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Abstract
We reported an intra-cavity pumped Q-switched laser with dual-wavelength synchronous output at 2066.7 nm and 1940nm. Ho:YLF crystal was pumped by a self-Q-switched Tm:YAP laser, which was served as both a gain medium and a saturable absorber simultaneously. For Ho:YLF laser, under 11.4-W incident pump power, a stable pulse laser was achieved at 2066.7 nm with the highest peak power of 69.65 W and the pulse repetition rate of 42.14 kHz. Under the same incident pump power, the highest peak power and pulse repetition rate of Tm:YAP laser were 17.85 W and 50.82 kHz, corresponding to the central wavelength of 1940nm. These results suggested that Q-switching without additional absorber element were effective way to obtain high-efficiency and compact 2.1 µm pulsed laser.
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1. Introduction
The advent of lasers has significantly expanded the scope of development in modern industries, military applications, and biomedicine [1–3]. Notably, the 2µm band laser operates within the atmospheric weak absorption range and ensures human eye safety [4–6], thereby holding immense value for atmospheric environmental monitoring and medical treatment, such as wind speed measurement, CO2 monitoring, as well as hard tissue and soft tissue resection. Additionally, it serves as an efficient pump source for 3-5 µm optical parametric oscillator (OPO) and represents an ideal laser source for the generation of infrared guided missile [7,8]. Consequently, there is a substantial societal demand for high-power and high-energy 2 µm laser in the current society.
LD-pumped Tm, Ho co-doped laser is a very important way to achieve the mid-infrared 2µm laser output, but they must be cooled to lower temperature suffering from high up-conversion effects and complex energy transfer process between Tm3+ and Ho3+ [9–11]. The attractive method to realize an efficient 2µm laser at room temperature is to place Tm-doped and Ho-doped crystals in a linear cavity, which is known as an intra-cavity pumping scheme. This results in a compact structure that allows efficient commercial LD near 800 nm to be used directly without another Tm laser leaking out of the cavity, improving the conversion efficiency. In 1992, Stoneman et al. demonstrated the first intra-cavity pumped Tm:YAG/Ho:YAG laser system with an output power of about 130 mW at 2.09 µm [12]. In 1998, Bollig et al. reported the 2.1 W Ho:YAG laser output at 2.097 µm for 9.2 W incident pump power based on a Tm:YAG rod [13]. In 2000, Hayward et al. demonstrated the maximum output power of 7.2 W in the same laser system with a slope efficiency of 17.5% [14]. In 2003, Schellhorn et al. obtained pulse mode Ho:YAG laser with end-pumping design [15]. In 2006, So et al. demonstrated a Ho:YAG laser intra-cavity side-pumped by a Tm:YLF laser which separated the Ho radiation from Tm radiation, and generated 14 W of continuous wave (CW) output power at 2.09 µm [16]. In 2013, when the injection pump power exceeded 100W, the amplification power of Tm:YLF intra-cavity pumped Ho laser by Zhu et al. was 8 W, and the corresponding slope efficiency was 10.9% [17]. In 2014, Shen et al. demonstrated a Ho:LuAG ceramic laser intra-cavity pumped by a Tm:YAG ceramic laser, and the Ho:LuAG laser exhibits a pulse mode behavior [18]. In 2016, Huang et al. demonstrated an efficient Ho:YAG laser intra-cavity pumped by a narrowband-diode-pumped Tm:YAG laser. In the Ho laser experiment, a maximum output power of 8.03 W at 2122 nm with a slope efficiency of 38% [19]. In 2017, Yang et al. reported a low-threshold intra-cavity pumped continuous-wave Ho:SSO laser is realized, and a tunable output wavelength is demonstrated by using a birefringent filter in the laser resonant cavity [20].
Compared with the oxide crystals described above, Yttrium lithium fluoride (YLF) belongs to tetragonal crystal system, with two identical a-axis and one c-axis. It has negative dn/dt, which can counteract the curvature effect caused by the temperature expansion of crystal axis, so it can effectively suppress the thermal lens effect [21–23]. At the same time, the trivalent lanthanide ions doped in fluoride matrix generally have higher laser upper level lifetime, which is conducive to energy storage and high power laser. Therefore, the Ho:YLF crystal which can generate 2.1µm laser has been attracting much attention [24–26]. When selecting the Tm laser as the pump source of Ho: YLF, we hope that the Tm laser has a large emission cross section near 1940nm and the output laser is linearly polarized. At the same time, it has good mechanical and thermal properties to obtain high power and high beam quality pump laser. The stimulated emission cross section of Tm:YAP (5.5 × 10−21 cm2) near 2µm is twice as high as that of Tm:YAG (2.2 × 10−21 cm2), and The Tm:YAP crystal has a low sensitivity to the temperature as compared to the Tm:YAG [27]. The absorption peak wavelength of Tm:YAP is around 795 nm, which is closer to the output wavelength of commercial diode lasers than that of Tm:YAG (785 nm) [28,29]. Up to now, the Tm laser pumped Ho:YLF continuous wave (CW) laser and cascaded MIR Ho:YLF laser have been investigated [30–32]. However, as far as we know, there is no report on the intra-cavity pumped Ho:YLF automatic pulsed laser.
In this paper, we demonstrated an efficient Ho:YLF laser intra-cavity pumped by a diode-pumped Tm:YAP laser. Interestingly, we observed stable Q-switching operation for both Tm and Ho lasers. The two pulse lasers were studied experimentally in detail. In Tm:YAP laser, when using an output mirror with a transmission rate of 10%, we obtained the minimum pulse width and the maximum repetition frequency were 1.29 µs and 50.82 kHz, respectively. In Ho:YLF laser, the maximum average output power was achieved using an output mirror with 5% transmission. The maximum single pulse energy and peak power of Ho: YLF laser are 42.76 µJ and 69.65 W, respectively.
2. Experimental setup
The Ho:YLF laser intra-cavity pumped by Tm:YAP laser experimental setup was shown in Fig. 1. We employed a commercial fiber-coupled diode laser as the pump source that emits at 794 nm, with a fiber core diameter of 200 µm and numerical aperture of 0.22. To achieve a high laser output, the pump laser was focused into the Tm:YAP and Ho:YLF crystal by a 1:1 coupling system. The Tm:YAP crystal was 3 × 3 × 8 mm3 in dimension and both ends are coated with 1.9 µm antireflection film. The Ho:YLF crystal was 3 × 3 × 10 mm3 in dimension and both ends are coated with 1.9 - 2.1 µm antireflection film. Both crystals were wrapped with indium foils and tightly mounted in different water-cooled copper blocks. The cooling temperature for both crystals was maintained at 13°C. The emission wavelength of b-cut Tm:YAP crystal is 1.94 µm. Additionally, the absorption peak of the a-cut Ho:YLF crystal is also at 1940nm, which matches well with the emission wavelength of the Tm:YAP laser. As a result, the Tm:YAP laser can be effectively utilized as a pump source for the Ho:YLF laser. M1 was a plane mirror coated with a high reflection in a range of 1.9-2.0 µm and a high transmission at 780 - 810 nm. The concave mirror M2 worked as the output coupling (OC) with a radius of 200 mm and transmissions of 2%, 5% and 10% for 1.9-2.1 µm. M3 was a dichroic 45°-mirror with a high reflection at 2.1 µm and a high transmission at 1.9 µm.
3. Experimental results and discussion
The Tm:YAP laser was studied first before operation of the Ho laser, and the Ho:YLF crystal was removed. The high output power is required for Tm:YAP CW laser experiment to provide sufficiently high excitation for subsequent Ho:YLF laser experiment. As shown in Fig. 2, when the incident pump power was increased to 14.0 W, the maximum output power of 4.79 W, 5.12 W and 4.81 W were obtained with the 2%, 5% and 10% OCs, corresponding to the slope efficiencies of 35.6%, 38.7% and 42.1%, respectively. We used an optical spectrum analyzer with a wavelength resolution of 0.34 nm (SOL-MS3504i) to measure the emission spectrum, the wavelengths obtained with 2%, 5% and 10% OCs, are at 1943.5 nm, 1940.7 nm and 1939.7 nm, respectively (Fig. 3).
In the intra-cavity pumping arrangement, Tm:YAP and Ho:YLF crystal samples were placed close together. By adjusting the length of the resonator and the angle of the resonator mirror carefully, 1.9 µm of Tm:YAP Q-switched laser and 2.1 µm of Ho:YLF pulsed laser were obtained simultaneously by using the output mirror with different transmissions. Both pulsed lasers exhibit a typical Q-switching phenomenon, that is, as the pump power increased, the pulse width decreased, and the repetition rate, single pulse energy and peak power increased. For the Tm:YAP laser, the Q-switched pulse output was realized by using the saturable reabsorption effect of Tm3+ ground state level [28], which was served as both a gain medium and a saturable absorber. However, the Ho:YLF can also be used as a saturable absorber to generate pulsed laser. The generation of 2µm band pulse laser is the result of the interaction of Tm:YAP and Ho:YLF, and also has a certain relationship with the resonator parameters, which contains a very complex modulation process [33]. The absorption cross section of the Ho:YLF saturable absorber at the laser wavelength is roughly the same as that of the emission cross section of the Tm:YAP laser gain medium. The maximum average output power of 1170 mW with a slope efficiency of 17.5% was obtained by transmission of 10% OC. The pulse width and repetition rate were measured by photodetector and oscilloscope 1.29 µs and 50.82 kHz, respectively (Fig. 4). The minimum pulse width of 1.1 µs was obtained using an output mirror with a transmission of 5%. Figure 5 shows the pulse sequence diagram at the highest pump power with OC transmission of 5%. Detailed pulse laser parameters obtained by different transmission are listed in Table 1. At the same time, the spectra of the pulse laser were measured. Compared with the Tm:YAP CW laser, the wavelength of the pulse laser has a blue shift, which reason is the loss caused by Ho:YLF gain medium in the cavity.
Fig. 4. (a-d) Repetition rate, pulse duration, single pulse energy and peak power for different OCs versus the incident pump power of Tm:YAP laser.
Table 1. Performance of The Tm:YAP Pulse Laser Under Different OCs
For the Ho:YLF pulse laser, when the incident pump power is 11.4 W, the maximum output power of 1.8 W was obtained by using the OC with a transmittance of 5%, and the corresponding efficiency is 23.5%. The pulse width and repetition rate measured at the highest output power were 614 ns and 42.14 kHz, respectively (Fig. 4). The pulse width was slightly smaller than that of Tm:YAP laser was attributed to the difference in intra-cavity loss. And the maximum single pulse energy and peak power at this time were 42.76 µJ and 69.65 W (in Fig. 6). The central wavelength of the Tm:YAP laser obtained by using the OC with transmission of 2% deviated from the absorption peak of Ho:YLF crystal, so the experimental results are not satisfactory. Figure 7 is the pulse sequence diagram of Ho:YLF laser with an output mirror of 5% transmission at the highest pump power. Detailed pulse laser parameters obtained by different transmissions are listed in Table 2. The output spectra of Ho:YLF pulsed laser were measured, as shown in Fig. 8. The output wavelengths of pulses obtained by using output mirrors with transmission of 2%, 5% and 10% are not much different, and they are 2067.21 nm, 2066.74 nm and 2066.64 nm, respectively (Fig. 8).
Fig. 6. (a-d) Pulse duration, repetition rate, single pulse energy and peak power for different OCs versus the incident pump power of Ho:YLF laser.
Table 2. Performance of The Ho:YLF Pulse Laser Under Different OCs
4. Conclusion
In conclusion, we demonstrated a pulsed Ho:YLF laser intra-cavity pumped by a Q-switched Tm:YAP laser. The Ho and Tm laser exhibited pulse mode behavior simultaneously. We believed that the pulsing behavior of our lasers can be explained by considering Q-switched Tm:YAP laser as a pumping source for the Ho:YLF laser. The pulse behaviors of the two pulse lasers were characterized in detail by using three different transmission output mirrors, keeping other conditions unchanged. For Tm:YAP laser, the maximum output power of 1170 mW was obtained by T = 10% OC, corresponding the maximum single pulse energy and peak power are 23.02 µJ and 17.85 W, respectively. For Ho:YLF laser, the maximum output power of 1802 mW was obtained by T = 5% OC, corresponding the maximum single pulse energy and peak power are 42.76 µJ and 69.65 W, respectively. This demonstration shows that the intra-cavity pumped and passively Q-switched configuration should be promising way to get dual-wavelength compact pulsed laser.
Funding
National Natural Science Foundation of China (12104271, 12374401); Natural Science Foundation of Shandong Province (ZR2021LLZ008, ZR2021QA030); China Postdoctoral Science Foundation (2021M691981).
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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