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Abstract
The compact dual-crystal orthogonally polarized Tm,Ho:YLF laser with balanced orthogonal polarization output power was reported for the first time, to the best of our knowledge. Two pieces of a-cut Tm,Ho:YLF crystals were placed compactly with their c-axis perpendicular. The maximum multimode total output power of 460 mW was obtained with the slope efficiency of 28.2%, and the M2 factor value was measured to be 1.07. By adjusting the pump focus position along the optical axis in the process of changing pump power, the balanced output powers in the two orthogonal polarization directions were successfully achieved, and the maximum output powers of the P-polarization and S-polarization states were both 215 mW. Inserting two uncoated solid state etalons into the resonator, the dual-wavelength single-longitudinal-mode laser with orthogonal polarization was obtained. The output powers at P-polarized 2052.1 nm and S-polarized2065 nm were 71mW and 62mW, respectively, and the M2 factor was 1.26. The orthogonally polarized dual-wavelength laser at 2 µm can be used as a seed laser for differential absorption lidars and coherent THz wave generation.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Orthogonally polarized dual-wavelength solid state lasers have attracted a wide range of investigations because of its various applications, such as differential absorption lidars, coherent THz radiation generation, precision metrology, self-sensing metrology, heterodyne interferometry, and so on [1–5]. The orthogonally polarized lasers have been successfully demonstrated with additional intracavity birefringent element, etalon, and stress-induced birefringence [6–10]. Y. P. Huang et al. reported the orthogonally polarized dual-wavelength laser at 1086 nm in σ-polarization and 1089 nm in π-polarization with a single Nd:LuVO4 crystal, but the output power of 1086 nm laser decreased at more than 7.5 W pump power due to the gain competition between the two orthogonal polarization directions [6]. The orthogonally polarized dual-wavelength single-longitudinal-mode (SLM) laser at 2 µm was realized with a single c-cut Tm,Ho:LLF crystal by X. L. Zhang et al., however gain competition and polarization rotation appeared with the increasing of the absorbed pump power [7]. By introducing an intracavity etalon, B. Xu et al. demonstrated an orthogonally polarized dual-wavelength laser at π-polarized 1342 nm and σ-polarized 1345 nm with a maximum output power of 1.35 W, but the mode competition between the two wavelengths was observed because they shared the same upper level ions [10]. Therefore, when the dual-wavelength lasers were realized with a single laser crystal, it was very difficult to obtain balanced output powers at the two wavelengths, moreover, the output power of each wavelength was generally unstable.
In recent years, the method, eliminating mode competition between the two orthogonal polarization directions by using two independent gain media, has been proved feasible [11–16]. X. P. Yan et al. presented a 26.2 W orthogonally linearly polarized Nd:YVO4 laser in 2010 by using two pieces of a-cut composite YVO4/Nd:YVO4 crystals with their c-axis orthogonal, and each composite crystal was end-pumped with a fiber-coupled laser diode [13]. In 2016, an orthogonally polarized dual-wavelength passively Q-switched laser was demonstrated by combining two Nd-doped crystals with the c-axis of the two crystals perpendicular to each other. When the incident pump power was 7.66 W, the output powers at 1064 nm and 1047 nm were 0.98 W and 0.83 W, respectively [16]. In 2020, M. T. Chen et al. used two Nd:YLF crystals in a double resonator structure, sharing one Cr:YAG saturable crystal, to realize a 8.7 W orthogonally polarized dual-wavelength passively Q-switched laser at 1047 nm and 1053 nm [12]. These orthogonally polarized dual-wavelength lasers mentioned above successfully eliminated gain competition due to the application of dual gain media. It is worth noting that the orthogonally polarized dual-wavelength laser mainly focused on the wavelengths of 1 µm and 1.3 µm to date [10–19]. The orthogonally polarized dual-wavelength laser has potential application for the coherent THz generation by using nonlinear crystals to realize difference frequency [20–23]. However, some excellent nonlinear crystals such as ZnGeP2 and OP-GaAs are not transparent at the wavelength of less than 2 µm [24,25]. Therefore, it is very meaningful to realize the 2 µm orthogonally-polarized dual-wavelength laser with balanced output power.
In this paper, we use two pieces of a-cut Tm,Ho:YLF crystals as the gain media with their c-axis orthogonal to explore the output performances of the orthogonally polarized multimode and SLM lasers at 2 µm. For the 5% output coupler, the orthogonally polarized multimode laser operated at 2064.1 nm in S-polarization, and at 2052.3 nm and 2064.1 nm in P-polarization, with the maximum total output power of 460 mW. However, the output powers of the two orthogonal polarization directions showed significant difference in the process of changing pump power. By careful adjusting the pump focus position along the optical axis in the process of changing pump power, the equal output powers were obtained in the two orthogonal polarization directions, and the maximum output power in each polarization direction was approximately 215 mW. Furthermore, the orthogonally polarized SLM Tm,Ho:YLF laser with dual-wavelength emission at P-polarized 2052.1 nm and S-polarized 2065nm was also realized by carefully adjusting the tilt angles of the two intracavity uncoated solid state Fabry-Perot (F-P) etalons.
2. Experimental setup
The setup of dual-crystal Tm,Ho:YLF laser is presented in Fig. 1. The gain media consisted of two a-cut Tm,Ho:YLF crystals with the dimension of 4 mm×4 mm×1.5 mm. The dopant concentrations were 5 at.% for Tm3+ and 0.5 at.% for Ho3+. The end faces of the two laser crystals with high transmittance coating at 792 nm and 2050∼2070nm. The c-axes of the two crystals were placed perpendicularly to each other. The c-axis of the first crystal was parallel to the optical platform, which was defined as the P-polarization direction, while the c-axis of the other crystal was perpendicular to the optical platform, which was defined as the S-polarization direction. Therefore, the two polarization directions were orthogonal to each other. The laser crystals were wrapped with indium foil and held in a brass heat sink. The temperature of the crystals was maintained at 283 K with a water circulation system and a temperature control system to reduce the influence of thermal effect on the laser performances. The pump source was a fiber-coupled laser diode temperature-tuned to 792 nm which was located in the absorption peak of Tm,Ho:YLF laser crystal, with a maximum output power of 3 W. The core diameter and numerical aperture of fiber were 105 µm and 0.22, respectively. A 1:1 coupling lens group was used to reimage the pump beam into the laser crystals. The pump spot diameter in the laser crystal was approximately 100 µm. In the experiment, the single-end-pumped method was adopted. The two relative short crystals were chosen and placed compactly in the resonator, so they all can obtain enough pump power to realize laser output. A simple plane-concave cavity was adopted, and the length of resonator was set to be around 50 mm. The input mirror M1 was a plate mirror with high transmission at 792 nm and high reflection at 2050∼2070nm. The output coupler was a concave mirror with a curvature radius of 51.88 mm. For the SLM operation, two uncoated fused quartz F-P etalons with thicknesses of 0.1 mm and 1 mm were inserted into the resonator [26–28]. Regulating the tilt angle of 0.1 mm thick etalon, the laser wavelength could be coarsely selected. Combining the 0.1 mm thick etalon, by finely adjusting the tilt angle of 1 mm thick etalon, a certain SLM could be selected. The output laser was separated into two orthogonally polarized beams after passing through a polarizing beam splitter (PBS), and the output powers in the two orthogonal polarization directions were measured separately by using a power meter (MolectronPM10).
3. Results and discussion
To begin with, the orthogonally polarized dual-crystal Tm,Ho:YLF laser in the multimode operation was investigated for three different output coupler transmittances. Under the condition of obtaining the highest output power at the maximum absorbed pump power, the variation of the output power with the absorbed pump power for the output coupler transmittances of 2%, 5%, and 10% was measured, respectively, as shown in Fig. 2. It can be seen that the best result was achieved with the 5% output coupler, so the output coupler with the transmittance of 5% was chosen in the following experiments. When the absorbed pump power was increased to 152 mW, the laser began oscillating. Further increasing the absorbed pump power, the output power increased almost linearly. Increasing the absorbed pump power to the maximum value of 1.83 W, the total output power reached to 460 mW, corresponding to the slope efficiency of 28.2%. In this case, the output powers of the P-polarization and S-polarization lasers also were measured after the 2 µm laser passed the PBS.
Fig. 2. The output powers in the multimode operation as functions of absorbed pump power for three different output couplings of 2%, 5%, 10%.
Figure 3 shows the total, S-polarization, and P-polarization output powers as functions of the absorbed pump power for the 5% output coupler. When the absorbed pump power was 0.15 W, the S-polarization and P-polarization lasers oscillated simultaneously. In the process of increasing the absorbed pump power from the threshold to 0.74 W, the output powers of the S-polarization and P-polarization lasers increased with the increase of the absorbed pump power, and the output power of the P-polarization laser maintained a higher level than that of the S-polarization laser, until their output powers were equal at the absorbed pump power of 0.74 W. When the absorbed pump power was over 0.74 W, the output power of the S-polarization laser was higher than that of the P-polarization laser. Increasing the absorbed pump power to 1.83 W, the maximum output powers of the P-polarization and S-polarization lasers were 219 mW and 241 mW, respectively. It is worthy to note that the output power of the S-polarization laser showed obvious saturation when the absorbed pump power increased over 1.32 W, however, the output power of the P-polarization laser still increased with the absorbed pump power.
Fig. 3. The total, S-polarization, and P-polarization output powers in the multimode operation as functions of absorbed pump power for the 5% output coupler.
At the absorbed pump power of 1.83 W, the output wavelengths of the orthogonally polarized Tm,Ho:YLF laser in the multimode operation were measured with a monochrometer (WDG-30) and an InGaAs PIN photodiode, as shown in Fig. 4. It can be seen that the central wavelength of the S-polarized laser was 2064.1 nm, while the P-polarized laser had two wavelengths at 2052.3 nm and 2064.1 nm, respectively, because the inversion population ratio and the gain of the P-polarized laser maintained at a higher level, comparing to the S-polarized laser. According to the analysis of the π-polarized gain spectrum of the a-cut Tm,Ho:YLF crystal [28–30], the laser at the single wavelength of 2064.1 nm oscillated when the population inversion ratio was at a lower level. Increasing the inversion population ratio appropriately, the gains at the two wavelengths of 2052.3 nm and 2064.1 nm were approximately equivalent, so the lasers at 2052.3 nm and 2064.1 nm both realized oscillating. For realizing the orthogonally polarized dual-wavelength output, the P-polarized 2064.1 nm laser can be effectively suppressed by suitably increasing the transmittance of output coupler or slightly inclining the cavity mirrors.
Fig. 4. Optical spectra of the orthogonally polarized Tm,Ho:YLF laser in the multimode operation, (a) S-polarization at 2064.1 nm and (b) P-polarization at 2052.3 nm and 2064.1 nm.
The spatial beam distributions of the orthogonally polarized Tm,Ho:YLF laser in the multimode operation were measured with a beam analyzer (Electrophysics, Micron Viewer 7290A) under the absorbed pump power of 1.83W, as shown in Fig. 5(a) and Fig. 5(b). The output laser beam was close to the fundamental transverse electromagnetic mode (TEM00). Figure 5(c) shows the M2 factor value calculated to be 1.07, by fitting the standard Gaussian beam propagation expression to the measured beam radius at different positions along the beam propagation direction. Thus, the orthogonally polarized Tm,Ho:YLF laser with high beam quality was realized.
Fig. 5. (a) Two dimensional and (b) three dimensional far field beam profiles of the orthogonally polarized Tm,Ho:YLF laser in the multimode operation. (c) The result of the beam quality measurement.
It should be seen from Fig. 3 that the output powers were generally different at the P-polarization and S-polarization states. However, the output powers in the two orthogonal polarization directions could be equalized under each absorbed pump power by only adjusting the pump focus position along the optical axis in the process of increasing pump power. The relationship between the output power and the absorbed pump power was obtained, as shown in Fig. 6. The output powers in the two orthogonal polarization directions linearly increased with the increase of the absorbed pump power. When the absorbed pump power was increased to the maximum of 1.83 W, the total output power was 430 mW with the slope efficiency of 26.8%, and the maximum output powers in the S-polarization and P-polarization directions were both 215 mW.
Fig. 6. The output powers in the multimode operation as functions of absorbed pump power, showing equal output powers in the P-polarization and S-polarization states.
On the foundation of multimode operation, inserting two uncoated solid fused quartz F-P etalons with thicknesses of 0.1 mm and 1 mm into the laser resonator, the orthogonally polarized SLM Tm,Ho:YLF laser was successfully realized when the tilt angles of the 0.1 mm and 1 mm thick etalons were approximately 10.2° and 6.4°, respectively. Figure 7(a) shows the output power as a function of absorbed pump power in the SLM operation for the 5% output coupler. Since the position of the crystal that generated the P-polarization laser was in front of the other, the oscillation threshold could be reached at a lower pump power. Thus, when the absorbed pump power was 0.28 W, the P-polarization laser began to oscillate first. It can be found that the output laser was in the P-polarization state when the absorbed pump power increased from 0.28 W to 0.85 W. When the absorbed pump power reached to 0.85 W, the S-polarization laser also started to oscillate. The output power of the S-polarization laser increased linearly with the increase of absorbed pump power, while the output power of the P-polarization laser decreased gradually. However, the total output power maintained a linear increase trend. When the absorbed pump power increased over 1.12 W, the output powers in the two orthogonal polarization directions increased linearly with approximately equal slope efficiency. The output power of the P-polarization laser maintained at a higher level throughout, because the losses of the P-polarization laser were lower than that of the S-polarization laser. Under the absorbed pump power of 1.48 W, the maximum total output power was 133 mW, corresponding to the slope efficiency of 10.6%. The maximum SLM output powers of the P-polarization and S-polarization lasers were 71 mW and 62 mW, respectively. It should be seen from Fig. 7(a) that the output powers of the two orthogonal polarization lasers showed evident saturation due to thermal effect. Further enhancement of output power may be realized by suitably reducing the doping concentration of Tm3+ and increasing the length of laser crystal.
Fig. 7. (a) The output powers as functions of absorbed pump power and (b) optical spectra of the orthogonally polarized dual-wavelength Tm,Ho:YLF laser in the SLM operation.
Figure 7(b) shows the optical spectra of the orthogonally polarized SLM Tm,Ho:YLF laser at the absorbed pump power of 1.48 W. Compared with the multimode operation, the output laser wavelengths in the SLM operation became different due to inserting the etalons into the resonator [26]. It can be noted from Fig. 7(b) that the central wavelength of the S-polarized laser was 2065 nm, while, the output laser wavelength was 2052.1 nm for the P-polarized laser. The reason was that the P-polarized 2052.1 nm laser had a higher net gain than the P-polarized 2064 nm laser, and the 2064 nm laser was suppressed, so the central wavelength of the P-polarized laser was 2052.1 nm. The output wavelengths of the orthogonally polarized SLM Tm,Ho:YLF laser were measured multiple times over an hour, and the central wavelengths always remained at P-polarized 2052.1 nm and S-polarized 2065nm.
When the absorbed pump power was 1.48 W, a scanning F-P interferometer with the free spectrum scope of 3.75 GHz and an InGaAs photodiode connected with a oscilloscope (Tektronix TDS3032B) were used to measure the mode spectra of the orthogonally polarized dual-wavelength Tm,Ho:YLF laser in the SLM operation. The F-P spectra of the P-polarization, dual-polarization, and S-polarization lasers were shown in Fig. 8. The results from Fig. 8 show that the orthogonally polarized Tm,Ho:YLF laser worked in the SLM operation, moreover, the output laser mode was very stable. The output frequencies of the orthogonally polarized SLM Tm,Ho:YLF laser could be tuned continuously over 1.5 GHz by only changing the tilt angle of the 1 mm thick etalon.
Fig. 8. F-P spectra of the orthogonally polarized dual-wavelength Tm,Ho:YLF laser in the SLM operation. (a) P-polarization at 2052.1 nm, (b) dual-polarization at 2052.1 nm and 2065nm, and (c) S-polarization at 2065nm.
Figure 9 shows the transverse output beam profile of the orthogonally polarized dual-wavelength SLM Tm,Ho:YLF laser at the absorbed pump power of 1.48 W. The output beam was close to the TEM00 mode. The beam radius at different positions along the beam propagation direction was measured, as shown in Fig. 9(c), and the M2 factor was calculated to be 1.26. Although the beam quality of the SLM laser was somewhat worse than that of the multimode laser, it was still relatively good. The orthogonally polarized dual-wavelength Tm,Ho:YLF laser in the SLM operation is an ideal seed source of the differential absorption lidars at 2 µm, furthermore it can also be used to generate coherent THz wave.
Fig. 9. (a) Two dimensional and (b) three dimensional far field beam profiles of the orthogonally polarized dual-wavelength Tm,Ho:YLF laser in the SLM operation. (c) The result of the beam quality measurement.
4. Conclusion
We have investigated the output performances of the dual-crystal orthogonally polarized multimode and SLM Tm,Ho:YLF lasers. Two pieces of a-cut Tm,Ho:YLF crystals were placed compactly with their c-axis perpendicular. For the 5% output coupler, the maximum multimode total output power of 460 mW was obtained at the absorbed pump power of 1.83 W, with the slope efficiency of 28.2%, and the M2 factor was measured to be 1.07. The output powers at S-polarized 2064.1 nm and P-polarized 2052.3 nm and 2064.1 nm were 241 mW and 219 mW, respectively. In addition, the balanced output powers in the two orthogonal polarization directions were achieved by only adjusting the pump focus position along the optical axis in the process of increasing pump power. The maximum total output power was 430 mW at the absorbed pump power of 1.83 W, and the slope efficiency was 26.8%. By inserting two uncoated solid state F-P etalons into the resonator, the orthogonally polarized dual-wavelength SLM Tm,Ho:YLF laser was successfully realized. At the absorbed pump power of 1.48 W, the output powers at P-polarized 2052.1 nm and S-polarized 2065 nm were 71 mW and 62 mW, respectively, and the beam quality M2 factor was 1.26. The orthogonally polarized dual-wavelength SLM Tm,Ho:YLF laser at 2 µm is potential to be applied indifferential absorption lidars and coherent THz wave generation.
Funding
National Natural Science Foundation of China (61275138, 61775166); Natural Science Foundation of Tianjin City (19JCZDJC32600); Program for Innovative Research Team in University of Tianjin (TD13-5035).
Disclosures
The authors declare no conflicts of interest.
Data availability
No data were generated or analyzed in the presented research.
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