Abstract
1 Introduction
Lasers with 2 μm waveband have been widely used in various technical fields, including medical laser application[
A reflecting volume Bragg grating (VBG) is a kind of narrow-band-pass filter element based on the Bragg condition. It can be used to replace the laser resonator mirror of Tm:YLF laser to achieve narrow-linewidth laser output[
At present, there are few reports about Tm:YLF lasers with an output power level of 200 W, but the previous reports are based on Tm crystals dual-end-pumped by LD stacks, where the beam quality factor M2 in one direction is very poor. In 2013, Li et al. reported that a 200 W Innoslab Tm:YLF laser output had a strong elliptical beam shape with a Gaussian distribution along the semi-minor axis and a top-hat-like distribution beam shape along the semi-major axis[
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In this paper, a 202 W Tm:YLF laser with relatively good beam quality and narrow linewidth near 1908 nm was demonstrated by using two crystals in series and a dual-end-pumped structure for each crystal. To the best of the authors’ knowledge, this is the first report on such high-power output with VBG as an output coupler for a Tm:YLF slab laser. To suppress the wavelength shift to the water absorption line near 1908.9 nm, the active heat dissipation methods were used to cool the VBG, including microchannel cooling and temperature control with a thermoelectric cooler (TEC). The results show that active cooling can effectively restrain the growth rate of the laser output wavelength. Without active cooling, the laser wavelength was 1908.8 nm at 160 W; with TEC, the laser wavelength was 1908.64 nm at 201 W; with microchannel cooling, the laser wavelength was 1908.5 nm at 202 W. In this way, the Tm:YLF laser was operated at 1908.5 nm with linewidth (full width at half maximum, FWHM) 0.57 nm. Under an incident pump power of 553 W, the maximum continuous wave (CW) output power of 202 W at 1908.5 nm was obtained, corresponding to the slope efficiency of 39.7% and optical-to-optical conversion efficiency of 36.5%, where the beam quality factors M2 were 2.3 and 4.0 for horizontal and vertical directions, respectively.
2 Experimental setup
The experimental setup of the Tm:YLF slab laser is shown in Figure 1. A double-crystal series and a dual-end-pump configuration were utilized in this experiment. The four pump lasers were 150 W fiber-coupled LDs with core diameter of 400 μm, numerical aperture of 0.2, and central wavelength of 792 nm at the highest power. The radiation of each LD was coupled into the laser crystal by the same focusing optical system composed of two spherical lenses with focal lengths f1 = 13 mm and f2 = 55 mm, generating a pump beam diameter of about 1.7 mm within the laser crystals. Between f1 and f2, a 45° high-reflection (HR) mirror M2 was used to change the direction of the 792 nm radiation. In this work, two 2 at.%, 2 mm × 6 mm × 40 mm and a-cut Tm:YLF crystals were wrapped with indium foil and mounted in copper blocks cooled by water at a temperature of 290 K. The two crystals were connected in series in the resonant cavity. The whole cavity consisted of four 45° dichroic mirrors M1, a VBG, and a reflector M3. One side of the mirror M1 was coated with 792 nm anti-reflection coating, the other side was coated with HR material at 1908–2090 nm (R > 97%) and highly transmitting at 792 nm. The mirror M3 was a concave mirror with a radius of curvature of 200 mm and HR coated at 1.9 μm (R > 99.7%). As the output coupler of the laser, the VBG had a clear aperture of 3 mm × 4 mm and a thickness of 2.4 mm and its diffraction efficiency was about 58.4% at 1907.3 nm, whose spectral selectivity was less than 0.7 nm. It was wrapped in indium foil and mounted on a copper heat sink. In addition, the physical cavity length of the Tm:YLF laser was 230 mm.
Figure 1.Diagrammatic sketch of the experimental setup.
3 Results and discussion
First, without active cooling, the emission wavelength shifted from 1907.3 to 1908.8 nm and the corresponding volume grating temperature increased from 302 to 391 K, when the output power was increased from 7 to 160 W, as shown in Figure 3(a). The temperature was measured three times and averaged at the same output power, recorded by a temperature-measuring infrared thermal imager (DALI LT7-P). The wavelength was shifted 1.5 nm. This meant that if we continued to increase the output power of the laser, its wavelength was likely to drift to the water absorption line near 1908.97 nm as shown in Figure 2, which would cause damage to the laser. The reason for the wavelength shift is that VBG has a small absorption coefficient (less than 0.01 cm–1[
Figure 2.Water absorption spectrum near 1908 nm (plotted using HITRAN data[11]) and laser wavelength shift.
Figure 3.Dependence of the laser wavelength on temperature without active cooling in the VBG: (a) wavelength at different output power and corresponding temperature under different output power; (b) fitting of the relationship between wavelength and temperature, and corresponding theoretical curve.
Owing to the change in spatial period caused by the non-uniform thermal expansion and the change in refractive index caused by the thermal dispersion coefficient, some wavelength components of the incident laser deviate from the Bragg condition,
In Equation (2), ρ is the thermal expansion coefficient and dn0/dT is the thermal dispersion coefficient. For PTR glass, the values are
The general solution of Equation (3) is
Here
Equation (5) shows that the dependence of Bragg wavelength on temperature can be approximately linear, described by the slope of the expression with dλB/dT. We assume that the Bragg wavelength of the VBG is 1907.3 nm at 293 K to calculate the dependence. The constant
The output characteristics with different heat dissipation methods of the VBG are compared in Figure 4. There were two active cooling methods, the first was a microchannel cooler and the second was a TEC. The ambient temperature of the VBG was controlled at 290 K with these two methods. As shown in Figure 4(a), the slope efficiencies under different conditions were 39.7%, 38.2%, and 39.9% and the corresponding thresholds were 54.7, 51.5, and 57.6 W, respectively. According to the existing literature, it can be shown that a VBG used as a cavity end mirror of laser has the thermal lensing effect[
Figure 4.Comparison of CW laser performance, including (a) output power and (b) wavelength under different heat dissipation methods for the VBG.
The 10/90 knife edge method was used to measure the spot radius at different positions, and the beam quality factor M2 was further obtained by fitting the spot change curve. As shown in Figure 5, the beam quality factor M2 was compared under different output power. When the CW output power was 160 W without active heat dissipation for the VBG, the beam quality factor in x direction was M2
Figure 5.Beam quality at different average power levels: (a) beam quality of 160 W Tm:YLF laser without active cooling; (b) beam quality of 202 W Tm:YLF laser with microchannel cooling.
In this paper, the thermal lensing effect in the Tm:YLF crystal is not negligible under high pump power[
Microchannel cooling was used in the follow-up experiment. From Figure 3(a), the slope efficiency of the laser was 39.7%, and the optical-to-optical conversion efficiency was 36.5%. The central wavelength was 1908.5 nm recorded by a wavemeter (721A, Bristol) and the linewidth (FWHM) was less than 0.6 nm at 202 W, as shown in Figure 6. In addition, the laser was not damaged during the 40 min free operation of the laser at room temperature, and the power fluctuation was less than 1% of the maximum power.
Figure 6.Spectrum of the Tm:YLF laser.
4 Conclusion
In summary, we have studied a 202 W two-crystal-in-series and dual-end-pumped Tm:YLF slab laser with a reflecting VBG as an output coupler. In order to suppress the growth of wavelength, the active heat dissipation methods were used for the VBG. The results show that the shift of wavelength was better suppressed by active heat dissipations; in particular, the effect of microchannel cooler was more prominent. When a microchannel cooler was used, the maximum CW output powers of 202 W at 1908.5 nm with linewidth (FWHM) 0.57 nm were obtained under an incident pump power of 553 W, corresponding to a slope efficiency of 39.7% and optical-to-optical conversion efficiency of 36.5%. We could also see the wavelength from 1907.5 nm at 30 W to 1908.5 nm at 202 W only shifted by 1.0 nm, which was much better than that without active cooling. The beam quality factors M2 were 2.3 and 4.0 for horizontal and vertical directions at 202 W, respectively.
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