Objective  KTA crystal as Raman gain medium has attracted increasing attention. Its larger Raman gain and smaller Raman shift make it more advantageous for cascade Raman conversion to obtain new wavelength Stokes laser of 1 178 nm and 1 212 nm. The frequency doubling of both two Stokes laser could realize yellow and orange emissions which have important applications in remote sensing, laser medicine and so on. In particular, 1 212 nm laser has an absorption affinity for lipid-rich tissues and can be used to stimulate adipose cells and mesymal fine tissue in subcutaneous tissues, which is an ideal source for laser assisted skin healing and prevention of excessive scar formation. In view of the important applications of 1 178 nm and 1 212 nm laser, KTA cascade Raman operation driven by passive Q-switched laser for these two wavelengths generation was further investigated.

  Methods  The laser system for KTA cascade Raman operation is shown (Fig.1). Nd:YAG/Cr⁴⁺:YAG composite was used for passive Q-switched laser generation. Cr⁴⁺:YAG crystal, with an initial transmittance of about 85%, was used as saturable absorber crystal and diffusion bonded to Nd:YAG crystal to make the laser system more compact and easy to dissipate heat. An x-axis cut KTA crystal with 25 mm in length was used as the Raman crystal. The plano-concave cavity with the total cavity length of about 50 mm guarantees the effective oscillation of the fundamental laser and Raman laser. The specific coating parameters are shown (Fig.2). The transmittance of Stokes wavelengths based on the Raman shift of 234 cm⁻¹ and 671 cm⁻¹ is also given (Fig.2).

  Results and Discussions  With the increase of Raman laser output power, we found the Stokes laser was accompanied by a small amount of yellow laser output, and the corresponding yellow light spectrum was shown (Fig.3). Under an incident pumping power of 10.05 W, an average output power of 280 mW was obtained, and the conversion efficiency is 2.8%. The output power and laser wavelength are shown (Fig.4-5). From the threshold to 8 W incident pump power, the main intensity laser wavelength was 1 178 nm, accompanied by a weak 1 146 nm wavelength. With the further increase of incident pump power, 1 212 nm wavelength laser appeared. As the incident pump power gradually further increased, the proportion of 1 212 nm line in the total output intensity also increased. A dual-wavelength laser with 1 178 nm and 1 212 nm output was obtained under the incident pump power of 10.05 W. The results show that the third- and fourth- Stokes laser generation with a Raman shift of 234 cm⁻¹, as well as the yellow laser produced by frequency doubling of 1 146 nm would lead to gain competition, and reduce the conversion efficiency of Stokes laser at 1 178 nm and 1 212 nm. The recorded pulse profile and pulse train at the highest output power are shown (Fig.7). The pulse repetition frequency was 10.3 kHz and the pulse width was about 1.2 ns. The compact passive Q-switched Raman laser cavity increased the power density, and resulted in the improvement of the Raman conversion, so as to realize the high-order Stokes waves.

  Conclusions  In this study, a diode end-pumped passive Q-switched cascade Raman laser with the Raman shifts of 671 cm⁻¹ and 234 cm⁻¹ based on KTA crystal is reported. Nd:YAG/Cr⁴⁺:YAG composite crystal is used to generate pulsed fundamental laser and then to drive KTA crystal. The output power, spectrum and pulse characteristics of cascade Raman laser with different incident pump power are studied. With the increasing pump power, the output laser wavelength shifted from single wavelength of 1 178 nm based on 671 cm⁻¹ and 234 cm⁻¹ cascaded Raman shifts to dual-wavelength output of 1 178 nm and 1 212 nm. Under an incident pump power of 10.05 W, a dual-wavelength laser with an average output power of 280 mW and a conversion efficiency of 6.2% was obtained. The corresponding pulse width and pulse repetition frequency are 1.2 ns and 10.3 kHz, respectively. The single pulse energy and peak power are 27.2 μJ and 22.7 kW, respectively. The results show that rich Stokes laser wavelengths could be obtained based on two comparable gain Raman shifts of KTA crystal with the coating control of the cavity mirror.