Open Access
Add to BibSonomy
Abstract
A diode-pumped passively Q-switched 1065 nm laser with a novel Nd:Gd0.89La0.1NbO4 mixed crystal was demonstrated for the first time to the best of our knowledge. First, the crystal characteristics were investigated. In the continuous-wave (CW) operation, when the absorbed pump power was 12.24 W, the maximum CW output power of 3.06 W was obtained with the optimum transmission (20%) of the output plane coupler. In the passively Q-switched situation, when Cr4+:YAG crystal with an initial transmission of 90% served as the Q-switched saturable absorber, the maximum repetition rate, pulse peak power and minimum pulse width were 24.5 kHz, 1.13 kW and 32 ns, respectively, at the absorbed pump power of 12.24 W.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Diode-pumped Q-switched lasers owing to the narrow pulse width and high output power are widely used in many areas, such as laser processing, laser radar and laser ignition [1–4]. Passively Q-switched lasers do not need additional artificial control and have the advantages of low power consumption, very compact structure and easy operation. Cr4+:YAG crystal is used as saturable absorber with many advanced properties, for example, low saturable intensity, large absorption cross section and high damage threshold [5,6]. Because of its advantage of high conversion efficiency, Nd:YAG serves as a common operation material for solid-state lasers in the near-infrared region. However, the absorption bandwidth of Nd:YAG at 808 nm is only 1.5 nm [7]. The narrow absorption bandwidth means that Nd:YAG crystal is sensitive to the pump light bandwidth. Recently, Nd-doped niobates (Nd:GdYNbO4 and Nd:GdLaNbO4) and tantalate (Nd:GdYTaO4) mixed laser crystals were successfully grown by using Czochralski method [8–10]. The absorption bandwidth for c-cut Nd:GdYTaO4, Nd:GdYNbO4 and Nd:GdLaNbO4 at 808 nm are 6 nm, 5.11 nm and 13 nm [8,10,11], respectively, which are much wider than the 1.5 nm of Nd:YAG. Furthermore, compared to the reported mixed crystals of garnets (Nd:LuYAG, 9 nm; Nd:LaGGG, 8.6 nm) and vanadates (Nd:GdYVO4, 9.5 nm; Nd:GdLaVO4, 3.6 nm) [12–15], a boarder absorption bandwidth of 13 nm for Nd:GdLaNbO4 shows advantage. Besides, compared with the above mixed crystals of garnet of Nd:LuYAG (262 μs) and Nd:LaGGG (243 μs) [12,13], Nd:GdLaNbO4 has a shorter fluorescence time τ (176.4 μs) [11], which is suitable for producing high repetition rate laser. Theoretical analysis shows that Nd:GdLaNbO4 mixed laser crystal is beneficial to reduce the requirements for pumping sources and improve the laser efficiency.
Moreover, Lanthanum (La) is the first element in the lanthanide series and has a larger atomic radius than Yttrium (Y). Hence, Nd:GdLaNbO4 has more disordered structure and broader fluorescence bandwidth than other Y-doped mixed crystals. In addition, there is no component volatilization during crystal growth and the external helicity of Nd:GdLaNbO4 mixed crystal is small, so it is easy to get large size and high quality Nd:GdLaNbO4 mixed crystal. But at present, there is a lack of research on the Q-switched Nd:GdLaNbO4 laser.
In this paper, as far as we know, a diode-pumped passively Q-switched 1065 nm laser with a novel Nd:Gd0.89La0.1NbO4 mixed crystal was demonstrated for the first time. The crystal characteristics were investigated firstly. The laser resonator was optimized and the output characteristics of continuous-wave (CW) laser using output coupler with different transmission were studied. Cr4+:YAG crystals with different initial transmission (T0) were selected as saturable absorbers and the performance of pulsed laser was investigated.
2. Crystal characteristics
Nd:GdLaNbO4 mixed crystal, in which Gd ions were replaced by La ions partly, was grown by Czochralski method in Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences in China. Raw materials for this crystal preparation were Nd2O3 (5N), Gd2O3 (5N), La2O3 (4N), and Nb2O5 (4N). When the concentration of La3+ is higher than 10%, the as-grown crystal will crack and be non-transparent (showing in Fig. 1 (a)), which is caused by the large ionic radius of La3+. Generally, the higher concentration of La3+ in Nd:GdLaNbO4 crystal should be corresponding to a higher disorder structure and the higher disorder structure is benefit to obtain broader spectral band. Therefore, a Nd0.01:Gd0.89La0.1NbO4 crystal with the highest La3+-doping of 10% was used in this work. The raw materials for growth were weighed according to the following equation: 0.005Nd2O3 + 0.445Gd2O3 + 0.05La2O3 + 0.5Nb2O5 → Nd0.01:Gd0.89La0.1NbO4. The doping concentration of Nd3+ ion in the mixed crystal was 1 at%. The as-grown crystal is shown in Fig. 1 (b). From Fig. 1 (b), we can see that there are no cracks in the mixed crystal.
Fig. 1 The photograph of the as-grown Nd:GdLaNbO4 mixed crystal with different concentration of La3+: (a) Concentration of La3+ is 15%; (b) Concentration of La3+ is 10%.
The absorption cross section σa can be calculated by the formula σa = α/N, where α is the absorption coefficient and N is the concentration of Nd3+ ions in Nd:GdLaNbO4 crystal. The Nd3+ concentration in Nd0.01:Gd0.89La0.1NbO4 crystal was determined to be 9.1 × 1019 cm−3 using a X-ray Fluorescence Spectrometer (XRF). The absorption spectrum of Nd0.01:Gd0.89La0.1NbO4 mixed crystal in three directions from 320 nm to 950 nm was measured by a Perkin-Elmer lambda-950 spectrophotometer and shown in Fig. 2. Since Nd0.01:Gd0.89La0.1NbO4 crystal belongs to monoclinic system, the absorption cross section of Nd0.01:Gd0.89La0.1NbO4 crystal showed difference in three directions. The absorption in c direction was the strongest. At around 808 nm, Nd0.01:Gd0.89La0.1NbO4 crystal appeared a strong absorption peak with an absorption cross-section of 10.5 × 10−20 cm2 and a full width at half maximum (FWHM) of 13 nm in c direction. The large absorption bandwidth means that Nd0.01:Gd0.89La0.1NbO4 can match with the 808 nm LD pumping source easily, and thus improve laser efficiency.
Fig. 2 The absorption spectrum of Nd0.01:Gd0.89La0.1NbO4 mixed crystal in three directions. Inset: Enlarged spectrum at around 808 nm.
The stimulated emission cross section σe can be estimated from the fluorescence spectra using the Fuchtbauer–Ladenburg equation [16]. The emission spectrum of Nd0.01:Gd0.89La0.1NbO4 mixed crystal is shown in Fig. 3, which was recorded by an FSLP-920 Edinburgh fluorescence spectrometer. There was an intense emission peak at around 1065 nm with an emission cross section of 18 × 10−20 cm2, corresponding to the 4F3/2 → 4I11/2 transition of Nd ions.
3. Experimental setup
The experimental setup of the diode-pumped passively Q-switched Nd:Gd0.89La0.1NbO4 laser is shown in Fig. 4.
The pumping source was a fiber coupled 808 nm diode laser. The output fiber of the diode laser was unpolarized. NA of the fiber was 0.22. The fiber core diameter was 400 μm. L1 and L2 were the collimating and focusing lenses, and the focus length of which were 26.7 mm and 34.2 mm, respectively. A c-cut Nd:Gd0.89La0.1NbO4 mixed crystal was used. The dimension of the mixed crystal was 2 × 2 × 5 mm3. The doped concentration was 1 at%. The mixed crystal was wrapped by indium and placed in a copper heat sink refrigerated by a water cooling system, which had been proved to have a better capability of thermal dissipation. The cooling temperature of the crystal was set to 20 °C. The input plane mirror M1 had a full reflection at 1.06 μm and high transmission at 808 nm. The output plane mirror M2 had a series of transmission of 15%, 20% and 25%. The Cr4+:YAG crystals with different T0 of 90% and 95% were selected as the saturable absorbers. It was determined that the length of the laser resonator was 40 mm.
4. Results and discussions
Before the experiments, the absorbed efficiency of pump power was measured and it was about 90%. CW laser performance was investigated firstly. In the CW operation, the experimental system was set up without Cr4+:YAG crystal. From Fig. 5, we can see that the output power increased linearly with increasing of absorbed pump power. When the absorbed pump power was 12.24 W, the maximum output power of 3.06 W was obtained using output coupler with transmission of 20%. The corresponding slope efficiency was 29.1%. The absorbed threshold power for the output plane mirrors with transmission of 15%, 20% and 25% were 1.63 W, 2.02 W and 2.31 W, respectively.
As shown in Fig. 6, we utilized a CCD to shoot the beam profiles of the CW Nd:Gd0.89La0.1NbO4 laser under different absorbed pump power. It can be seen from Fig. 6, with the increase of absorbed pump power, the beam quality of output laser became worse. This was because of higher-order modes oscillation when the absorbed pump power increased. As illustrated in Fig. 7, the laser beam quality M2 factor on different directions were measured by knife-edge method at the maximum output power of 3.06 W. On the parallel and vertical directions, M2 factors were 8.67 and 6.25, respectively. The beam quality can be improved by using longer resonant cavity or direct pumping.
The optical and laser properties of Nd:Gd0.89La0.1NbO4 crystal were compared to those of Nd:GdNbO4 crystal, which were listed in Table 1. In the mixed laser crystal, the difference of ion radius in the host varies the crystal field and causes the inhomogeneous broadening of the absorption spectrum. The broader spectrum bandwidth indicates the higher absorption efficiency for the pump energy. From the Table 1, we can see that, due to the larger absorption bandwidth FWHM, absorption cross section σa and stimulated emission cross section σe, the maximum output power Pmax and the slope efficiency ηs of Nd:Gd0.89La0.1NbO4 laser were much higher than those of Nd:GdNbO4 laser. The information of other usually used mixed crystals of garnet Nd:LaGGG and vanadate Nd:GdLaVO4 was also added in the Table 1. Because of the mature crystals and concave stable cavities used in these reports, ηs of Nd:LaGGG and Nd:GdLaVO4 lasers was high.
Table 1. Comparison of optical and laser properties between Nd:GdLaNbO4 and other crystals
Cr4+:YAG crystals with T0 = 90% and 95% were used as passively Q-switched saturable absorbers. The output coupler with transmission of 20% was employed. The relation between average output power and absorbed pump power for Cr4+:YAG Q-switched Nd:Gd0.89La0.1NbO4 laser was experimentally studied. As shown in Fig. 8, the average output power increased at a near-liner trend with increasing absorbed pump power. Because Cr4+:YAG crystal with T0 = 95% had a lower insertion loss, the output power was higher than that for Cr4+:YAG crystal with T0 = 90%. At the absorbed pump power of 12.24 W, the obtained maximum average output power for Cr4+:YAG crystals with T0 = 90% and 95% were 0.89 W and 1.51 W, respectively.
The pulsed laser performance was measured by a high speed Si-detector and a digital oscillograph. The measured results of the pulse width and the repetition rate were shown in Fig. 9 and Fig. 10, respectively. From Fig. 9, we can see that the pulse width decreased firstly for Cr4+:YAG with T0 = 90%. And when the absorbed pump power got high, the pulse width became constant. For Cr4+:YAG with T0 = 95%, the pulse width was insensitive to the absorbed pump power. This is because of the different fully bleached light intensity for Cr4+:YAG with different T0. The obtained minimum pulse width were 32 ns and 39 ns for Cr4+:YAG with T0 = 90% and 95%, respectively. The narrower pulse width was resulted from the lager modulation depth for Cr4+:YAG with T0 = 90%. The calculated modulation depth for Cr4+:YAG crystals with initial transmission of 90% and 95% were 8% and 4%, respectively. A temporal pulse profile with a minimum pulse width of 32 ns is inserted in Fig. 9. As we can see from Fig. 10, when the absorbed pump power was 12.24 W, the maximum repetition rate of 24.5 kHz and 50.5 kHz were obtained for Cr4+:YAG crystals with T0 = 90% and 95%, respectively. Because Cr4+:YAG with T0 = 95% had a high initial transmission, it was easy for it to bleach. Consequently, the repetition rate for Cr4+:YAG with T0 = 95% was larger than that with T0 = 90% at the same absorbed pump power. The jitter of pulse width and pulse repetition rate was less than 5%.
As shown in Fig. 11, the pulse energy and the pulse peak power increased with absorbed pump power. As depicted in Fig. 11(a), the maximum pulse energy for Cr4+:YAG crystals with T0 = 95% and 90% were 30.0 μJ and 36.4 μJ, respectively, at the absorbed pump power of 12.24 W. Correspondingly, from Fig. 11(b), we can see that the maximum peak power for Cr4+:YAG crystals with T0 = 95% and 90% were 0.83 kW and 1.13 kW, respectively, at the same absorbed pump power.
Fig. 11 Pulse performance as a function of absorbed pump power: (a) Pulse energy; (b) Pulse peak power.
5. Conclusion
In this paper, a diode-pumped passively Q-switched Nd:Gd0.89La0.1NbO4 laser was demonstrated for the first time, best to our knowledge. The crystal characteristics were investigated firstly. In the following CW operation, the transmission of output coupler was optimized. The maximum CW output power of 3.06 W was obtained at the absorbed pump power of 12.24 W with the optimum transmission of 20%. The beam profiles and M2 factor of the CW laser were measured by a CCD and knife-edge method, respectively. M2 factor were 8.67 and 6.25, on the parallel and vertical directions, respectively, at the maximum output power of 3.06 W. Using a longer resonant cavity or direct pumping can improve the beam quality. In the passively Q-switched situation, Cr4+:YAG crystals with T0 = 90% and 95% severed as saturable absorbers. The maximum average output power, repetition rate, pulse energy, peak power and minimum pulse width for Cr4+:YAG with T0 = 90% were 0.89 W, 24.5 kHz, 36.4 μJ, 1.13 kW and 32 ns, respectively, at absorbed pump power of 12.24 W. Correspondingly, for Cr4+:YAG with T0 = 95% they were 1.51 W, 50.5 kHz, 30.0 μJ, 0.83 kW and 35.9 ns, respectively, at the same absorbed pump power. We believe that the laser performance can be further improved when a Nd:GdLaNbO4 crystal with better quality is used.
Funding
The National Natural Science Foundation of China (61505041, 51502292 and 51702322); the Natural Science Foundation of Heilongjiang Province of China (F2015011); the Fundamental Research Funds for the Central Universities, the Application Technology Research and Development Projects of Harbin (2016RAQXJ140); the National Key R&D Program of China (2016YFB0402101).
References and links
1. Y. Ma, X. Li, X. Yu, R. Fan, R. Yan, J. Peng, X. Xu, R. Sun, and D. Chen, “A novel miniaturized passively Q-switched pulse-burst laser for engine ignition,” Opt. Express 22(20), 24655–24665 (2014). [CrossRef] [PubMed]
2. W. Strek, B. Cichy, L. Radosinski, P. Gluchowski, L. Marciniak, M. Lukaszewicz, and D. Hreniak, “Laser-induced white-light emission from graphene ceramics–opening a band gap in grapheme,” Light Sci. Appl. 4(1), 237 (2015). [CrossRef]
3. Y. Ma, Y. He, X. Yu, X. Li, J. Li, R. Yan, J. Peng, X. Zhang, R. Sun, Y. Pan, and D. Chen, “Multiple-beam, pulse-burst, passively Q-switched ceramic Nd:YAG laser under micro-lens array pumping,” Opt. Express 23(19), 24955–24961 (2015). [CrossRef] [PubMed]
4. T. L. Feng, S. Z. Zhao, K. J. Yang, G. Q. Li, D. C. Li, J. Zhao, W. C. Qiao, J. Hou, Y. Yang, J. L. He, L. H. Zheng, Q. G. Wang, X. D. Xu, L. B. Su, and J. Xu, “Diode-pumped continuous wave tunable and graphene Q-switched Tm:LSO lasers,” Opt. Express 21(21), 24665–24673 (2013). [CrossRef] [PubMed]
5. J. Liu, C. Wang, S. Liu, W. Tian, L. Li, S. Liu, and M. Liu, “Characterization of passively Q-switched mode-locking diode-pumped Nd:GdVO4 laser with Cr4+:YAG saturable absorber,” J. Mod. Opt. 55(12), 1971–1980 (2008). [CrossRef]
6. Y. Sato and T. Taira, “Model for the polarization dependence of the saturable absorption in Cr4+:YAG,” Opt. Mater. Express 7(2), 577–586 (2017). [CrossRef]
7. W. Koechner, Solid-State Laser Engneering (Springer, 2006).
8. S. Ding, Q. Zhang, F. Peng, W. Liu, J. Luo, R. Dou, G. Sun, X. Wang, and D. Sun, “Crystal growth, spectral properties and continuous wave laser operation of new mixed Nd:GdYNbO4 laser crystal,” J. Alloys Compd. 698, 159–163 (2017). [CrossRef]
9. S. Ding, Q. Zhang, J. Luo, W. Liu, W. Lu, J. Xu, G. Sun, and D. Sun, “Crystal growth, structure, defects, mechanical and spectral properties of Nd0. 01:Gd0. 89La0. 1NbO4 mixed crystal,” Appl. Phys., A Mater. Sci. Process. 123(10), 636 (2017). [CrossRef]
10. F. Peng, H. Yang, Q. Zhang, J. Luo, D. Sun, W. Liu, G. Sun, R. Dou, X. Wang, and X. Xing, “Growth, thermal properties, and LD-pumped 1066 nm laser performance of Nd3+ doped Gd/YTaO4 mixed single crystal,” Opt. Mater. Express 5(11), 2536–2544 (2015). [CrossRef]
11. S. Ding, Q. Zhang, W. Liu, J. Luo, F. Peng, X. Wang, G. Sun, and D. Sun, “Crystal growth and characterization of a mixed laser crystal: Nd-doped Gd0.89La0.1NbO4,” RSC Advances 7(57), 35666–35671 (2017). [CrossRef]
12. X. Xu, S. Cheng, J. Meng, D. Li, D. Zhou, L. Zheng, J. Xu, W. Ryba-Romanowski, and R. Lisiecki, “Spectral characterization and laser performance of a mixed crystal Nd:(LuxY1-x)3Al5O12.,” Opt. Express 18(20), 21370–21375 (2010). [CrossRef] [PubMed]
13. X. Fu, Z. Jia, Y. Li, D. Yuan, C. Dong, and X. Tao, “Crystal growth and characterization of Nd3+:(LaxGd1-x)3Ga5O12 laser crystal,” Opt. Mater. Express 2(9), 1242–1253 (2012). [CrossRef]
14. Y. He, X. Hou, L. Qin, Y. Sun, Y. Li, H. Qi, and L. Pan, “Laser-diode pumped passively Q-switched Nd: YxGd1− xVO4 laser with a GaAs saturable absorber,” Opt. Commun. 234(1-6), 305–308 (2004). [CrossRef]
15. V. Ostroumov, G. Huber, A. Zagumennyi, Y. Zavartsev, P. Studenikin, and I. Shcherbakov, “Spectroscopic properties and lasing of Nd:Gd0. 5La0. 5VO4 crystals,” Opt. Commun. 124(1-2), 63–68 (1996). [CrossRef]
16. I. Sokolska, E. Heumann, S. Kuck, and T. Lukasiewicz, “Laser oscillation of Er3+:YVO4 and Er3+, Yb3+:YVO4 crystals in the spectral range around 1.6 µm,” Appl. Phys. B 71(6), 893–896 (2000). [CrossRef]
17. S. Ding, F. Peng, Q. Zhang, J. Luo, W. Liu, D. Sun, R. Dou, J. Gao, G. Sun, and M. Chen, “Crystal growth, spectral properties, and continuous wave laser operation of Nd:GdNbO4,” J. Alloys Compd. 693, 339–343 (2017). [CrossRef]
18. S. Ding, X. Yang, Q. Zhang, W. Liu, J. Luo, G. Sun, Y. Ma, and D. Sun, “Diode-pumped passively Q-switched Nd:GdNbO4 laser with Cr4+:YAG saturable absorber,” Opt. Eng. 56(8), 086111 (2017). [CrossRef]
19. H. Jiang, J. Wang, H. Zhang, X. Hu, and H. Liu, “Optical-transition properties of the Nd3+ ion in Gd0.8La0.2VO4 crystal,” J. Appl. Phys. 92(7), 3647–3650 (2002). [CrossRef]
