The effective nonlinear coefficient and temperature acceptance bandwidth of three Lu and Sc co-doped GdCa4O(BO3)3 type nonlinear crystals were measured. NCPM for SHG in to the blue-UV spectral region can be obtained by controlling the co-dopant concentration. Measurements were based on intra-cavity SHG of a CW Ti:Sapphire laser, and the effective nonlinear coefficients were found to be in the range of 0.5 to 0.6 pm/V for the three crystals used. The FWHM temperature acceptance bandwidth was measured to be more than 35°C using a 6 mm long Gd0.871Lu0.129Ca4O(BO3)3 crystal. A maximum of 115 mW at 407.3 nm in a single direction was measured using a 6.5 mm long Gd0.96Sc0.04Ca4O(BO3)3 crystal.
© 2007 Optical Society of America
In recent years lasers in the blue-UV region have found an increasing number of applications. They are used e.g. for optical data storage, color display, medical diagnostic (bio-markers) and photodynamic therapy. To achieve coherent light in this spectral region, frequency conversion of near infrared (NIR) diode-pumped solid-state lasers (DSSL) has proven to be an efficient, compact and less power consuming alternative to gas lasers. Previously crystals like BBO [1, 2, 3, 4] and LBO [3, 4, 5, 6] have been used in this spectral region due to the large transparency range of these materials. Recently, a new nonlinear crystal, BiBO, has proven to be efficient in generating blue-UV light [7, 8, 9, 10]. BBO has a large effective nonlinear coefficient (deff ≈ 2.0 pm/V), but also has large walk-off, and furthermore BBO is weakly hygroscopic . LBO has a lower deff (≈ 0.75 pm/V) but is widely used for blue-UV generation due to its high damage threshold and low losses. BiBO has deff ≈ 3.2 pm/V, but high walk-off may be a limiting factor in second harmonic generation (SHG) . Optical damage of BiBO has been reported at even modest CW power levels [7, 11]. A new type of nonlinear crystal, with a great potential for SHG to the blue-UV spectral range through non critical phase-matching (NCPM), has been suggested by Gheorghe et al. . It was shown that NCPM can be achieved for a broad range of wavelengths in the blue-UV by appropriate co-doping of GdCa4O(BO3)3 with either Lutetium (Lu) or Scandium (Sc). Doping with Yttrium (Y) has also been reported by Wang et al. . In the present paper measurements of the nonlinear coefficients and the temperature acceptance bandwidths of Lu and SC co-doped crystals are presented for the first time. The measurements are performed using a single-pass configuration for evaluation of the acceptance parameters and intra-cavity NCPM SHG for evaluation of the nonlinear coefficients. In both measurements a continuous wave (CW) Ti:Sapphire (Ti:S) laser was used.
2. Experimental setup
A schematic of the setup is shown in Fig. 1. The Ti:S laser was pumped with a Verdi V5 laser capable of delivering up to 5.5 W at 532 nm. The pump was focused by a lens, f = 100 mm, through mirror M2 into a beam waist of 50 μm in the Ti:S laser crystal (LC).
Mirrors M1 - M5 were all high reflecting (HR) for the fundamental NIR wavelength. Mirrors M1 (r = ∞), M2 and M3 (r = -100 mm) were anti reflection (AR) coated for the pump wavelength, and mirrors M4 and M5 (r = -50 mm) were broadband AR coated for the generated blue-UV light.
The distance from M1 to M2 was 200 mm, from M2 to M3 113.5 mm, from M3 to M4 208 mm and from M4 to M5 the distance was 77 mm, resulting in a circular beam waist of 23 μm between M4 and M5, where the nonlinear crystal (NLC) was inserted. A 3-plate birefringent filter (BRF) inserted between M1 and M2 was used for tuning of the fundamental wavelength of the laser.
For the single-pass experiment a plane output coupler was inserted in the beam waist between mirror M3 and M4. The output coupler had a transmission of 10 % at the NIR wavelengths. Figure 2 shows a typical spectrum of the Ti:S laser. The FWHM was measured to 31 pm at 811.98 nm with the Ti:S laser configured for the single-pass experiment.
3. Crystals parameters
Three crystals with different co-doping were investigated and the results will be presented below. These crystals are similar to the ones described by Gheorghe et al. . When inserted into the Ti:S laser the direction of propagation in the crystal, was along the y-axis and polarized in the z-direction. NCPM then occurs around 800 nm generating light in the blue-UV region. The dimensions of the crystals are shown at Fig. 3: (left) Gd0.871Lu0.129Ca4O(BO3)3, (center) Gd0.93Lu0.07Ca4O(BO3)3 and (right) Gd0.96Sc0.04Ca4O(BO3)3. All crystals were AR coated for both the fundamental and the second harmonic wavelengths. Using the Sellmeier equations published in  the NCPM wavelength and spectral acceptance bandwidth can be calculated. The results are shown in Table 1. Comparing the measured spectrum of the Ti:S laser shown in Fig. 2. to the calculated data, it is seen that the measured spectrum is more than one order of magnitude smaller than the acceptance parameters of the nonlinear crystals, it is therefore appropriate to assume the laser operates at a single-frequency when calculating the conversion efficiency of the nonlinear materials.
4. Non critical phase-matching and temperature acceptance bandwidth
First the phase-matching wavelength and temperature acceptance bandwidth were measured for the NLCs. This was done in the single-pass configuration. The output of the Ti:S laser was then focused into the nonlinear crystal. The output power of the Ti:S was measured to be approximately 700 mW during the single-pass measurement.
The Gd0.871Lu0.129Ca4O(BO3)3 crystal was mounted in a temperature controlled oven and adjusted for 40°C, and the fundamental wavelength of the Ti:S laser was optimized for maximum blue light generation by adjusting the BRF. For this crystal the optimum fundamental wavelength was found to 799.0 nm, as oppose to the calculated 792 nm found in table 1. This, however, was calculated at 25°C. In order to see if the difference could be due to the elevated temperature of the NLC, a temperature scan was performed while monitoring the conversion efficiency. This measurement is shown in Fig. 4. To the author’s knowledge no temperature dependent Sellmeier equations has been published for these materials. Similar measurements are shown on Fig. 5. for the two other NLCs. These measurements were obtained using a fundamental wavelength of 812.0 nm and 814.8 nm respectively. It should be noted that the lengths of these crystal are not identical. It is seen that the values of the phase-matching wave-lengths measured are slightly different from the calculated values, especially in the case of the Gd0.96Sc0.04Ca4O(BO3)3 crystal where the greatest deviation of 12.8 nm was obtained. As a consequence, the deviations cannot be explained by the temperature. It is known that a minor change of the compositional parameter x can lead to a relatively large change in the phase-matching wavelength. Thus a possible explanation of the obtained differences could be the presence of small non-uniformities in the composition of the grown crystals. The small values of the segregation coefficient of the Sc ions in the GdCa4O(BO3)3 crystals (k = 0.38) , could explain the relative large deviation obtained for the Gd0.96Sc0.04Ca4O(BO3)3 crystal. The measured parameters of the three crystals are summarized in table 2.
5. Blue light generation
The Gd0.871Lu0.129Ca4O(BO3)3 NLC was then placed intra-cavity in the Ti:S laser as shown on Fig. 1 and temperature stabilized at the optimum phase-match temperature. The crystal was again orientated for beam propagation along the y-axis and the fundamental polarized in the z-direction. The generated power was measured as function of pump power, see Fig. 6 (left). More than 37 mW of Blue-UV light was measured through mirror M5. To measure the circulating power, which is essential for calculating the effective nonlinear coefficient, it was necessary to exchange one of the very highly reflecting mirrors with a 1 % output coupler (M1). In this way it was possible to measure the generated second harmonic power as a function of fundamental power squared, see Fig. 6 (right). It is seen, that a linear relation exists between the square of the fundamental power and the generated second harmonic power. This relation will be used for calculation of the effective nonlinear coefficient of the material in the following section. A similar experiment was made using the two other NLCs, and the results of these measurements are shown in Figs. 7 and 8. By comparing the graphs in Figs. 6, 7 and 8 (left), it is seen that the Gd0.93Lu0.07Ca4O(BO3)3 crystal has a much higher threshold than the other two crystals. This is believed to be due to thermal lensing in the Ti:S crystal. When optimizing for blue-UV light generation, the pump power was set at its maximum (5.5 W). As the Ti:S laser shown in Fig. 1 had a very small stability range, the laser was not stable before a certain thermal lensing was present in the Ti:S LC, hence the high threshold. The setup was optimised for highest generation of blue-UV power, and not for lowest threshold. At Fig. 6 (right) the circulating power for the Gd0.871Lu0.129Ca4O(BO3)3 crystal is very low compared to the two other crystals shown in Figs. 7 and 8. By closer examination of this crystal in a microscope, small impurities could be seen, giving rise to higher scattering losses.
6. Calculating the nonlinear coefficient
From the above measurement the effective nonlinear coefficient, deff , can be calculated using Eq. (1).
where P 1 is the fundamental power, P 2 is the generated power, n 1 and n 2 are the refractive indices for the fundamental and the generated light respectively. w 0 is the beam waist of the fundamental inside the NLC, ε 0 is the permittivity of free space, μ 0 is the permeablility of free space, c is the speed of light, ω is the frequency of the fundamental and h(B,ξ) is the Boyd-Kleinman factor defined in Ref. . This factor is estimated based on simulations done for the setup shown at Fig. 1. The calculation is strait forward, given the value listed below. It is seen that the nonlinear coefficients for these crystal is of the same size as for LBO. However, the ability to dope pure GdCa4O(BO3)3 crystals with either Lutetium (Lu) or Scandium (Sc) and thereby achieve noncritical phase-match for a variety of wavelengths, makes these crystals very attractive in applications where a specific wavelength is required.
The nonlinear coefficients for SHG of NIR light for three differently doped GdCa4O(BO3)3 crystals have been measured. The light was propagating along the y-axis with the fundamental polarized along the z direction. For Gd0.871Lu0.129 the nonlinear coefficient was found to be 0.51 pm/V at a fundamental wavelength of 799 nm. The temperature dependence acceptance bandwidth of the phase-matching was measured to have a FWHM of 38.1°C using a 6 mm long crystal. For Gd0.93Lu0.07 the nonlinear coefficient was calculated to 0.6 pm/V, and the temperature dependent FWHM acceptance temperature was measured to 32.5°C. The third crystal was Gd0.96Sc0.04 for which a nonlinear coefficient of 0.55 pm/V was found and the temperature dependent FWHM was measured to 28.4°C. A total of 115 mW of CW power at 407 nm was generated using the 6.5 mm long Gd0.96Sc0.04 crystal. Using NCPM it should be possible to scale this system further, both using a longer crystal and by increasing pump power. No signs of degradation of the NLCs heave been seen throughout the experiments. The nonlinear coefficients are seen to be comparable to those of LBO, and the ability to obtain NCPM at a large range of wavelengths makes this a promising candidate for nonlinear frequency conversion into the blue-UV spectral range for a number of applications.
This work was supported be the Danish Technical Research Council, grant 26-02-0210 and 274-05-0377.
References and links
1. C. Zimmermann, V. Vuletic, A. Hemmerich, and T. W. Hänsch, “All solid state laser source for tunable blue and ultraviolet radiation,” Appl. Phys. Lett. 66, 2318–2320 (1995). [CrossRef]
2. C. Schwedes, E. Peik, J. V. Zanthier, A. Y. Nevsky, and H. Walther, “Narrow-bandwidth diode-laser-based blue and ultraviolet light source,” Appl. Phys. B 76, 143–147 (2003). [CrossRef]
3. A. Friedenauer, F. Markert, H. Schmitz, L. Petersen, S. Kahra, M. Herrmann, T. Udem, T. W. Hänsch, and T. Schtz, “High power all solid state laser system near 280 nm,” Appl. Phys. B 84, 371–373 (2006). [CrossRef]
4. S. Bourzeix, B. de Beauvoir, F. Nez, F. de Tomasi, L. Julien, and F. Biraben, “Ultra-violet light generation at 205 nm by two frequency doubling steps of a cw titanium-sapphire laser,” Opt. Commun. 133, 239–244 (1997). [CrossRef]
5. S. Makio, T Miyai, M. Sato, and T. Sasaki, “67-mW Continuous-Wave Blue Light Generation by Intracavity Frequency Doubling of a Diode Pumped Cr:LiSrAlF6 Laser,” Jpn. J. Appl. Phys. 39, 6539–6541 (2000). [CrossRef]
6. Y. Chen, H. Peng, W. Hou, Q. Peng, A. Geng, L. Guo, D. Cui, and Z. Xu, “3.8 W of cw blue light generated by intracavity frequency doubling of a 946-nm Nd:YAG laser with LBO,” Appl. Phys. B 83, 241–243 (2006). [CrossRef]
7. M. Thorhauge, J.L. Mortensen, P. Tidemand-Licthenberg, and P. Buchhave, “Tunable intra-cavity SHG of CW Ti:Sapphire lasers around 785 nm and 810 nm in BiBO-crystals,” Opt. Express 14, 2283–2286 (2006). http://www.opticsinfobase.org/abstract.cfm?URI=oe-14-6-2283 [CrossRef] [PubMed]
8. T. Schmitt, A. Deninger, F. Lison, and W. Keanders, “Recent advances in non-linear frequency conversion of high-power, single-mode diode lasers,” Proceedings of. SPIE , 5707, 16–22 (2005). [CrossRef]
9. J. E. Hastie, L. G. Morton, A. J. Kemp, M. D. Dawson, A. B. Krysa, and J. S. Roberts, “Tunable ultraviolet output from an intracavity frequency-doubled red vertical-external-cavity surface-emmitting laser,” Appl. Phys. Lett. 89, 0611141–0611143 (2006). [CrossRef]
10. H. Hellwig, J. Liebertz, and L. Bohat, “Exceptional large nonlinear optical coefficients in the monoclinic bismuth borate BiB3O6 (BiBO),” Solid State Commun. 109, 249–251 (1999). [CrossRef]
12. L. Gheorghe, V. Lupei, P. Loiseau, G. Aka, and T. Taira, “Second-harmonic generations of blue light in nonlinear optical crystals of Gd1-XLuXCa4O(BO3)3 and Gd1-XScXCa4O(BO3)3 through noncritical phase matching,” J. Opt. Soc. Am. B 23, 1630–1634 (2006). [CrossRef]
13. Z. Wang, X. Xu, K. Fu, R. Song, J. Wang, J. Wei, Y. Liu, and Z. Shao, “Non-critical phase matching of Gd1-XYXCa4O(BO3)3(Gd1-XYXCOB) crystal,” Solid State Commun. 120, 397–400 (2001). [CrossRef]
14. G. D. Boyd and D. A. Kleinman, “Parametric Interaction of Focused Light Beams,” J. Appl. Phys. 39, 3597–3639 (1968). [CrossRef]