The wavelengths of the Tm:YLF laser were measured in the experiment by a Fourier transform infrared spectrometer (Nicolet iS50 FTIR), as shown in Fig. 2. The wavelength of the Tm:YLF laser varied with the operating temperature of the VBG. The wavelength increased from 1906.04 to 1908.83 nm, and the corresponding VBG operating temperature increased from 70℃ to 210℃ at the output power of 39.8 W. The wavelength tuning range was 2.79 nm.
The output powers of the Tm:YLF laser were measured at different VBG temperature. The initial operating temperature of the VBG was set to 70°C and high output power experiment was not carried out at 110°C to avoid crystal damage due to the water absorption wavelength in the near-infrared band. The output powers of the Tm:YLF laser at different VBG temperature and at pump power of 105.5 W were shown in Fig. 2. The minimum and maximum output powers in the entire tuning range were 39.8 and 42.9 W, and the corresponding output wavelengths were 1906.04 and 1907.96 nm, respectively.
The linewidths of the Tm:YLF laser at different VBG temperature are shown in Fig. 3.
The color curves represent the Tm:YLF laser output spectra at different VBG temperature of 70℃, 90℃, 130℃, 150℃, 170℃, 190℃, and 210℃; where the center wavelengths were 1906.04, 1906.48, 1907.36, 1907.55, 1907.96, 1908.40, and 1908.83 nm, respectively. The corresponding linewidths were 0.39, 0.38, 0.35, 0.41, 0.34, 0.34, and 0.35 nm. The Tm:YLF laser exhibited narrow linewidth and high stability, and the linewidths were between 0.34 nm and 0.41 nm in the entire tuning range.
The 10/90 knife edge technology was used to measure the beam radius of the Tm:YLF laser, and the beam quality factor was calculated using Gaussian fitting. The beam quality factors in the horizontal and vertical directions at different output wavelengths were approximately 3.3 and 3.1, respectively, as shown in Fig. 4.
The beam quality remained almost constant over the entire tuning range, which was favorable for pumping the Ho:YAG laser. Therefore, a narrow linewidth 1.91 µm source was obtained with a stable beam quality, tuning range of 1906.04–1908.83 nm, and output power of 39.8 W.
The measured absorption spectrum of the Ho:YAG crystal in the 1902 to 1912 nm range at room temperature (20℃) is represented by the black line in Fig. 5. The red squares represent the maximum output powers of the Ho:YAG laser corresponding to different pump wavelengths under the pump power of 39.8 W. The output powers aligned much better with the Ho:YAG absorption spectrum in the tuning range of 1906.04–1908.83 nm. The results demonstrated that the transmittance at different pump wavelengths had an effect on the Ho:YAG laser output power.
The output characteristics of the Ho:YAG laser versus the tunable pump wavelength are shown in table. 1. By comparing the output power of Ho:YAG laser at different pump wavelengths, it was found that the output power was at 21.04–23.53 W and the corresponding slope efficiency was at 64.08% − 68.26%. In addition, the conversion efficiency was between 52.86% and 59.12% over the entire tuning range. Additionally, the laser had high conversion efficiency at different pump wavelengths, and the higher power of Ho:YAG laser could be obtained by fine-tuning the pump wavelength.
Table 1
Output characteristics of Ho:YAG laser.
Pump wavelength (nm) | Output power (W) | Slope Efficiency | conversion efficiency |
1906.04 | 22.55 | 66.70% | 56.66% |
1906.48 | 22.84 | 68.08% | 57.39% |
1907.36 | 23.53 | 68.26% | 59.12% |
1907.55 | 23.18 | 68.01% | 58.24% |
1907.96 | 22.35 | 66.10% | 56.16% |
1908.40 | 21.37 | 64.24% | 53.69% |
1908.83 | 21.04 | 64.08% | 52.86% |
The spectrum of the Ho:YAG laser was measured using a Fourier transform infrared spectrometer (Nicolet iS50 FTIR). The full width at half maximum (FWHM) was 0.65 nm and the wavelength peak at 2097.38 nm, as shown in Fig. 6.
The beam radius was measured at an output power of 23.53 W using 90/10 knife edge technology. The beam quality factor M2 was calculated with Gaussian fitting, as shown in Fig. 7. The beam quality factors in the horizontal and vertical directions were 2.4 and 2.8, respectively. The inset in Fig. 7 shows the transverse beam profile recorded by a pyroelectric camera (Pyrocam III, Spiricon).