Trivalent ytterbium ions (Yb³⁺)-doped crystals have been considered one of the most promising active medium for solid-state lasers because it has a small quantum defect, a simple two-manifold structure, a low thermal load, a longer energy-storage lifetime, no upconversion and cross relaxation processes and excited state absorption compared to trivalent neodymium ions (Nd³⁺) [1–5]. Yb³⁺-doped tungstates such as Yb:NaY(WO₄)₂ [6–10], Yb:NaGd(WO₄)₂ [11–14], NaLu(WO₄)₂ [15], Yb:NaLa(WO₄)₂ [16–18] and Yb:KLu(WO₄)₂ [19–21] have played an important role in the development of solid-state lasers due to the broader emission and absorption linewidths. Yb³⁺-doped calcium tungstate (CaWO₄) crystal as an excellent host medium for rare-earth ions, has been widely used in solid-state lasers. Recently, the absorption and emission spectra of Yb:CaWO₄ crystal and its CW lasing properties have been investigated [22–24]. The absorption spectra of the Yb:CaWO₄ crystal from 875 nm to 1075 nm in two polarizatios were carried out in a UV-Vis-IR absorption spectrophotometer (Cary 5000, VARIAN USA). The emission spectra of the Yb:CaWO₄ crystal from 875 nm to 1075 nm in two polarizatios were measured at 875–1075 nm by a steady-state time-resolved fluorescence spectrometer (FLS-980, Edinburgh England) under 976 nm. The emission cross-sections can be calculated from the measured fluorescence spectra by the Füchtbauer-Landenburg equation [25] σ λ = λ 5 ⋅ I λ 8 π n 2 c τ r I λ d λ (1) where λ is the wavelength, I(λ) is the fluorescence intensity, n = 1.91 [26] is the refractive index of the Yb:CaWO₄ crystal, с is the velocity of light, τ _r = τ _f = 428 μs [22], τ _r and τ _f are the radiative lifetime and the fluorescence lifetime, respectively. Calculate using Equation 1, the cross-section curve is shown in Figure 1. It can be seen that there were three absorption peaks in π-polarization, which were 965, 976 and 994 nm respectively, and the corresponding absorption cross-sections (σ _abs,π ) were 2.03, 1.28 and 1.34 × 10⁻²⁰ cm² respectively. Two absorption peaks in σ-polarization were 934 and 975 nm, respectively, and the corresponding absorption cross-sections (σ _abs,σ ) were 1.48 and 1.27 × 10⁻²⁰ cm² respectively. Two emission peaks in π-polarization were 967 and 997 nm respectively, and the corresponding emission cross-sections (σ _em,π ) were 1.97 and 5.61 × 10⁻²⁰ cm² respectively. There was a wide emission spectrum (from 976 to 1024 nm) in σ-polarization, and corresponding to an emission cross-section (σ _em,σ ) of about 2.0 × 10⁻²⁰ cm². The gain cross-sections of the Yb:CaWO₄ crystal from 875 nm to 1075 nm in two polarizations were calculated by σ _g,i = βσ _em,i − (1 − β)σ _abs,i [27], where β is the fraction of Yb³⁺ excited to the upper state, and i = π, σ represents the π- and σ-polarization respectively, as shown in Figure 2. It can be seen from Figure 2 that the Yb:CaWO₄ crystal had a wide gain spectrum in both directions, which made it suitable for tunable laser output. In this work, we realized the first tunable Yb:CaWO₄ laser in NIR spectral range. The laser tuning ranges in π- and σ-polarizations were 42 nm and 41.8 nm, respectively. Continuously broadband tunable wavelengths are obtained in two polarizations by rotating the Lyot filter, which have the potential applications in some fields, such as mid-infrared laser absorption spectroscopy [28], wavelength modulation spectroscopy [29] and photoacoustic spectroscopy [30], etc.

A schematic setup for the diode-pumped tunable Yb:CaWO₄ laser is shown in Figure 3. In our experiment, we used a 9 mm long Yb:CaWO₄ was crystal with a doping concentration of 1.2 at% Yb³⁺, which supplied by Fujian Institute of Material Structure, Chinese Academy of Sciences. The thermal effect of the laser crystal will affect the spectral width of the tunable Yb:CaWO₄ laser, because the increase of the Yb:CaWO₄ crystal temperature will lead to changes in the refractive index and absorption coefficient of the crystal, which will directly affect the output spectral characteristics of the laser. The narrower the spectral line width, the higher the output power will be, because the narrower the spectral line, the lower the intracavity loss, the higher the photon number density, the higher the output power. Therefore, in order to reduce the thermal effect of the Yb:CaWO₄ crystal, we choose the Yb:CaWO₄ crystal with low doping concentration of Yb³⁺, which can reduce the probability of possible nonradiative cross-relaxation processes and the reabsorption of the laser emission. The Yb:CaWO₄ crystal was wrapped in indium foil and mounted on water-cooled copper blocks. The temperature of the water was controlled at 15^oC. The pump source of the tunable Yb:CaWO₄ laser is a diode array with fiber-coupled output, a maximum output power of 20 W and a radius of the pump beam waist of 200 μm. The two identical convex lenses with the focal length of 150 mm, L ₁ and L ₂, coupled the pump beam to the Yb:CaWO₄ crystal, which were antireflection (AR) coated at 976 nm. The plane mirror (M₁) was the input mirror, which was AR coated at 976 nm and high reflectivity (HR) coated at 1000–1050 nm. The concave mirror (M₂) with the radius of curvature of −150 mm was the output mirror, which was with a transmittance of about 3.0% at 1000–1050 nm. The concave mirror (M₃) with the radius of curvature of −300 mm was the reflector, which were HR coated at 1000–1050 nm. To achieve wavelength tuning, a Lyot filter (quartz crystal, thickness d = 2 mm) was inserted into the cavity, which was AR coated at 1000–1050 nm and was which supplied by Jiangyin Yunxiang Optoelectronic Technology Co. , Ltd, China. Figures 3A, B are Lyot filter placed in the π- and σ-polarization, respectively.

The transmittance of Lyot filter, T, can be written as [31]: T = 1 − 4 ⁡ cot 2 ⁡ γ ⁡ tan 2 ⁡ θ 1 − ⁡ cot 2 ⁡ γ ⁡ tan 2 ⁡ θ sin 2 δ / 2 (2) cos ⁡ α = cos ⁡ γ − ⁡ sin ⁡ θ ⁡ sin ⁡ φ cos ⁡ θ ⁡ cos ⁡ φ (3) where γ is angle between the internal ray and the optic axis, θ is incident angle (θ = θ _B = 57.2^o in the experiment), β is angle between the crystal axis and the surface of Lyot filter (φ = 0 in the experiment), δ = 2πd (n _o –n _e )sin² γ/λsinθ is the optical phase difference. According to Equations 2, 3, the angle, α (rotation angle) is changed by rotating the Lyot filter, the transmittance of Lyot filter, T, is also changed. Therefore, by rotating the Lyot filter, we could change its transmittance to different wavelengths in the NIR region, resulting in the continuously tunable laser output. The relationship between the rotation angle and the laser wavelength is calculated (T = 1), as shown in Figure 4. As can be seen from Figure 4, the different rotation angle, α, corresponds to different laser wavelength (the maximum transmittance, T = 1), thus the corresponding tunable wavelength output can be realized.

At an absorbed pump power of 15.6 W (or an incident pump power of 20 W), the output powers of the Yb:CaWO₄ laser for output wavelengths in the π-polarization are shown in Figure 5. As can be seen from Figure 5, the peak power is 5.2 W at 1026.2 nm in the π-polarization. The input-output performance of the CW 1026.2 nm Yb:CaWO₄ laser is shown in Figure 6. The oscillation threshold is 0.52 W. The slope efficiency and the optical-to-optical efficiency with respect to the absorbed pump power are 34.6% and 33.3%, respectively. The quality factor of the laser beam M² = 1.21. The stability of the output power is about 3.2% in 1 h. Using a LABRAM-UV spectrum analyzer to scan the output beam and dealing with the data with software, the tuning spectra of the Yb:CaWO₄ laser at the absorbed pump power of 15.6 W is shown in Figure 7. As can be seen from Figure 7, the Yb:CaWO₄ laser realized tuning wavelength from 1005.2 nm to 1047.2 nm in the π-polarization. The width of wavelength tuning in the NIR spectral range reached 42 nm.

Similarly, at an absorbed pump power of 15.6 W, the output powers of the Yb:CaWO₄ laser in σ-polarization are also shown in Figure 5. As can be seen from Figure 6, the peak power is 4.7 W at 1028.1 nm in the σ-polarization. The input-output performance of the CW 1028.1 nm Yb:CaWO₄ laser is also shown in Figure 6. The oscillation threshold is 0.67 W. The slope efficiency and the optical-to-optical efficiency with respect to the absorbed pump power are 30.3% and 30.1%, respectively. The quality factor of the laser beam M² = 1.26. The stability of the output power is about 2.7% in 1 h. The spectra of the Yb:CaWO₄ laser at the absorbed pump power of 15.6 W is shown in Figure 8. As can be seen from Figure 8, the Yb:CaWO₄ laser realized tuning wavelength from 1005.1 nm to 1046.9 nm in the σ-polarization. The width of wavelength tuning in the NIR spectral range reached 41.8 nm. At the highest output power, the output beam profile of each tuned wavelength in both polarized directions was measured, which exhibited almost Gaussian distribution along both axes.

In conclusion, we first demonstrate a diode pumped continuously tunable Yb:CaWO₄ laser in NIR spectral regions. The tuning widths of the output wavelengths in the π- and σ-polarizations are 42 nm (from 1005.2 nm to 1047.2 nm) and 41.8 nm (from 1005.1 nm to 1069.9 nm), respectively. Continuously broadband tunable wavelengths are obtained in two polarizations by rotating the Lyot filter, respectively. At an absorbed pump power of 15.6 W at 976 nm, the maximum output powers in the π- and σ-polarization are 5.2 W at 1026.2 nm and 4.7 W at 1028.1 nm, respectively. To the best of our knowledge, this is the first tunable laser operation by using Yb:CaWO₄ crystal. We believe that the same technology can be applied to other Yb³⁺-doped tungstate crystals to realize tunable laser output.