*Article* **A High-Energy, Narrow-Pulse-Width, Long-Wave Infrared Laser Based on ZGP Crystal**

**Chuanpeng Qian 1,2, Ting Yu 1,2,3,\*, Jing Liu 1,2, Yuyao Jiang 1,2,4, Sijie Wang 1,2,3, Xiangchun Shi 1,2,3, Xisheng Ye 1,2,3,\* and Weibiao Chen 1,2,3**


**Abstract:** In this paper, we present a high-energy, narrow pulse-width, long-wave infrared laser based on a ZnGeP2 (ZGP) optical parametric oscillator (OPO). The pump source is a 2.1 μm three -stage Ho:YAG master oscillator power-amplifier (MOPA). At a repetition frequency of 1 kHz, the Ho:YAG MOPA system outputs the maximal average power of 52.1 W, which corresponds to the shortest pulse width of 14.40 ns. By using the Ho:YAG MOPA system as the pump source, the maximal average output powers of 3.15 W at 8.2 μm and 11.4 W at 2.8 μm were achieved in a ZGP OPO. The peak wavelength and linewidth (FWHM) of the long-wave infrared laser were 8156 nm and 270 nm, respectively. At the maximal output level, the pulse width and beam quality factor *M*<sup>2</sup> were measured to be 8.10 ns and 6.2, respectively.

**Keywords:** long-wave infrared; ZnGeP2 crystal; Ho:YAG MOPA

#### **1. Introduction**

As an important atmospheric transmission window, long-wave infrared lasers (8–12 μm) have been extensively applied in many fields, such as lidar, spectroscopy, and national defense [1,2]. Among the many ways to obtain a long-wave infrared laser, the optical parametric oscillator is an attractive approach due to its wide wavelength-tuning range and high conversion efficiency [3]. As the core component, the characteristics of nonlinear optical crystals determine the performance of nonlinear frequency conversion. At present, nonlinear crystals suitable for generating a long-wave infrared laser mainly include OP-GaAs, AgGaSe2, CdSe, BaGa4Se7, and ZnGeP2 (ZGP).

The nonlinear coefficient of OP-GaAs is very large (d14 = 94 pm/V), and it was used to achieve an average pulse energy of 0.18 mJ at 8.5 μm [4] and 16.2 μJ at 10.6 μm [5], corresponding to the repetition frequencies of 2 and 50 kHz, respectively. AgGaSe2 has a low damage threshold (18 MW/cm2), which limits its ability to obtain a large-energy long-wave infrared laser. The highest energy of a long-wave infrared laser by AgGaSe2 was about hundreds of microjoules [6–8]. CdSe has a small nonlinear coefficient (d31 = 18 pm/V) and a moderate damage threshold (56 MW/cm2). Due to its weak walk-off effect, the disadvantage of a small nonlinear coefficient can be compensated by increasing the crystal length. The pulse energy of a long-wave infrared laser achieved 1.05 mJ at 10.1 μm [9] and 0.8 mJ at 11 μm [10]. BaGa4Se7 has a very large damage threshold (557 MW/cm2) and an acceptable nonlinear coefficient (d11 = 24.3 pm/V), but its low thermal conductivity makes it unsuitable for obtaining a high-power long-wave infrared laser. In 2018, Zhao et al.

**Citation:** Qian, C.; Yu, T.; Liu, J.; Jiang, Y.; Wang, S.; Shi, X.; Ye, X.; Chen, W. A High-Energy, Narrow-Pulse-Width, Long-Wave Infrared Laser Based on ZGP Crystal. *Crystals* **2021**, *11*, 656. https:// doi.org/10.3390/cryst11060656

Academic Editors: Xiaoming Duan, Renqin Dou, Linjun Li and Xiaotao Yang

Received: 17 May 2021 Accepted: 8 June 2021 Published: 9 June 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

obtained an average pulse energy of 0.31 mJ at 8.92 μm [11] with a repetition frequency of 1 kHz. The nonlinear coefficient of a ZGP crystal is high (d14 = 75 pm/V), and its thermal conductivity and damage threshold perform well among these nonlinear crystals. Using a ZGP crystal, the largest pulse energy of 45 mJ at 8.0 μm was achieved [12]. This result was achieved under a low repetition frequency (1 Hz). At a kilohertz frequency, the ZGP crystal obtained pulse energy of 1.26 mJ at 8.2 μm [13] and 0.35 mJ at 9.8 μm [14], with a repetition frequency of 10 kHz.

We demonstrate a high-energy, narrow-pulse-width, long-wave infrared laser with repetition frequency of 1 kHz based on a ZGP crystal. The pump source is a 2.1 μm Ho:YAG MOPA laser that can output a highest average energy of 52.1 mJ at 1 kHz. The pulse width and beam factor *M*<sup>2</sup> were measured to be 14.40 ns and 1.3, respectively. Then, a ZGP OPO with a four-mirror ring-cavity structure was used. The output energy of the long-wave infrared ZGP OPO was 3.15 mJ for the idler and 11.4 mJ for the signal. The pulse width and beam factor *M*<sup>2</sup> were measured to be 8.10 ns and 6.2 at the maximal output level.

#### **2. Experimental Setup**

The experimental setup of the Ho:YAG MOPA system is shown in Figure 1, which contains a Q-switched Ho:YAG oscillator and a three-stage Ho:YAG MOPA system. The acousto-optical modulator (AOM) Q-switched Ho:YAG oscillator had a compact L-shaped structure that consisted of an input mirror (M1) with antireflection (AR) for 1.9 μm and high-reflection (HR) for 2.1 μm, an output coupler (M2) with transmittance of 60% at 2.1 μm, and a thin-film polarizer (M3) with AR for s-polarized 2.1 μm and HT for p-polarized 2.1 μm. Its physical-cavity length was 150 mm. The Ho:YAG crystal in the oscillator with dopant concentration of 0.5%, diameter of 5 mm, and length of 30 mm was single-ended pumped by a Tm:YLF laser with the 1/*e*<sup>2</sup> beam-waist diameter of 0.34 mm and maximal power of 28 W.

**Figure 1.** Experimental setup of three-stage Ho:YAG MOPA system.

The three-stage Ho:YAG MOPA system was designed and operated at a PRF of 1 kHz. The three Ho:YAG crystals with a dopant concentration of 0.3%, diameter of 5 mm, and length of 70 mm were dual-ended pumped by six Tm:YLF lasers. The output powers of the six Tm:YLF lasers were about 40 W. The beam diameters of the pump in the three Ho:YAG crystals were 0.83, 1.4, and 1.9 mm, respectively.

The experimental setup of ZGP OPO is shown in Figure 2. The ZGP OPO had a fourmirror ring cavity structure, including two mirrors (M5) with an AR p-polarized pump laser and HR idler laser, a mirror (M6) with an AR pump and signal laser and an HR idler laser, and an output coupler (M7) with transmittance of 45% for the idler laser. In order to more accurately measure the power of the idler and the signal laser two M8 mirrors and one M7 mirror were used. The oscillator resonated with the single idler laser, and the ambient humidity of the experiment facility was kept at about 10%. Both measures were designed to avoid damaging the coating of the ZGP crystal caused by water absorption around the wavelength of the signal laser. The physical length of the ring cavity was about

118 mm. With the ZGP crystal, the wavelength of the OPO was continuously tunable from 3.8 to 12.4 μm [15]. However, the optical-to-optical efficiency of the OPO decreased with the increase in wavelength. In this experiment, we adjusted the wavelength of the idler laser to 8.2 μm. The ZGP crystal had an aperture of 6 mm × 6 mm and a length of 30 mm, cut at an angle of θ = 50.8◦ with respect to Type I phase matching. Both ends of the ZGP crystal (School of Chemical Engineering and Technology, HIT, Ha'erbin, China) were coated with HT for the pump, signal, and idler laser. The absorption coefficients of the ZGP crystal at the 2.1 μm pump laser and 8.2 μm idler laser were measured to be 0.03 and 0.01 cm−1, respectively. The pump laser from the Ho:YAG MOPA system was focused onto the ZGP crystal with a 1/*e*<sup>2</sup> beam diameter of 3.6 mm. The crystals of the entire experimental apparatus, including Tm:YLF, Ho:YAG, and ZGP, were all wrapped in indium foil and installed into copper blocks that were cooled by the chiller. The temperatures of the Tm:YLF, Ho:YAG, and ZGP crystals were controlled to be 16, 18, and 20 ◦C, respectively.

**Figure 2.** Experimental setup of ZGP OPO.

#### **3. Results and Discussion**

In this experiment, the output powers were measured by the same power meter (Ophir PM 150). The output performances of the Ho:YAG oscillator are shown in Figure 3. The threshold pump power of the Ho:YAG oscillator was about 8 W. Pump power of 28.0 W and a maximal average output power of 5.42 W were achieved, corresponding to the opticalto-optical conversion efficiency of 19.4%. The optical-to-optical conversion efficiency was low because we used a small mold volume to achieve the 2.1 μm narrow-pulse-width laser output. Using an InGaAs detector and a 1 GHz digital oscilloscope (Tektronix DPO4102B), we measured the minimal full-width half maximum (FWHM) of the pulse profile to be 11.56 ns, which is shown in Figure 3a. Figure 3b shows the beam-quality factor *M*<sup>2</sup> of the oscillator that was measured by the 90/10 knife-edge method. Under the maximal output condition of the Ho:YAG oscillator, the beam quality factor *M*<sup>2</sup> in the x and y directions was 1.19 and 1.26, respectively.

**Figure 3.** (**a**) Output power and pulse width, and (**b**) beam quality factor of Ho:YAG oscillator.

The 2.1 μm laser produced by the oscillator was injected into the Ho:YAG crystal in the primary amplifier after being transformed by a set of coupling lenses, as shown in Figure 4a. The output powers for each amplifier stage were 18.14, 37.5, and 52.4 W, corresponding to slope efficiencies of 24.7%, 36.0%, and 33.4%, respectively. When the amplifier moved from the first stage to the third, the pulse width of the 2.1 μm laser slightly increased, which was measured to be 12.72, 13.38, and 14.40 ns. Compared to the Ho:YAG oscillator, the beam quality factor *M*<sup>2</sup> of the Ho:YAG amplifier very slightly deteriorated, with 1.20 and 1.28 for the x and y directions, respectively. The final pulse width and beam quality of the Ho:YAG MOPA system are shown in Figure 4b,c respectively.

**Figure 4.** (**a**) Output powers, (**b**) pulse width, and (**c**) beam quality factor *M*<sup>2</sup> of three-stage Ho:YAG amplifier.

As shown in Figure 5a, the average output power of ZGP OPO was measured with an incident pump power of 52.4 W. The pump laser was injected into the crystal in a divergent way to avoid damaging the end face with the thermal-lens effect. The divergence angle was about 6 mrad. During the experiment, we gradually reduced the size of the pump spot to obtain the highest pulse energy of the long-wave infrared laser. Lastly, the beam diameter at the front-end face of the ZGP crystal was ~3.6 mm. Threshold pump power was about 21.8 W and the maximal average output power of the ZGP OPO was about 3.15 W at 8.2 μm and 11.4 W at 2.8 μm, corresponding to the slope efficiency of about 10.1% and 37.0%. The beam quality factor *M*<sup>2</sup> was measured and calculated to be 6.2 at the maximal output power, which is shown in Figure 5b.

**Figure 5.** (**a**) Output power, and (**b**) beam quality factor *M*<sup>2</sup> of ZGP OPO.

In the experimental process, the damage threshold of the ZGP crystal had great correlation with the repetition frequency of the pump laser. In our previous work, which used a 3 kHz Ho:YAG MOPA system to pump ZGP OPO [16], the ZGP crystal was damaged when the pump power was ~73 W and the spot radius was 1.28 mm, corresponding to peak power density of 54.9 MW/cm2. We also measured the damage threshold of the ZGP crystal at 10 kHz repetition frequency, and it was about 25.7 MW/cm2. However, under the condition of 1 kHz repetition frequency, the ZGP crystal remained undamaged when the peak power density of the pump reached 60 MW/cm2. For the same ZGP crystal, the damage threshold increased by more than two times under the same heat-dissipation conditions as the repetition frequency of the pump laser decreased from 10 to 1 kHz. This phenomenon could have been related to the time during which the laser was acting on the coating film. At a high repetition rate, a longer treatment time led to a higher film temperature, and this made the coating film of the ZGP crystal more vulnerable to damage.

Because the InGaAs detector could not respond to a long-wave infrared laser, we employed an HgCdTe detector combined with a signal amplifier to measure the pulse width of the 8.2 μm idler laser, which is shown in Figure 6a. The FWHM pulse width was 8.10 ns with the peak power of 0.39 MW. Using a monochromator spectrograph (Zolix, omni-λ 300i), the idler spectrum was measured and is shown in Figure 6b. Peak wavelength was 8156 nm. The corresponding linewidth (FWHM) was approximately 270 nm.

**Figure 6.** (**a**) Pulse width, (**b**) ZGP OPO spectrum.

#### **4. Conclusions**

The Ho:YAG oscillator was Q-switched at 1 kHz, and the pulse width was ~12 ns. The successive three-stage Ho:YAG amplifier stages increased the maximal average output power up to 52.1 W with a pulse width of ~14 ns, corresponding to the beam quality factors *M*<sup>2</sup> of 1.20 and 1.28 for the horizontal and vertical directions, respectively. With the above Ho:YAG MOPA system, maximal output powers of 3.15 W at 8.2 μm and 11.4 W at 2.8 μm were produced in ZGP OPO, with an idler laser output energy of 3.15 mJ and a pulse width of 8.10 ns. The linewidth of the long-wave infrared laser was 270 nm at a central wavelength of 8156 nm. Its beam quality factors *M*<sup>2</sup> were 6.2. As far as we know, this is the largest reported amount of pulse energy in a long-wave infrared laser at a kilohertz repetition-frequency band.

**Author Contributions:** Experiment and writing original draft preparation: C.Q., Y.J. and S.W.; formal analysis: T.Y., J.L. and X.S.; review and editing: X.Y. and W.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the National Natural Science Foundation of China (NSFC) (62005300).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


### *Article* **High-Efficiency Ho:YAP Pulse Laser Pumped at 1989 nm**

**Chao Niu 1, Yan Jiang 1, Ya Wen 1, Lu Zhao 1, Xinyu Chen 1, Chunting Wu 1,\* and Tongyu Dai <sup>2</sup>**


**Abstract:** A Tm:YAP laser with an output wavelength of 1989 nm was selected for the first time as the pump source of a Q-switched Ho:YAP laser. When the absorbed power was 30 W, an average power of 18.02 W with the pulse width of 104.2 ns acousto-optic (AO) Q-switched Ho:YAP laser was obtained at a repetition frequency of 10 kHz. The slope efficiency was 70.11%, and the optical-optical conversion efficiency was 43.03%. The output center wavelength was 2129.22 nm with the line width of 0.74 nm.

**Keywords:** 1989 nm; Ho:YAP; AO Q-switched laser

#### **1. Introduction**

2 μm holmium (Ho3+) doped solid-state lasers have important application prospects in the fields of laser ranging, laser medical treatment, environmental monitoring, and optical communication due to its near-infrared window and safety to the human eye [1–8]. In addition, 2 μm Ho3+ doped lasers were considered as good pump sources for mid-far infrared optical parametric oscillator (OPO) [9]. Compared with Ho:YAG, Ho:YLF and Ho:GdVO4 crystals, Ho:YAP crystal has obvious advantages, such as wide absorption line, large absorption cross-section, anisotropy, short growth period, the output power was not easy to saturate, and so on. There are many absorption peaks in Ho:YAP crystal at 1.9 μm. For an a-cut Ho:YAP crystal, the absorption peaks included 1872 nm, 1907 nm, 1931 nm, 1970 nm and 2045 nm. For a b-cut Ho:YAP crystal, the absorption peaks included 1884 nm, 1923 nm, 1946 nm, 1984 nm, 2023 nm and 2059 nm [10]. For a c-cut Ho:YAP crystal, the absorption peaks included 1915, 1941, 1980, and 1996 nm [11]. A maximum absorption peak for a-, b-, and c-cut Ho:YAP crystals was about 1976 nm [12].

In recent years, there are many reports on Ho:YAP lasers. In 2009, a Tm:YLF laser with the output wavelength of 1900 nm was used to pump the continuous wave Ho:YAP laser, was reported by Duan et al. [13]. The output power was 10.2 W, with the slope efficiencies of 64.0%, the optical-optical conversion efficiencies of 52.6%, and the output wavelength of 2118 nm. In 2011, a Tm:YLF laser with output wavelength of 1910 nm was used to pump the Ho:YAP (b-cut) Q-switched laser at room temperature, was reported by Yang et al. [14]. When the Q-switched repetition frequency was 5 kHz, the output power was 18.1 W, the slope efficiencies was 45.9%, the optical-optical conversion efficiencies was 36.5%, and the output wavelength was 2118 nm. In 2012, the theoretical and experimental analysis of a Ho:YAP (a-cut) crystal of 2 μm laser was reported by Yang et al. [15]. The pump wavelength was 1900 nm. The CW output power was 15.6 W. The slope efficiencies was 63.7%, the optical-optical conversion efficiencies was 54.5%, and the output wavelength was 2118 nm. In 2012, a Tm:YLF laser with an output wavelength of 1910 nm was used to pump the Q-switched Ho:YAP (a-cut) ring laser, was reported by Dai et al. [16]. When the Q-switched repetition frequency was 1 kHz, the output power of 10.17 W was obtained, the slope efficiencies was 60%, the optical-optical conversion efficiencies was 29.5%, and the output

**Citation:** Niu, C.; Jiang, Y.; Wen, Y.; Zhao, L.; Chen, X.; Wu, C.; Dai, T. High-Efficiency Ho:YAP Pulse Laser Pumped at 1989 nm. *Crystals* **2021**, *11*, 595. https://doi.org/10.3390/ cryst11060595

Academic Editors: Xiaoming Duan, Renqin Dou, Linjun Li and Xiaotao Yang

Received: 19 April 2021 Accepted: 18 May 2021 Published: 24 May 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

wavelength was 2119 nm. In 2014, a Tm fiber laser with the output wavelength of 1910 nm was used to pump the Q-switched Ho:YAP (a-cut) laser, was reported by Wang et al. [17]. When the Q-switched repetition was 10 kHz, the output power of 11.0 W was obtained with the slope efficiencies of 62.1%, the optical-optical conversion efficiencies of 26.3%, and the output wavelength of 2118.0 nm. In 2014, Tm:YLF laser with an output wavelength of 1910 nm was used to pump Ho:YAP (a-cut) Q-switched laser, was reported by Duan et al. [18]. When the Q-switched repetition frequency was 10 kHz, 17.2 W output power was obtained, the slope efficiencies was 63.2%, the optical-optical conversion efficiencies was 29%, and the output wavelength was 2118 nm. In 2016, a Tm fiber laser with the output wavelength of 1910 nm was used to pump a mode-locked Ho:YAP laser, was reported by Duan et al. [19]. The output power of 2.87 W was obtained, with a slope efficiency of 15%, an optical–optical conversion efficiency of 11.9%, and an output wavelength of 2118 nm. In 2017, a Tm fiber laser with an output wavelength of 1941 nm was used to pump a Ho:YAP (c-cut) laser, as reported by Ting et al. [11]. The output power of 29 W was obtained with the slope efficiency of 42.8%, the optical–optical conversion efficiency of 60.67%, and the output wavelength of 2118 nm. In 2018, a Tm fiber laser with an output wavelength of 1910 nm was used to pump a Ho:YAP (b-cut) laser, as reported by Duan et al. [20]. The output power was 10.5 W, the slope efficiency was 53.2%, the optical–optical conversion efficiency was 41%, and the output wavelength was 2115 nm. In 2020, a Tm:YAP laser with output wavelength of 1940 nm was used to pump the electro-optic Q-switched Ho:YAP (a-cut) laser reported by Lei et al. [21]. When the repetition frequency was 4 kHz, the output power was 6.5 W, the slope efficiency was 50.6%, the optical-to-optical conversion efficiency was 28%, and the output wavelength was 2118 nm.

As mentioned above, Tm-doped solid-state lasers or Tm fiber lasers with output wavelength of 1900 nm, 1910 nm, or 1940 nm are often used as the pumping sources of Ho:YAP lasers. Although there was no pump source whose wavelength matches the strongest absorption peak of Ho:YAP crystal, the slope efficiency of Ho:YAP lasers was quite high under all kinds of situations, such as continuous or Q-switch or mode-locked operation, which means that the Ho:YAP crystal was one of the most promising Ho3+ doped lasers.

Under the premise of ensuring that the Ho:YAP crystal absorbs enough pumping power, the closer the wavelength of output laser and pumping laser, the smaller the quantum loss. However, there is not report on a Ho:YAP laser pumped by a 1989 nm laser, to our best knowledge.

In this paper, a Tm:YAP laser with the output wavelength of 1989 nm was selected for the first time as the pump source of Q-switched Ho:YAP laser. When the absorbed power was 30 W, the output power of acousto-optic (AO) Q-switched Ho:YAP laser was 18.02 W, and the pulse width was 104.2 ns at repetition frequency of 10 kHz. The corresponding slope efficiency was 70.11%, and the optical-optical conversion efficiency was 43.03%. The output center wavelength was 2129.22 nm.

#### **2. Materials and Methods**

The experimental configuration was shown in Figure 1.

To achieve high output power of Q-switched Ho:YAP laser, four semiconductor lasers (Type: SHCC-FCP-60-200-795-S, Shanghai Chuchuang Optical Machinery Technology Co., Ltd., Shanghai, China) with a central wavelength of 795 nm were used as pumping sources of Tm:YAP laser.

Two Tm:YAP crystals with the same parameter were used in the experiment. The Tm:YAP crystal had a cross-section size of 4 mm × 4 mm, a length of 12 mm, and Tm3+ doping concentration of 3 at.%. Both ends of the crystal were coated with high transmissivity at 1989 nm and 795 nm. The crystal was wrapped in thick indium foil with the thickness of 0.1 mm and placed in a copper heat sink. The heat sink was cooled by water, which was kept at 18 ◦C.

**Figure 1.** Acousto-optic Q-switched Ho:YAP laser pumped by 1989 nm laser.

The output power of Tm:YAP laser was improved by using a laser diode (LD) doubleended pump structure. The cavity was formed by flat mirrors M1, M2, M3, M4, and a concave mirror M5. M1, M3 and M4 were 45◦ mirrors coated with high transmissivity at 795 nm and high reflectivity at 1989 nm. M2 was a 0◦ mirror coated with high transmissivity at 795 nm and high reflection at 1989 nm. Curvature radius of the output coupler M5 was 300 mm and coated with transmissivity of 10% at 1989 nm.

Good mode matching between the pumping beam and oscillating beam of the Tm:YAP laser was achieved by adjusting the focus coupling mirrors, f1 = f4 = f5 = f8 = 25 mm, f2 = f3 = f6 = f7 = 50 mm. The focus lenses were anti-reflection coated at 795 nm. We measured the pump power before the lenses and after M1, M2, M3 and M4, and we calculated the pump transmission to be about 90%.

The resonator of Ho:YAP was a straight cavity composed of M7 and M8. M7 was coated with high transmissivity at 1989 nm and high reflectivity at 2118 nm. Curvature radius of the output coupler M8 was 100 mm and coated with high transmissivity at 1989 nm and transmissivity of 20% at 2118 nm. The cavity length of Ho:YAP was 70 mm.

The size of Ho:YAP crystal was 4 mm × 4 mm × 25 mm, and the Ho3+ doped concentration was 0.8 at. %. Both ends of the crystal were coated with high transmissivity at 1989 nm and 2118 nm. The crystal was wrapped in thick indium foil with the thickness of 0.1 mm and placed in a copper heat sink. The heat sink was cooled by water, which was kept at 18 ◦C.

A quartz acousto-optic Q-switch (QS041-10M-HI8 and the drive model MQH041- 100DM-A05, Gooch&Housego Co., Ltd, Ilminster, Somerset, UK) with a length of 46 mm and aperture of 2.0 mm was employed for Q-switching operation. Both ends of the Qswitch crystal were coated with high transmissivity at 2118 nm. The radio frequency was 40.68 MHz, and the maximum radio frequency power was 50 W. The threshold of damage was larger than 500 MW/cm2. The AO Q-switch crystal was cooled by a water cooler at 18 ◦C.

In order to facilitate the adjustment and realize the good mode matching between the pump light and the oscillating light, plat mirror M6 and focus lenses f9 and f10 were used. M6 was a 45◦ full mirror coated with high-reflection at 1989 nm. The focus lens f9 = 50 mm was anti-reflection (AR) coated at 1989 nm.

#### **3. Results and Discussion**

The absorptance of Tm:YAP crystal to pump light was 92%. The output power of Tm:YAP laser was measured with the power meter F150A (OPHIR, Jerusalem, Israel), as shown in Figure 2. The output power of the laser varied linearly with the absorbed power. The threshold power of the laser was 11 W. The maximum output power of the Tm:YAP laser was 50 W, and the slope efficiency was 41.32%. The central wavelength at the maximum output power was 1989.01 nm, which was measured using the spectrometer (AQ6370 of Yokogawa, Musashino, Tokyo, Japan), as shown in Figure 3.

**Figure 2.** Output power versus absorbed power of Tm:YAP laser.

**Figure 3.** Output spectrum of Tm:YAP laser.

The central wavelengths of Tm:YAP laser versus output power were shown in Figure 4. When the output power of continuous Tm:YAP laser varied from 0.5 W to 50 W, the center wavelength of the Tm:YAP laser remained between 1986.00 nm and 1990.00 nm. The fluctuation of the center wavelength was affected by the accuracy of temperature control of the Tm:YAP crystal. However, the output wavelength of the Tm:YAP laser was always in the absorption line width of the Ho:YAP crystal, which means that it can be used as the pump source of the Ho:YAP laser.

**Figure 4.** Output wavelength of Tm:YAP laser versus output power.

With Tm:YAP laser as the pump source with the maximum output power of 50 W, the experimental study of acousto-optic Q-switched Ho:YAP laser was carried out. The average power of the laser output was measured by a power meter (30A-BB-18, OPHIR, Jerusalem, Israel), and the pulse width of the laser output was measured by an oscilloscope (DPO3054, Tektronix, Beaverton, Oregon, U.S.) and a pulse width detector (PCI-3TE-12, VIGO System S.A., Warsaw, Poland).

As shown in Figure 5, the average output powers of an AO Q-switched Ho:YAP laser versus pump power were achieved under repetition frequency of 1 kHz, 5 kHz and 10 kHz. At pump power of 50 W, the maximum average output powers of AO Q-switched Ho:YAP laser were 14.2, 15.84, and 18.02 W, with the slope efficiencies of 55.25, 61.66, and 70.11%, respectively. The output pulse width of the Q-switched Ho:YAP laser versus the pump power was achieved under different repetition frequencies, as shown in Figure 6. At absorbed power of 30 W, the narrowest output pulse widths were 101.7 ns, 103.1 ns and 104.2 ns under repetition frequency of 1 kHz, 5 kHz, and 10 kHz, respectively.

**Figure 5.** Average output power of Q-switched Ho:YAP laser versus absorbed power.

**Figure 6.** Output pulse width of Q-switched Ho:YAP laser versus absorbed power.

The central wavelength of the AO Q-switched Ho:YAP laser was measured using the spectrometer (AQ6370, Yokogawa, Musashino, Tokyo, Japan). The central wavelengths at the maximum output average power were 2129.29 nm, 2129.47 nm and 2129.22 nm, with output linewidths of 0.77 nm, 0.75 nm and 0.74 nm under repetition frequency of 1 kHz, 5 kHz and 10 kHz, respectively.

Figure 7 showed the output spectrum of a Q-switched Ho:YAP laser at an output average power of 18.02 W and repetition frequency of 10 kHz. While, Figure 8 gave the output width of Q-switched Ho:YAP laser under the same condition.

**Figure 7.** Output spectrum of AO Q-switched Ho:YAP laser.

**Figure 8.** Output pulse width of AO Q-Switched Ho:YAP laser.

#### **4. Conclusions**

We demonstrated an AO Q-switched Ho:YAP laser pumped by the 1989 nm laser for the first time. Under the pump power of 50 W, at a PRF of 1 kHz, the average output power of 14.2 W Ho:YAP laser was obtained, with the slope efficiency of 55.25%, the pulse width of 101.7 ns, and the central wavelength of 2129.29 nm. At a PRF of 5 kHz, the average output power of 15.84 W laser was obtained, with the slope efficiency of 61.66%, the pulse width of 103.1 ns, and the central wavelength of 2129.47 nm. At a PRF of 10 kHz, the average output power of 18.02 W laser was obtained, with the slope efficiency of 70.11%, the pulse width of 104.2 ns, and the central wavelength of 2129.22 nm.

**Author Contributions:** Conceptualization, C.N., Y.W. and C.W.; methodology, T.D. and C.N.; software, C.N., Y.W. and L.Z.; validation, C.N. and Y.W.; formal analysis, X.C. and L.Z.; investigation, Y.J., C.N., L.Z. and X.C.; resources, C.W. and T.D.; data curation, Y.W., Y.J. and C.N.; writing—original draft preparation, C.N.; writing—review and editing, C.W. and Y.W.; visualization, C.N.; supervision, T.D.; project administration, C.W.; funding acquisition, C.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Study on the radiation mechanism and output characteristics of single longitudinal mode laser with tunable injection frequency locked 2 μm pulses, grant number 202002041JC and the APC was funded by Science and Technology Department of Jilin Province in China.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Acknowledgments:** This work is supported by Science and Technology Department of Jilin Province in China (Grant No. 202002041JC).

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

