Objective The 1197 nm laser is located in the photoacoustic imaging window of the C--H bond, which can be used for the clinical diagnosis and treatment of biological tissues, such as tumors and fats. Additionally, its frequency-doubled yellow light has a wide range of applications in the field of fluorescence detection. However, the 1197 nm Raman laser suffers from low energy and poor reliability, making it unsuitable for engineering applications. In this paper, we conducted experiments to investigate the ways to improve the energy and efficiency of the 1197 nm Raman laser. On the one hand, the beam distribution and energy of fundamental light were improved to demonstrate their effect on Raman efficiency. On the other hand, the Raman laser was optimized theoretically and experimentally. A high-efficiency and high-energy solid-state Raman laser is studied and a prototype of this laser is developed to test its performance.
Methods In this study, we investigated an external resonator stimulated by Raman scattering technology and demonstrated a Ba(NO3)2 Raman laser pumped by a Q-switched 1064 nm laser. First, a xenon-lamp-pumped Nd∶YAG laser was constructed. Experiments were conducted using three different cavity length conditions to determine the best cavity length of the resonator and simultaneously increase the output energy. To improve the uniformity of the 1064 nm beam, the laser output coupler adopted a variable reflectivity output mirror. Then, the beam diameter of the Nd∶YAG laser was compressed to 5 mm using a 1.5× telescope to increase the peak power density of the fundamental light. Thereafter, we optimized the parameters of the external-resonator Raman laser according to the rate equation and radiation transmission theories, such as the optimum reflectivity, the threshold of first-order Stokes light, and the highest conversion efficiency. Moreover, the output coupler of the Raman laser was selectively coated to suppress the oscillation of high-order Stokes light. Then, the external-cavity Raman laser experiment was performed and the differences between the experimental results and theoretical values were compared. Finally, a prototype of a 1197 nm Raman laser was built to verify its reliability and stability.
Results and Discussions The experimental results of the 1064 nm and 1197 nm resonators are shown in this study. Figure 3(a) shows that 305 mm is the best cavity length under the three cavity length conditions. The maximum output energy, wavelength, and pulse width are 334 mJ, 1064.3 nm, and 10.5 ns, respectively. When the variable reflectivity output coupler is used, the 1064 nm beam is uniform with a flat top distribution [Fig. 3(b)] and the divergence angle is 1.1 mrad. The fundamental beam passed through a 1.5× telescope, and its peak power density was increased to 145.5 MW/cm 2. According to theoretical calculations of the external-resonator Raman laser, the best reflectivity of the output coupler is 55% and the highest conversion efficiency is 52%. Figures 4 and 5 show the output characteristics of the 1197 nm Raman laser. The best transmittance of the output coupler is 40%, which is consistent with the theory. When a 300 mJ 1064 nm laser was injected into the barium nitrate crystal, a steady laser at 1197.81 nm with an output energy of 135 mJ, a pulse width of 7.4 ns, and a divergence angle of 5 mrad was obtained. The maximum conversion efficiency was 46.6%, which was slightly different from the theory. This is because the second-order Stokes light starts to oscillate, and the thermal effect of Raman crystal becomes more severe. A prototype of the 1197 nm Raman laser was developed, as shown in Fig. 6(b). The energy stability (RMS) within 1 h is 0.48% [Fig. 6(a)], indicating that both the output characteristics and reliability meet the requirements of engineering applications.
Conclusions We study the solid-state laser technology based on external-cavity stimulated Raman scattering and fabricate a high-efficiency and large energy 1197 nm pulse laser. By varying the resonant cavity length and using a variable reflectivity output coupler, the energy and beam distribution of the fundamental light are optimized. The maximum output wavelength of the 1064 nm laser is 334 mJ, and the pulse width is 10.5 ns. The fundamental beam is compressed by a 1.5×telescope to increase the peak power density and improve the Raman conversion efficiency. In addition, the selective coating on the output coupler suppresses the oscillation of high-order Stokes light, increasing the output energy of the first-order Stokes light. When the injected energy reaches 300 mJ, we obtained a 1197.81 nm laser of 137 mJ with a pulse width of 7.4 ns. The maximum Raman conversion efficiency is 46.6%, and the energy stability (RMS) within 1 h is 0.48%. Generally, the prototype of the 1197 nm Raman laser has been successfully used in medical photoacoustic imaging. The system has the advantages of high output energy and conversion efficiency, good energy stability, and low cost. It can provide high-quality light sources for photoacoustic imaging and fluorescence detection and has broad application prospects.
