用BaWO4晶体的925、332 cm-1和YVO4晶体的890 cm-1拉曼频移峰实现了1103.6、1145.7、1175.9、1180.7、1228.9 nm五个波长的一阶和二阶拉曼激光同时输出。实验中通过对泵浦光斑大小、腔镜曲率半径、晶体长度的一系列优化,获得了最高输出功率为1.24 W的多波长拉曼激光输出,相应的光光转换效率为5.4%。该工作发表在Chinese Optics Letters 2020年第18卷第11期(Li Fan et al., First-Stokes and second-Stokes multi-wavelength continuous-wave operation in Nd:YVO4/BaWO4 Raman laser under in-band pumping)。

研究人员还对不同晶体或同一晶体的不同拉曼频移峰之间的竞争机制进行了详细的理论和实验研究。研究结果表明,可通过控制不同晶体的长度比及基频光的偏振方向来控制不同激光谱线间的竞争。该多波长激光不仅拓宽了现有固体激光器输出波长的范围,并且表明基于固体SRS的激光系统还有很广泛的发展潜力,将在光谱分析、激光干涉仪、差分吸收激光雷达及太赫兹波产生等领域有着潜在的重要应用。

Multi-wavelength continuous-wave operation in Nd:YVO4/BaWO4 Raman laser under in-band pumping

With the wide and expanding applications of laser in medical treatment, communication, industrial development, military and many other fields, existing laser wavelengths are inadequate to meet the growing needs of various applications. The development of new special laser wavelengths has attracted an increasing interest.

Stimulated Raman Scattering (SRS) has been widely recognized as an efficient method of laser frequency conversion. Raman laser is a kind of laser which uses the SRS effect of medium to convert the existing laser wavelength to obtain new laser wavelength output. All-solid-state Raman laser with laser diode pumped and crystal as Raman medium has many excellent characteristics, such as simple structure, high efficiency, high beam quality, and good stability. At present, it has obtained a variety of new wavelength laser output in the ultraviolet to mid infrared spectral ranges, and has found a broad range of applications in many areas, including information technology, transportation, measurement, medicine, military and industrial fields.

Currently, all-solid-state Raman lasers can be divided into continuous-wave (CW) and pulsed operation modes. Due to the high peak power of fundamental laser in the pulsed laser, it is easier to reach the threshold of SRS. Hence continuous-wave Raman laser is more difficult to be realized than pulsed Raman laser. Moreover, thermal effects are more severe when the laser works in continuous operation mode, which limits the improvement of laser performance. Therefore, in order to achieve high efficiency and stable CW Raman laser output, it is necessary to carefully optimize laser structure and carry out good thermal management.

Experimental setup of continuous-wave all-solid-state Raman laser

Recently, the research group led by Prof. Li Fan from Yangzhou University studied the composite Nd:YVO4 laser crystal which is in-band pumped by a wavelength-locked narrow-linewidth 879 nm laser diode (LD) (Li Fan et al., First-Stokes and second-Stokes multi-wavelength continuous-wave operation in Nd:YVO4/BaWO4 Raman laser under in-band pumping). The in-band pumping technology and composite crystal are used to improve thermal effect. At the same time, accurate matching of the emission spectrum of LD and absorption peak of the crystal is used to improve the absorption of pump light, so as to improve the fundamental laser power in the laser cavity.

In order to further improve thermal effect and avoid damaging the crystal, the LD pump beam is amplified and incident on the Nd:YVO4 laser crystal by couplers with different magnification ratios, generating the fundamental laser at 1064 nm. In addition, BaWO4 crystal with high Raman gain is directly put into fundamental laser cavity, using the high power density of fundamental laser in the cavity to achieve the threshold of Raman conversion. Finally, three first-Stokes lasers at 1103.6, 1175.9, and 1180.7 nm and two second-Stokes lasers at 1145.7 and 1228.9 nm are obtained simultaneously using the Raman shifts of 925 and 332 cm-1 in BaWO4 and 890 cm-1 in YVO4. Through a series of optimization of pump spot size, curvature radius of cavity mirror and crystal length, a maximum output power of multi-wavelength Raman laser up to 1.24 W is obtained, and the corresponding optical conversion efficiency is 5.4%.

Besides, competition among different Raman vibrational modes has been investigated theoretically and experimentally. The results show that competition among different laser spectral lines can be managed by controlling the length ratio of different crystals and the polarization direction of fundamental laser. The multi-wavelength lasers not only broaden the range of the existing solid-state laser output wavelengths, but also show that the laser system based on SRS has a wide range of development potential, which may find important applications in spectral analysis, laser interferometer, differential lidar and terahertz wave generation.