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.
