Open Access
Add to BibSonomyAbstract
Single-longitudinal-mode operation is achieved in a twisted-mode-cavity c-cut Nd:GdVO4 laser. With a semiconductor saturable absorption mirror as an intracavity saturable absorber to launch passive Q-switching, no mode-locked spikes are observed on the temporal envelopes of the Q-switched output pulses due to the complete elimination of spatial hole burning in the gain medium to suppress longitudinal multi-modes. The maximal average output power is 1.24 W with the repetition rate of 76.3 kHz, and the single pulse energy is 16.0 µJ. The pulse width and polarization ratio of the output laser beam are measured about 150 ns and 53:1, respectively.
©2005 Optical Society of America
1. Introduction
Q-switched single-longitudinal-mode (SLM) lasers offer advantages such as high-peak output powers as well as smooth Q-switched pulses with well-resolved laser spectra that can be of broad applications in many fields. It has been demonstrated that a twisted-mode-cavity (TMC) laser could be operated for a SLM output [1,2], and that Q-switched operation can be achieved passively by using intracavity saturable absorbers such as semiconductor saturable absorption mirrors (SESAMs) [3,4]. Compared with other techniques which have been widely used for SLM lasers such as unidirectional ring cavities [5,6] and microchip lasers [7], a TMC laser configuration offers a competitive solution to eliminate the troublesome spatial hole-burning for SLM operation with a relatively simple arrangement of laser cavity, particularly applicable to high-power SLM laser operation. In a TMC laser, the polarization state of the intracavity beam is controlled by several polarization optics in the cavity, typically, a Brewster plate and a pair of quarter-wave plates (QWPs) sandwiching the laser active material. In order to eliminate the spatial hole burning in the gain medium, it is of critical importance that laser gain and intracavity absorption are isotropic over all the polarization components and that there exists negligible thermal birefringence in the gain medium. The TMC configuration may be broken by any thermal birefringence of the gain medium under high-power pumping, which leads to incomplete elimination of spatial hole-burning effects and some undesirable mode-locked spikes on the passively Q-switched pulses. In order to achieve a stable SLM operation of a TMC laser, the gain medium should possess excellent thermal conductivity thus negligible thermal birefringence. Many of existent laser active materials exhibit birefringence and anisotropies in absorption and polarization-dependent gain. It seems that they can not work in the TMC configuration. However, in order to get a polarization-independent gain for a TMC laser operation with an anisotropic crystal, Y. Louyera and coworkers used a c-cut Nd:YLF as laser gain material [8]. This work gives us a helpful clew how to avoid anisotropic gain in some birefringent laser crystals. Unfortunately, there exists a mismatch between the absorption band of Nd:YLF (around 796 nm) and the standard wavelength of commercially available diodes. Some TMC single-frequency lasers were also demonstrated by employing other laser crystals [9,10]. Nd:YVO4 is one of the most popular laser crystals which favor direct diode pumping with commercially available GaAlAs laser diodes. However, as limited by the thermal conductivity, it is still a challenge to get SLM operations in TMC Nd:YVO4 lasers under high-power diode laser pumping.
As a new laser crystal and an isomorph of Nd:YVO4 crystal, Nd:GdVO4 is also suitable for high-power laser-diode pumping since its absorption band matches well the emission band of GaAlAs laser diode [11]. Its large absorption cross section, which is even larger than that of Nd:YVO4, makes it quite suitable to take the full advantages of laser-diode pumping. Moreover, Nd:GdVO4 has a much higher thermal conductivity (11.7 W/mK along the <110> direction [12]) than Nd:YVO4, even comparable to that of Nd:YAG, which enables its use as a laser gain material to be high-power pumped [13–16]. Its large thermal conductivity guarantees a weak thermal birefringence. Its thermal expansion coefficient, 7.42×10-6/K along <001> direction and 1.05×10-6/K along <100> direction, is much smaller than that of Nd:YVO4 (11.37×10-6/K and 4.43×10-6/K, respectively). And the temperature birefringence coefficient of Nd:GdVO4 crystal is as low as 4.33×10-6/K [17]. Its good thermal properties, especially its weak thermal birefringence, show that c-cut Nd:GdVO4 crystal is suitable for high-power laser-diode pumping and TMC laser operation.
Semiconductor saturable absorbed mirrors (SESAMs) have been extensively used for passive Q-switching of solid-state lasers. Using a SESAM as a passive Q-switch has a variety of advantages over other approaches [18], including its low inserted loss. Because of its isotropic features, it is appropriate to use a SESAM as a high-reflection mirror in a TMC configuration. In this letter, we demonstrate a TMC laser with a c-cut Nd:GdVO4 crystal as the gain medium and a SESAM as the intracavity saturable absorber, which can be operated to output passively Q-switched SLM pulses. The experimental results indicate that the TMC technique can be applied to efficiently suppress mode-locked spikes within the Q-switched pulses.
2. Experiments and results
Our compact passively Q-switched TMC laser with a SESAM is arranged as shown in Fig.1. M1 is a concave mirror (R=100 mm) with anti-reflection coating at 808 nm on both sides and high-reflection coating at 1064 nm on the concave surface. The folded concave mirror M2 (R=100 mm) is high-reflection coated at 1064 nm. The transmission of the output coupler (M3) is 5% at 1064 nm, so the total output is approximately 10% since M3 is used as a folded mirror. Here, a SESAM is set at the end of the resonator as a flat cavity mirror to launch passive Q-switching. Its designed center wavelength and the unsaturable loss are 1064±5 nm and less than 0.3%, respectively. The total cavity length is approximately 250 mm. A fiber coupled 13W laser-diode array operating at the wavelength of 808nm is employed as the pump source, which is controlled by a thermal regulator. The core diameter and the numerical aperture of the multimode fiber are 400 µm and 0.22, respectively. Out of the multimode fiber, the pump beam is focused by a series of lenses onto the laser crystal with a beam size approximately 600 µm. The effective pump power is about 11.5 W. A 3mm-long 1-at.%- dopped c-cut Nd:GdVO4 crystal with anti-reflection coating at 1064 and 808 nm on both sides is used as laser gain material. It is wrapped with indium foil and fixed in a water-cooled copper heat-sink. The crystal is sandwiched by two QWPs (QWP1 and QWP2), both of which are anti-reflection coated at 1064 nm on both sides in order to reduce their induced loss. Before being set into the cavity, the c-cut Nd:GdVO4 crystal is examined by placing it between two crossed polarizers to confirm that the c-cut crystal induces no birefringence to the passing laser beams. A 0.5mm-thick fused silica Brewster plate is inserted between the QWP2 and the folded concave mirror.
Firstly, the laser is operated in continuous-wave mode, and the SESAM is taken place by a piece of mirror with HR coating at 1064nm. The maximum total output power is 3.3W with the pumping power 11.5W, corresponding to the slope efficiency 35.2%. Then the SESAM launches the laser under a passively Q-switched operation. In the absence of intracavity polarization elements, there exists unavoidable spatial-hole burning in the laser crystal, which brings about multi-longitudinal-mode oscillations. As saturable absorption from SESAM can not only cause passive Q-switching but also incomplete mode-locking in a laser cavity oscillating multi-longitudinal modes, mode-locked spikes appear within the Q-switched pulse envelopes. Figure 2(a) presents the measured Q-switched pulse envelope by using a digital oscilloscope (Hewlett Packard 54616C), which clearly indicates the existence of incompletely mode-locked spikes within the Q-switched pulses. The repetition rate of the mode-locked spikes is measured as 598 MHz, which is consistent with the total cavity length. As the pump power increases, the repetition rate of the Q-switched pulses increases. Under a pumping power of 11.5 W, the repetition rate and the pulse width are obtained as 128 kHz and 186 ns, respectively. The polarization ratio of the output laser is measured as 7:1. The total output power and the slope efficiency are 2.0 W and 29.2%, respectively. As limited by pumping power in our experiments, only Q-switched mode-locking is launched in the c-cut Nd:GdVO4 laser, and no continuous-wave mode-locking is observed.
Fig. 2. (a) The Q-switched pulse with mode-locked spikes. (b)The Q-switched pulse without any mode-locked spikes under SLM operation by using the TMC technique.
In order to eliminate the mode-locked spikes, the intracavity polarization elements are inserted to form a TMC laser. The laser crystal is sandwiched by two QWPs whose fast axes are well fixed perpendicularly to each other, and at 45° to the polarization direction determined by the Brewster plate. The two QWPs and the Brewster plate works together to produce two counter-propagating orthogonally circularly-polarized laser beams inside the laser crystal. As a consequence, the intracavity light intensity is independent of the position along the direction of laser propagation thus the troublesome spatial-hole burning is eliminated. The absence of spatial-hole burning suppresses the mode-locked oscillation and results in single-longitudinal-mode operation [9,19]. Under such a circumstance, the TMC laser outputs possess smooth Q-switched pulse profiles without any mode-locked spikes as shown in Fig. 2(b). The output spectra are scanned by a home-made Fabry-Perot interferometer with a free spectral range of 1.5 GHz. According to the cavity length, the adjacent longitudinal modes are separated by 0.6 GHz. Figure 3 shows the output laser exhibits a clean single-frequency spectrum with the narrow output line-width of 0.16 GHz approximately, and the Q-switched output laser is free of any mode-locked spikes. Figure 4 displays the relationship between the pumping power and the repetition rate of the Q-switched pulse train, as well as the average output power. The maximal output power is 1.24 W, and the corresponding repetition rate and pulse width are 76.3 kHz and 150 ns, respectively. Due to the elimination of the spatial hole-burning effect, the noise of the SLM laser output is measured less than 0.5%.
Fig. 3. The output spectra of the TMC single-frequency laser. (a) The TMC laser is under continuous-wave operation. (b) The TMC laser is passively Q-switched by using a piece of SESAM. The free spectral range of the Fabry-Perot scanning interferometer is approximately 1.5 GHz.
Fig. 4. The repetition rate and total output power of the SLM Q-switched TMC laser (free of mode-locked spikes) as a function of the pumping power. The inset shows the repetition rate as 76.3 kHz when the pumping power was 11.5 W.
While the output Q-switched pulses of our SLM laser possess completely smooth profiles, the corresponding spectra show little spectral variation from pulse to pulse. As required by TMC lasers for stable SLM operations, the intra-cavity birefringence, including the thermal birefringence even under high-power pumping, other than those of the QWPs and Brewster plate, should be negligible. The spectral stability of our Q-switched SLM laser implies that the Nd:GdVO4 crystal has sufficiently high thermal conductivity for TMC laser operation. On the other hand, the Q-switched TMC laser suffers some extra round-trip loss that is caused by the intracavity polarization elements, as a result of which the average output power is smaller than that of a Q-switched mode-locked laser under the same pumping condition, and the slope efficiency unavoidably decreases. However, also determined by these polarization elements, a high polarization ratio of the output beam is obtained as high as 53:1.
3. Conclusion
In summary, we demonstrate that a single-frequency laser operation with a smooth output pulse profile can be obtained in an Nd:GdVO4 laser, which is passively Q-switched by a SESAM. Here, single-frequency laser oscillation is guaranteed by elimination of spatial-hole burning and consequent suppression of multi-longitudinal-mode oscillation in a TMC configuration with a c-cut laser crystal sandwiched by two QWPs. The experimental results indicate that the TMC technique works efficiently to obtain single-frequency Q-switched output laser free of any mode-locked spikes. Some Yb-ion doped crystals have been shown to possess small quantum defects and high quantum efficiencies that can dramatically reduce the thermal birefringence. It can be predicted that the output power and the spectral stability of the TMC single-frequency laser is possibly impelled to some higher level by using these crystals.
Acknowledgments
This work is supported in part by Key Project from Science and Technology Commission of Shanghai Municipality (Grant 04dz14001), National Key Project for Basic Research (Grant TG1999075204), and National Natural Science Fund (Grant 60478011).
References and links
1. V. Evtuhov and A. E. Siegman, “A ‘twisted-mode’ technique for obtaining axially uniform energy density in a laser cavity,” Appl. Opt. 4, 142–143 (1965). [CrossRef]
2. K. Nakagawa, Y. Shimizu, and M. Ohtsu, “High power diode-laser-pumped twisted-mode Nd:YAG,” IEEE Photon. Technol. Lett. 6, 499–501 (1994). [CrossRef]
3. U. Keller, D. A. B. Miller, G. D. Boyed, T. H. Chiu, J. F. Ferguson, and M. T. Amos, “Solid-state low-loss intracavity saturable absorber for Nd:YLF laser: an antiresonant semiconductor Fabry-Perot saturable absorber,” Opt. Lett. 17, 505–507 (1992). [CrossRef] [PubMed]
4. Sanjun Zhang, E Wu, Haifeng Pan, and Heping Zeng, “Passive mode locking in a diode-pumped Nd:GdVO4 laser with a semiconductor saturable absorber mirror,” IEEE J. Quantum. Electron. 40, 505–508(2004). [CrossRef]
5. T. J. Kane and R. L. Byer, “Monolithic, unidirectional single-mode Nd:YAG ring lasers,” Opt. Lett. 10, 65–67 (1985). [CrossRef] [PubMed]
6. R. Scheps and J. Myers, “A single frequency Nd:YAG ring laser pumped by laser diodes,” IEEE J. Quantum. Electron. 26, 413–416 (1990). [CrossRef]
7. J. J. Zayhowski and A. Mooradian, “Single-frequency microchip Nd lasers,” Opt. Lett. 14, 24–26 (1989). [CrossRef] [PubMed]
8. Y. Louyer, F. Balembois, M.D. Plimmer, T. Badr, P. Georges, P. Juncar, and M.E. Himbert, “Efficient cw operation of diode-pumped Nd:YLF lasers at 1312.0 and 1322.6 nm for a silver atom optical clock,” Opt. Commun. 217, 357–362 (2003). [CrossRef]
9. C. S. Adams, J. Vorberg, and J. Mlynek, “Single-frequency operation of a diode-pumped lanthanum-neodymium-hexaaluminate laser by using a twisted-mode cavity,” Opt. Lett. 18, 420–422 (1993). [CrossRef] [PubMed]
10. T. Y. Fan and J. Ochoa, “Tunable single-frequency Yb:YAG laser with 1-W output power using twisted-mode technique,” IEEE Photon. Technol. Lett. 6, 1137–1138 (1995). [CrossRef]
11. T. Jensen, V. Ostroumov, J. Meyn, G. Huber, A. Zagumennyi, and I. Shcherbakov, “Spectroscopic characterization and laser performance of diode-laser-pumped Nd:GdVO4,” Appl. Phy. B 58, 373–379 (1994). [CrossRef]
12. P A Studenikin, A I Zagumennyi, Yu D Zavartsev, P A Popov, and I A Shcherbakov, “GdVO4 as a new medium for solid-state lasers: some optical and thermal properties of crystals doped with Cd3+, Tm3+, and Er3+ ions,” Quantum Electron. 25, 1162–1165 (1995). [CrossRef]
13. J. Liu, B. Ozygus, S. Yang, J. Erhard, U. Seelig, A. Ding, H. Weber, X. Meng, L. Zhu, L. Qin, C. Du, X. Xu, and Z. Shao, “Efficient passive Q-switching operation of a diode-pumped Nd:GdVO4 laser with a Cr4+:YAG saturable absorber,” J. Opt. Soc. Am. B 20, 652–661 (2003). [CrossRef]
14. B. Zhang, G. Li, M. Chen, Z. Zhang, and Y. Wang, “Passive mode locking of a diode-end-pumped Nd:GdVO4 laser with a semiconductor saturable absorber mirror,” Opt. Lett. 28, 1829–1831 (2003). [CrossRef] [PubMed]
15. J. He, C. Lee, J. Huang, S. Wang, C. Pan, and K. Huang, “Diode-pumped passively mode-locked multiwatt Nd:GdVO4 laser with a saturable Bragg reflector,” Appl. Opt. 42, 5496–5499 (2003). [CrossRef] [PubMed]
16. A. Agnesi, A. Guandalini, G. Reali, S. Dell’Acqua, and G. Piccinno, “High-brightness 2.4-W continuous-wave NdGdVO4 laser at 670nm,” Opt. Lett. 29, 56–58 (2004). [CrossRef] [PubMed]
17. L. J. Qin, X. L. Meng, H. Y. Shen, L. Zhu, B. C. Xu, L. X. Huang, H. R. Xia, P. Zhao, and G. Zheng, “Thermal conductivity and refractive indices of Nd:GdVO4 crystals,” Cryst. Res. Technol. 38, 793–797 (2003). [CrossRef]
18. R. Häring, R. Paschotta, R. Fluck, E. Gini, H. Melchior, and U. Keller, “Passively Q-switched microchip laser at 1.5 mm,” J. Opt. Soc. Am. B 18, 1805–1812 (2001). [CrossRef]
19. D. Draegert, “Efficient single-longitudinal-mode Nd:YAG laser,” IEEE J. Quantum. Electron. QE-8, 235–239 (1972). [CrossRef]
