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Abstract
Single crystal fiber (SCF) lasers have attracted extensive attention recently. In this paper, Tm:SrF2 SCF was successfully grown by the temperature gradient method. With a compact resonator, the acousto-optically Q-switched properties of a diode-pumped Tm:SrF2 SCF laser was investigated first. At the repetition rate of 500 Hz, a highest pulse energy of 1.32 mJ and a narrowest pulse width of 51 ns were acquired with the absorbed pump power of 2.60 W, resulting in the peak power of 25.85 kW. These results indicate that Tm:SrF2 SCF is a promising laser gain material for generating ∼2 µm region pulsed lasers.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Laser diode (LD) pumped Tm3+-doped all-solid-state pulsed laser emits the wavelength of near 2 µm region, which has widespread applications in remote sensing, coherent radar, material processing, atmospheric detection, biomedical and other aspects [1–4]. In order to obtain this widely used laser, actively Q-switching is one of the main technologies, which has the benefits of controllable pulse period, high pulse energy and large peak power [5,6]. As an important component of laser, gain medium determines the capacity and efficiency of the laser output. The traditional bulk crystal and glass fiber are two of the most commonly used gain media at present, but their defects limit the further development in high power and high energy pulsed laser [7,8].
Single crystal fiber (SCF) is a novel gain medium between bulk crystal and glass fiber, which has received much attention due to its excellent performance. Compared with the bulk crystal, SCF has the advantages of small size, light weight and good pump guidance, which provides an opportunity to realize compact all-solid-state laser [9,10]. In addition, the high aspect ratio of one-dimensional material makes SCF have great heat dissipation performance, which is beneficial to the thermal management of laser system [11]. Meanwhile, SCF has lower stimulated Brillouin scattering cross-sections and better thermal conductivity than glass fiber [12–14]. Combining the advantages of high gain of traditional bulk crystal and good heat dissipation of glass fiber, SCF has a good prospect in realizing high-power and high-energy compact pulsed laser. Rare earth ions doped fluoride crystals have the advantages of simple fluorite cubic structure, wide transparency range and long radiation life [15–18]. As the most excellent fluorides, CaF2 and SrF2 crystals possess low melting point, small thermal expansion coefficient and high thermal conductivity, which were considered as promising matrix material [19,20]. In 2017, Liu et al. have realized an acousto-optically (AO) Q-switched laser on Tm,Y:CaF2 crystal, and obtained a pulsed laser with 280 ns pulse width and 0.335 mJ pulse energy [21]. Compared to CaF2 crystal, SrF2 has a lower phonon energy (∼280 cm−1) [22]. Low phonon energy leads to weak multi-phonon non-radiative quenching, resulting in higher optical efficiency and lower heat loss of rare earth ions doped fluoride [23,24]. Up to now, only the continuous-wave (CW) laser has been reported based on Tm:SrF2 crystal [25]. By improving the thermal management of the laser system, SCF is expected to increase the output power of Tm:SrF2 laser, thus achieving high power laser output. At present, the research on laser characteristics in ∼2 µm region is still in a preliminary stage. Song et al. have achieved passively Q-switched laser output using Tm:LuAG SCF as the gain medium with Cr:ZnSe saturable absorber, and Zu et al. have successfully demonstrated self-Q-switched pulsed laser output of Tm:CaF2 SCF [26,27]. However, actively Q-switched laser based upon Tm3+-doped SCF has not been studied.
In this work, 3 at.% Tm:SrF2 SCF with excellent optical and physical properties was grown by temperature gradient method. By employing acousto-optic-modulator (AOM), actively Q-switched Tm:SrF2 SCF lasers at different modulation frequencies were firstly generated. Pumped by a 792-nm-LD, 51 ns pulse width and 1.32 mJ single pulse energy were realized under the absorbed pump power of 2.60 W.
2. Experimental setup
The schematic diagram was presented in Fig. 1. In this experiment, the Tm:SrF2 SCF without coating was used as gain medium, in which the doping concentration of Tm3+-ions was 3 at.%. The shape of Tm:SrF2 SCF was a transparent cylinder with a diameter of 2 mm and a length of 5 mm. A fiber-coupled (105 µm core diameter, NA = 0.22) LD emitting at 792 nm was acted as the excitation source. Through a 1:2 coupling system, the pump light was focused on the Tm:SrF2 SCF. To remove generated heat, the Tm:SrF2 SCF was wrapped in thin indium and placed in a copper block fixture connected to a 12 °C water-cooled circulation system. A 95.5 mm plane-concave cavity was used to generate the laser. The input mirror M1 was anti-reflection coated at 0.78-0.81 µm and high-reflection coated at 1.9-2.0 µm. An output coupler (OC) with a curvature radius of 100 mm was used as M2 with transmittances of 2% and 5% in 1.9-2.0 µm. Then, the AOM (QS027-4M-AP1) with a length of 46 mm was inserted into the cavity to achieve pulsed laser, which were coated at ∼2 µm anti-reflection film on two ends, and driven by a radio frequency was 27.12 MHz at the power of 50 W.
3. Results and discussion
The characteristics of laser were studied with different transmittances. Firstly, the CW laser was operated of Tm:SrF2 SCF without modulator. Among them, the gain medium has an absorption efficiency of 37% for pump light. The output power was measured by a 30 A-SH-V1 (Israel) laser power meter, which varies with the pump power as shown in Fig. 2. Using an OC with a transmittance of 2%, the maximum output power was 1.44 W under the absorbed pump power of 2.60 W, corresponding to the slope efficiency of 60.7%. The 1.37 W output power and the 61.3% slope efficiency were obtained at 5% OC. Such a high slope efficiency should be caused by a cross relaxation phenomenon of Tm3+ ions in the transition process. This individual phenomenon means one pumped photon was able to excited two active ions, thus improving the quantum efficiency. Placed AOM in the cavity, by controlling the pulse repetition frequency (PRF), a stable AO Q-switched pulsed laser was obtained at the absorbed pump power of 373 mW with 2% OC. The low oscillation threshold of pulsed laser was 548 mW with transmittance of 5% OC. Under the transmittances of T = 2% and 5% OCs, the maximum outputs reached 563 mW and 658 mW at the repetition rate of 500 Hz. The corresponding maximum output powers were 639 mW and 732 mW with the repetition rate of 1 kHz. In experiment, there is no obvious fluctuation in the power meter, which proves that the stability of the Q-switched pulsed laser. With the absorbed pump power further increasing, pulse train became unstable and the pulse width was no longer narrowed. In order to protect the AOM, we did not continue to increase the absorbed pump power.
Through an optical spectrum analyzer (SOL-MS3504i) with a resolution of 0.34 nm, we measured the output spectra of CW and pulsed lasers. As shown in Fig. 3, the central wavelengths of CW lasers at the transmittances of 2% and 5% OCs were 1971.8 nm and 1935.1 nm, corresponding to the spectra center were recorded as 1957.5 nm and 1934.2 nm in Q-switched lasers respectively. Both on CW and pulsed lasers, it was obvious that a blue shift of the central wavelengths occurred when we used higher OC transmittance, which may be caused by the larger loss of higher transmittance OC. Due to the loss resulted by the insertion of AOM, the central wavelengths of the pulsed lasers have a blue shift relative to the CW lasers.
With the increase of absorbed pump power, the number of inversion particles in the gain medium will augment at the same repetition rate, resulting in a narrower pulse width, as exhibited in the Fig. 4(a). Therefore, a laser with narrowest pulse width was obtained when the absorption pump power was 2.6 W. For T = 5% OC, pulsed lasers with pulse width of 119 ns and 51 ns were demonstrated under the PRFs of 1 kHz and 500 Hz. The maximum single pulse energy was shown in Fig. 4(b), which were 0.73 mJ and 1.32 mJ, corresponding the peak power were 6.16 kW and 25.85 kW respectively. By employing T = 2% OC, the shortest pulse widths were acquired as 174 ns and 64 ns with the highest single pulse energies of 0.64 mJ and 1.13 mJ at 1 kHz and 500 Hz PRFs. The peak power of 3.68 kW and 17.68 kW were obtained, respectively. The experimental results under different frequency and transmittances were summarized in Table 1.
Table 1. Summarizes the characteristics of Tm:SrF2 SCF AO Q-switched laser.
The pulse trains were recorded through a 1 GHz digital oscilloscope (Tektronix DPO4104). Compared with the data in Table 1, we can clearly observe that the power of the laser is higher and the pulse width is narrower when the OC has 5% transmittance. When the absorbed pump power was 2.60 W, the pulse train at different modulation frequencies and different scales were shown in Fig. 5. The laser M2 were measured by the 90/10 knife-edge method, which were 1.81 and 1.85 in the horizontal and vertical directions, respectively, as shown in Fig. 6. Further modified the resonator, the beam quality may be improved. The illustration shows the laser-beam profile and the 3D light-intensity distribution were recorded by a detector (NS2-Pyro/9/5-PRO, Photon).
4. Conclusion
We have reported a near 2 µm pulsed laser based on Tm:SrF2 SCF material. As a novel gain medium, SCF combines the strengths of traditional bulk crystal and glass fiber, so it has a good prospect in the realization of high energy compact pulse lasers. In this paper, a 1.32 mJ AO Q-switched laser was demonstrated with the repetition rate of 500 Hz, corresponding the narrowest pulse width of 51 ns and maximum peak power of 25.85 kW. These results show that Tm:SrF2 SCF has great potential to achieve high energy and short pulse durations in near 2 µm region pulsed laser.
Funding
National Natural Science Foundation of China (11974220, 61925508); Strategic Priority Program of the Chinese Academy of Sciences (XDA25020312); CAS Interdisciplinary Innovation Team (JCTD-2019-12); Science and Technology Commission of Shanghai Municipality (20501110300, 20511107400).
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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