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Abstract
In this paper, a continuous-wave Nd:YAG InnoSlab laser at 1319 nm with high output power and high beam quality is demonstrated. The maximum output power of 170 W at 1319-nm single wavelength is obtained with an optical-to-optical efficiency of 15.3% from absorbed pump power to laser output and the corresponding slope efficiency of 26.7%. The beam quality factors of M2 are 1.54 and 1.78 in the horizontal and vertical directions, respectively. To the best of our knowledge, this is the first report on Nd:YAG 1319-nm InnoSlab lasers with such high output power and good beam quality.
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1. Introduction
The 1319-nm lasers own important applications in surgery, military, optical fiber communication, information storage, and these radiations can be converted into red and yellow light by non-linear frequency conversion [1–9]. Usually, Nd:YAG material is used to generate 1064-nm laser radiations and emission at 1319-nm is also possible. However, the effective stimulated emission cross section for transition of 1319 nm is about one fourth that of the 1064-nm emission line, and the 1319-nm operation has much larger quantum defect [10,11]. Therefore, it is quite difficult to get high-power 1319-nm laser output with high optical conversion efficiency and excellent beam quality. Until now, various geometries have been reported for high-power laser operation at 1319 nm, such as non-plannar ring oscillator, rod, and slab [10–13]. In 2007 [12], a diode-side-pumped high-power Nd:YAG rod continuous wave (CW) laser at 1319 nm was described. The output power was 131 W for pump power of 555 W with the optical-optical conversion efficiency of 23.6%, and the beam quality factor of M2 of ∼51. In 2022 [13], Tianyuan Zang et al. obtained 109-W CW laser output at 1319 nm with an 808 nm diode-end-pumped Nd:YAG slab medium by using a plano-concave stable cavity. The optical-optical efficiency was 20.9%, and the better beam quality factor of M2 of 3.06 was obtained in the thickness-direction. Because of serious thermal problems [14], it is a great challenge for high-power laser operation at 1319 nm with good beam quality by using the traditional laser geometries.
Partially end-pumped slab (InnoSlab) lasers are characterized by the stable-unstable hybrid resonator, which can effectively alleviate thermal effects. Compared to traditional solid-state lasers, Innoslab lasers have higher power output and better beam quality due to their cavity design and optical coupling characteristics [15,16]. Recently, the researchers have shown both InnoSlab amplifier and resonator schemes to generate 1319-nm laser radiations [17–19]. In 2013 [18], a high-power high-beam-quality diode-end-pumped Nd:YAG slab amplifier at 1319 nm was reported. In a five-pass configuration, the amplifier yielded a 42.3-W output with the optical-optical efficiency of 6.5%, and the beam quality factors of M2x = 1.13 and M2y = 2.16 in the orthogonal directions. In 2022, Hengli Zhang et al. [19] described a Nd:YAG InnoSlab laser resonator at 1319 nm with the maximum output power of 23.2 W, the optical-optical efficiency was 11.36% and the slope efficiency was 17.8%. How to scale the output power and optical conversion efficiency while maintaining the good beam quality still needs a deeper research.
In this paper, we demonstrate a laser-diode (LD) dual-end-pumped Nd:YAG InnoSlab laser for high-power CW operation of 1319 nm. The maximum output power of 170 W at 1319-nm single wavelength is reported for the first time with the optical-optical efficiency of 15.3% and the slope efficiency of 26.7%. The beam quality factors of M2 are 1.54 and 1.78 in the horizontal and vertical directions, respectively.
2. Experimental setup
Figure 1 shows the structure of the 1319-nm InnoSlab laser resonator. The gain medium used in the experiment is a Nd:YAG slab crystal with the dimension of 24 mm × 1 mm × 12 mm and the Nd3+-ions concentration of 1 at.%. Two CW 808-nm LD arrays are employed as the pump source. Each LD array is composed of 8 LD bars which are vertically encapsulted and collimated with micro-lenses in the fast axis direction. The spacing between each LD bar is 1.8 mm. Each LD array owns a maximum output power of 800 W. The pump light is focused thickly into the planar waveguide to make the beam more uniform in the slow axis direction. Two polarizers (P1, P2) and a half wave plate (HWP) in the coupling systems are used to protect the LDs from the residual radiation of the LDs on the other side. The LD pump light is injected into the crystal through two end surfaces with a uniformly distribution of 24-mm width in the slow-axis direction and a Gaussian distribution of 0.4-mm width in the fast-axis direction. The two large surfaces of the crystal are welded by Indium, dissipating waste heat through the water-cooled copper and reducing the crystal temperature, which can help to improve the laser output power and beam quality.
Fig. 1. Schematic of the LD dual-end-pumped Nd:YAG InnoSlab laser resonator. LD: laser diode, HWP: half wave plate, (P1, P2): polarizer.
M1 and M2 are 45° mirrors with anti-reflection (AR) coated at 808 nm and 1064 nm and high-reflection (HR) coated at 1319 nm, and they serve to fold the resonant cavity. M3 and M4 are concave and convex cylindrical mirrors with the radius of curvature of R1 and R2, respectively. They are used as the resonant cavity mirrors and both coated with 1319-nm HR and 1064-nm AR. In the experiments, two distinct sets of resonant cavity mirrors are adopted. They are R1 = 1000 mm & R2=−800 mm and R1 = 700 mm & R2=−500 mm, respectively. The equivalent transmittance of the resonant cavity is determined by the curvature of the two cavity mirrors, with the expression of equivalent coupling transmittance T = 1-|R2|/R1, and the corresponding transmittance values are 20% and 28%, respectively. The geometric distance between the two cavity mirrors is 100 mm, forming a positive-branch confocal unstable resonator in the 24-mm width direction of the Nd:YAG crystal, and the laser beam is coupled out from the M4 edge.
Figure 2(a) shows the intensity distribution presented in the Nd:YAG crystal after the 808-nm LD pump light passes through the coupling systems. It can be seen that the pump light is generally a rectangular spot with a uniform distribution in the horizontal direction. In the vertical direction, the intensity distribution shows a Gaussian pattern. In addition, the total coupling efficiency of the pump light is about 90% and the absorption efficiency by the crystal is ∼80% in the experiments. Figure 2(b) shows the temperature rising estimation of the crystal under maximum absorbed pump power of 1230 W, and the maximum temperature can be controlled at about 76 °C. No large temperature fluctuation is found under long-time operation, indicating that the cooling system is normal and supports the stable operation of the 1319-nm laser.
Fig. 2. (a) Pump beam intensity distribution in the Nd:YAG crystal and (b) temperature rising estimation of the crystal at the absorbed pump power of 1230 W.
3. Results and discussion
In our experiments, the two LD arrays and the Nd:YAG crystal were all cooled by circulating water at the same temperature of 18 °C. Two pairs of cavity mirrors were employed in the experiments with the radius of curvature of R1 = 1000 mm & R2=−800 mm and R1 = 700 mm & R2=−500 mm. The equivalent transmittance was 20% and 28%, respectively.
The 1319-nm output power and the optical to optical efficiency versus the absorbed pump power is shown in Fig. 3. It can be seen that the output power is always higher for the equivalent transmittance of 20% than that of 28% under the same pump power. When the equivalent transmittance is 28%, the output power gradually slows down with the increase of pump power. When the equivalent transmittance is 20%, the laser threshold is at the absorbed pump power of about 450 W. The maximum output power is 206 W, the optical-optical efficiency is 16.8% and the corresponding slope efficiency is 27.5% at the maximum absorbed pump power of 1230 W. Further increasing the pump power, the laser output power has no obvious increase. The output power is mainly limited by the design of the crystal and energy up-conversion, resulting in the crystal saturation of absorption [20]. The output power is 170 W under the absorbed pump power of 1110 W, with the optical conversion efficiency of 15.3% and the slope efficiency of 26.7%, respectively. To the best of our knowledge, this is the first report on Nd:YAG InnoSlab lasers at 1319 nm with such a high output power and efficiency.
For the equivalent transmittance of 20% and the output power of 170 W, the power fluctuation is measured and shown in Fig. 4. The average output power of the laser in 30 min is recorded by a power meter, and the power fluctuation is measured to be ±2.5%. As the measurement time increases, the laser output power decreases linearly. The reason for this phenomenon is that during the operation of the laser, the mirror holder of the resonant cavity continuously absorbs stray light, causing the holder to heat up and resulting in a slight decrease in laser output power. By optimizing the mirror holder structure in the future, the cooling and heat dissipation effect can be helpful to reduce the impact of temperature on it.
For a laser resonator, the output power is determined by the transmittance of the cavity. We use Rigrod Analysis to simulate the output power of the laser to derive the optimum transmittance [21,22]. The simulation results are shown in Fig. 5. When the values of absorbed pump power are 500 W, 800 W and 1200 W, the corresponding optimum transmittance is 15%, 18% and 22%, respectively. At a pump power of 1200 W, its optimal equivalent transmittance is 22%, which is similar to the 20% available in the laboratory. This is the reason why the output power is always higher for the equivalent transmittance of 20% than that of 28% under the same pump power, as described above.
Fig. 5. Simulation of optimum transmittance under different absorbed pump power (the vertical axis represent Normalized Intensity of the laser output power at 1319 nm).
For the radius of curvature of R1 = 1000 mm & R2=−800 mm (the equivalent transmittance of 20%), Fig. 6 depicts the measured spectra by an optical spectrum analyzer (YOKOGA WA, AQ6370D) at the output power of 170 W and 206 W, respectively. The spectral property of the output beam is scanned and analyzed over the range of 1000 nm to 1400 nm with the resolution of 20 pm. As can be seen from Fig. 6(a), we obtain 1319-nm CW laser operation with a single wavelength when the output power is below 170 W. And increasing the pump power further, the 1339-nm wavelength begins to oscillate obviously, as shown in Fig. 6(b). It is because the laser gain medium has two strong transitions in the 1.3 µm region from the 4F3/2 to the 4I13/2 manifold which have almost the same stimulated emission cross section. Thus, the laser output typically contains both wavelengths. One is the R2→X1 transition at 1319 nm and the other is the R2→X3 line at 1339 nm. We can further reduce the reflectivity of the cavity mirrors (M3 and M4) at 1339 nm to suppress its oscillation [10,23].
For the output power of 170 W, the two-dimensional profile of the laser beam is measured by a beam quality analyzer (SPIRICON, M2−200), as shown in Fig. 7. The beam quality factors of M2 are 1.54 and 1.78 in horizontal and vertical directions, respectively.
4. Conclusion
In summary, we demonstrate a high output power continuous-wave Nd:YAG InnoSlab laser at 1319 nm/1339 nm for different equivalent transmittance, T = 20% (1000 mm & −800 mm) and T = 28% (700 mm & −500 mm). When the equivalent transmittance is 20%, a maximum output power of 206 W is achieved with the optical-optical efficiency of 16.8% and the slope efficiency of 28%. When the output power is 170 W, the single-wavelength operation at 1319 nm is obtained with the optical conversion efficiency of 15.3% and the slope efficiency of 26.7%, respectively. The corresponding beam quality factors of the horizontal axis and the vertical axis are M2x = 1.54 and M2y = 1.78, respectively.
Funding
China Academy of Engineering Physics (YZJJLX2019015).
Disclosures
The authors declare no conflicts of interest.
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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