A high performance eye-safe 1.55 µm microchip laser was fabricated by tightly pressing two sapphire crystals with high thermal conductivity and an Er:Yb:YAl3(BO3)4 laser crystal between them. Temperature distribution inside the Er:Yb:YAl3(BO3)4 was simulated by the finite element analysis method. 1.55 µm continuous-wave and passively Q-switched pulse laser properties were investigated. At an incident pump power of 7.2 W, a 1550 nm continuous-wave microchip laser with the maximum output power of 2.05 W and slope efficiency of 39.8% was realized. When a Co2+:MgAl2O4 saturable absorber with an initial transmission of 97% was placed between the Er:Yb:YAl3(BO3)4 and a sapphire crystal, a 1522 nm passively Q-switched microchip laser with pulse energy of about 10 µJ, repetition frequency of 77 kHz, width of 7 ns, and output peak power of 1.43 kW was realized at an incident pump power of 7.2 W.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Due to the strong penetration ability to smoke, excellent transparency in atmosphere and high sensitivity for the room-temperature Ge as well as InGaAs photodiodes, eye-safe 1.55 µm laser can be widely used in many applications, such as lidar, laser ranging, three-dimensional imaging and target recognition [1,2]. Compared to other techniques, such as the optical parametric oscillator (OPO) and the stimulated-Raman shift (SRS) [3,4], the 976nm-diode-pumped Er3+/Yb3+ co-doped materials is a more direct and convenient method to obtain compact and low cost 1.55 µm solid state laser. When the resonator mirror films were directly deposited on the end faces of an Er3+/Yb3+ co-doped material, a compact 976 nm diode-end-pumped 1.55 µm solid-state microchip laser can be fabricated. In recent years, with the rapid development of lidar used in the fields of autonomous vehicle, unmanned aerial vehicle (UAV) and industrial robot, the 1.55 µm solid-state microchip laser with the advantages of simplicity, maintenance-free and easy integration has attracted wide attention.
Gain media of the 1.55 µm solid-state microchip laser investigated up to now are mainly focused on the Er3+/Yb3+ co-doped borate crystals and phosphate glass [2,5–8]. 1.53 µm continuous-wave (cw) microchip laser with the maximum output power of 220 mW and slope efficiency of 38% was realized in the commercial Er:Yb:phosphate glass . Based on an Er:Yb:glass rigidly joined to a Co2+:MgAl2O4 saturable absorber, 1.53 µm passively Q-switched pulse microchip laser with average output power of 150 mW, energy of 6 µJ, repetition frequency of 27 kHz, and width of 5 ns was obtained . However, due to the limitation of the low thermal conductivity of the host glass, the maximum average output power of the Er:Yb:glass microchip laser was hardly to be increased further. Recently, Er:Yb:YAl3(BO3)4 (Er:Yb:YAB) crystal has been considered as an excellent 1.55 µm laser material, because it has high thermal conductivity, high Yb3+→Er3+ energy transfer efficiency and weak upconversion loss . 1.602 µm cw microchip laser with the maximum output power of 800 mW and slope efficiency of 16%, as well as 1.522 µm passively Q-switched pulse microchip laser with average output power of 315 mW, energy of 5.25 µJ, repetition frequency of 60 kHz, and width of 5 ns were realized in the Er:Yb:YAB crystal, respectively . However, the performances of the 1.55 µm solid-state microchip lasers must be further improved for meeting the application requirement. In this work, a 1.55 µm microchip laser is constructed when two sapphire crystals with high thermal conductivity are closely attached to two ends of an Er:Yb:YAB laser crystal. Then, the performances of the cw and passively Q-switched pulse lasers are investigated in detail.
2. Laser experimental arrangement
The used experimental setup is shown in Fig. 1. The gain medium was a c-cut Er(1.5 at.%):Yb(12 at.%):YAB crystal with a cross-section of 3×3 mm2 and a thickness of 1.5 mm. There was no antireflection coating on the polished crystal surfaces. The room-temperature absorption spectrum of the crystal in 875–1075 nm was recorded by a spectrophotometer (Lambda 950, Perkin Elmer), and is shown in Fig. 2. The absorption coefficient of the crystal at the peak absorption wavelength of 975 nm was 17.7 cm−1. A cw fiber-coupled laser diode (LD) with a core diameter of 100 µm and a numerical aperture of 0.15 was used as the pumping source. The emission wavelength of the LD was stabilized at 975.4 nm by the volume Bragg grating (VBG) technique, and the full width at half maximum (FWHM) of its emission band is less than 1.0 nm, as also shown in Fig. 2. It can be seen that the emission wavelength of the LD basically coincides with the peak absorption wavelength of the crystal. So, the single-pass absorption efficiency of the Er:Yb:YAB crystal to incident pump power was about 93%. By using a telescopic lens system (TLS) consisting of the collimating and focusing lens with the same focal length of 45 mm, pump beam was focused to a spot with waist radius of about 50 µm in the Er:Yb:YAB crystal. The Er:Yb:YAB crystal was placed between two sapphire crystals with the same dimensions of 3×3×1.2 mm3. In order to obtain a high optical quality of the interfaces between the crystals, both Er:Yb:YAB and sapphire crystals were polished to achieve a high surface quality with scrath/dig specification of 20/10 (MIL-PRF-13830B), flatness of less than one-quarter wave at 633 nm, and parallelism of better than 20 arc sec. Then, all the crystals were mounted in a copper holder cooled by water at about 20 °C and tightly pressed together by screws. There is a hole with radius of about 0.75 mm in the center of the holder to permit the passing of laser beams. Input mirror (IM) film with transmission of 90% around 975 nm and reflectivity of 99.8% in 1.5–1.6 µm was directly deposited on the input end of the Er:Yb:YAB crystal. Output mirror (OM) film was directly deposited on the output end of the rear sapphire crystal. Four OMs with transmissions of 1.5%, 2.5%, 4% and 6% in 1.5–1.6 µm were used in the experiment. The cavity length was 2.7 mm. Output power of the laser was measured by a PM100D power meter associated with a S314C thermal power head from Thorlabs Inc. Laser spectrum was recorded by a monochromator (Triax550, Jobin-Yvon) with a TE-cooled Ge detector.
3. Results and discussion
Based on the finite element analysis method, the influence of sapphire crystal on the temperature distribution inside the Er:Yb:YAB crystal was simulated by the commercial COMSOL Multiphysics software. Assuming that the pump light is a Gaussian beam, the heat source q(r, z) at position (r, z) inside the Er:Yb:YAB crystal induced by the absorption of the pump light can be roughly described as :
Figure 3 shows the temperature distribution along the direction of light propagation at the center of the pump region (r = 0) inside the Er:Yb:YAB crystal at incident pump power of 7.2 W. When the sapphire crystals were not used in the experiment, the highest temperature of 613 K was appeared at the front surface of the Er:Yb:YAB crystal, and the lowest temperature of 354 K was appeared at the back surface of the crystal. When the two sapphire crystals were respectively attached to the front and rear surfaces of the Er:Yb:YAB crystal, the maximum temperature of 500 K was appeared at z = 0.3 mm in the Er:Yb:YAB crystal, and the temperatures of the front and rear surfaces of the crystal were 330 and 300 K, respectively. In addition, when the incident pump power was 7.2 W, the temperature distributions at the center cross-section inside the Er:Yb:YAB crystal with and without sapphire cooling are shown in Figs. 4(a) and (b), respectively. Combining with Figs. 3 and 4, it can be seen that when the sapphire crystals with high thermal conductivity were closely contacted with the front and rear surfaces of the Er:Yb:YAB crystal, temperature gradient inside the Er:Yb:YAB crystal can be reduced and then the thermal effect of the crystal can be weakened obviously.
Figure 5 shows the dependences of the output power and spectrum on the incident pump power for the Er:Yb:YAB cw microchip laser at OM transmission of 2.5%. Due to the broad gain bandwidth (about 20 nm FWHM around 1550 nm and 10 nm FWHM around 1600 nm) of the Er:Yb:YAB crystal , the cavity length used in this work was still too long to realize single-longitudinal-mode laser oscillation. When the incident pump power was lower than 4.35 W, the output laser wavelength was located around 1600 nm, and the slope efficiency was 22.2%. When the incident pump power was higher than 4.35 W, the output laser wavelength was blue-shifted to 1550 nm, and the slope efficiency was increased to 39.8%. When the incident pump power was 7.2 W, the maximum cw output power of the laser was up to 2.05 W. The maximum output power and slope efficiency obtained in this experiment are higher than those of the 1.55 µm cw microchip lasers reported previously, such as Er:Yb:phosphate glass (output power of 0.22 W and slope efficiency of 38%) and Er:Yb:YAB (output power of 0.8 W and slope efficiency of 16%) [2,8]. The output performance parameters of the Er:Yb:YAB cw microchip laser for different OM transmissions at incident pump power of 7.2 W are listed in Table 1. For the experimental conditions used in this work, the optimal OM transmission may be between 1.5% and 2.5%. In addition, with the increment of the OM transmission, the slight blue-shift of the output laser wavelength was also observed. The variations of output laser wavelength with the pump power and OM transmission are often observed in the Er3+ quasi-three-level 1.55 µm laser, which is mainly caused by the change of gain curve of laser crystal with the gain and loss of laser cavity [7,9,13].
When a convex lens with a focal length of 100 mm was used to focus the output beam, the spatial profile of the focused beam of the Er:Yb:YAB microchip laser was recorded with a Pyrocam III camera from Ophir Optronics Ltd. The beam radius at various distances from the focusing lens was calculated by the 4-sigma method and then the beam quality factor M2 can be estimated by fitting these data to the Gaussian beam propagation expression. Figure 6 shows the spatial profile and quality factor M2 of the laser beam. A near circular symmetric laser beam was always observed for the various pump power. At an incident pump power of 7.2 W, the values of Mx2 and My2 were 2.96 and 2.88, respectively. Due to the reduction of the thermal effect, the beam quality of the output laser was improved with the decrement of the pump power. When the incident pump power was reduced to 1.67 W, the values of Mx2 and My2 were both close to 1.1.
When a 1.0 mm thick Co2+:MgAl2O4 crystal with an initial transmission of 97% was inserted between the Er:Yb:YAB and sapphire crystals, the passively Q-switched pulse microchip laser was realized. The experimental setup is shown in the inset of Fig. 7. When the OM transmission was 2.5%, the dependence of the average output power of the pulse laser on the incident pump power is shown in Fig. 7. Due to the increment of the cavity loss, the output laser wavelength was blue-shifted to 1522 nm. When the incident pump power was 7.2 W, the maximum average output power of the pulse laser was up to 790 mW, which is much higher than those of the 1.55 µm passively Q-switched pulse microchip laser reported previously, such as the Er:Yb:phosphate glass (150 mW) and Er:Yb:YAB (315 mW) [5,8]. The values of Mx2 and My2 of the pulse laser were both close to 2.5 at an incident pump power of 7.2 W.
Pulse profiles were measured by a 5 GHz InGaAs photodiode (DET08C, Thorlabs) connected to a digital oscilloscope with a bandwidth of 1 GHz (DSO6102A, Agilent). For the 2.5% OM transmission, the repetition frequency of the Er:Yb:YAB passively Q-switched pulse microchip laser was 77 kHz at incident pump power of 7.2 W, as shown in Fig. 8(a). It can be seen from the figure that the amplitude between various pulses was changed obviously and the output pulse energy was unstable due to both the thermal effect of the crystal and the multimode laser operation. When the incident pump power was 6.07 W, the pulse repetition frequency was 67 kHz and the amplitude variation between various pulses was generally kept within 7%, which demonstrates that stable pulse energy can be obtained, and is shown in Fig. 8(b). Figure 9 shows the dependences of the repetition frequency and energy of the pulse laser on the incident pump power. When the incident pump power was reduced to 2 W, the repetition frequency of the pulse laser was 13.2 kHz. For all the incident pump power, the pulse width was kept at about 7.0 ns and the pulse energy was about 10 ± 1 µJ, as shown in Figs. 8(c) and 9. Then, the peak output power of the Er:Yb:YAB passively Q-switched pulse microchip laser was estimated to be about 1.43 kW. The pulse energy realized in this work is higher than those of the Er:Yb:glass (6 µJ) and Er:Yb:YAB (5.25 µJ) passively Q-switched pulse microchip laser reported previously [5,8]. Furthermore, due to the shorter fluorescence lifetime of upper laser level 4I13/2 of the Er:Yb:YAB crystal (0.3 ms) than that of the Er:Yb:phosphate glass (about 7–8 ms), the pulse repetition frequency (77 kHz) realized in this work is also higher than that of the Er:Yb:glass microchip laser (27 kHz) .
Lasers at 761 and 507 nm were also observed along with the Er:Yb:YAB passively Q-switched pulse microchip laser by using a high resolution spectrometer (HR4000, Ocean Optics), as shown in Fig. 10. The generation of above visible lasers was caused by the YAB host crystal being an excellent nonlinear optical material [14,15]. At the high peak power density, the 761 and 507 nm lasers can be realized by the second-order and third-order nonlinear frequency conversions of the 1522 nm fundamental laser, respectively. However, because the Er:Yb:YAB crystal used in this experiment was not cut according to the phase matching angle of the corresponding laser wavelength, above nonlinear frequency conversions were very inefficient. Therefore, the output powers of the 761 and 507 nm lasers were very low (lower than 1.0 mW).
The temperature gradient inside the Er:Yb:YAB laser crystal can be effectively reduced when the sapphire crystals with high thermal conductivity are closely contacted with the laser crystal. Based on the effective reduction of the thermal effect, high performance 1.55 µm cw and passively Q-switched pulse microchip lasers can be realized. The output powers of the lasers realized in this work are obviously higher than the reported ones for 1.55 µm solid-state microchip lasers. This eye-safe 1.55 µm solid-state microchip laser has the characteristics of easy integration and compactness, which is expected to be a lidar source used in the fields of autonomous vehicle, UAV and industrial robot.
Ministry of Science and Technology of the People's Republic of China (MOST) (2016YFB0701002); Chinese Academy of Sciences (CAS) (XDB20000000).
1. M. Eichhorn, “Quasi-three-level solid-state lasers in the near and mid infrared based on trivalent rare earth ions,” Appl. Phys. B 93(2-3), 269–316 (2008). [CrossRef]
2. B. Denker, B. Galagan, S. Sverchkov, and A. Prokhorov, “Erbium (Er) glass lasers,” in Handbook of Solid-State Lasers, B. Denker and E. Shklovsky, eds. (Woodhead, 2013).
3. Y. M. Duan, H. Y. Zhu, Y. L. Ye, D. Zhang, G. Zhang, and D. Y. Tang, “Efficient RTP-based OPO intracavity pumped by an acousto-optic Q-switched Nd:YVO4 laser,” Opt. Lett. 39(5), 1314–1317 (2014). [CrossRef]
4. Y. J. Huang, Y. F. Chen, W. D. Chen, and G. Zhang, “Dual-wavelength eye-safe Nd:YAP Raman laser,” Opt. Lett. 40(15), 3560–3563 (2015). [CrossRef]
5. G. Karlsson, F. Laurell, J. Tellefsen, B. Denker, B. Galagan, V. Osiko, and S. Sverchkov, “Development and characterization of Yb-Er laser glass for high average power laser diode pumping,” Appl. Phys. B 75(1), 41–46 (2002). [CrossRef]
6. B. Denker, B. Galagan, L. Ivleva, V. Osiko, S. Sverchkov, I. Voronina, J. E. Hellstrom, G. Karlsson, and F. Laurell, “Luminescence and laser properties of Yb-Er:GdCa4O(BO3)3: a new crystal for eye-safe 1.5-μm lasers,” Appl. Phys. B 79(5), 577–581 (2004). [CrossRef]
7. P. Burns, J. M. Dawes, P. Dekker, J. A. Piper, H. Zhang, and J. Wang, “CW diode-pumped microlaser operation at 1.5-1.6 μm in Er, Yb:YCOB,” IEEE Photon. Technol. Lett. 14(12), 1677–1679 (2002). [CrossRef]
8. V. Kisel, K. Gorbachenya, A. Yasukevich, A. Ivashko, N. Kuleshov, V. Maltsev, and N. Leonyuk, “Passively Q-switched microchip Er,Yb:YAl3(BO3)4 diode-pumped laser,” Opt. Lett. 37(13), 2745–2747 (2012). [CrossRef]
9. N. A. Tolstik, V. E. Kisel, N. V. Kuleshov, V. V. Maltsev, and N. I. Leonyuk, “Er,Yb:YAl3(BO3)4 —efficient 1.5 μm laser crystal,” Appl. Phys. B 97(2), 357–362 (2009). [CrossRef]
10. R. Weber, B. Neuenschwander, M. Mac Donald, M. Roos, and H. Weber, “Cooling schemes for longitudinally diode-pumped Nd:YAG rods,” IEEE J. Quantum Electron. 34(6), 1046–1053 (1998). [CrossRef]
11. T. Y. Fan, “Heat generation in Nd:YAG and Yb:YAG,” IEEE J. Quantum Electron. 29(6), 1457–1459 (1993). [CrossRef]
12. C. Rothhardt, J. Rothhardt, A. Klenke, T. Peschel, R. Eberhardt, J. Limpert, and A. Tünnermann, “BBO-sapphire sandwich structure for frequency conversion of high power lasers,” Opt. Mater. Express 4(5), 1092–1103 (2014). [CrossRef]
13. Y. Chen, Y. Lin, X. Gong, Q. Tan, Z. Luo, and Y. Huang, “2.0 W diode-pumped Er:Yb:YAl3(BO3)4 laser at 1.5–1.6 μm,” Appl. Phys. Lett. 89(24), 241111 (2006). [CrossRef]
14. P. Becker, “Borate materials in nonlinear optics,” Adv. Mater. 10(13), 979–992 (1998). [CrossRef]
15. A. Brenier, “Tunable coherent infrared generation near 2.5 μm from self-difference frequency mixing in YAl3(BO3)4:Nd3+,” Appl. Opt. 43(32), 6007–6010 (2004). [CrossRef]