Open Access
Add to BibSonomyAbstract
A multi-pass pumping scheme for thin disk lasers consisting of dual parabolic mirrors with conjugated relationship is presented. The anti-disturbance ability of pumping is analyzed by ray tracing method under different kinds of disturbances. Both theoretical and experiment results show that disturbances in this system won’t lead to a misalignment of each pumping spot, but only the position of superposed pumping spot on disk crystal will be changed. Compared with the multi-pass pumping scheme consisting of parabolic mirror and folding prisms, this pumping scheme has a better anti-disturbance ability.
© 2015 Optical Society of America
1. Introduction
With the geometry of a thin disk crystal for the laser active medium, the thermal lensing effect is greatly reduced in thin disk lasers compared to traditional rod or slab lasers. However, the thin disk crystal results in an insufficient absorption length in the end-pumped configuration, so that an efficient multi-pass pumping scheme is required to improve the absorption efficiency. Since the first demonstration of the thin disk laser by A. Giesen et al. at IFSW [1], many works have been carried out for the designing and optimizing of multi-pass pumping schemes [2–4].
Nowadays, the multi-pass pumping scheme for a thin disk laser presented by IFSW [5] realizes at least 24 pumping passes with a parabolic mirror and a pair of folding prisms, and nearly 94% pumping light can be absorbed by the laser active medium [6]. In order to reach a higher absorption efficiency, 44 pumping passes have been achieved by the same pumping method [7]. With the excellent pumping technology, CW laser output power of 10kW in multi-mode operation, 4kW in fundamental-mode operation and 8kW with a beam quality of 3 mm·mrad are achieved from a single Yb:YAG disk crystal [7–10]. In addition, other crystals have been investigated in multi-pass pumping thin disk configuration as well, showing their properties in ultrafast lasers and mid-infrared lasers [11–15]. Even for industrial products, laser power of 6kW are extracted from one disk and 16kW are emitted from four disks by TRUMPF Laser GmbH [7,16]. Now thin disk lasers have been accepted as important and versatile tools in many industrial applications.
Based on the multi-pass pumping scheme, many research works have been carried out in thermal management, resonator design, power scaling, ultra-short pulse operation and so on [17–20]. However, not many attentions are paid on the anti-disturbance ability issue of pumping. The properties of pumping stability, such as the overlap of each pumping spot on disk crystal, are very important for the efficient and stable laser output, especially in industrial applications.
In this paper, a multi-pass pumping scheme consisting of dual parabolic mirrors with conjugated relationship [21] is presented for thin disk lasers to realize high efficiency absorption. The anti-disturbance ability of pumping is analyzed by ray tracing method under different kinds of disturbances, including the deflection and displacement of two parabolic mirrors and deflection of incident collimating pumping beam. Theoretical analysis shows that disturbances in this system will only change the position of superposed pumping spot on disk crystal, which is also confirmed by the experiment result. Compared with the multi-pass pumping scheme consisting of parabolic mirror and folding prisms, this pumping scheme has a better anti-disturbance ability.
2. Multi-pass pumping scheme
2.1. Principle and structure
A multi-pass pumping scheme for thin disk lasers consisting of dual parabolic mirrors with conjugated relationship is shown in Fig. 1(a). Three axes of X, Y and Z are defined. Figures 1(b)-1(d) are the side views in YZ-plane, XZ-plane and XY-plane. Two parabolic mirrors I and II with equal focal length are placed opposite, their optical axes are collinear and focal point of one parabolic mirror is just on the vertex of the other one. A thin disk crystal and an adjusting mirror are placed on the focal points of parabolic mirror II and I respectively. On parabolic mirror I, aperture A and B are set for the injection of pumping beams, and on parabolic mirror II, aperture C is designed for the V-shaped resonator.
Fig. 1 A multi-pass pumping scheme for thin disk lasers consisting of dual parabolic mirrors with conjugated relationship. (a) Schematic diagram. (b) Side view in YZ-plane. (c) Side view in XZ-plane. (d) Side view in XY-plane.
A collimating pumping beam B is injected into the system through aperture B, and then is focused on the disk crystal by the parabolic mirror II. After propagating through the disk crystal, the unabsorbed pumping beam is reflected by the HR coating at crystal’s backside and then is transformed into a collimating beam by the parabolic mirror II. Since the disk crystal is set at a small angle around Y-axis, shown in Fig. 1(c), the collimating beam reflected by the parabolic mirror II is shifted a small distance along the X-axis direction. So it will be focused on the adjusting mirror instead of escaping from the system through aperture A when propagating to parabolic mirror I. The rotating angle of disk crystal is approximately decided by Eq. (1).
Where is the displacement of collimating beam and is equal or greater to the diameter along X-axis of aperture A and B, is the focal length of two parabolic mirrors, and is the distance between the center of collimating pumping beam and the optical axis of parabolic mirror.After focusing on adjusting mirror, the beam is reflected and gets collimated again by the parabolic mirror I. Hereto, the first propagating cycle of pumping beam is finished. The unabsorbed pumping beam returns to a position on the parabolic mirror I, which is shifted along X-axis direction from aperture B. Then the collimating beam starts the second cycle from parabolic mirror I. During the subsequent multi-pass pumping process, every time a propagating cycle is finished, the collimating pumping beam adds a displacement along X-axis direction until it’s outside the effective aperture of parabolic mirror. In this way, a multi-pass pumping scheme for disk crystal is achieved. Figure 1(d) shows the position of each pumping spot on the parabolic mirrors and their moving directions during multi-pass pumping process. It is worth of mentioning that the number of pumping passes can be doubled if a mirror is placed at the end of the optical path. It is because the unabsorbed pumping beam will be reflected back and then propagates through the multi-pass pumping scheme in the opposite direction.
As shown in Fig. 2(a), in addition to the collimating pumping beam B mentioned above, the aperture A allows the injection of the other collimating pumping beam simultaneously, which enhances the pumping power of the thin disk laser. Aperture A is located next to the disk crystal and is symmetric with the aperture B. The collimating pumping beam A propagates the similar multi-pass pumping paths as aforementioned. Figure 2(b) shows the position of each pumping spot on two parabolic mirrors and their moving directions during multi-pass pumping process. They are distributed into two columns, which improves the area utilization of parabolic mirrors. On parabolic mirror II, the aperture C around the adjusting mirror is a no-pumping area, and it’s enough and spare for the V-shaped resonator.
Fig. 2 A multi-pass pumping scheme for thin disk lasers consisting of dual parabolic mirrors with conjugated relationship with two pumping beams injected simultaneously. (a) Schematic diagram. (b) Position of each pumping spot on two parabolic mirrors and their moving directions during multi-pass pumping process.
2.2. Analysis of the anti-disturbance ability
The multi-pass pumping system requires a good anti-disturbance ability to ensure the overlap of each pumping spot on disk crystal. Otherwise a misalignment of each pumping spot can reduce the pumping uniformity, which will decrease the output laser power and even damage the crystal by thermal stress. In general, disturbances come from the deflection and displacement of two parabolic mirrors, as well as the deflection of incident collimating pumping beam.
Figure 3 shows the characteristics of the pumping spot under the deflection of parabolic mirror II. The schematic diagram in YZ-plane is shown in Fig. 3(a). The incident pumping beam P1P2 is parallel with the optical axis of parabolic mirror I, and the optical axis of parabolic mirror II has a small deflection angle . So the incident angle between beam P1P2 and the optical axis of parabolic mirror II is . Therefore, the first pumping spot on disk crystal is located at F1 with a distance to the center, and are decided by Eq. (2).
Fig. 3 Characteristics of the pumping spot under the deflection of parabolic mirror II. (a) Schematic diagram in YZ-plane. (b) Deviation of each pumping spot on disk crystal and adjusting mirror to the center as a function of pumping number with different α. (c) Simulated superposed pumping spots on disk crystal and adjusting mirror with α = 6mrad.
The deflection of parabolic mirror II is so small that F1 can be considered still on the focal plane of parabolic mirror II. So P3P4 is parallel with P1P2 as well as the optical axis of parabolic mirror I, which makes P3P4 focused on the center point F2 of the adjusting mirror. Similarly, since F2 is on the focal plane of parabolic mirror I, P5P6 is parallel with P3P4, too. Consequently, P1P2 and P5P6 are also parallel with a small distance of . That is to say the second pumping beam P5P6 has the same incident angle with P1P2, i.e. . As a result, P5P6 will be focused on F1 as well. What’s more, so will all the pumping beams after that. In consequence, each pumping spot can overlap well at F1 on disk crystal and F2 on adjusting mirror.
Figure 3(b) is the deviation of each pumping spot on disk crystal and adjusting mirror to the center as a function of pumping number with different deflection angle α. It’s shown that deviation of each pumping spot on disk crystal gets larger along with the increase of deflection angle. However, under a same deflection angle, each pumping spot on disk crystal keeps an equal deviation, which makes a good overlap. On the other hand, deviation of each pumping spot on the adjusting mirror has nothing to do with the deflection angle. For parabolic mirrors with larger focal length, a same deflection angle of parabolic mirror II will result in a larger position displacement of the superposed pumping spot on disk crystal according to Eq. (2). However, the overlap of each pumping spot won’t be affected by the disturbance all the same.
The superposed pumping spots on the disk crystal and the adjusting mirror with deflection angle α = 6mrad are simulated by ray tracing method. All the pumping spots overlap well and the pumping uniformity is not affected by the disturbance, shown in Fig. 3(c).
The anti-disturbance ability of pumping under the other three disturbances is researched by the same method as mentioned above. The schematic diagrams in YZ-plane and the simulated superposed pumping spots on the disk crystal and the adjusting mirror are shown in Fig. 4, in which Fig. 4(a) is under the deflection of parabolic mirror I, Fig. 4(b) is under the displacement of the parabolic mirror II and Fig. 4(c) is under the deflection of incident collimating pumping beam. It’s shown that disturbances have no effect on the overlap of each pumping spot and a good pumping uniformity is kept.
Fig. 4 Characteristics of the pumping spot under the other three disturbances. (a) Under the deflection of parabolic mirror I. (b) Under the displacement of parabolic mirror II. (c) Under the deflection of incident collimating pumping beam.
In some possible situations such as the differences in the focal lengths of the two parabolic mirrors or the errors in the distance between two mirrors, by making sure that the disk crystal is located on the focal point of parabolic mirror II and the adjusting mirror is located on the focal point of parabolic mirror I when assembling the system, the good anti-disturbance ability can also be achieved as mentioned above.
2.3. Compare with the multi-pass pumping scheme consisting of one parabolic mirror and a pair of folding prisms
The anti-disturbance ability of pumping in the multi-pass pumping scheme consisting of one parabolic mirror and a pair of folding prisms is also analyzed under the angle deviation of folding prism I, shown in Fig. 5.
Fig. 5 Characteristics of the pumping spot under the angle deviation of folding prism I. (a) Schematic diagram of the multi-pass pumping scheme consisting of one parabolic mirror and a pair of folding prisms. (b) Side view in the vertical plane of the ridge of folding prism I. (c) Deviation of each pumping spot to disk crystal center as a function of pumping number with different Δθ/2. (d) Simulated superposed pumping spot on disk crystal with different Δθ/2.
Figure 5(a) shows the schematic diagram of the multi-pass pumping scheme consisting of one parabolic mirror and a pair of folding prisms, and Fig. 5(b) is the side view in the vertical plane of the ridge of folding prism I. The angle of folding prism I is 90° + Δθ/2. The incident pumping beam P1P2, paralleling with the optical axis of parabolic mirror, is focused on the center point F1 on disk crystal. After leaving the parabolic mirror, the beam P3P4 propagates through the folding prism I. Due to the angle deviation Δθ/2, the incident angle of the next pumping beam P5P6 (defined as the acute angle between P5P6 and the optical axis of parabolic mirror) will increase by Δθ compared to P1P2. So P5P6 will be focused on a position F2 apart away from F1, resulting in a misalignment of two pumping spots. Then the pumping beam continues propagating in the multi-pass pumping system. Every time the beam passes the folding prism I, the incident angle of next pumping beam will increase by Δθ compared to the last one. The incident angle of each pumping beam in Fig. 5(a) is shown in Table 1.
Table 1. Incident angle of each pumping beam when the angle of folding prism I and II are 90° + Δθ/2 and 90° respectively
The deviation of each pumping spot to the center of disk crystal can be calculated by Eq. (3).
Where is the focal length of the parabolic mirror, and is the distance between the center of collimating pumping beam and the optical axis of parabolic mirror.The deviation of each pumping spot to the center of disk crystal as a function of pumping number with different angle deviations is shown in Fig. 5(c). It’s observed that the deviation of each pumping spot increases as multi-pumping process goes on under the same angle deviation. This is going to induce the misalignment of each pumping spot. The larger the angle deviation is, the faster the deviation of each pumping spot increases, and the more serious the misalignment will be. Figure 5(d) shows the ray tracing results of superposed pumping spot on the disk crystal with different angle deviations. It’s seen that different levels of misalignments reduce the pumping uniformity on disk crystal.
In addition, if both the folding prism I and II have the angle deviations, disturbances will come from two directions vertical to the prism ridges. Superposition of two disturbances may even make the misalignment worse.
Comparing the two multi-pass pumping schemes, with similar cost and size for the same number of passes, a better anti-disturbance ability of pumping is acquired in the scheme consisting of dual parabolic mirrors with conjugated relationship. In this pumping scheme, the disturbances only change the position of superposed pumping spot on disk crystal. This is because the deviation of each pumping spot has the same size and direction, which makes them still overlap well. Furthermore, the position change of superposed pumping spot can be adjusted easily by moving the crystal slightly or adjusting the direction of incident collimating pumping beam.
3. Experiment
The experiment setup of the multi-pass pumping scheme for thin disk lasers is shown in Fig. 6(a). Two pumping beams emitted from square core fibers are injected into the system after collimation. Two same parabolic mirrors with the focal length of 200mm and the diameter of 250mm are used and 14 pumping passes are achieved for each pumping beam. In addition, 28 passes can be realized if a mirror is utilized to reflect the unabsorbed pumping beam back. Figure 6(b) shows the superposed pumping spot on disk crystal after the multi-pass pumping system is well adjusted. It can be seen that a square pumping spot with good uniformity is located at the center of disk crystal since two square core fibers are used for homogenization. When a small deflection of parabolic mirror II is set as shown in Fig. 3(a), the superposed pumping spot on disk crystal is shown in Fig. 6(c). It is observed that there is no misalignment of each pumping spot and a good uniformity is kept, but only the position of superposed pumping spot is changed. The experiment result consists with the theoretical analysis and proves the good anti-disturbance ability of this pumping scheme.
Fig. 6 Experiment on the anti-disturbance ability of pumping. (a) Experiment setup of the multi-pass pumping thin disk lasers based on dual parabolic mirrors with conjugated relationship. (b) Superposed pumping spot on disk crystal after the multi-pass pumping system is well adjusted. (c) Superposed pumping spot on disk crystal under a deflection about 3.5mrad of parabolic mirror II.
4. Conclusion
A multi-pass pumping scheme for thin disk lasers consisting of dual parabolic mirrors with conjugated relationship is presented. The anti-disturbance ability of pumping is analyzed by ray tracing method, and comparison with the multi-pass pumping scheme consisting of one parabolic mirror and a pair of folding prisms is carried out. Theoretical results show that the disturbances in this pumping scheme won’t lead to a misalignment of each pumping spot on the disk crystal, but only the position of superposed pumping spot on the disk crystal is changed. In experiments, the overlap of each pumping spot can be confirmed under a small deflection of parabolic mirror II, proving a good anti-disturbance ability.
Acknowledgment
We would like to acknowledge the financial support by the National Natural Science Foundation of China (Grant No. 61475057).
References and links
1. A. Giesen, H. Hügel, A. Voss, K. Wittig, U. Brauch, and H. Opower, “Scalable concept for diode-pumped high-power solid-state lasers,” Appl. Phys. B 58(5), 365–372 (1994). [CrossRef]
2. A. Voss, U. Brauch, K. Wittig, and A. Giesen, “Efficient high-power diode-pumped thin-disk Yb:YAG-laser,” Proc. SPIE 2426, 501–508 (1995). [CrossRef]
3. S. Erhard, A. Giesen, M. Karszewski, T. Rupp, C. Stewen, I. Johannsen, and K. Contag, “Novel pump design of Yb:YAG thin disc laser for operation at room temperature with improved efficiency,” in ASSL(OSA, Boston, Massachusetts, OSA Trends in Optics and Photonics26, 38–44 (1999).
4. S. Erhard, M. Karszewski, C. Stewen, A. Giesen, K. Contag, and A. Voss, “Pumping schemes for multi-kW thin disk lasers,” in ASSL(OSA, Davos, Switzerland, OSA TOPS 34, 78–84 (2000).
5. S. Erhard, A. Giesen, and C. Stewen, “Laser amplifier system,” US 6891874 B2, May.10, 2005.
6. C. Stewen, K. Contag, M. Larionov, A. Giesen, and H. Hügel, “A 1-kW CW thin disc laser,” IEEE J. Sel. Top. Quantum Electron. 6(4), 650–657 (2000). [CrossRef]
7. T. Gottwald, C. Stolzenburg, D. Bauer, J. Kleinbauer, V. Kuhn, T. Metzger, S. Schad, D. Sutter, and A. Killi, “Recent disk laser development at TRUMPF,” Proc. SPIE 8547, 85470C (2012). [CrossRef]
8. T. Gottwald, V. Kuhn, S. Schad, C. Stolzenburg, and A. Killi, “Recent developments in high power thin disk lasers at TRUMPF Laser,” Proc. SPIE 8898, 88980P (2013). [CrossRef]
9. S. Schad, V. Kuhn, T. Gottwald, V. Negoita, A. Killi, and K. Wallmeroth, “Near fundamental mode high-power thin-disk laser,” Proc. SPIE 8959, 89590U (2014). [CrossRef]
10. J. Negel, S. Piehler, B. Weichelt, K. Wentsch, A. Loescher, B. Dannecker, M. Eckerle, M. Rumpel, J. Wolter, A. Voss, T. Graf, and M. A. Ahmed, “Recent progress in thin-disk lasers based on various Yb-doped materials,” in ASSL, OSA Technical Digest (online) (Optical Society of America, 2014), paper AM2A.1.
11. K. S. Wentsch, L. Zheng, J. Xu, M. A. Ahmed, and T. Graf, “Passively mode-locked Yb3+:Sc2SiO5 thin-disk laser,” Opt. Lett. 37(22), 4750–4752 (2012). [CrossRef] [PubMed]
12. B. Dannecker, X. Délen, K. S. Wentsch, B. Weichelt, C. Hönninger, A. Voss, M. A. Ahmed, and T. Graf, “Passively mode-locked Yb:CaF2 thin-disk laser,” Opt. Express 22(19), 22278–22284 (2014). [CrossRef] [PubMed]
13. S. Ricaud, A. Jaffres, K. Wentsch, A. Suganuma, B. Viana, P. Loiseau, B. Weichelt, M. Abdou-Ahmed, A. Voss, T. Graf, D. Rytz, C. Hönninger, E. Mottay, P. Georges, and F. Druon, “Femtosecond Yb:CaGdAlO4 thin-disk oscillator,” Opt. Lett. 37(19), 3984–3986 (2012). [CrossRef] [PubMed]
14. K. L. Schepler, R. D. Peterson, P. A. Berry, and J. B. McKay, “Thermal effects in Cr2+:ZnSe thin disk lasers,” IEEE J. Sel. Top. Quantum Electron. 11(3), 713–720 (2005). [CrossRef]
15. R. W. Stites, G. Cook, P. A. Berry, and K. L. Schepler, “Power scaling a Cr:ZnSe thin disk laser by increasing pump diameter,” in CLEO, OSA Technical Digest (online) (Optical Society of America, 2014), paper JTu4A.124.
16. D. Havrilla and M. Holzer, “High power disk lasers - advances & applications,” Proc. SPIE 7912, 79120F (2011). [CrossRef]
17. M. D. Nixon and M. C. Cates, “High energy high brightness thin disk laser,” Proc. SPIE 8547, 85470D (2012). [CrossRef]
18. S. Piehler, B. Weichelt, A. Voss, M. A. Ahmed, and T. Graf, “Power scaling of fundamental-mode thin-disk lasers using intracavity deformable mirrors,” Opt. Lett. 37(24), 5033–5035 (2012). [CrossRef] [PubMed]
19. C. J. Saraceno, F. Emaury, C. Schriber, M. Hoffmann, M. Golling, T. Südmeyer, and U. Keller, “Ultrafast thin-disk laser with 80 μJ pulse energy and 242 W of average power,” Opt. Lett. 39(1), 9–12 (2014). [CrossRef] [PubMed]
20. J. P. Negel, A. Voss, M. Abdou Ahmed, D. Bauer, D. Sutter, A. Killi, and T. Graf, “1.1 kW average output power from a thin-disk multipass amplifier for ultrashort laser pulses,” Opt. Lett. 38(24), 5442–5445 (2013). [CrossRef] [PubMed]
21. Xiao Zhu, Guangzhi Zhu, Changhong Zhu, Lijun Qi, Jianli Shang, Xingyun Duan and Peng Chen, “Solid state thin disk laser,” US 8755416 B2, June 17, 2014.
