# Demonstration of Diode-Pumped Yb:LaF\({}_{3}\) and Tm,Ho:LaF\({}_{3}\) Lasers
## 3 Continuous-Wave Laser Experiment Operation  
The respective properties of CW lasers were researched with the Yb:LaF\({}_{3}\) and Tm,Ho:LaF\({}_{3}\) samples by building laser systems, which are presented in Figure 3. Two commercial LDs were used as the pump source. Matching their absorption wavelengths, an LD with a center wavelength of 974 nm was applied to the Yb:LaF\({}_{3}\) laser, and the Tm,Ho:LaF\({}_{3}\) laser was used with an LD with a central wavelength of 793 nm. The LDs both had a fiber core diameter of 200 \(\upmu\)m and a numerical aperture of 0.22. The pump source laser they sent was focused by a 1:1 optics coupling system. In order to effectively dissipate the heat, the Yb:LaF\({}_{3}\) and Tm,Ho:LaF\({}_{3}\) samples were both wrapped with indium foil and mounted in a water-cooled system where the water was maintained at 20 \({}^{\circ}\)C temperature. Two mirrors (M1 and M2) formed a laser cavity with a length of 35 mm. One of them (M1) had a curvature of 100 mm, and the other (M2) was a flat mirror. For the Yb:LaF\({}_{3}\) experiment, M1 was the input mirror having high transmission at the pump wavelength and high reflection at 1030-1090 nm. M2 was the output mirror, having transmission values of 1%, 2%, or 5% at 1030-1090 nm. M3 was the same type of mirror as M1 and was used to separate the pump laser and the Yb:LaF\({}_{3}\) laser. The pump laser of 974 nm was transmitted in Direction 2 (D2), and the Yb:LaF\({}_{3}\) laser was reflected in Direction 1 (D1). For the Tm,Ho:LaF\({}_{3}\) laser, M1 had high transmission at a 793 nm wavelength and high reflection at 1.9-2.1 \(\upmu\)m. There were two kinds of mirrors (M2) with transmission values of 2% and 5% at 1.9-2.1 \(\upmu\)m. M3 was a beam splitter mirror. At a 45\({}^{\circ}\) angle, the 793 nm laser was reflected in D1, whereas the target laser was obtained in D2. The Yb:LaF\({}_{3}\) and Tm,Ho:LaF\({}_{3}\) samples were each placed near the M1 mirrors. Figure 3: Experimental setup of the continuous-wave (CW) laser. LD: laser diode; M1 and M2: two cavity mirrors; M3: beam splitter mirror; D1 and D2: direction 1 and 2. Figure 2: Spectral information for Tm,Ho:LaF\({}_{3}\) materials: (**a**) absorption spectrum, (**b**) emission spectrum. First, the absorption efficiency was measured for the Yb:LaF\({}_{3}\) and Tm,Ho:LaF\({}_{3}\) crystals in non-lasing conditions. As shown in Figure 4, as the injection pump power was increased and the absorbed pump power was added linearly. The absorption efficiency of the Yb:LaF\({}_{3}\) crystal was found to be 22.9%, and the maximum injection pump power was 20.13 W. For the Tm,Ho:LaF\({}_{3}\) crystal, the absorption efficiency and the maximum injection pump power were found to be 21.9% and 15.85 W, respectively. Considering that a large amount of pump laser was not absorbed, the M3 mirror was used to split the pump laser and target laser. After the cavity mirrors were adjusted, the CW lasers of the Yb:LaF\({}_{3}\) crystal were obtained in D1. The relationship between the average output power and the absorbed pump power for different transmittance values is illustrated in Figure 5. When the transmittance of M2 was 1% and the absorbed pump power was 1.54 W, the Yb:LaF\({}_{3}\) laser was obtained. When the absorbed pump power was increased to 4.91 W, a maximum average output power of 904 mW was achieved, corresponding to a slope efficiency of 28.2%. With the highest transmittance of 5%, a maximum output power of 656 mW with a slope efficiency of 32.2% was obtained. The optimal average output power was 1.19 W, when the transmittance of output coupler was 2%. The highest slope efficiency was 37.1%. The spectra of the Yb:LaF\({}_{3}\) lasers were recorded using a spectrometer (USB2000, Ocean Optics, Largo, Florida, FL, United States). For different transmittance values of M2, the experimental results are shown in Figure 6 at the maximum average output power. When M2 had transmittance values of 1% and 5%, the output spectra both exhibited a single peak.