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In this paper, a high power blue laser at 447 nm was obtained by intracavity sum-frequency-mixing of a diode-side-pumped Q-switched Nd:YAlO3 (Nd:YAP) laser operating at 1341.4 nm. A type-I critical phase matching LiB3O5 (LBO) crystal and type-II critical phase matching KTiOPO4 (KTP) crystal were used for second harmonic generation and third harmonic generation, respectively. The phase matching condition of the KTP crystal was researched. The results show that the KTP has superiority in intracavity sum-frequency-mixing blue light generation. 4.76 W blue light output was achieved at 4.6 kHz with the pulse width of 190ns. The fluctuation of output power was better than 3% at the output power of 4.76 W during half an hour.
©2008 Optical Society of America
1. Introduction
Multi-watt power blue lasers have been widely investigated for its important applications, such as underwater communications, laser color display, high-density data storage, laser spectroscopy and medical diagnostics. Non-linear optical conversion technology of solid-state laser is one of the most attractive ways to produce an efficient and reliable blue light source for the applications mentioned above.
Recently, continue-wave blue light lasers generated by frequency doubling of the diodeend-pumped neodymium doped lasers operating at the 4F3/2→4I9/2 transition have been extensively explored [1–5]. But this way is limited by the considerable re-absorption loss caused by thermal population of the lower laser level for the oscillation of quasi-three-level laser. Another efficient way to obtain blue radiation is based on frequency tripling of the neodymium doped laser operating at the 4F3/2→4I13/2 transition. Unlike the three-level system of the 4F3/2→4I9/2 transition, stimulated emission at the 4F3/2→4I13/2 transition is a four-level system that can provide a low-threshold and stable laser output due to the lack of sensitive temperature dependence of the transition rate [6]. High power blue light has been achieved in this way. However, most of the research results were focused on using long LBO crystals with noncritical phase matching cut (NCPM) as third-harmonic generation (THG) crystals [7–9]. Hitherto, the highest output power of 7.6 W at wavelength of 440 nm with slop efficiency of 2.1% was achieved by use of a 4 cm length NCPM LBO [7].
In this paper, frequency tripling of the diode-side-pumped Nd:YAP laser at 1341.4 nm has been demonstrated. A type-I critical phase matching cut (CPMI) LBO crystal was chosen for the second harmonic generation (SHG). For comparison, both type-II critical phase matching cut (CPMII) LBO and KTP have been utilized for THG by means of sum-frequency mixing (SFM, 1341.4 nm+670.7 nm→447.1 nm). The results show that the KTP has superiority in intracavity SFM blue light generation. 4.76 W blue light output was obtained at 4.6 kHz repetition rate with the pulse width of 190 ns.
2. Experiments design
Nd:YAP is an important candidate for high power 1.3 µm laser because of possessing the merits of both Nd:YVO4 and Nd:YAG. Its big natural birefringence has the advantage of overcoming the depolarization losses caused by thermally induced stress birefringence and bifocals produced at high average power [10]. Nd:YVO4 is also a natural birefringence, but it used to be utilized for low power laser because of its lower thermal conductivity. Moreover, Nd:YAP possesses large stimulated emission cross section for 4F3/2–4I13/2 transition and high thermal conductivity, which make it well-suited for high power LD side-pumped solid laser. For Nd:YAG, the 4F3/2–4I13/2 transition contains two intense overlapped stark transitions eradiating 1318.8 nm and 1338.2nm lines. In order to achieve stable and high output, it is necessary to inserting a solid etalon and a Brewster plate to avoid them oscillating simultaneously and make it polarized, which will increase the insert loss [7,8]. Whereas for baxis cut Nd:YAP, 4F3/2→4I13/2 transition used to oscillate with single wavelength 1341.4 nm polarized along c-crystalline axis. Making use of linearly polarized Nd:YAP laser at 1341.4nm, the experimental arrangement can be simplified for the absence of solid etalon and Brewster plate, and the insertion loss also was reduced. More recently, the maximum cw linearly polarized 1341.4 nm output power of 121 W was obtained by a side-pumped Nd:YAP crystal[11].
The schematic diagram of the experimental setup for intracavity- frequency-tripling laser system is illustrated in Fig. 1. A b-axis cut Nd:YAP rod with 0.9 at.% Nd3+ doped, 100 mm in length and 4 mm in diameter was used as activation medium. Both end surfaces of it were well polished and coated with antireflective film at both 1079.5 nm and 1341.4 nm. It was mounted inside the flow tube of a LD side-pumping module. The maximum output power of LD module is 555 W at the input current of 45 A. The module consists of three pump units in a three-fold symmetry around the Nd:YAP rod to produce uniform pump light distribution within the cross section of the rod. Each pump unit contained five 10-mm long CW diode bars and the effectively pumped length is approximately 50 mm. Both the diode bars and Nd:YAP rod were cooled by re-circulating filtered water whose temperature was controlled to 25 °C. Pump light with the center wavelength about 808 nm was directly coupled into the Nd:YAP rod. The light which had not been absorbed by the Nd:YAP rod could be reflected back into the rod again by the reflector. An acousto-optic Q-switch modulator (from Gooch & Housego Co.) with high diffraction loss at 1341.4nm was placed between mirror M1 and Nd:YAP rod.
The folded resonator consisting of three mirrors with a tight intracavity beam waist and a total resonator length of about 72 cm has been designed for effective frequency conversion. The folding angle was kept as small as possible to be about 20°to minimize the astigmatism. The flat mirror M1 is HR (high reflective) coated at 1341.4 nm (R>99.9%). The radius of the concave face for two flat-concave mirrors M2 and M3 are 150 mm and 300 mm, respectively. The concave face of M3 is HR coated at both 1341.4 nm and 670.7 nm. Both output coupler M2 and filter mirror M4 are HR coated at 1341.4 mm (T<0.3%) and 670.7 nm (T=0.4%), and AR(anti-reflective) coated at 447 nm (T=92.6%). In order to restrain the oscillation of 1079.5 nm strongest line, all of the mirrors are AR coated at 1079.5 nm (T>70%).
A LBO crystal (4×4×15 mm3, θ=85.9°, ϕ=0°) was chosen as SHG crystal for its high anti-damage threshold (18 GW/cm2) and much smaller walk off angle (about 3.1mrad). Both end faces were coated with AR films at 1341.4 nm and 670.7 nm wavelength.
Table 1. Phase matching parameter of KTP and LBO for SFM calculated [12]
Table 2. PM directions for both KTP and LBO under different crystal temperature[12]
Up to now, most of the research results for high power blue light generation had been use the long NCPM LBO crystals as THG crystals. Using KTP crystal as THG crystals still has not been reported. The phase matching(PM) parameters of KTP and LBO for SFM were displayed in Table 1. Though the walk-off angle is bigger than that of LBO and the acceptance angle is smaller, the deff of KTP is more than six times larger. What’s more, from Table 2, we can see that PM direction for KTP has the much less variation from the temperature, so the precision of temperature control is not so strictly as LBO, which will contribute for more stable blue laser output. Though SFM conversion efficiency using NCPM LBO can be raised by using longer crystal for large acceptance angle and no walk-off angle, it is sensitive to the change of temperature, and the temperature should be controlled at 455K. All those results show that the KTP will superiority in SFM blue light generation.
For comparison, both CPMII LBO (3×3×14 mm3, θ=15.5°, ϕ=0°) and KTP (4×4×10 mm3, θ=78.7°, ϕ=0°) provided by Fujian CASTECH crystal, INC. have been used for SFM. Both end faces of the SFM crystals were coated with AR films at 1341.4 nm, 670.7 nm and 447 nm wavelength. They were wrapped with a thin indium foil and mounted in a copper holder which was cooled by thermoelectric cooler for an active temperature control with stability of ±0.1°C. Based on the design of the cavity, the SHG crystal and the SFM crystal are set close to each other and beside two side of the beam waist in the folded arm of the resonator, which is in favor of taking the advantage of the high power density of the laser beam in the cavity.
3. Experimental results and discussion
Thermal lens focal lengths of Nd:YAP at different pumping powers had been measured in Ref.11. As the pumping power is increased from 350 W to 520 W, the thermal focal length is decreasing from 18 cm down to 8.5 cm. Figure 2 shows the radius of the laser mode in the center of Nd:YAP rod and beam waist of folded arm. The resonator used in our experiment contains two stability zones which have been designed close to each other.
Fig. 2. Radius of the laser mode in the center of Nd:YAP rod and beam waist of folded arm versus thermal lens focal lengths
The second harmonic wave was generated by frequency doubling of the fundamental wave in SHG-LBO. The unconverted fundamental wave and the second harmonic were mixed in the SFM crystal to generate the third harmonic wave. By SHG with the type-I phase matching, the polarizations of the second harmonic wave and the fundamental wave are orthogonal, so the type-II phase matching should be used for SFM. For SHG with the type-I phase matching and SFM with type-II phase matching, all the fundamental and second harmonic waves in the cavity can take part in the SFM process. When the LBO crystal with CPMII cut was used for SFM, the highest output power of only 0.9 W was obtained. We replaced the SFM-LBO with SFM-KTP. The output power increased remarkably. Their measured average output powers were shown in Fig. 3.
The blue-light output started at a pumping power around 350 W, and the output power increased slowly with the increasing of the pumping power at beginning. At the pumping power around 440W, the thermal lens focal length of c-axis polarized laser reached the interval of two stability zones, so the loss of 1341.4 nm laser in the resonator increased and output power of blue laser turned down. After this region, the output power rose rapidly for the laser entering into the second stability zone. The maximum blue output power of 4.76W was obtained at pumping power of 513 W, which has been limited by thermal lens effects as being shown in Fig. 2. The average output power has been optimized at the repetition rate of 4.6 kHz. Due to the transmission at 447 nm is 92.6% for both output coupler M2 and the filter mirror M4, the blue light in the cavity was actually about 5.5 W. When we replaced the M2 with the same curvature concave mirror coated for high reflection at 1341.4 nm and high transmission at 670.7 nm (T1341.4nm≈0.1%, T670.7nm≈95%) and tuned the angle of the SFM-KTP out of the phase-match value, the obtained maximum power of red light is about 12 W. So the red-to-blue conversion efficiency is about 46%.
The blue laser pulse signal was detected by a PIN photodiode, and observed by a Model TDS3052B (500 MHz) dual-line oscilloscope. The measured pulse width is about 190 ns, and the temporal pulse profile of the blue light is shown in Fig. 4. The blue light spot and spectrum are displayed in Fig. 5 and Fig. 6, respectively.
The average power stability of the blue laser was investigated and shown in Fig. 7. The fluctuation of output power is better than 3% at the output power of 4.76 W in half an hour. By changing the temperature of KTP in ±2°C, the fluctuation of the output power is still within 3%. This small fluctuation may attribute to the precise control of the LBO crystal temperature and the small variation from the temperature for KTP crystal.
4. Conclusion
In summary, Multi-watt power blue laser at 447nm achieved by intracavity sum-frequency-mixing of a diode-side-pumped Nd:YAP laser operating at 1341.4 nm has been demonstrated. A type-I critical phase matching LBO crystal was chosen for SHG. For comparison, LBO and KTP both cut at type-II critical phase matching direction have been used for SFM, respectively. The result shows the KTP has the superiority in intracavity SFM blue light generation. Making use of optical anisotropic Nd:YAP and KTP as the laser rod and SFM crystal, the laser system was simplified, and the stability was improved. 4.76W blue light output power was achieved at 4.6 kHz repetition rate with the pulse width of 190 ns. The fluctuation of output power was better than 3% at the output power of 4.76 W in half an hour.
Acknowledgment
This work was supported by the Natural Science Foundation of China (60208001), the Special Project on Science and Technology of Fujian Province (2004HZ01-1) and Guangzhou Bureau of Science and Technology (2006Z3-D0111).
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