In atmospheric transmission windows, ~1.6 μm is a significant waveband, and lasers with emission wavelengths longer than 1.4 μm are called eye-safe wavebands. Based on the above advantages, lasers of 1.6 μm waveband are used as laser sources for aerosol lidar, laser range finder system, and other applications. The pulse energy (or average power) of the laser is a key parameter in determining the measurement distance of this type of instrument, and the large pulse energy of a 1.6 μm waveband laser is imminently required. Several methods for generating lasers with a 1.6 μm waveband, such as Er∶YAG and 1064 nm pumped OPO lasers, are available; each has its advantages and applications. Stimulated Raman scattering (SRS) is a convenient method for generating lasers with new wavelengths. Gaseous Raman lasers have high conversion efficiency, high damage threshold, and high peak power (large pulse energy), among others. In this study, we present a 1.56 μm Raman laser generated by 1064 nm laser-pumped high-pressure deuterium.

A Q-swithed Nd∶YAG laser of 1064 nm is used as the pump source. The laser beam diameter is 8 mm, the maximum output energy is 900 mJ, the full width at half maximum (FWHM) is approximately 10 ns, and the repetition frequency is 10 Hz. Pressurized deuterium gas is used as a laser Raman active medium. The gas pressure varies 0.53.5 MPa. A double focus configuration is used for the SRS experiment. Further, the relationships between the conversion efficiency under different gas pressures, focus numbers, and focus positions are investigated. To study the beam quality of Raman laser, the frequency is doubled to 780 nm.

A double focus configuration is used for the SRS experiment. Experimental parameters, such as the pressure of deuterium and focus condition are optimized under the assistance of theoretical simulation, and the maximum photon conversion efficiency of the Raman laser is 82.4% (Fig. 6). In this study, the hole-burning effect of the Raman gain coefficient is identified and explained. This indicates that a higher pressure of Raman active gas is beneficial for maintaining a high conversion efficiency under the condition of a big pulse energy pump laser. Although a high conversion efficiency can be achieved using double focus configuration, the pulse energy is limited. To increase the pulse energy of the Raman laser, the pressure of the Raman active gas is decreased to 1 MPa, the beam waist size and pump laser energy are increased, and a maximum of 245 mJ Raman laser is achieved, whereas the photon conversion efficiency decreases to 48.8% (Fig. 8). A temperature-matched LBO crystal is used to double the frequency of a 1.56 μm Raman laser, and a 780 nm laser with a maximum conversion efficiency of 36.6% (Fig. 9) is obtained. By comparing the beam qualities of the pump and 780 nm lasers, it is inferred that the thermal effect of the SRS process caused the limited frequency doubling efficiency.

In this study, the highest photon conversion efficiency of the first Stokes laser is 82.4%, which is also the highest conversion efficiency of deuterium gas stimulated Raman scattering pumped by Nd∶YAG 1064 nm wavelength in free space. The phenomenon of Raman gain hole-burning is discovered and explained. Appropriately increasing the gas pressure of Raman medium is conducive for inhibiting Raman gain hole-burning, which is useful in maintaining pump laser high conversion efficiency at the stimulated Raman wavelength shifting of large pulse energy laser. To further improve the pulse energy of the Raman laser, we reduce the pressure of the Raman medium and increase the waist size of the pump laser beam to achieve maximum single pulse energy of 245 mJ. Additionally, we employ LBO crystal to obtain the second-harmonic generation with an output wavelength of 780 nm, maximum energy of 57.3 mJ, and conversion efficiency of 36.1%. The measurement of beam quality confirms that the gas thermal effect degrades the Raman laser beam quality, which reduces the laser energy conversion efficiency of frequency doubling. Raman medium circulating or cooling technology is a method for increasing the pulse energy of the laser.