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Abstract
A power balanced orthogonally polarized dual-wavelength Ho:GdVO4 laser was demonstrated for the first time. Without inserting any other devices into the cavity, the power balanced simultaneous orthogonally polarized dual-wavelength laser at π-polarization 2048nm and σ-polarization 2062nm was successfully achieved. At the absorbed pump power of 14.2 W, the maximum total output power was 1.68 W, and the output powers of 2048nm and 2062nm were 0.81 W and 0.87 W, respectively. The interval between the two wavelengths in the orthogonally polarized dual-wavelength Ho:GdVO4 laser was nearly 14nm, corresponding to the frequency separation of 1 THz. This power balanced orthogonally polarized dual-wavelength Ho:GdVO4 laser can be applied to generate the terahertz wave.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Lasers with simultaneous emission of two orthogonally polarized wavelengths have attracted interest in recent years due to their wide range of applications in differential absorption lidars [1], medical treatment [2], holographic interferometry [3], nonlinear optical mixer [4], and precision laser spectroscopy [5]. In addition, with a 45° tilted polarizer to change the orthogonal polarization direction to the same polarization direction for both wavelengths, the orthogonally polarized dual-wavelength lasers can be applied to generate terahertz (THz) waves by using photoconductive switches [6,7] or nonlinear crystals [8–13]. Coherent terahertz waves, with a frequency range including frequencies between 100 GHz and 10 THz, are promising applications for terahertz communications, detection, imaging systems, and sensing applications [14,15]. In the past decades, many dual-wavelength solid state lasers were reported in 1 µm waveband by using the Nd- and Yb-doped laser crystals [16–22]. Meanwhile, many dual-wavelength lasers are used as pump sources for the generation of THz waves by difference frequency generation (DFG) [23–25]. However, the two-photon absorption of 1 µm region laser radiation in GaSe might lead to pump dissipation and the generation of free carriers that increase the THz absorption. The power dependence significantly diverged from the quadratic power law caused by the strong two-photon absorption. Besides, employing the pump laser in the 2 µm waveband will proportionally increase the quantum efficiency according to the Manlye-Rowe relations, which doubles the quantum efficiency compared with DFG using the pump laser in the 1 µm waveband. At present, many research groups are firstly using the optical parametric oscillator to achieve a 2 µm dual-wavelength laser output, followed by a DFG to produce a THz wave [26–28]. However, this system is more complex and costly. Therefore, it is very meaningful to investigate the directly generated orthogonally polarized dual-wavelength lasers in the 2 µm waveband.
Recently, the orthogonally polarized dual-wavelength lasers in the 2 µm waveband have been obtained in Tm-doped and Tm,Ho co-doped lasers [29–32]. In 2013, M. Segura et al. observed the polarization switching in the Tm:KLu(WO4)2 laser between the Nm-1948nm and Np-1917nm states oscillating [29]. In the next year, X. L. Zhang et al. reported an orthogonally polarized dual-wavelength single-longitudinal-mode Tm,Ho:LLF laser. In continuous wave (CW) multimode operation, the maximum output powers of the π-polarized laser and σ-polarized laser were 206 mW and 170 mW, respectively [30]. Meanwhile, the single-longitudinal-mode laser output was realized by inserting two 0.1 mm and 1 mm thick uncoated etalons into the cavity, with the maximum output powers of 76 mW and 32 mW in the π-polarized direction and the σ-polarized direction. In 2018, X. L. Zhang et al. investigated the polarization coexistence and switching in an optical bistability Tm,Ho:LLF laser in the theory and experiment [31]. In 2021, X. L. Zhang et al. reported a dual-crystal Tm,Ho:YLF laser with balanced orthogonal polarization output power, with the maximum output power of 215 mW in both the P-polarized and S-polarized states [32]. Similarly, a dual-wavelength orthogonal polarization single-longitudinal-mode laser was obtained by inserting two etalons into the resonator, with the output powers of 71 mW in the S-polarized direction and 62 mW in the P-polarized direction. Overall, power balanced orthogonally polarized dual-wavelength laser output was only achieved for the Tm,Ho co-doped lasers in the wavelength beyond 2 µm. However, the Tm,Ho co-doped lasers needed cooling with liquid nitrogen to realize the high power laser output due to the severe thermal effect [33,34]. Thus, the Tm,Ho co-doped crystals are not suitable as a gain medium to realize the high power orthogonally polarized dual-wavelength laser output at room temperature. Ho-doped solid state lasers pumped by Tm lasers are particularly suitable for high power operation in the 2 µm waveband at room temperature, because of the large emission cross section and low quantum defect [35]. Among various laser host materials, gadolinium vanadate (GdVO4) crystal doped with various ions represents a promising host material. The GdVO4 crystal has a large damage threshold (>500 MW/cm2) and a large thermal conductivity (11.7 W/m/K) [36], which makes it an excellent laser crystal for achieving high power output. At present, CW and Q-switched Ho:GdVO4 lasers have been widely investigated in the 2 µm region [37–47]. However, the orthogonally polarized dual-wavelength Ho:GdVO4 laser has not been reported to date.
In this paper, we report a power balanced, continuous wave, orthogonally polarized dual-wavelength Ho:GdVO4 laser working at 2048nm and 2062nm with a frequency difference of 1 THz. To the best of our knowledge, this is the first orthogonally polarized dual-wavelength solid-state laser in the 2 µm waveband using a Ho:GdVO4 crystal. The employed linear concave-plano cavity was as compact as 25 mm. The maximum total output power of 1.68 W was obtained for the absorbed pump power of 14.2 W, corresponding to the π-polarization and σ-polarization powers of 0.81 W and 0.87 W, respectively. The power balanced orthogonally polarized dual-wavelength Ho:GdVO4 laser can be used to the generation of terahertz wave.
2. Experimental setup
The experiment setup of the orthogonally polarized dual-wavelength Ho:GdVO4 laser is shown in Fig. 1. The pump source was a 1940nm Tm-doped fiber laser (TDFL) with a core diameter of 25 µm, the numerical aperture of 0.11, and the maximum output power of 30 W. The Tm fiber laser (Connet Laser Technology Ltd.) was a single transverse mode fiber laser with a beam quality factor M2 of less than 1.1. A 10 mm collimating lens (L1) and a 100 mm converging lens (L2) were used to reimage the pump laser. By using a transverse beam profiler to measure the pump beam, the radius of the pump waist in the crystal was approximately 100 µm. The gain medium was a 0.5 at.% doped Ho:GdVO4 crystal (Beijing Kegang Photoelectric Technology Ltd.) and the crystal was cut with dimensions 3 × 3 × 20 mm3 along the optical principle direction of the a-axis. The Ho:GdVO4 crystal has two equal a-axes and a c-axis. In our experiments, the a-axis of the crystal was parallel to the optical platform surface and the c-axis was vertical to the optical platform surface. The crystal was carefully wrapped with a thin indium foil and mounted in a copper heat sink, which was kept at 15 °C with a thermoelectric cooler. The resonator was a simple two-mirror cavity with a physical length of 25 mm, and the total size of the laser system was approximately 500 × 480 × 200 mm3. The input mirror M1 with a curvature radius of 200 mm was anti-reflection coated at 1940nm and high reflection at 2.05-2.6 µm. For the output mirror M2, three plane mirrors with the transmission of 6%, 10%, and 15% for 1.9-2.1 µm were used to explore the output performances of Ho:GdVO4 laser. The output laser was separated into two orthogonally polarized beams after passing through a polarizing beam splitter (PBS). A 45° flat dichroic mirror was used to separate the residual pump laser and output laser, which had high transmittance at 1.94 µm and high reflection at 2.05-2.06 µm respectively. The output power of the π-polarized laser was first measured and the σ-polarized laser was measured when the PBS was rotated by 90°. The output power was measured with a power meter (PM30, Coherent Inc.). The laser spectrum was measured with a 300 mm monochromator (Omni-λ 300, Zolix) and an InGaAs detector. The transverse beam profile of the output laser was measured with a laser beam profiler (WinCamD-IR-BB, DataRay Inc.).
3. Results and discussion
To begin with, the output performances of Ho:GdVO4 laser were investigated for three different output couplers. The evolutions of output power with the absorbed pump power for the three different output couplers are shown in Fig. 2. The maximum output powers of 2.89 W, 3.36 W, and 2.01 W were obtained for the output couplers of 6%, 10%, and 15% at the absorbed pump power of 15.56 W, respectively. The highest output power of 3.36 W was obtained for the output coupler of 10%, with a slope efficiency of 44.3% and an optical-to-optical conversion efficiency of 21.6%.
The power stability was measured at the maximum output power under different output couplers, as shown in Fig. 3(a). The root-mean-square (RMS) instability during 30 min was 1.33%, 3.36%, and 2.01% for the output couplers of 6%, 10%, and 15%, indicating good output power stability. Figure 3(b) shows the output spectra of Ho:GdVO4 laser at the maximum output power for three different output couplers. The central wavelengths were around 2062nm for the output couplers of 6% and 10%. However, the central wavelength shifted to 2048nm from 2062nm for the output coupler of 15%. It is worth noting that the output wavelengths were located around 2062nm and 2048nm at all absorbed pump powers when the output couplers were 6% and 15%, respectively. However, when the output coupler of 10% was used, the output wavelengths switched between 2048nm and 2062nm with increasing the absorbed pump power.
Fig. 3. (a) Stability measurement of output power and (b) output spectra of Ho:GdVO4 laser at maximum output power for three different output couplers.
When the laser was dual-wavelength output, the dual-wavelength lasers were linearly polarized and their polarization directions were orthogonal to each other. According to the emission cross-section of the Ho:GdVO4 crystal [36], the laser at 2048nm was π-polarized and the laser at 2062nm was σ-polarized. Figure 4 shows the output powers of the total and each polarization as functions of the absorbed pump power. It can be noted that the π-polarization laser began oscillating at the threshold absorbed pump power of 4.63 W. As increasing the absorbed pump power, the σ-polarization laser began oscillating at the absorbed pump power of 7.93 W. Further increasing the absorbed pump power, the π-polarization and σ-polarization lasers increased simultaneously. When the absorbed pump power increased to 9.1 W, the π-polarization reached a maximum output power of 0.41 W. Further increasing the absorbed pump power, the π-polarization laser began to decrease and disappeared at the absorbed pump power of 11.37 W, while the σ-polarization laser kept increasing with the increase of absorbed pump power.
Fig. 4. The relative dual-wavelength output powers at 2048nm and 2062nm versus absorbed pump power for the output coupler of 10%.
At the absorbed pump power of 9.55 W, a relatively balanced orthogonally polarized laser was obtained. In this case, the output powers of the π-polarization laser and σ-polarization laser were 0.35 W and 0.44, respectively. The power stability was measured in this case, and the RMS instability during 30 min was 1.09%, 1.4%, and 0.92% for the total, π-polarized laser, and σ-polarized laser, respectively, as shown in Fig. 5(a). Meanwhile, the output spectrum of the orthogonally polarized dual-wavelength Ho:GdVO4 laser was measured at the absorbed pump power of 9.55 W, as shown in Fig. 5(b). The output wavelengths of the π-polarized laser and σ-polarized laser were 2048nm and 2062nm, respectively. It should be noted from Fig. 4 that the output powers of the π-polarized laser and σ-polarized laser can achieve an equal value of 0.37 W at the absorbed pump power of 9.39 W.
Fig. 5. (a) Stability measurement of output power and (b) output spectra of orthogonally polarized dual-wavelength Ho:GdVO4 laser at the absorbed pump power of 9.55 W.
Meanwhile, for the output coupler of 10%, slightly misaligning the input mirror and rotating the gain medium to regulate the gain competition between the two-wavelength lasers, the power balanced orthogonally polarized dual-wavelength laser can be realized in a large absorbed pump power range. The output powers of the total, π-polarization laser, and σ-polarization laser as functions of absorbed pump power are shown in Fig. 6(a). The maximum total output power of 1.68 W was obtained at the absorbed pump power of 14.2 W, corresponding to the slope efficiency of 22.9% and optical-to-optical efficiency of 11.9%. Further increasing the absorbed pump power, the total output power showed significant saturation. This phenomenon should be attributed to the strong thermal effect due to tilting the input mirror and rotating the gain medium to regulate the gain competition between the two-wavelength lasers. In addition, there exists a considerable range of absorbed pump power from 9.1 to 14.7 W, in which the output powers of the two-wavelength lasers were quite close, as shown in Fig. 6(a). In this absorbed pump power region, the output power ratios of two polarized lasers were between 0.7 and 1.4. At the maximum output power, the output powers of π-polarized laser and σ-polarized lasers were 0.81 W and 0.87 W, respectively. The power stability was also measured at the maximum output power, and the RMS instability during 30 min was 2.18%, 3.28%, and 7.12% for the total, π-polarized laser, and σ-polarized laser, respectively. As can be seen that the RMS instability of power balanced orthogonally polarized dual-wavelength laser was larger than the unadjusted orthogonally polarized dual-wavelength laser. This phenomenon may be caused by more intense gain competition within the cavity when the powers of the two-wavelength lasers were closed by adjusting the input mirror and rotating the gain medium.
Fig. 6. (a) Output power versus absorbed pump power for the power balanced orthogonally polarized dual-wavelength laser. (b) Stability measurement of π-polarization, σ-polarization, and total output powers for the absorbed pump power of 14.2 W.
The two-dimensional beam profiles of total, π-polarized laser, and σ-polarized laser at the absorbed pump power of 14.2 W were measured, as shown in the insets of Fig. 7. It can be seen that the total output beam was a good fundamental transverse electromagnetic (TEM00). However, the energy distributions of the π-polarized laser and σ-polarized laser were not standard Gaussian distributions. This phenomenon may be caused by the laser passing the double-glued PBS. To determine the beam quality factor M2 of the output laser, the beam radii along the beam propagation direction were measured through a biconvex lens with a 100 mm focal length, as shown in Fig. 7. By fitting the standard Gaussian beam propagation expression to the measured data, the M2 factor values in the x and y directions were calculated to be 1.84 and 1.86 for the total output laser, 1.84 and 2.06 for the π-polarized laser, and 1.78 and 1.61 for the σ-polarized laser.
Fig. 7. The laser beam qualities of (a) total, (b) π-polarized, and (c) σ-polarized output lasers at the absorbed pump power of 14.2 W. The insets show the output beam transverse intensity profiles.
The output spectrum of the power balanced orthogonally polarized dual-wavelength Ho:GdVO4 laser was recorded, as shown in Fig. 8. The central wavelengths were 2048nm for the π-polarized laser and 2062nm for the σ-polarized laser with respective spectra widths (FWHM) of 2.3 nm and 2.6 nm. The interval between two wavelengths in the orthogonally polarized dual-wavelength Ho:GdVO4 laser was nearly 14 nm. Changing the orthogonal polarization direction to the same polarization direction for both wavelengths with a 45° tilted polarizer, this orthogonally polarized dual-wavelength Ho:GdVO4 laser can be used to produce coherent terahertz waves of 1 THz by the technology of photomixing using a photoconductive antenna.
Fig. 8. Output spectrum of orthogonally polarized dual-wavelength Ho:GdVO4 laser with balanced output powers.
It should be noted that the line widths of two-wavelength lasers were rather large in our experiment, which would make the spectral line width of the generated THz wave very large. In future research, an orthogonally polarized dual-wavelength laser with narrow line widths and a certain range tunability of frequency difference will be achieved by inserting the etalons into the resonator with a reasonable cavity length. Meanwhile, the output power stability of the orthogonally polarized dual-wavelength Ho:GdVO4 laser can be effectively improved by using the dual-crystal gain medium [48].
4. Conclusion
In conclusion, a power balanced orthogonally polarized dual-wavelength laser working at 2048nm and 2062nm in a Ho:GdVO4 crystal was reported for the first time, to our knowledge. A single wavelength laser with a maximum output power of 3.36 W at 2062nm was obtained at an absorbed pump power of 15.6 W, corresponding to a slope efficiency of 44.3% and an optical-to-optical efficiency of 21.5%. By slightly tilting the laser cavity mirror and rotating the laser crystal, a power balanced orthogonally polarized dual-wavelength laser was realized with a total output power of 1.68 W, and the output powers of π-polarized laser and σ-polarized laser were 0.81 W and 0.87 W at the absorbed pump power of 14.2 W, respectively. The power balanced orthogonally polarized dual-wavelength Ho:GdVO4 laser is a potential pump source for generating THz wave with a frequency difference of 1 THz.
Funding
National Natural Science Foundation of China (62275194, 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
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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