Keywords: 
• Raman laser /
• diamond /
• multi-wavelength /
• pulse /
• high-power

Abstract:   Objective  The all-solid-state multi-wavelength red laser has significant applications in laser color large-screen displays, high-density holographic storage, measurement, and medical treatment. Its multi-wavelength characteristics also enable it to serve as a terahertz light source through difference-frequency generation. Currently, the multi-wavelength red laser can be generated by combining the emission spectrum of an inversion particle gain medium with second-order nonlinear effects. However, these methods typically have lower conversion efficiency. Stimulated Raman scattering (SRS) is a high-intensity third-order nonlinear effect that offers flexible wavelength conversion, automatic phase matching, and beam cleanup. The cascaded frequency shift property of Raman crystals is an effective method for achieving multi-wavelength output using a single pump wavelength. Diamond crystals have a high Raman gain coefficient in the visible wavelength range compared to conventional Raman crystals. Pumping diamond with a well-established 532 nm laser has great potential for obtaining efficient, high-energy, high-beam quality multi-wavelength red laser output. In this study, we investigate the generation of multi-wavelength red laser output using cascaded diamond Raman oscillators pumped by a 532 nm laser and explore their output characteristics.   Methods  The setup of the multi-wavelength red diamond Raman laser is shown (Fig.1). The pump source is a self-built 532 nm frequency doubled nanosecond laser. The pump beam is collimated by the lens group F1 and F2. A half-wave plate (HWP) is used to adjust the polarization direction of the pump to be parallel to the <111> axis of the diamond crystal for the maximum Raman gain. The diamond Raman oscillator uses a plane-concave cavity with a curvature radius of 200 mm as the output mirror. The diamond size is 2 mm× 4 mm× 7 mm. The coating parameters of the two cavity mirrors are shown (Tab.1). The cavity mirrors are high reflection coated at first-order Stokes to increase the conversion efficiency and obtain pure higher-order Stokes output. The lens F3 is used to control the pump radius in the diamond crystal to about 350 μm. The total length of the Raman cavity is 60 mm, and the distance from the output coupler to the end surface of the diamond is 7 mm. The intrinsic modes of the Raman cavity for each order of Stokes are shown (Fig.2), with a diamond between the purple dashed lines. The radius of the TEM₀₀ modes of the first, second, third and fourth-order Stokes are 128, 133, 139, 146 μm, respectively.   Results and Discussions  The spectra of second-order Stokes, second- and third-order Stokes, and second- to fourth-order Stokes were collected at pump energies of 343, 437, 1165 μJ, respectively (Fig.3). The frequency shift between each Stokes order was 1 332 cm⁻¹, consistent with the inherent Raman frequency shift of diamond. With a maximum pump energy of 1 738 μJ (Fig.4(a)), three wavelength lasing in red with energies of 143, 425, 65 μJ were obtained, with slope efficiencies of 9.7%, 31.3%, and 8.7%, respectively. The conversion efficiency increases with pump energy and levels off (Fig.4(b)). A multi-wavelength red laser output energy of 633 μJ was obtained at a maximum pump energy of 1 738 μJ, with a slope efficiency of 45.3% and an optical-to-optical conversion efficiency of 36.4%. The temporal waveform of the incident pump at 532 nm and the output Stokes of each order at maximum pump energy were measured to be 11.43, 10.41, 3.75, 2.45 ns, respectively (Fig.5). The pulse width of each Stokes order is compressed compared to the pump, with more evident compression as the Raman order increases. The near-field spot of each Stokes order has no obvious distortion. The optical-to-optical conversion efficiency can be improved by optimizing the Raman cavity mode-matching degree, and the energy ratio of each wavelength in the multi-wavelength output can be controlled by designing the mirror coating.   Conclusions  In this study, we developed a 532 nm pumped multi-wavelength diamond Raman laser and investigated its cascaded Raman laser output energy, spectrum, and pulse characteristics at different pump energies. Cascaded Raman outputs of 620, 676, and 743 nm were successfully demonstrated. With a maximum pump energy of 1 738 μJ, the output energies of 143 μJ at 620 nm, 425 μJ at 676 nm, and 65 μJ at 743 nm were achieved, with pulse widths of 10.41, 3.75, and 2.45 ns, respectively. Meanwhile, the near-field beams of all the orders exhibit good spatial distribution. The output energy of the combined multi-wavelength red laser was 633 μJ, with an optical-optical conversion efficiency of 36.4%. The results show that the visible light-pumped diamond Raman laser has tremendous potential for efficient all-solid-state miniaturized multi-wavelength lasers in red due to its extremely high Raman gain coefficient and excellent photothermal properties. This study can also provide guidance for the development of multi-wavelength Raman lasers pumped by other wavelengths.