Growth, Structure, Spectroscopy, and Laser Performance of Nd : GSAG Crystals

Objective Atmospheric water vapor has a significant impact on the greenhouse effect, water cycle, weather phenomena, atmospheric physical and chemical reactions, and air quality; therefore, the detection of atmospheric water vapor profiles is crucial. Differential absorption lidar (DIAL) is a high-precision, high-spatiotemporal-resolution atmospheric water vapor detection system with important application prospects for airborne and satellite platforms. The absorptivity values of light with wavelengths in the vicinity of 940 nm, 935 nm/936 nm, 942 nm/943 nm, and 944 nm are high in water vapor and are less affected by interference from other gases, making it suitable for lidar emission light sources. Nd:GSAG crystals exhibit excellent radiation resistance and are therefore suitable for use in space environments. It can directly generate the laser with wavelength of 942 nm pumped by laser diode (LD) and has advantages such as high efficiency and stability, long lifespan, light weight, and small volume. It is suitable for use on airborne and satellite platforms. Differential absorption lidar for water vapor detection requires high wavelength stability, linewidth, and spectral purity of the emitted laser. Therefore, further research on the spectral and laser performances of Nd:GSAG, as an excellent 942 nm laser working material, is warranted. In addition, reducing the doping concentration of Nd3+ in garnet laser crystals is expected to increase the fluorescence lifetime, reduce the thermal lensing effect, and improve the laser beam quality. Therefore, optimizing the doping concentration of Nd:GSAG is expected to improve its 942 nm laser efficiency and beam quality. Methods According to the stoichiometric ratio of Nd0.045Gd2.955Sc2Al3O12, the raw materials Gd2O3, Sc2O3, Al2O3, and Nd2O3 are weighed, evenly mixed, pressed into circular blocks, and calcined in a muffle furnace to obtain polycrystalline raw materials. Finally, a single-crystal furnace is used to grow crystals with a size of 50 mm x 70 mm, and the laser ablation (LA)-inductively coupled plasma mass spectrometry (ICP-MS) is used to measure the crystal composition. Single-crystal rocking (XRC) and X-ray powder diffraction (XRD) tests are performed on the crystals using an X-ray diffractometer. The transmittance spectra are measured using an ultraviolet (UV)/visible/near-infrared spectrophotometer. The fluorescence lifetime and emission spectra are obtained using a steady-state/ transient fluorescence spectrometer, wherein the fluorescence lifetime is excited by an optical parametric oscillator and the fluorescence emission spectrum is excited by an 808 nm fiber coupled laser. The pump source in the laser experiment is an 808 nm fiber-coupled laser, and the resonant cavity is a 10-mm long flat cavity. The dimensions of the laser gain medium are 2 mmx 2 mmx 6 mm. Results and Discussions The crystal formula is Nd0.025Gd2.64Sc1.79Al3.28O11.60, in which the Nd3+ doping atomic fraction is 0.94%. Further, the full width at half maximum (FWHM) of the XRC curve is 0.019 degrees , and the XRD peak is consistent with that in the standard card ICSD78052. At the strongest absorption peak of 808.5 nm, the absorption coefficient is 3.79 cm(-1), the absorption cross section is 3.41x10(-20) cm(2), and the FWHM of the absorption peak is 3.23 nm, which is better than that (2.79 nm) of Nd:YAG crystal with doping atomic fraction of 0.6%. Moreover, 1060 nm is the strongest emission wavelength excited at 808 nm, with emission cross-sections of 5.62x10(-20) cm(2) and 2.33x10(-20) cm(2) at 1060 nm and 942 nm, respectively. The fluorescence lifetime is 275 mu s, which is 22 mu s longer than that of Nd : GSAG crystal with doping atomic fraction of 1.20%. The FWHM of the spectrum of the 942 nm laser is 0.53 nm, with a maximum output power of 0.54 W, a conversion efficiency of 5.6%, a slope efficiency of 9.1%, and a laser threshold of 3.35 W. At a laser power of 0.4 W, the beam quality factors Mx2 and My2 in the horizontal and vertical directions are 2.72 and 3.45, respectively. The waist diameter is small and the waist diameters dx and dyin the horizontal and vertical directions are 0.1048 mm and 0.1185 mm, respectively. All indicators are better than those of the 466 nm laser of Nd:YAG crystal with doping atomic fraction of 0.6%. Conclusions The grown Nd: GSAG crystal with doping atomic fraction of 0.94% has good crystal quality. The Nd doping increases the cell parameters and crystal density. At 808.5 nm, the absorption coefficient of the Nd:GSAG crystal with doping atomic fraction of 0.94% is less than that of Nd:GSAG with doping atomic fraction of 1.20%, and the thermal lensing effect can be reduced by increasing the crystal length. The FWHM of the absorption peak is greater than that of Nd:YAG, which has a lower requirement for a pump source. The fluorescence lifetime and emission cross-section at 942 nm are better than those of high-concentration crystals; the grown crystal is therefore more conducive to 942 nm laser output and energy storage. The maximum laser output power, optical conversion efficiency, slope efficiency, laser threshold, and beam quality of the 942 nm laser are superior to those of the 946 nm laser of Nd:YAG crystal with doping atomic fraction of 0.6%. The 942 nm waist diameter and laser spectral FWHM are the smallest among those of the four wavelengths (i.e., 942 nm, 946 nm, 1060 nm, 1064 nm), indicating good monochromaticity. The results indicate that the Nd:GSAG crystals with low doping concentrations exhibit excellent laser performances at 942 nm.