By use of CW diode laser stacked arrays, side-pumping Q-switched composite ceramic Nd:YAG rod laser based on a type II KTP crystal intracavity frequency-doubled, a high power high stability green laser has been demonstrated. Average output power of 104 W is obtained at a repetition rate of 10.6 kHz with a diode-to-green optical conversion efficiency of 10.9%. For the average output power of about 100 W, the measured pulse width is 132 ns with power fluctuation of less than 0.2%. The experimental results show that the green laser system using this novel ceramic Nd:YAG offers better laser performance and output stability than the traditional single Nd:YAG crystal green laser system with the same operating conditions and experimental configuration.
©2007 Optical Society of America
High power all-solid-state green laser source perform better than IR lasers in many applications, such as marking, precision micro fabrication and trimming due to its lower spot size limit and larger absorbance for most materials. It also has applications for ocean exploration, laser probes and underwater communication due to its lower absorption in water. A high power green laser also has been used for medical treatment. An 80W green laser has been used clinically for laser vaporization of the prostate . It is one of the most promising methods to obtain an efficient and reliable green-beam source by an intracavity-frequency-doubled of a near 1 micron solid-state laser. High power >100 W solid state green lasers have been demonstrated in same fashion [2–8]. However, Nd:YAG single crystals are used for in these experiments as the laser gain to generate the fundamental 1064 nm laser source for frequency doubling. Comparing to the method of growing ceramic media, the conventional Czochralski method of growing single YAG crystal is relatively expensive and time-consuming. It also has its own insurmountable disadvantages such as limited crystal size and low doping concentration (typically 1 at. %) for Nd:YAG . However, the novel ceramic media not only contains good thermal, mechanical and spectra properties as fine as single crystal but also can be made with large size (1 m × 1 m × 0.02 m), high concentration (uniformly doped up to 4 at.%) and it is also suitable for mass-production . Due to those advantages mentioned above, the ceramic YAG is expected to be a popular host material of the laser gain media in the future.
The ceramic laser media has become a rapid developing research field since the polycrystalline material CaF2 was first obtained in 1960s. High efficiency kilowatts Nd:YAG ceramic laser has been reported [10–11]. It shows that these kilowatts 1064 nm Nd:YAG ceramics laser gives close optical to optical conversion efficiency (42%) relative to the single crystal laser (49%) . However, the research on nonlinear optical frequency conversion based on the ceramic Nd:YAG laser has only been demonstrated up to 30 mW green power with diode-to-green optical efficiency of 4% previously . In this paper, we report generation of 104 W of stable pulse green laser by intracavity type II KTP crystal frequency doubling of a diode side pumped composite Nd:YAG ceramic laser with a diode-to-green optical conversion efficiency of 10.9%. To the best of our knowledge, it is the highest green power and efficiency obtained by intracavity frequency doubling of this novel composite ceramic Nd:YAG laser. Both efficiency and output power have been improved greatly compare to the previous reports. The measured output power fluctuation is less than 0.2% up to two hours. The measured stability of the green laser is higher than that of the same pumping conditions and experimental configuration setting but using single Nd:YAG crystal instead.
2. Experimental setup
The experimental arrangement is shown schematically in Fig. 1. The laser consists of a pump head to couple the diode laser beam to the composite ceramic Nd:YAG rod (Baikowski Japan Co. Ltd.), a KTP nonlinear crystal (CSK Photonics Co. Ltd., Jinan, China) for intracavity frequency doubling, an Acousto-Optic, AO, modulator (Gooch & Housego Inc., UK) and a harmonic separator and a resonator.
The schematic of the pump module assembly is shown in Fig. 2. The optical pumping module consisted of five laser diode stacked arrays (Cutting Edge Optronics Co.), which were arranged in a pentagonal shape around the rod to reach better pumping uniformity. Each stacked array consisted of 16 laser diodes. The composite ceramic Nd:YAG rod was mounted inside a flow-tube whose outer face was coated with an AR coating at the laser diode wavelength. The size of the rod was 6.35 mm in diameter and 144 mm in length. The 0.7 ± 0.02% Nd-doped middle part was 114 mm in length and side-polished, while undoped parts of the rod were about 15 mm long extended from both end. The end faces of the rod were polished and antireflection-coated at 1064 nm. This undoped end designs has many advantages, including reduced thermal stress on the end surfaces coating and also improve output power stabilization. Considering the thermal lens effect introduced by the Nd:YAG rod and the KTP crystal in high power operation, we employed a plano-concave cavity structure in order to enhance stability and efficiency. The total cavity length is 550 mm. An acousto-optic modulator with high diffraction efficiency is used as a Q-switcher. Its repetition rate was optimized to be 10.6 kHz with respect to the green average power. The KTP crystal with dimension of 6 mm×6 mm×10 mm, was antireflection coated at 532nm (Transmission > 98%) and at 1064 nm (Transmission > 99.5%) and it was placed in between the flat output coupler (Transmission > 99.5% at 532 nm, Reflection > 99% at 1064 nm) and harmonic separator mirror (Reflection > 99.5% at 532 nm, Transmission > 98% at 1064 nm). KTiOPO4 (KTP) was chosen as a nonlinear crystal for frequency doubling in these experiments for its high nonlinear conversion coefficient, higher temperature control tolerant, small walking-off angle and relatively high damage threshold. At the temperature of 23 °C, the phase matching angles of the KTP crystal are φ= 23.27°, θ = 90°, which indicates type II phase matching. The KTP crystal was placed inside the 10°C water cooled copper holder. Although the boundary temperature of KTP crystal can be kept constant in this way, the temperature in the middle of KTP crystal (the pumped volume) will still getting higher as the 1064 nm pumping power increases. In this case, the phase-matching condition is violated and degrades the green power. In order to compensate this thermal-induced phase mismatching, the incident angle to the KTP crystal was adjusted to satisfy noncollinear phase matching condition. In this way, the condition of phase matching of type II KTP SHG can be kept stable as pump power increases.
3. Results and discussions
Figure 3 shows the power versus current of LD module. The total pumping power available from the five diode stacked array is about 1600 W at the operational current of 25 A. The center wavelength of the diode laser at 24 A, 25°C is about 803 nm with FWHM of 2.3 nm. At diode laser current of 25 A, a 511 W 1064 nm CW laser has been demonstrated previously in a linear ceramic Nd:YAG plano-plano cavity with optical-to-optical efficiency of 31.9% . Referring to our former experimental record of the Nd:YAG single crystal high power green laser , which is done under the same optimum operating conditions and experimental configuration, we can make direct comparison between two green laser performance based on the composite ceramic rod and the general single crystal rod. The average green laser output power and optical-to-optical conversion efficiency of the two systems against pump diode current is shown in Fig. 4. Generally the slope efficiency of both systems is very close but the composite ceramic Nd:YAG system shows slightly lower pump threshold. When the pump current is 16.5 A (956 W), the maximum average output powers of the green laser and optical-to-optical efficiencies were 104 W (10.9%) and 96 W (10.2%) for the composite ceramic rod and the single crystal rod, respectively.
At the pump diode current of 16 A, the output power of the ceramic green laser were measured up to 2 hours and the results is presented in Fig. 5. Base on data shown in Fig. 5, the calculated output power fluctuation is less than 0.2% and also the measured pulse to pulse instability is 3%. However, the ~100 W green laser generated from the system based on Nd:YAG single crystal has output power fluctuation about 2% . According to Ref. , the thermal conductivity of ceramic and single crystal Nd:YAG rods is 10.7WT-1m-1 and 10.5WT-1m-1 respectively. We believe that the slightly higher thermal conductivity features of the composite ceramic Nd:YAG laser rod and the construction of composite ceramic Nd:YAG rod (with both end undoped) enhances the stability of the fundamental 1064 nm generation and lead to more stable SHG output.
The pulse shape of 100 W average green output power at repetition rate of 10.6 kHz is measured by a fast photo-detector and given in Fig. 6. The measured pulse width from Fig. 6 is 132 ns and gives the calculated pulse peak power to be about 74 kW.
The beam quality of the green laser has been characterized by a computer control CCD camera. Two dimensional beam profiles of the 100 W green laser is recorded and presented in Fig. 7. The beam shape of green laser shown in Fig. 7 has Gaussian-like distribution. The divergence angle of the laser beam was measured by laser trepan method. So the M2 beam quality of green laser is about 30 by conversion.
In conclusion, we have successfully demonstrated a 104 W high power green laser with high stability by using diode-side-pumped intracavity-frequency-doubling of a Q-switched composite ceramic Nd:YAG rod laser. The diode-to-green optical conversion efficiency is 10.9%. The experimental results demonstrated in this paper show much higher output power stability than previous reports. At around 100 W average output power, the fluctuation measured is less than 0.2% with pulse to pulse instability of 3%. The measured green laser pulse width is 132 ns with repetition rate of 10.6 kHz. The M2 beam quality of the 100 W green lasers is 30. The experimental results show that the green laser system using composite ceramic Nd:YAG offers better laser performance and output stability than the single Nd:YAG crystal green laser system with the same operation conditions and experimental configuration.
References and links
1. A. Bachmann, S. Wyler, R. Ruszat, R. Casella, T. Gasser, and T. Sulser, “80W high-power KTP laser vaporization of the prostate clinical results after 110 consecutive procedures,” Eur Urol. 3 (suppl 2),145–145 (2004). [CrossRef]
3. E. C. Honea, C. A, Raymond, and J. Beach, et al., “Analysis of an intracavity-doubled diode-pumped Q-switched Nd:YAG laser producing more than 100 W of power at 0.532μm,” Opt. Lett. 23,1203–1205 (1998). [CrossRef]
4. J. J. Chang, E. P. Dragon, and C. A. Ebbers, et al., “An efficient Diode-Pumped Nd:YAG Laser with 451 W of CW IR and 182 W of pulsed green output,” in Advanced Solid State Lasers, C. R. Dragon and W. R. Bosenberg, eds., Vol.10 of OSA Trends in Optics and Photonics Series (Optical Society of America, Washington, D.C., 1998), pp.300–304.
5. T. Kojima, S. Fujikawa, and K. Yasui, “Stabilization of a high-power diode-side-pumped intracavity-frequency-doubled CW Nd:YAG laser by compensating for thermal lensing of a KTP Crystal and Nd:YAG Rods,” IEEE J. Quantum Electron. 35,377 (1999). [CrossRef]
7. A. C. Gong, Y. Bo, and Y. Bi, et al., “High beam quality green generation with output 140W based on a thermally-near-unstable fla-flat resonator,” Chin. Phys. Lett. 22,125–127 (2005). [CrossRef]
8. D. G. Xu, J. Q. Yao, and B. G. Zhang, et al., “110 W high stability green laser using type II phase matching KTiOPO4 (KTP) crystal with boundary temperature control,” Opt. Commun. 245,341–347 (2005). [CrossRef]
9. R. R. Monchamp, “The distribution coefficient of neodymium and lutetium in Czochralski grown Y3Al5O12,” J. Cryst. Growth. 11,310–312 (1971). [CrossRef]
10. G. A. Kumar, J. Lu, A. A. Kaminskii, K. Ueda, H. Yagi, T. Yanagitani, and N. V. Unnikrishnan, “Spectroscopic and stimulated emission characteristics of Nd3+ in transparent YAG ceramics,” IEEE J. Quantum Electron. 40,747–758 (2004). [CrossRef]
11. J. Lu, T. Murai, K. Takaichi, T. Uematsu, and K. Ueda, et al., “Development of Nd:YAG ceramic lasers,” in Advanced Solid-State Lasers, M. Fermann and L. Marshall, eds., Vol.68 of Trends in Optics and Photonics Series (Optical Society of America, 2002), pp.507–517.
12. A. Ikesue, T. Taira, and K. Yoshida, “SHG laser using YAG ceramics for light source of photofabrication,” J. Photopolym. Sci. Technol. 13,687–690 (2000). [CrossRef]
13. H. F. Li, D. G. Xu, and Y. Yang et al., “Experimental 511W composite Nd:YAG ceramic laser,” Chin. Phys. Lett. 22,2565–2567 (2005). [CrossRef]
14. D. G. Xu, J. Q. Yao, and R. Zhou, et al., “104W all solid state Nd:YAG intracavity frequency doubled laser,” Acta Opt. Sin. 24,925–928 (2004).