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Abstract
In this paper, we present the acousto-optical (AO) Q-switched performance of a holmium (Ho):gadolinium tantalate (GdTaO4) (Ho:GTO) laser pumped by a thulium (Tm)-fiber laser emitting at 1.94 µm. In the efficient continuous wave (CW) regime, a maximum output power of 30.5 W at 2068.8 nm was achieved, corresponding to a slope efficiency of 74.9% with respect to the absorbed pump power. In the Q-switching regime, pulse energies of 2.4 mJ, 1.2 mJ, and 0.9 mJ were obtained with pulse repetition frequencies of 10 kHz, 20 kHz, and 30 kHz, respectively. The minimum pulse widths were 18 ns, 23 ns, and 26 ns, corresponding to peak powers of approximately 133.3 kW, 52.2 kW, and 34.6 kW, respectively.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Holmium (Ho) lasers operating in the 2-µm spectral range are attractive for many applications such as ranging, medicine, and environmental monitoring [1–3]. In particular, using them as the pump source enables high-performance middle infrared (IR) laser radiation to be achieved with nonlinear optical conversion methods. Using an in-band pumping method, we observed a weak thermal load and a low upconversion loss in the Ho-doped materials, which makes the Ho laser working at 2 µm more likely to obtain high efficiency and high output power. In the past two decades, numerous Ho-doped oxide [4–11] and fluoride [12–15] materials have been successfully used to produce 2-µm laser radiation. Usually, the output wavelengths of the Ho-doped laser based on oxide and fluoride materials are located around 2.1 µm and 2.05 µm, respectively. The Ho-doped laser emission in the range from 2.07 µm to 2.08 µm has been hardly reported, although Ho-doped tungstates [16,17] and Ho:CaF2 [18,19] lasers can produce laser radiation around 2.08 µm. Unfortunately, under rare-earth doping conditions, low thermal conductivity seems unsuitable for achieving high output power.
Orthotantalate LnTaO4 (Ln = Sc, Y, La, Gd, Lu) has a high photoluminescence efficiency. In particular, gadolinium tantalate (GdTaO4, GTO) is a suitable host for Bi, Eu, and Tb ions [20–22], which are used to detect high-energy radiation. Because of its low symmetry and strong symmetrical crystal field, the GTO crystal is a promising host for doping of rare-earth elements. Recently, continuous wave (CW) and pulsed Nd-doped GTO lasers have been demonstrated [23–27]. A maximum CW output power of 7.7 W was reported in Nd:GTO laser recently [27]. Ho-doped gadolinium tantalate (Ho:GTO) crystal is a promising choice to obtain 2-µm laser output. However, there are few studies on the lasing performance of the Ho:GTO crystal. In 2019, our group presented the spectral properties of Ho:GTO crystal at room temperature and the CW lasing performance with an output power of 11.2 W at 2.07 µm [28]. In addition, the single-longitudinal-mode characteristics of the Ho:GTO laser was also demonstrated [29]. However, to the best of our knowledge, the Q-switching performance of Ho:GTO lasers has not been reported to date.
In this paper, we demonstrate the acousto-optical (AO) Q-switching performance of the Ho:GTO laser for the first time. Under an efficient CW regime, a maximum output power of 30.5 W at 2068.8 nm was realized with a slope efficiency of 74.9% and a beam quality factor (M2) of approximately 1.5. Under the AO Q-switching regime, maximum pulse energies of 2.4 mJ, 1.2 mJ, and 0.9 mJ, corresponding to minimum pulse widths of 18 ns, 23 ns, and 26 ns, were achieved with pulse repetition frequencies (PRFs) of 10 kHz, 20 kHz, and 30 kHz, respectively, resulting in calculated peak powers of approximately 133.3 kW, 52.2 kW, and 34.6 kW, respectively.
2. Experimental setup
Figure 1 schematically shows the experimental setup of the AO Q-switched Ho:GTO laser. The pump source is a custom-designed thulium (Tm)-fiber laser that emits at 1940nm and has a maximum output power of 70 W and an M2 factor of 2.2. The pump spot radius was measured to be 0.35 mm around the laser crystal, which means that the pump Rayleigh length can be calculated to be about 190 mm inside the laser crystal. A c-cut of 1.0 at.%- M-type Ho:GTO crystal with dimensions of 4×4 mm2 (in cross section) and 24 mm (in length) was used as the gain medium, whose two end faces were polished and antireflection coated for the pump and laser wavelengths. Under the no-lasing conditions, the single-pass pump absorption efficiency was measured to be 83.7%. The laser crystal was mounted on a copper heatsink, which was wrapped with 0.1-mm-thick indium foils, aiming to achieve better thermal conductivity. A thermoelectric cooler was employed to control the heatsink temperature, which was 20 °C in our experiment. A three-mirror folded cavity was used to study the output characteristics of the Ho:GTO laser under the CW and Q-switching regimes. The input flat mirror M1 was coated with high transmission (∼98%) for the pump wavelength and with high reflectivity (∼99.8%) for the laser wavelength. The 45° dichroic mirror M2 was flat with high transmission (∼94%) for the pump wavelength and high reflectivity (∼99.8%) for the laser wavelength with any polarizations. The output coupler M3 was a plano-concave mirror. Three curvatures of R=200 mm, 300 mm, and 500 mm were used in this work. With the ABCD matrix method, we calculated that this cavity can endure a thermal focal length above 45 mm from Ho:GTO crystal. For the Q-switching mode, a water-cooled AO Q-switch (SGQ41-2000-2QB, CETC) was inserted into the laser cavity, which was driven by a radiofrequency (RF) driver with an RF power of 50 W at 41 MHz, resulting in more than 80% loss of modulation.
3. Experimental results
3.1 High-efficiency CW operation
First, without an AO Q-switch, the physical cavity length was 67 mm. We investigated the output characteristics of a highly efficient Ho:GTO laser under the CW regime. Five output couplers with different parameters were used in this experiment, as shown in Fig. 2(a). With the same transmittance of 30%, three curvatures of R=200 mm, 300 mm, and 500 mm were employed. Clearly, the output power and slope efficiency improved with the increase in the curvature of the output coupler. The same phenomenon was also observed in the case with a transmittance of 40%. In the case of T=40% and R=500 mm, the Ho:GTO laser achieved the best output performance, corresponding to a maximum output power of 30.5 W under 51.4 W of absorbed pump power, when the slope efficiency was 74.9% with respect to the absorbed pump power. According to the ABCD matrix method, the resonant beam radii in the laser crystal were calculated to be 0.24 mm, 0.27 mm, and 0.31 mm, corresponding to R=200 mm, 300 mm, and 500 mm, respectively. For this experiment, the overlap efficiency between the pump and the resonant beam can be evaluated by the ratio between them because the pump Rayleigh length is much longer than the crystal length. The ratios between the resonant and pump beam were 0.69, 0.77, and 0.89 for R=200 mm, 300 mm, and 500 mm, respectively, indicating that the overlap efficiency was the best with R=500 mm in this experiment, which means that the Ho:GTO laser worked in the best condition in our experiment. In addition, the polarization state of Ho:GTO laser beam was linear verified by a Glan-Taylor prism. Compared with well-known Ho:YLF, Ho:LLF and Ho:YAG lasers, the output power of new-type Ho:GTO laser was lower, but its slope efficiency was acceptable. Actually, the optical quality of Ho:GTO crystal is not perfect because of immature growth techniques. We believe that the output characteristics of Ho:GTO laser could be increased with improving of crystal quality.
The output spectrum of the Ho:GTO laser was recorded using a wavemeter (Bristol 721A). Only the oscillation line was recorded for all output couplers, and it was centered at approximately 2068.8 nm, as shown in Fig. 2(b). In addition, the 2D and 3D far-field beam intensity distributions of the Ho:GTO laser were recorded by a camera (Spiricon Pyrocam III) under output powers of 5 W, 10 W, 20 W, and 30 W, as shown in Fig. 3, verifying the TEM00 propagation. With the most efficient output coupler, the M2 factor of the Ho:GTO laser was measured using the 90/10 knife-edge method. A focal lens with a 150-mm focal length was employed to transform the output beam. The beam radii were measured at different positions along the beam propagation direction, as shown in Fig. 4. The beam parameter was achieved by fitting the measured data. Then, the M2 factor was calculated to be 1.5 at the maximum output power. This result is like to other Ho lasers such as Ho:YAG, Ho:YLF and Ho:YAP.
3.2 Acousto-optical Q-switching regime
To achieve the Q-switching output, we inserted an AO Q-switch between M2 and M3, which extended the physical cavity length to 110 mm. Employing the most efficient output coupler (T=40%, R=500 mm), we obtained the output characteristics of the Ho:GTO laser and these are shown in Fig. 5(a). After the RF driver was shut down, at an absorbed pump power of 51.4 W, the Ho:GTO laser yielded an output power of 26.5 W and a slope efficiency of 70.7%. Under the Q-switching regime, the Q-switching performance of the Ho:GTO laser was investigated with PRFs of 10 kHz, 20 kHz, and 30 kHz. A maximum average output power of 25.9 W and a slope efficiency of 69.1% were obtained at a PRF of 30 kHz. With the decrease in PRF to 20 kHz and 10 kHz, the slope efficiencies were reduced to 65.9% and 63.4%, respectively, corresponding to average output powers of 24.8 W and 23.5 W, respectively. In addition, the polarization state of Q-switched Ho:GTO laser was same as CW operation.
An InGaAs photodetector (ET-5000, EOT) connected to a digital oscilloscope (DPO4000, Tektronix) was employed to record the pulse profiles of the AO Q-switched Ho:GTO laser. Figure 5(b), 5(c), and 5(d) shows the pulse energies, pulse widths, and peak powers of the AO Q-switched Ho:GTO laser under PRFs of 10 kHz, 20 kHz, and 30 kHz, respectively. Maximum pulse energies of 2.4 mJ, 1.2 mJ, and 0.9 mJ were achieved. The average minimum pulse widths of 18 ns, 23 ns, and 26 ns were recorded (Fig. 6), corresponding to calculated maximum peak powers of approximately 133.3 kW, 52.2 kW, and 34.6 kW, respectively, for the above three PRFs. Table 1 summarizes the representative works on tantalate lasers (Nd- or Ho-doping only at present). Clearly, our work realized the highest output power and slope efficiency in tantalate lasers.
Fig. 6. The minimum pulse widths of AO Q-switched Ho:GTO laser with PRFs of 10 kHz, 20 kHz and 30 kHz
Table 1. Comparison of output characteristics of tantalite lasers
Where λp is the pump wavelength, λl is the lasing wavelength, Pout is the maximum output power, ηs is the slope efficiency, PQS is the passive Q-switching, ML is the mode-locking, SLM is the single-longitudinal-mode, * with respect to absorbed pump power.
4. Conclusions
In summary, the AO Q-switched performance of a Tm-fiber-pumped Ho:GTO laser was demonstrated. In the high-efficiency CW regime, the Ho:GTO laser produced 30.5 W of output power at 2068.8 nm, corresponding to a slope efficiency of 74.9% with respect to the absorbed pump power. In the AO Q-switching regime, the Ho:GTO laser produced pulse energies of 2.4 mJ, 1.2 mJ, and 0.9 mJ at PRFs of 10 kHz, 20 kHz, and 30 kHz, respectively. Minimum pulse widths of 18 ns, 23 ns, and 26 ns were obtained, resulting in calculated maximum peak powers of approximately 133.3 kW, 52.2 kW, and 34.6 kW, respectively. The AO Q-switched Ho:GTO laser is a good choice to replace the Ho:YAG or Ho:YLF laser, which is used to pump the mid-IR optical parametric oscillator. In addition, with an output wavelength of 2.07 µm, the Ho:GTO laser could be applied to gas detection or materials processing.
Funding
National Natural Science Foundation of China (51802307).
Disclosures
The authors declare no conflicts of interest.
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