Multinational research team achieves breakthrough in diamond Raman laser oscillator

Recently, the team led by Professor Lv Zhiwei and Professor Bai Zhenxu from Hebei University of Technology, in collaboration with Professor Richard Mildren from Macquarie University in Australia and Professor Takashige Omatsu from Chiba University in Japan, successfully achieved direct output of Raman vortex optical rotation with large wavelength extension in a diamond Raman laser oscillator. This breakthrough research achievement was recently published as a cover paper in the authoritative journal ACS Photonics in the field of optics (Chen H., Bai Z., Chen J., Li X., Zhu Z.H., Wang Y., Omatsu T., Mildren R.P., and Lu Z. Diamond Raman Vortex Lasers. ACS Photonics, 2025, 12 (2): 864-869).

Diamond, with its wide spectral transmission range and excellent thermophysical properties, exhibits unique advantages and enormous potential in expanding the wavelength of vortex light. The team innovatively combines a simple and efficient method of generating vortex rotation inside the cavity - off-axis pumping - with a traditional external cavity dual mirror standing wave diamond Raman oscillator, using a 1 μ m laser as the pump light source. By precisely adjusting the off-axis angle of the resonant cavity output mirror, Raman laser outputs of 1.2 μ m and 1.5 μ m were obtained in first-order and second-order diamond Raman oscillators, respectively, and high-quality beam control of Gaussian fundamental mode, Hermite Gaussian (HG) mode, and Laguerre Gaussian (LG) mode was successfully achieved.

First-order Raman vortex beam
In the first-order diamond Raman conversion experiment, the research team used an output mirror with a transmittance of less than 0.5% at the first-order Raman wavelength and constructed a quasi concentric resonant cavity structure. By precisely adjusting the rotation angle of the output mirror in different directions, they successfully obtained multiple modes of 1.2 μ m laser output as shown in Figure 1 (a). When the resonant cavity is in a collimated state, the laser output exhibits Gaussian fundamental modes; When the output mirror rotates off-axis in the horizontal and vertical plane directions, it produces HG1,0 and HG0,1 mode outputs; When the output mirror rotates along a 45 ° diagonal direction, an LG mode output with a hollow intensity distribution is obtained. Furthermore, by performing interferometric measurements on the LG mode beam (as shown in Figure 2 (b)), it was confirmed that it has a spiral phase distribution, indicating that it is a vortex beam. The corresponding spectral characteristics are shown in Figure 2 (c). At the maximum pump power, the experiment achieved a Gaussian fundamental mode laser output of 65.5 W and an LG mode output of 42.2 W, with corresponding conversion efficiencies of 23.8% and 15.3%, respectively.

Figure 1. First order Raman vortex rotation results: (a) First order Stokes output modes at different off-axis angles of the output mirror (b) LG mode interference results (c) First order Stokes output spectra

Second-order Raman vortex beam
In order to further expand the working wavelength range of Raman vortex rotation, the research team used the same off-axis control method in a second-order diamond Raman oscillator and successfully obtained 1.5 μ m laser outputs of different modes. The experimental results are shown in Figure 2. At the maximum pump power, a Gaussian fundamental mode output of 119.4 W and an LG mode second-order Stokes output of 22.2 W were achieved, further verifying the effectiveness and scalability of this method in high-order Raman conversion.

Figure 2. Second order Raman vortex optical rotation results: (a) Second order Stokes output modes at different off-axis angles of the output mirror (b) Second order Stokes output spectra and corresponding LG mode interference results

Summary and Outlook
As a new type of optical crystal with excellent performance, diamond has received widespread attention and achieved rapid development in recent years due to its wide spectral transmission range and outstanding thermal properties. The team from Hebei University of Technology innovatively combined a simple off-axis pumping method with traditional external cavity diamond Raman oscillators, achieving direct output of 1.2 μ m and 1.5 μ m diamond vortex rotation in first-order and cascaded diamond Raman oscillators for the first time. This study not only demonstrates the unique advantages of diamond in expanding the wavelength of vortex light, but also further broadens the application boundaries of diamond laser technology, providing new ideas and technical support for the efficient generation of high-power, multi wavelength vortex light.

Chen Hui, a doctoral student at Hebei University of Technology, is the main author of this achievement, and the team leader is Professor Lv Zhiwei, the director of the university's academic committee and the director of the Advanced Laser Technology Research Center. This work has received funding from the National Natural Science Foundation of China and the Outstanding Youth Science Foundation of Hebei Province. The team conducted systematic research on diamond Raman laser, diamond Brillouin laser, diamond structured light, and thermal management of diamond lasers. The team members have led over 100 scientific research projects, including the National Natural Science Foundation of China's major research instrument development projects, the National Key R&D Program projects, the National Defense 173 Key Project, and the Equipment Development Department's field funds. They have won more than 10 scientific and technological awards, including the First Prize for Military Science and Technology Progress of the Central Military Commission, the Second Prize for Technical Invention of Hebei Province, the Natural Science Award of the Chinese Optical Engineering Society, the Excellence Award of the "Insight Action" Equipment Competition, as well as more than 10 academic awards, such as the Individual Award of the International Optical Engineering Society, the First Prize of "Rising Stars of Light", and the Excellent Young Scholar Award of Functional Diamond.

Source: Yangtze River Delta Laser Alliance