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Abstract
Based on Faraday effect we demonstrate a thulium fiber pumped continuous-wave single-longitudinal-mode laser with a new Ho:GdTaO4 crystal. By inserting a faraday rotator and a half-wave plate into the laser cavity, the single-longitudinal-mode output power of 392 mW at wavelength of 2068.33 nm was obtained in unidirectional Ho:GdTaO4 ring laser, corresponding to a slope efficiency of 60.2% respect to the absorbed pump power. Furthermore, utilizing the Ho:GdTaO4 power amplifier, the maximum single-longitudinal- mode output power of 1.02 W was achieved.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The interest in solid-state single-longitudinal-mode laser operating in the 2-µm spectral range is acknowledged for many applications such as lidar [1,2], laser remote sensing [3], high resolution spectroscopy [4] and so on. Well known, holmium (Ho)-doped solid-state laser (5I7→5I8 transition) is a prime way to obtain the 2-µm laser radiation. Recently, several techniques have been used to obtain 2 µm single-longitudinal-mode lasers, such as microchip laser, twisted-mode cavity, nonplanar ring oscillator (NPRO), unidirectional ring laser, coupled cavity and intra-cavity Fabry-Perot (F-P) etalons [5–9]. Among the above technical approaches, the NPRO is an effective method to obtain high efficiency 2 µm single- longitudinal-mode lasers. However, the system of the NPRO laser is complex and expensive for achieving a laser with a single-longitudinal-mode output, and this technique is only applicable for isotropic crystals. In addition to the NPRO, the use of a ring cavity configuration and enforcing unidirectional operation with acousto-optic effect or Faraday effect is an attractive and potential low-loss method for achieving of high efficiency single-longitudinal-mode laser, offering flexibility in the choice of crystal materials and resonator design. Recently the unidirectional ring technique has been successfully applied to Ho-singly-doped ring lasers to achieve 2 µm single-longitudinal-mode laser. In 2004, based on the acousto-optic effect, Shen et al. reported a single-longitudinal-mode 2114-nm Ho:YAG unidirectional ring laser with a slope efficiency of 47% [10]. In 2017, our group reported a single-longitudinal-mode 2051-nm Ho:YLF unidirectional ring laser based on the Faraday effect, which reaches 528 mW output power and 39.5% slope efficiency [11].
The gadolinium tantalite GdTaO4 (GTO) crystal was used as promising host for doping of rare earth. The GTO crystal has low symmetry and strong symmetrical crystal field, which is beneficial for obtaining polarized laser output and enhancing the photoluminescence efficiency. Compared with traditional YAG crystal, the GTO crystal has low sensitivity for thermal-optical effect. Owing to this advantage, the GTO crystal was successfully applied to dope the Nd and Ho ions to obtain high-power 1-µm and 2-µm laser radiations [12–15]. In 2018, the continuous-wave and actively Q-switched Nd:GTO laser was presented with a maximum continuous-wave output power of 1.93 W at 1066 nm and a pulse duration of 28 ns at repetition rate of 10 kHz. In 2019, our group demonstrated a thulium fiber-pumped Ho:GTO laser with the maximum continuous-wave output power of 11.2 W at 2068.39 nm, corresponding to a slope efficiency of 72.9% with respect to the absorbed pump power. However, single-longitudinal-mode lasing performance of the Ho:GTO crystal in the 2µm spectral range is never reported up to present.
In this paper, to the best of our knowledge, we demonstrate the single-longitudinal-mode lasing performance of Ho:GTO laser for the first time. Using a faraday rotator and a half- wave plate, the unidirectional single-longitudinal-mode operation of the continuous-wave Ho:GTO ring laser was realized. The single-longitudinal-mode output power of 392 mW at 2068.33 nm was achieved in the Ho:GTO unidirectional ring laser, corresponding to a slope efficiency of 60.2% respected to the absorbed pump power. In addition, with the M2 factor of 1.1 was measured with 90/10 knife-edge method. Furthermore, utilizing the Ho:GTO power amplifier, the maximum single-longitudinal-mode output power was up to 1.02 W.
2. Experimental setup
The schematic diagram of the single-longitudinal-mode Ho:GTO unidirectional ring laser was shown in Fig. 1. A 20-W Tm-fiber laser, which has central wavelength of 1.94-µm (linewidth of about 0.2 nm) and M2 factor of 1.4, was employed as the pump source. By using two lenses (collimated lens f1 and focus lens f2) and a polarizer P1 (high reflectivity for 1.94 µm pump laser, horizontal polarization high transmittance and vertical polarization high reflectivity for resonant wavelength at 45°), the pump beam was re-focused into the Ho:GTO crystal with a beam radius of 0.18 mm. The pump Rayleigh length was calculated to be about 79 mm inside the Ho:GTO crystal with the refraction index of 2.11. The resonator consists of two plane mirrors and two curved mirror. 300-mm curvature radius M1 and 400-mm curvature radius M2 were coated for high reflected resonant wavelength. Plane mirror M3 was coated for high reflectivity at resonant wavelength. The plane mirror M4 was used as the output coupler. A faraday rotator (Optics For Research) and a half-wave plate (antireflection coated at resonant wavelength) were inserted into the resonator for unidirectional operation. The faraday rotator was surrounded by a magnetic field with a polarization rotation angle of 45°. The laser beam propagation of one direction was horizontal polarization while the other was vertical polarization by combining the faraday rotator and a half-wave plate. Thus only one direction laser can be produced. The single-longitudinal-mode operation can be realized without the spatial hole-burning effect. In addition, in order to avoid unabsorbed pump laser damaging the faraday rotator, the polarizer P2 was inserted into the resonator. The physical cavity length of ring laser was approximately 1300 mm (corresponding to FSR of 225 MHz). The c-cut Ho:GTO crystal had a dimension of 4×4×10 mm3 and 1.0 at.% Ho3+ doping concentration. Both end-faces of Ho:GTO crystal were polished and antireflection coated for pump and resonant wavelength. The indium foil-wrapped crystal was mounted in a copper heat sink maintaining at the temperature of 12 °C by a thermoelectric cooler (TEC).
3. Experimental results
In this experiment we use a Coherent PM30 power meter to record the output powers. Firstly, Without the faraday rotator and a half-wave plate, the free-running output characteristics of the Ho:GTO ring laser was investigated. Figure 2(a) shows the output powers of the free-running Ho:GTO ring laser versus the absorbed pump power with the output transmittances of 3.5%, 15%, and 30%. With output transmittance of 3.5%, the free-running Ho:GTO ring laser yielded 0.84 W output power with the absorbed pump power of 5.1 W, corresponding to a slope efficiency of 31.6% respected to the absorbed pump power. When the output transmittance increased to 15%, the output power increased to 1.2 W, corresponding to a slope efficiency of 47.9%. The highest output power was achieved at the output transmittance of 30%, and the laser started to work when the absorbed pump power is over 3 W. The maximum output power is 1.37 W, corresponding to a slope efficiency of 62.9% respected to the absorbed pump power.
Fig. 2. Output powers of the free-running Ho:GTO ring laser. (b)- Output wavelength of free-running Ho:GTO ring laser with different output transmittances.
The output wavelength of the free-running Ho:GTO ring laser were recorded by a wavemeter (Bristol 721A, 0.7 pm resolution) under difference output transmittances, as shown in Fig. 2(b). The output wavelength was centered at 2081.51 nm, 2068.85 nm and 2068.38 nm for T = 3.5%, 15% and 30%, respectively. The output wavelength of about 2081nm was achieved with lower transmittance. With increasing of transmittance, the output central wavelength has blue shifts from 2081nm to 2068nm. This change can be probably explained by the gain cross sections of Ho:GTO crystal at room temperature. In addition, we measured the Fabry-Perot spectrum of the free-running Ho:GTO ring laser by a scanned F-P interferometer with FSR of 1.5 GHz (SA200-18B, THORLABS) for the three output transmittance, and the typical Fabry-Perot spectrum were shown in the Fig. 3(a). The free-running Ho:GTO ring laser operated at multimode oscillation because the output coupler produce laser in two directions. In addition, the output beam of the free-running Ho:GTO ring laser was horizontal polarization measured by a Glan prism.
Fig. 3. (a)- Fabry-Perot spectrum of the free-running Ho:GTO ring laser. (b)–(d) Output wavelength and Fabry-Perot spectrum of the unidirectional operation Ho:GTO ring laser with the output transmittance of 30%, 15%, and 3.5%.
In order to achieve single-longitudinal-mode operation of the Ho:GTO ring laser, a faraday rotator and a half-wave plate was used to realize the Ho:GTO ring laser unidirectional operation. With regulating the angle of half-wave plate, the polarization of counterpropagation laser was changed. With combining of the faraday rotator and polarizer, the ring laser can be obtained the unidirectional operation. Figure 3(b), Fig. 3(c), and Fig. 3(d) shows the output wavelength and Fabry-Perot spectrum of the unidirectional operation Ho:GTO ring laser with the output transmittance of 30%, 15%, and 3.5%. No higher transverse modes were observed in the scanning F-P spectrum. As can be seen, the unidirectional operation Ho:GTO ring laser with the three output transmittance were all operating on a single-longitudinal-mode. The output central wavelength of the unidirectional operation Ho:GTO ring laser was located at 2068.33 nm, 2068.61 nm, and 2081.83 nm, respectively, corresponding to the linewidth of about 0.1 nm for the three transmittances. However, the real linewidth of single-longitudinal-mode laser is far less than the wavemeter resolution, so the linewidth of single-longitudinal-mode laser is broadened under the measurement of wavemeter. In addition, compared with the free-running Ho:GTO ring laser, the output beam of the single-longitudinal-mode Ho:GTO ring laser was same with horizontal polarization. However, the output wavelength of the unidirectional operation Ho:GTO ring laser was slightly changed. This may be attributed to the change of the cavity gain caused by the insertion of faraday rotator and a half-wave plate.
For the unidirectional operation Ho:GTO ring laser with the output transmittance of 30%, the maximum single-longitudinal-mode output power of 392 mW was obtained at the absorbed pump power of 3.9 W, corresponding to a slope efficiency of 60.2%, as shown in Fig. 4(a). With insertion of faraday rotator and a half-wave plate, the cavity loss was slightly increased, which leads to higher pump threshold and lower slope efficiency under unidirectional operation conditions. However, compared with the unidirectional ring laser with acousto-optic device, there are some advantages, for example, easy adjustment, no electric, etc. Therefore, the ring laser with faraday rotator and a half-wave plate can reach high efficiency and high stability. In addition, it can be clearly see the fact that, from Fig. 4(a), the single-longitudinal-mode output power of unidirectional operation Ho:GTO ring laser increases linearly with the increasing absorbed pump power. It is implied that single- longitudinal-mode output power of the unidirectional operation Ho:GTO ring laser can be further increased if increasing pump power is available. However, in this work, to avoid damaging the faraday rotator, we did not obtain higher single-longitudinal-mode output power over 392 mW. Over a period of 20 minutes, the wavelength fluctuation of the single-longitudinal-mode Ho:GTO laser was approximately 2.3 pm, which may be caused by thermal/mechanical perturbations. For some applications, the wavelength fluctuation can be improved by passive/active frequency stabilization methods such as high-precision controlling of the laser pump current, temperature and cavity length. To determine the beam quality factor M2 of the unidirectional operation Ho:GTO ring laser, the 90/10 knife-edge technique was used to measure the 1/e2 beam radius along the propagation direction at the highest single-longitudinal-mode output power, as shown in Fig. 4(b). By fitting Gaussian beam standard expression to these data, the M2 factor of single-longitudinal-mode Ho:GTO laser was calculated to be approximately 1.1.
Fig. 4. (a)- Output powers of the single-longitudinal-mode Ho:GTO ring laser with T = 30%. (b)- Beam quality measurement of the unidirectional operation Ho:GTO ring laser.
In order to improve the single-longitudinal-mode power level of the unidirectional operation Ho:GTO ring laser, the output characteristics of the Ho:GTO power amplifier was investigated. The schematic diagram of the Ho:GTO power amplifier was shown in Fig. 5. Another Ho:GTO crystal with parameters same as the oscillator one was used as the amplification medium. The 1.94-µm pump laser and the Ho:GTO single-longitudinal-mode seed laser were focused into the Ho:GTO crystal with a beam radius of 0.18 mm by lens f1 and f2, respectively. The 1.94-µm pump laser was injected into the Ho:GTO crystal through the polarizer P3 with parameters same as P1 and P2. The flat mirror M with high transmission for pump light and high reflectivity for laser was used as the 45° dichroic mirror.
The output wavelength of the Ho:GTO amplifier was shown in Fig. 6(a). The amplified laser is still single-longitudinal-mode operation at 2068.31 nm tested by the wavelength meter and scanned F-P interferometer. The output power of the Ho:GTO amplifier as a function of the absorbed pump power was shown in Fig. 6(b). The maximum output power of 1.02 W was obtained at an absorbed pump power of 5 W when the inject seed power was 390 mW. Limited by the length of gain medium, we cannot demonstrate more output power in Ho:GTO amplifier.
Fig. 6. (a)- Output wavelength of the Ho:GTO amplifier. (b)- Output power of the Ho:GTO amplifier with different seed power.
4. Conclusion
In this paper, we reported a single-longitudinal-mode 2068.33-nm Ho:GTO ring laser pumped by a 1.94-µm Tm fiber laser. Unidirectional operation of Ho:GTO ring laser was obtained by inserted a faraday rotator and a half-wave plate into resonator. The maximum single- longitudinal-mode output power was 392 mW at the absorbed pump power of 3.9 W, corresponding to a slope efficiency of 60.2% respected to the absorbed pump power. Furthermore, with a Ho:GTO power amplifier, up to 1.02 W single-longitudinal-mode output power was obtained. These results indicate that the Ho:GTO laser is a promising candidate for highly efficient 2.1-µm single-longitudinal-mode lasers.
Funding
National Natural Science Foundation of China (51572053, 51802307).
References
1. T. Y. Dai, Y. L. Ju, B. Q. Yao, Y. J. Shen, W. Wang, and Y. Z. Wang, “Single-frequency, Q-switched Ho:YAG laser at room temperature injection-seeded by two F-P etalons-restricted Tm, Ho:YAG laser,” Opt. Lett. 37(11), 1850–1852 (2012). [CrossRef]
2. T. Y. Dai, Y. L. Ju, X. M. Duan, W. Liu, B. Q. Yao, and Y. Z. Wang, “Single-frequency, injection-seeded Q-switched operation of a resonantly pumped Ho:YAlO3 laser at 2,118 nm,” Appl. Phys. B: Lasers Opt. 111(1), 89–92 (2013). [CrossRef]
3. J. Li, S. H. Yang, C. M. Zhao, H. Y. Zhang, and W. Xie, “Coupled-cavity concept applied to a highly compact single-frequency laser operating in the 2 µm spectral region,” Appl. Opt. 50(10), 1329–1332 (2011). [CrossRef]
4. B. Q. Yao, F. Chen, C. H. Zhang, Q. Wang, C. T. Wu, and X. M. Duan, “Room temperature single-frequency output from a diode-pumped Tm,Ho:YAP laser,” Opt. Lett. 36(9), 1554–1556 (2011). [CrossRef]
5. Z. Y. You, Y. Wang, J. L. Xu, Z. J. Zhu, J. F. Li, H. Y. Wang, and C. Y. Tu, “Single-longitudinal-mode Er:GGG microchip laser operating at 2.7 µm,” Opt. Lett. 40(16), 3846–3849 (2015). [CrossRef]
6. C. Gao, R. Wang, Z. Lin, M. Gao, L. Zhu, Y. Zheng, and Y. Zhang, “2 µm single-frequency Tm:YAG laser generated from a diode-pumped L-shaped twisted mode cavity,” Appl. Phys. B: Lasers Opt. 107(1), 67–70 (2012). [CrossRef]
7. L. Wang, C. Q. Gao, M. W. Gao, and Y. Li, “Resonantly pumped monolithic nonplanar Ho:YAG ring laser with high-power single-frequency laser output at 2122 nm,” Opt. Express 21(8), 9541–9546 (2013). [CrossRef]
8. C. T. Wu, Y. L. Ju, Z. G. Wang, Y. F. Li, H. Y. Ma, and Y. Z. Wang, “Lasing characteristics of a CW Tm:LuAG laser with a set of double cavity,” Laser Phys. Lett. 5(7), 510–513 (2008). [CrossRef]
9. B. Q. Yao, F. Chen, C. H. Zhang, Q. Wang, C. T. Wu, and X. M. Duan, “Room temperature single-frequency output from a diode-pumped Tm,Ho:YAP laser,” Opt. Lett. 36(9), 1554–1556 (2011). [CrossRef]
10. D. Y. Shen, W. A. Clarkson, L. J. Cooper, and R. B. Williams, “Efficient single-axial-mode operation of a Ho:YAG ring laser pumped by a Tm-doped silica fiber laser,” Opt. Lett. 29(20), 2396–2398 (2004). [CrossRef]
11. J. Wu, Y. L. Ju, T. Y. Dai, B. Q. Yao, and Y. Z. Wang, “1.5 W high efficiency and tunable single-longitudinal-mode Ho:YLF ring laser based on Faraday effect,” Opt. Express 25(22), 27671–27677 (2017). [CrossRef]
12. F. Peng, H. Yang, Q. Zhang, J. Luo, W. Liu, D. Sun, R. Dou, and G. Sun, “Spectroscopic properties and laser performance at 1,066 nm of a new laser crystal Nd:GdTaO4,” Appl. Phys. B: Lasers Opt. 118(4), 549–554 (2015). [CrossRef]
13. G. Wang, Q. Song, Y. Gao, B. Zhang, W. Wang, M. Wang, Q. Zhang, W. Liu, D. Sun, F. Peng, and G. Sun, “Passively Q-switched mode locking performance of Nd:GdTaO4 crystal by MoS2 saturable absorber at 1066 nm,” Appl. Opt. 54(18), 5829–5832 (2015). [CrossRef]
14. Y. Ma, Y. He, Z. Peng, H. Sun, F. Peng, R. Yan, X. Li, X. Yu, Q. Zhang, and S. Ding, “Continuous-wave and acousto-optically Q-switched 2018 nm laser performance of a novel Nd:GdTaO4 crystal,” Opt. Laser Technol. 101, 397–400 (2018). [CrossRef]
15. X. M. Duan, G. P. Chen, C. P. Qian, Y. J. Shen, R. Q. Dou, Q. L. Zhang, L. J. Li, and T. Y. Dai, “Resonantly pumped high efficiency Ho:GdTaO4 laser,” Opt. Express 27(13), 18273–18281 (2019). [CrossRef]
