Open Access
Add to BibSonomyAbstract
High efficiency and high power laser operation of highly-doped 2at.% crystalline Nd:YAG at 1.3μm is demonstrated in a diode-side pumped bounce amplifier configuration. A linearly polarized output power of 16.7W is obtained representing the highest power at 1.3μm achieved to date, to our knowledge, in a highly doped crystalline Nd:YAG laser. The system could deliver over 9W average output power in Q-switched operation with repetition rates from 20 – 160kHz. With quasi-continuous wave diode pumping 5.5mJ pulses at 100Hz repetition rate were achieved as well as 50ns pulses in Q-switched operation.
©2007 Optical Society of America
1. Introduction
Diode-pumped solid-state (DPSS) lasers have gained high prominence as compact, high quality infrared light sources. Neodymium-doped YAG (Nd:YAG) has been the predominant solid-state laser material used, although in recent years other materials such as Nd:YVO4 have challenged Nd:YAG as a preferred solid-state lasing material due to its stronger absorption for diode pump radiation at 808nm and higher stimulated emission cross-section at 1064nm.
However, Nd:YAG has better thermal and mechanical properties than Nd:YVO4 and is therefore better suited for high power laser operation. It also has a longer upper state lifetime, and hence higher energy storage capacity, making it suitable for high pulse energy operation.
The bounce amplifier configuration, involving diode side-pumping, has been demonstrated to be a viable design for producing higher power operation whilst also achieving high efficiency single mode laser output [1–4]. The bounce design requires a high absorption coefficient for the pump wavelength and has so far been demonstrated predominantly with Nd:YVO4 and Nd:GdVO4, which possess high absorption. The bounce geometry becomes an attractive option with the enhanced absorption of highly doped Nd:YAG. This has been verified by this group and co-workers with high efficiency laser operation with highly doped crystalline and ceramic Nd:YAG at 1064nm [5, 6].
High power laser radiation at the 1.3μm transition of Nd:YAG is also of considerable interest. As well as its fundamental wavelength, harmonic conversion can be used for the generation of red light and blue light [7, 8], which can be utilised for example in display technologies, laser therapeutics and biomedical applications. Another potential area of application is multi-wavelength operation which together with difference frequency mixing is a possible source for coherent terahertz radiation [9].
In this paper we report high power laser operation in Czochralski grown 2at.% doped Nd:YAG at 1.3μm by use of the bounce geometry, for the first time. We report CW multimode output powers of 16.7W and Q-switched TEM00 output powers of over 9W with up to 160kHz repetition rate. We also demonstrate QCW operation with 5.5mJ pulse energy at 100Hz and Q-switching with 50ns pulse duration.
2. Experimental work
2.1 Cavity design
The bounce amplifier laser geometry under investigation is depicted in Fig. 1. The laser crystal used was a Nd:YAG slab with 2at.% Nd doping grown by the Czochralski method [10]. The crystal dimensions were 30mm x 5mm x 2mm, and the two 5mm x 2mm end faces were angled to allow the laser cavity beam to be incident at Brewster’s angle to these faces and to experience total internal reflection from the diode pump face. This allows laser operation in a linearly polarized state. The slab was diode pumped on the 30mm x 2mm face, which was antireflection (AR) coated for the 808nm pump wavelength. Heat generated by the pumping process was removed by conduction cooling of the 30mm x 5mm faces of the slab.
In the experiments, a nominally 40W diode bar or a 100W diode bar, both with fast axis collimation and emitting at 808nm, were used for diode pumping. The pump radiation was focused onto the crystal with a 12.7mm focal length cylindrical lens (VCLD) producing a line focus on the pump face.
The Nd:YAG crystal was arranged in a grazing-incidence bounce geometry. The cavity was formed by a high reflectivity (HR@1342nm) back mirror and a partially reflective output coupler. Two vertical cylindrical lenses (VCL) with focal lengths f=50mm and AR coated for 1342nm were used to match the cavity laser mode size to the small gain region in the vertical.
2.2 CW operation
Figure 2 shows the result of output power of the 2at.% Nd:YAG bounce laser versus input pump power for pumping with the 40W diode bar. The oscillator had a compact symmetric cavity with total length 19cm and a 90% reflectivity output coupler. The output coupler reflectivity may not be optimal, but no other output couplers were available. Maximum output power of 10.1W was produced for 40W of diode pump power corresponding to an optical-to-optical conversion efficiency of 25% and a slope efficiency of 38%. Spatial quality of the output beam was single mode in the vertical and multimode in the horizontal.
To increase the output power, the crystal was pumped with a 100W diode bar. The results are shown in Fig. 3. A maximum output power of 16.7W for maximum diode pump power was obtained. A maximum conversion efficiency of 20% is reached at 50W of pumping before efficiency decreases towards maximum pumping. The beam was spatially multimode in the horizontal but single mode in the vertical. This is the highest power achieved, to our knowledge, from a highly doped (>1.1at.%) Nd:YAG laser at 1.3μm.
A single mode TEM00 cavity was constructed with a back mirror to crystal centre distance of 8cm and output coupler to crystal centre distance of 15.5cm. This asymmetric cavity configuration has two stability regions due to the pump power dependence of the thermal lens in the crystal [11]. The cavity has an unstable region of operation between pump powers ∼ 40W and 70W. A maximum TEM00 mode output of 11.0W was produced at 72W diode pumping, as shown in Fig. 3. Even though an overall maximum output power of 12.5W was achieved, the beam was no longer TEM00 mode.
The cavity was then Q-switched by inserting an acousto-optic Q-switch into the output coupler arm of the cavity, as depicted in Fig. 1. The average TEM00 output power and the Q-switched pulse widths as a function of pulse repetition rate are shown in Fig. 4. For a pump power of 72W the average output power was over 9W and the Q-switching frequency ranged from 20 – 160kHz with the corresponding pulse widths from 33 - 129ns.
Fig. 4. Average output power and pulse width as a function of repetition rate for Q-switched 2at.% Nd:YAG TEM00 laser.
2.3 QCW operation
An interesting advantage of Nd:YAG is its longer upper-state lifetime compared to Nd:YVO4 leading to improved energy storage capability. The upper state lifetime of 2at.% Nd:YAG has been measured to be 175μs [12], which compares to a lifetime of 90μs in the vanadate crystals. To investigate this capability we used quasi-CW pumping of the Nd:YAG laser cavity. The Nd:YAG crystal was diode pumped with 200μs long pulses at a repetition rate of 100Hz.
Figure 5 shows the output pulse energy as a function of pump pulse energy for a compact symmetric cavity, as described earlier. Maximum output pulse energy of 5.5mJ for 28mJ of diode pumping has been obtained at a conversion efficiency of 20% and a slope efficiency of 22%. The pumping threshold was 1.6mJ. To our knowledge, this is the first demonstration of QCW pumping of 2at.% Nd:YAG at 1.3μm.
2.4 QCW pumping and active Q-switching
The cavity was redesigned for Q-switched operation. The diode focusing lens was replaced with a 50mm focal length lens to increase the pump region vertically. The larger pump region made the intracavity lenses unnecessary. The cavity mirrors were positioned 2.5cm from the crystal centre.
Figure 6 shows the output pulse energy as a function of pump pulse energy for this cavity. With the diode focusing lens at focal length from the crystal, a maximum output pulse energy of 4.9mJ for 28mJ of diode pumping has been obtained at a conversion efficiency of 18% and a slope efficiency of 23%. With the diode lens defocused at 2.5cm from the crystal the output pulse energy fell to 4.3mJ with a conversion efficiency of 15% and a slope efficiency of 22%.
To fit an active acousto-optic Q-switch which was AR coated for 1342nm into the cavity, the output coupler was extended to 17cm from the crystal. The Q-switch was opened after 195μs of pumping at the end of the pulse to optimize the output pulse energy. Maximum pulse energy of 1.8mJ in a 50ns pulse was produced for 28mJ of pumping.
3. Conclusion
In conclusion, we have demonstrated the highest output power to date in a highly doped crystalline 2at.% Nd:YAG laser at 1.3μm, producing 16.7W of multimode output and 11W of TEM00 mode output in a bounce amplifier geometry. We have also demonstrated Q-switched TEM00 operation with repetition rates from 20 – 160kHz and an average output power of over 9W. Using QCW diode pumping, output pulses with 5.5mJ energy at 100Hz repetition rate were shown. Active Q-switching of the QCW diode-pumped laser produced pulses with 1.8mJ energy and pulse duration of 50ns.
Acknowledgments
The authors acknowledge support from the Engineering and Physical Sciences Research Council (UK) under grant number GR/T08555/01.
References and links
1. M. J. Damzen, M. Trew, E. Rosas, and G. J. Crofts, “Continuous-wave Nd:YVO4 grazing-incidence laser with 22.5 W output power and 64% conversion efficiency,” Opt. Commun. 196,237–241 (2001) [CrossRef]
2. A. Minassian, B. Thompson, and M. J. Damzen, “Ultrahigh-efficiency TEM00 diode-side-pumped Nd:YVO4 laser,” Appl. Phys. B. 76,341–343 (2003) [CrossRef]
3. A. Minassian, B. Thompson, and M. J. Damzen, “High-power TEM00 grazing-incidence Nd:YVO4 oscillators in single and multiple bounce configurations,” Opt. Commun. 245,295–300 (2005) [CrossRef]
4. A. Minassian, B. A. Thompson, G. Smith, and M. J. Damzen, “High-power scaling (>100W) of a diode-pumped TEM00 Nd:GdVO4 laser system,” IEEE J. Sel. Top. Quantum Electron. 11,621–625 (2005) [CrossRef]
5. D. Sauder, A. Minassian, and M. J. Damzen, “High efficiency laser operation of 2 at.% doped crystalline Nd:YAG in a bounce geometry,” Opt. Express. 14,1079–1085 (2006). http://www.opticsinfobase.org/abstract.cfm?URI=oe-14-3-1079 [CrossRef] [PubMed]
6. T. Omatsu, K. Nawata, D. Sauder, A. Minassian, and M. J. Damzen, “Over 40-watt diffraction-limited Q-switched output from neodymium-doped YAG ceramic bounce amplifiers,” Opt. Express. 14,8198–8204 (2006). http://www.opticsinfobase.org/abstract.cfm?URI=oe-14-18-8198 [CrossRef] [PubMed]
7. H. B. Peng, W. Hou, Y. H. Chen, D. F. Cui, Z. Y. Xu, C. Chen, F. D. Fan, and Y. Zhu, “28W red light output at 659.5nm by intracavity frequency doubling of a Nd:YAG laser using LBO,” Opt. Express. 14,3961–3967 (2006). http://www.opticsinfobase.org/abstract.cfm?URI=oe-14-9-3961 [CrossRef] [PubMed]
8. H. B. Peng, W. Hou, Y. H. Chen, D. F. Cui, Z. Y. Xu, C. T. Chen, F. D. Fan, and Y. Zhu, “Generation of 7.6W blue laser by frequency-tripling of a Nd:YAG laser in LBO crystals,” Opt. Express. 14,6543–6549 (2006). http://www.opticsinfobase.org/abstract.cfm?URI=oe-14-14-6543 [CrossRef] [PubMed]
9. A. Saha, A. Ray, S. Mukhopadhyay, N. Sinha, P. K. Datta, and P. K. Dutta, “Simultaneous multi-wavelength oscillation of Nd laser around 1.3um: A potential source for coherent terahertz generation,” Opt. Express. 14,4721–4726 (2006). http://www.opticsinfobase.org/abstract.cfm?URI=oe-14-11-4721 [CrossRef] [PubMed]
10. J. A. ĹHuillier, G. Bitz, V. Wesemann, P. von Loewis, R. Wallenstein, A. Borsutzky, L. Ackermann, K. Dupre, D. Rytz, and S. Vernay, “Characterization and laser performance of a new material: 2 at. % Nd:YAG grown by the Czochralski method,” Appl. Opt. 41,4377–4384 (2002) [CrossRef]
11. V. Magni, “Resonators for solid-state lasers with large-volume fundamental mode and high alignment stability,” Appl. Opt. 25,107–117 (1986) [CrossRef] [PubMed]
12. Y. Urata, S. Wada, H. Tashiro, and P. Z. Deng, “Laser performance of highly neodymium-doped yttrium aluminum garnet crystals,” Opt. Lett. 26,801–803 (2001) [CrossRef]
