High-power, acoustooptic Q-switched red laser output is obtained by intracavity frequency-doubling a Nd:YAG laser operating at wavelength of 1.319μm with a LiB3O5 (LBO) crystal. 28 W average power at 659.5 nm was demonstrated at 5 kHz and a pulse width of 250±10 ns (FWHM). The beam quality of M2 value is 22±3 in both horizontal and vertical directions. The conversion efficiency and the stability of the red laser output power at 659.5 nm were both improved through the suppression of 1.338 μm operation by means of a thin YAG etalon. The long-term stability of the red light is better than 1% at an output of 23 W during 200 hours.
©2006 Optical Society of America
High power red lasers can be used as a source for large area projection systems, laser therapy and pumping source for tunable lasers. Diode laser, optical parametric oscillator, frequency-doubling techniques was developed to obtain red lasers. Although the output power of red diode lasers is boosted and various kinds of beam-shaping techniques are under investigation, their brightness is not yet sufficient for some special applications. Optical parametric converters potentially represent the most flexible and tunable solution [1–4]. Felix Brunner reported the generation of 8 W red light at 603 nm, however they are complicated. Frequency doubling of a 1.3-μm all solid state laser is an efficient way to obtain the high-brightness red radiation. One method is frequency doubling of the output from Nd:YVO4 and Nd:GdVO4 around the 1.34-μm region. In 1999, Lee et al. reported that a CW 659.5 nm red laser radiation of 6.1 W was yielded by frequency doubling a side-pumped Nd:YAG laser using a KTP crystal as the nonlinear medium . More recently, Chenlin Du et al. reported the generation of Q-switched red light of 6.0 W by frequency doubling of Nd:GdVO4 laser in a LBO crystal . Our group reported the generation of 11.5 W Q-switched red light at 659.5nm and 664nm using linear shape resonator . However, no one has reported about the red power more than 20W output with diode-pumped all-solid state laser, so far.
Usually, two fundamental wavelengths can be obtained around 1.3-μm from a Nd:YAG laser. One is the R2→X1 transition at 1.319-μm and the other is the R2→X3 transition at 1.338-μm, their effective stimulated emission cross sections is one third of the R2→Y3 transition at 1.064-μm. In order to suppress lasing at the highest gain at 1.064-μm and enhance the operation at 1.3-μm, the reflectivity of one of the mirrors is selected such that it is higher than 99% at 1.3-μm and lower than 30% at 1.064-μm Generally, researchers figured out that the 1.319-μm transition can oscillate more easily than the 1.338-μm line [7,8], but We found that the 1.338-μm transition can oscillate simultaneously with the 1.319-μm line even though under lower pumping level, it would be very harmful for frequency doubling to obtain high power red laser output. From the view of coating engineering, the difference in the reflectivity at these two wavelengths has been measured to be less than 0.3% for the total reflective mirror. It is difficult to discriminate 1.319-μm transition and 1.338μm transition. In order to avoid the simultaneous existence of two wavelengths, some wavelength selector must be insert into laser cavity to suppress the unwanted transition.
In this paper, we report the generation of 28 W of acoustooptic Q-switched red beam at 659.5 nm by the intracavity frequency-doubling of a side-pumped Nd:YAG laser operating at 1.319-μm with a 4-cm LBO. A solid etalon was placed in the laser cavity as wavelength selector to suppress the 1.338-μm transition. The long-term stability of the red-light source is better than 1% at 23 W during 200 operating hours. The beam quality of M2 value at the maximum output is 22 ±3 in both horizontal and vertical directions. To the best of our knowledge, the red laser reported in this paper has the highest output power among the all-solid-state Q-switched red lasers ever reported. Especially, high power and high efficiency generation single line red light at 659.5 nm from 1.319-μm was realized at the first time.
2. Experimental setup
The experimental arrangement of the system is illustrated in Fig. 1. The Z shaped resonator was composed of four mirrors: the flat mirror M1 was coated for high reflection at 1.3-μm, the 400-mm radius of curvature of mirror M2 was an output coupler for 659.5 nm with high transmission at 659.5 nm and high reflection for 1319-μm, the150-mm radius of curvature of mirror M3 and mirror M4 were coated for high reflection at 1.3-μm and 659.5 nm. The total resonator length is about 1 meter. In order to suppress oscillation at 1064 nm, the coatings of M1, M2, M3, M4 have the transmission greater than 70% at 1064 nm. The high transmission at 1064 nm for the mirrors is necessary since the gain of the oscillation line at 1064 nm is much higher than that at 1.3-μm. The highly wavelength selective dielectric coatings suppressed the laser oscillation at the strongest transition at 1064 nm and provide optimum conditions at 1.3-μm. An acousto-optic modulator operating at 1.3-μm was used. the system is Q-switched and operated at a repetition rate of 5.0 kHz. A 4-cm long type II LBO crystal is used as the nonlinear optical medium for intracavity frequency-doubling. A quarter-wave plate specific to the 1319 nm wavelength is placed between the Nd:YAG rod and the harmonic mirror to reduce the so-called “green problem” .
The primary consideration of resonator design is how to obtain the minimal beam waist in the LBO, and how to expand the fundament mode volume. The image relationship between the LBO and YAG was not considered. The minimum beam diameter in the LBO at 1.3-μm is about 0.6 millimeter at high pumping level.
The laser system in our experiment contains only one diode-pumped Nd:YAG laser-modules. A Nd:YAG rod 4 mm in diameter and 100 mm in length, which is 0.6 at.% Nd3+ doping, is used as the laser medium. On both end faces, the antireflective films for 1319 nm were coated. The Nd:YAG rod is surrounded by a diffusive reflector and pumping diodes from radial directions. Pump lights from the diodes are coupled into the Nd:YAG rod directly. Figure 2 shows the schematic cross-section of the pumping laser module. It consists of five pumping modules arranged in fivefold symmetry around the laser rod. Five 1-cm-long linear laser diodes are arranged on each module, in which the cooling water flows parallel to the laser diodes. To match the Nd:YAG absorption band near 808.5 nm, we carefully selected the 25 laser diodes at a given drive current and a given cooling temperature to obtain the lowest spectral dispersion in the laser modules. The maximum output power of each laser diode is 20 W at the maximum input current of 25 A. The choose of Nd:YAG as the laser medium is due to the fact that it is favorable for laser operation and its relative strong transition at 1.3-μm.
A LBO crystal, used as the nonlinear crystal for intracavity frequency doubling in this experiment, was cut for type-II noncritical phase matching (θ=θ° φ = θ°). The size of the LBO is 4×4×40 mm3 and it was provided by Beijing center for crystal research and development. The antireflective films for 659.5 nm and 1319 nm were coated on both end faces .The LBO is well suitable for high-power SHG with long-term reliability owing to its high damage threshold and low absorption at both the fundamental and second harmonic outputs of the Nd:YAG laser at 1.3-μm. The crystal was placed in an oven, whose temperature was maintained by a precise temperature controller to a precision of ±0.1 K. The LBO crystal is located the minimum beam waist between M3 and M4 to take the advantage of the strongest power density of the fundamental beam.
3. Results and discussion
First, when etalon is absent from the laser cavity, two fundamental wavelengths was obtained around 1.3-μm in the output of a Nd:YAG laser, one is the R2→X1 transition at 1.319-μm and the other is the R2→X3 transition at 1.338-μm. Moreover we found that the 1.338-μm transition can oscillate simultaneously with the 1.319-μm line even just above threshold. Figure 3 shows the coexistence of two transitions and their relative laser performance under high pumping power level, which was measured by Anritsu optical spectrum analyzer MS9710B. It consists with the same effective stimulated emission cross sections data for both transitions as A. A. Kaminskii reported: 0.95×10-19 cm-2 and 1.0×10-19 cm-2 respectively .
Second, we analyzed the spectrum of the red beam under maximum output power and it is displayed in Fig. 4. The radiation at 659.5 nm and 669 nm were generated by frequency doubling of 1.319-μm and 1.338-μm, while the output at 664 nm can be attributed to the sum-frequency between 1.319-μm and 1.338-μm. It indicates that the Nd:YAG laser oscillated on the two transitions simultaneously under high pump power. Obviously, the coexistence of the two wavelengths is a big problem for the generation of high efficiency frequency doubling to get red light. First, the existence of 1.338-μm radiation, consuming the energy stored in the Nd:YAG material, reduces the peak power density of 1.319-μm radiation in the cavity; Second, the sum-frequency between 1.319-μm and 1.338μm consumes a larger portion of the energy oscillated at 1.319-μm in the cavity. Therefore, the coexistence of the two transitions holds back the efficient increase of total red output power at the high pump power level; Third, the mode competition between two transitions around 1.3-μm in the cavity makes the output unstable. Under this condition, the maximum output power of red light is about 15 W, the output instability is more than 7%.
Finally, in order to avoid the simultaneous existence of two waves and to select a wanted wavelength, we designed a solid etalon to be placed in the laser cavity. The etalon is made of undoped solid YAG and is not coated. YAG material, not silica, is selected because of high refractive-index and perfect laser performance. The thickness of the etalon is 0.6 mm. It can improve the transmission difference between 1319-μm and 1.338-μm from 12% to 28% than that of silica etalon. The insertion loss for the etalon at 1.319-μm has been calculated to be less than 0.1%. We measured the spectrum distribution of the red laser. Figure 5 shows that only single red spectrum at 659.5nm was obtained at 500 W pumping power. We try to increase the pumping power to excess the maximum permitted pumping power by 10%, other two red spectrums at 664nm and 669nm was observed.
Experimentally, we measured the output power of the 659.5nm red light as a function of the pump power of the diodes. Figure 6 shows the SHG output power versus the pump power at 808 nm under the repetition rate of 5 kHz. The red-light output started at a pumping power around 80 W. It increases rapidly as the pumping power of the diode laser increases and is not saturated at 500 W, the corresponding maximum red output power is 28 W, with pulse width 250±10 ns. The pulse-to-pulse stability is less than 2%(RMS).
In order to integrate a large screen laser projector, one high power red laser machine of 23 W was manufactured. It has been operating for 200 hours up to now. the time trace of the output power at the output power level of 23 W is shown in Fig. 7, the fluctuation of the red beam output power was better than 1.0%.
Beam quality is important for many applications. The beam quality factor M2 becomes poorer as the pump power increases. At maximum output of 28 W, the M2 value is 22±3 in both horizontal and vertical directions measured by Spiricon beam analyzer, it is enough for laser projecting application. At a lower output, the beam quality is much better. For example, at an output of 5 W, the value of M2 is reduced to about 15±2 in both directions. The poorer quality at higher output is mostly due to that the laser tends to oscillate in higher-order transverse modes under high pump power, while no any mode-selecting technique is used.
In conclusion, intracavity frequency doubling of a diode-pumped Nd:YAG laser at 1.319-μm has been demonstrated. A high power red laser beam of 28 W at 659.5 nm was obtained with M2=22±3 in Q-switched operation. The 1.338-μm wavelength was suppressed by using a thin YAG etalon in the laser cavity to improve stability and conversion efficiency. The power fluctuation of less than 1% at an output of 23W was obtained for 200 hours. This excellent performance of the laser system demonstrates that the side-pumped Nd:YAG laser with LBO intracavity frequency doubling is an promising method for generating high-power red light with high beam quality.
This work was supported by the Knowledge Innovation Programme of the Chinese Academy of Sciences (No. KJCX1-05), the National High Technology Research and Development Programme of China under Grant No. 2002AA311120 and 2005AA31120, the National Key Basic Research and Development Programme of China under Grant No. G1998061413, and the council for Science and Technology of Beijing under Grant No. H020420060060110.
References and links
2. N. A and R. E. Wallenstein, “Concepts and performance of solid state RGB laser sources for large-frame laser projection displays,” in Projection Displays 2000: Sixth in a Series, M. H. Wu, Ed., Proc. SPIE 3954, 163–166 (2000). [CrossRef]
3. D. Lee and P. F. Moulton, “A compact OPO-based RGB source,” in Projection Displays VII, M. H. Wu, Ed., Proc. SPIE 4294, 60–66 (2001). [CrossRef]
4. D. Lee, P. F Moulton, R. Bergstedt, and G. W. Flint, “OPO-based compact laser projection display,” in Cockpit Displays VII: Displays for Defense Applications, D. G. Hopper, Eds., Proc. SPIE 4362, 203–212 (2001). [CrossRef]
5. Y. Inoue, S. Konno, T. Kojima, and S. Fujikawa, “High-power red beam generation by frequency-doubling of a Nd:YAG laser,” IEEE J. Quantum Electron. 35, 1737–1740 (1999). [CrossRef]
6. C. Du, S. Ruan, Y. Yu, and F. Zeng, “6-W diode-end-pumped Nd:GdVO4/LBO quasi-continuous-wave red laser at 671 nm,” Opt. Express 13, 2013–2018 (2005) http://www.opticsinfobase.or g/abstract.cfm?URI=oe-13-6-2013. [CrossRef] [PubMed]
7. Z. Sun, R. Li, Y. Bi, X. Yang, Y. Bo, W. Hou, X. Lin, H. Zhang, D. Cui, and Z. Xu, “Generation of 11.5 W coherent red-light by intra-cavity frequency-doubling of a side-pumped Nd: YAG laser in a 4-cm LBO,” Opt Commun. 241, 167–172 (2004). [CrossRef]
8. W. Koechner, Solid-state laser engineering (5th ed, Berlin, Springer, 1999), Chap.2.
10. A. A. Kaminskii, Laser Crystals (2nd ed, Berlin, Springer-Verlag, 1990).