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Abstract
We demonstrated a compact and efficient Ho:YAG slab laser intra-cavity pumped by a Tm:YLF slab laser for the first time. In the Tm:YLF laser operation, the maximum power of 32.1 W with optical-to-optical efficiency of 52.8% was obtained. In the intra-cavity pumped Ho:YAG laser operation, the output power of 12.7 W at 2122 nm was obtained. The beam quality factors M2 in the vertical and horizontal directions were 1.22 and 1.11, respectively. The RMS instability was measured to be lower than 0.1%. To the best of our knowledge, this was the maximum power for the Tm-doped laser intra-cavity pumped Ho-doped laser with near-diffraction-limited beam quality.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The 2 µm laser is useful for coherent Doppler radars, and differential absorption lidars (DIALs), and it can also serve as a pump source for mid-infrared (3-5 µm) optical parametric oscillators [1]. Compared with the traditional Tm-doped laser in-band pumped Ho-doped laser to achieve 2 µm laser [2,3], the Tm-doped laser intra-cavity pumped Ho-doped laser has a more compact structure and achieves an efficient 2 µm laser easily.
In 2013, Zhu et al. proposed the structure of an intra-cavity pumped Ho:YAP laser, and obtained 8 W laser output with beam quality factors M2 of 2.2 [4]. In 2016, Huang et al. achieved the output power of 8.03 W with a wavelength of 2122 nm and beam quality factors M2 of 2.7 and 2.6 in the horizontal and vertical directions respectively, which used Tm:YAG laser intra-cavity pumped Ho:YAG laser [5]. In 2020, Huang et al. developed an intra-cavity pumped Ho:YLF laser based on Tm:YLF laser, achieving a maximum output power of 11.3 W at 2062nm with the beam quality factors M2 in the horizontal and vertical directions of 1.06 and 1.25, respectively [6]. In 2022, Hu et al. compared the laser performance of Ho:YLF, Ho:YAG and Ho:YAP lasers based on Tm:YLF laser, the output power corresponding to 8.63 W, 7.45 W and 3.55 W, respectively [7]. In previous research reports, 11.3 W was the maximum output power of the Tm-doped laser intra-cavity pumped Ho-doped laser with high beam quality.
In addition, other operations of Tm-doped laser intra-cavity pumped Ho-doped laser had also been studied gradually, including Tm:YAG and Ho:SSO lasers [8], Tm:YVO4 and Ho:YAG lasers [9], Tm:YAG and Ho:GTO lasers [10], Tm:YAP and Ho:YAG lasers [11] etc, however, the output powers were all less than 10 W.
In this letter, we demonstrated an efficient and compact Ho:YAG slab laser intra-cavity pumped by a Tm:YLF slab laser for the first time. Tm:YLF slab crystal was pumped by a 792 nm fiber-coupled laser diode (LD). When the incident LD pump power was 60.8 W, the maximum output power provided by Tm:YLF laser was 32.1 W, corresponding to the optical-to-optical efficiency was 52.8%. At the same incident pump power, the output power of Ho:YAG laser which intra-cavity pumped by the Tm:YLF laser was 12.7 W at 2122 nm with the beam quality factors M2 of 1.11 and 1.22 in the horizontal and vertical directions, respectively. As far as we know, 12.7 W was the reported maximum output power of the Ho-doped laser which intra-cavity pumped by Tm-doped laser with near-diffraction-limited beam quality.
2. Experimental setup
The experimental setup was shown in Fig. 1. A fiber-coupled LD (BWT K793DN1RN) which the fiber core diameter was 106.5 µm and the numerical aperture (NA) was 0.22 with a center wavelength of 792 nm was used as the pump source. The pump light was focused to a diameter of approximately 0.8 mm by the focusing lens F = 75 mm. The doping level of the Tm:YLF slab crystal was 3 at.% and the size was 14 mm x 20 mm x 1.5 mm. Both end faces (14 mm x 1.5 mm) were polished and high transmittance (HT) coated for the pump wavelength of 792 nm and the laser wavelength in the range of 1.88-2.15 µm. The doping level of the Ho:YAG slab crystal was 0.8 at.% and the size was 14 mm x 25 mm x 1.5 mm. Both end faces (14 mm x 1.5 mm) were polished and HT coated for the wavelength range of 1.85-2.15 µm. Both crystals were a-cut with the c-axis along 20 mm (or 25 mm). The distance between Tm:YLF and Ho:YAG crystals was 4 mm. And both crystals were installed between two large surfaces of copper heatsinks. The circulating water was passed through the water-cooled base of fiber-coupled LD and the heatsinks of the crystal to control the temperature. The temperature of the circulating water was kept at 16 °C.
In our experiment, a flat-concave structure with a physics cavity length of 85 mm was used for the Tm:YLF laser intra-cavity pumped Ho:YAG laser. The input mirror M1 was coated with HT at 792 nm and high reflection (HR) in the range of 1.88-2.15 µm. The output mirror M2 was coated with different transmittances at 1.88-2.15 µm. The 45° plane mirrors M3 and M4 were used to separate lasers of different wavelengths. The mirror M3 was coated with HR at 792 nm and HT in the range of 1.88-2.15 µm. And the M4 mirror was coated with HT in the range of 1.88-1.95 µm and HR in the range of 2.05-2.15 µm.
3. Intra-cavity thermal effects
The choice of YLF as the matrix material benefits from its remarkable advantages. Figure 2(a) showed the thermal lens focal lengths of Tm:YAP, Tm:YAG and Tm:YLF lasers at the same concentration. Table 1 provided a summary of the parameters utilized in the thermal lens focal simulation. At the incident pump power of 60.8 W, the thermal lens focal lengths of Tm:YAP, Tm:YAG and Tm:YLF lasers were 36 mm, 80 mm, and -122 mm, respectively. The thermal lens effect of the YLF-based crystal was significantly lower than that of YAP-based and YAG-based crystals. However, the fracture limit of Tm:YLF was 40 MPa which was lower than that of Tm:YAP and Tm:YAG (160 MPa for Tm:YAP [12] and 176 MPa for Tm:YAG [13]). Figure 2(b)-(d) showed the calculated temperature and stress distribution of Tm:YLF slab crystal at the incident pump power of 60.8 W. When the incident pump power was 60.8 W, the maximum temperature rise of the crystal was 54 K, and the maximum thermal stress was 39.1 MPa with the pump beam waist radius of 400 µm.
Fig. 2. (a) The calculated thermal focal lengths of Tm:YAP, Tm:YAG, and Tm:YLF lasers. (b) The calculated temperature distribution inside the Tm:YLF crystal at the incident pump power of 60.8 W. (c) The calculated stress distribution at the pump end of the Tm:YLF crystal. (d) The calculated stress distribution at the upper-end face of the Tm:YLF crystal.
Table 1. Parameters used in the thermal lens focal simulation.
The resonator structure of the intra-cavity pumped laser could be equivalently converted as shown in Fig. 3. G1 and G2 were used to describe the isometric Tm:YLF crystal, and FTm represented its negative thermal lens. The Ho:YAG crystal was described by G3 and G4, and FHo was used to describe its positive thermal lens. The thermal-optic coefficient of Tm:YLF crystal was -2.0 × 10−6 K-1 (σ-pol), which negative thermal lens focal length would be conducive to alleviating the thermal effect. The combined thermal lens focal length F could be expressed as
where FTm and FHo were the thermal lens focal lengths of the Tm:YLF and Ho:YAG crystals respectively, and D was the distance between the two crystals. In the cavity, the pump light of fiber-coupled LD was absorbed by Tm:YLF crystal, and the laser generated by Tm:YLF crystal was used as the pump light of Ho:YAG laser.The calculated three-dimensional surface of thermal lens focal lengths of Tm:YLF, Ho:YAG crystals and combined focal length were respectively shown in Fig. 4(a). The negative thermal lens focal length of Tm:YLF crystal weakened the overall thermal effect in the cavity, which was beneficial to the high power output of the intra-cavity pumped Ho:YAG laser. Figure 4(b) simulated the propagation diagram of the beam at different positions in the equivalent resonator when the radius of the pump beam waist was 400 µm under the assumption that the size of the pump spot ωp and the laser spot ωs was equal. The distance between Tm:YLF and Ho:YAG crystal was 4 mm. In the process of increasing the incident pump power, the change in thermal lens focal length led to the change of intracavity mode. In the simulation, at the maximum incident pump power, we set the thermal lens focal lengths of Tm:YLF and Ho:YAG crystals to -122 mm and 59 mm, respectively. The waist spot radius of the Ho:YAG crystal end face was approximately 364 µm which was slightly less than 400 µm. The higher incident pump power density was conducive to the emission of Ho:YAG laser.
Fig. 4. (a) The calculated three-dimensional surface of thermal lens focal lengths. (b) Schematic diagram of calculated intracavity beam propagation.
4. Results and discussions
Figure 5 showed the output characteristics of Tm:YLF slab laser with a physics cavity length of 85 mm. By using different output mirrors, the maximum output power of 32.1 W with a central wavelength of 1908.36 nm was obtained at the incident pump power of 60.8 W, which resulted in an optical-optical efficiency of 52.8% and a slope efficiency of 56.7%, with the OC of transmittance of 20% and radius of curvature of 200 mm.
Fig. 5. Relationship between output power and incident pump power with different output couplers (OCs) of Tm:YLF laser. The insert showed the wavelength of the maximum output power.
A pyroelectric array camera (Pyrocam III No.31804) was used to measure the beam quality by measuring the spot diameter at different positions behind the F = 300 mm spherical lens. The beam quality factors were shown in Fig. 6, when the output power was 32.1 W. The beam quality factors M2 in the vertical and horizontal directions were 1.55 and 1.84, respectively.
Fig. 6. The beam quality of Tm:YLF laser at maximum output power. The inset showed the typical 2D beam profile.
Under the same incident pump power, the relationship of intra-cavity pumped Ho:YAG laser between output power and incident pump power with different OCs was shown in Fig. 7. The maximum output power was 12.7 W, 9.4 W, and 8.0 W with an optical-to-optical efficiency of 20.9%, 15.5% and 13.2% and slope efficiency of 29%, 22% and 18.8%, when the transmittance of 10%, 15%, and 20% were used as OC, respectively. Under the condition of transmittance of 10%, the OCs with a curvature radius of 400 mm, 300 mm, and 200 mm were replaced, and the maximum output power was 12.5 W, 12.2 W, and 11.3 W, respectively. The maximum output power corresponded to the output mirror with a radius of curvature of 500 mm and a transmittance of 10%.
Fig. 7. (a) Relationship between output power and incident pump power with different transmittance of intra-cavity pumped Ho:YAG laser. (b) Relationship between output power and incident pump power with different curvature radii of intra-cavity pumped Ho:YAG laser.
The wavelengths of intra-cavity pumped Ho:YAG laser with transmittances of 10%, 15% and 20% were measured and shown in Fig. 8, respectively. The central wavelength of the left spectrum which was measured at the side of the mirror M4 was approximately 1897nm, corresponding to the output wavelength of the Tm:YLF laser. And the central wavelength of the right spectrum which was measured at the rear of the mirror M4 was approximately 2122.3 nm, corresponding to the output wavelength of the Ho:YAG laser. In the process of increasing the incident pump power, due to the temperature increase caused by heat accumulation, the wavelength would shift slightly to the long wavelength [17]. The central wavelength of the Tm:YLF laser was about 1897nm ± 0.1 nm, and the central wavelength of the Ho:YAG laser was about 2122.3 nm ± 0.1 nm.
According to the absorption and emission cross sections, the gain cross section σg(λ) of Tm:YLF crystal was evaluated by using the classical formula and it was given by [18]
where β was the inversion parameters, σem(λ) was the emission cross-section corresponding to the wavelength λ, and σabs(λ) was the absorption cross section. In addition, we also calculated the inversion parameters and gain spectrum of Ho:YAG crystal. Under steady-state circumstances, the effective single-pass gain coefficient could be given by [19]Figure 9(a) showed the calculated gain cross-section of the 3F4 → 3H6 transition in Tm:YLF laser for various population inversions. The inversion parameter β represented the ratio of the populations of the upper manifolds to the total doped particle number concentration. The calculated β was 0.23, 0.25 and 0.27, when the transmittance of 10%, 15%, and 20% were used as OC, respectively. When the inversion parameter β was in the range of 0.20 to 0.40, laser oscillation may occur at 1860-1940nm and the peak wavelength of the maximum gain cross section occurred at 1908nm. In the intra-cavity pumped laser scheme, the wavelength of the Tm-doped laser was determined by the ground state reabsorption, the cavity round-trip loss and the loss caused by the absorption in Ho:YAG crystal [21,22]. The similar phenomenon that the wavelength of Tm:YLF laser was 1897nm was also described in other reports (see [23,24]). From the absorption spectrum of Ho:YAG crystal in Fig. 9(a), there was strong absorption of Ho:YAG crystal near 1908nm. The increase of loss would result in the wavelength of Tm:YLF laser shift. The wavelength of 1897nm was more conducive to the oscillation in the Ho:YAG laser intra-cavity pumped by Tm:YLF laser, due to the minimum absorbed loss of Ho:YAG crystal and high gain of Tm:YLF crystal. Therefore, the intra-cavity pumped Ho:YAG laser finally exhibited a wavelength of 1897nm. We also calculated the gain spectrum of Ho:YAG laser at 2122 nm, see Fig. 9(b). When the transmittance of 10%, 15% and 20% were used as OC, the peak wavelengths of the maximum gain value were all at 2122 nm. It also could be seen that with the increase of transmittance, the peak wavelength tended to shift to the short wavelength, which was a classic feature of the quasi-three-level laser [25].
Fig. 9. (a) Calculated gain cross-section of the 3F4 → 3H6 transition in Tm:YLF laser for various population inversions. (b) Calculated gain spectrum of Ho:YAG laser.
Figure 10(a) showed the laser power with wavelengths of 792 nm and 1897nm at mirrors M3 and M4. It is worth noting that the 1897nm laser leaked from the resonator was 4.3 W when the incident pump power was 60.8 W. According to the maximum 32.1 W at the Tm:YLF laser operation, it showed that the laser generated by Tm:YLF crystal was well absorbed by Ho:YAG crystal. And in the experiment, we found that the 1897nm laser appeared earlier than the 2122 nm laser because the threshold pump power of Tm:YLF laser was lower than that of Ho:YAG laser. The beam quality when the output power was 12.7 W was shown in Fig. 10(b). The beam quality factors M2 in the vertical and horizontal directions were 1.22 and 1.11, respectively. As far as we know, 12.7 W was the maximum output power of the Tm-doped laser intra-cavity pumped Ho-doped laser structure with near-diffraction-limited beam quality. As shown in Fig. 11(a), the root mean square (RMS) instability of the laser at the maximum output power was measured as 0.08% within 25 minutes. A polarization splitting prism was used to measure the polarization characteristics of the Ho:YAG laser, which showed in Fig. 11(b). In the pumping process, the output laser was most of the s-polarized light, and only weak p-polarized light (approximately 0.1 W) was output when the incident pump power was maximum. When the incident pump power was 60.8 W, the power ratio of s-polarized light to p-polarized light was about 123, which showed that the polarization state of the output laser of intra-cavity pumped Ho:YAG laser was s-polarization.
Fig. 10. (a) The laser power of different wavelengths at mirrors M3 and M4. (b) The beam quality of intra-cavity Ho:YAG laser at maximum output power. The inset showed the typical 2D beam profile.
Fig. 11. (a) RMS instability of the intra-cavity Ho:YAG laser at the maximum output power. (b) The polarization state of intra-cavity Ho:YAG laser.
5. Conclusions
In conclusion, Ho:YAG laser at 2122 nm had been demonstrated intra-cavity pumped by a 792 nm fiber-coupled LD pumped Tm:YLF laser. With a 792 nm incident pump power of 60.8 W, the Tm:YLF laser produced 32.1 W output power and the corresponding slope efficiency was 56.7%. Under the same incident pump power, the output power of 12.7 W was achieved with near diffraction limited beam quality factors M2 of 1.22 and 1.11 in the vertical and horizontal directions, respectively. In future work, the measure of optimizing the cavity and pump light waist radius according to the relationship between power and fracture limit will be adopted to further improve the output power of the intra-cavity pumped Ho-doped laser.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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