Simultaneous optical parametric oscillation and intracavity second-harmonic generation based on a hexagonally poled lithium tantalate

Simultaneous optical parametric oscillation and intracavity second-harmonic generation based on a hexagonally poled lithium tantalate is reported. Both the optical parametric oscillation and the cascaded noncollinear second-order harmonic generation processes reach a high efficiency. A variety of possible self-doubling optical parametric oscillation processes indicate this hexagonally poled lithium tantalate has potential applications as a compact multi-wavelength light source. © 2009 Optical Society of America OCIS codes: (140.0140) Lasers and laser optics; (190.2620) Harmonic generation and mixing; ( 160.4330) Nonlinear optical materials. References and links 1. R. A. Andrews, H. Rabin, and C. L. Tang, “Coupled parametric downconversion and upconversion with simultaneous phase matching,” Phys. Rev. Lett. 25, 605-608 (1970). 2. J. M. Yarborough and E. O. Ammann, “Simultaneous Optical Parametric Oscillation, Second Harmonic Generation, and Difference Frequency Generation,” Appl. Phys. Lett. 18, 145-147 (1971). 3. V. Petrov and F. Noack, “Frequency upconversion of tunable femtosecond pulses by parametric amplification and sum-frequency generation in a single nonlinear crystal,” Opt. Lett. 20, 2171-2173 (1995). 4. T. Kartaloğlu, K. G. Köprülü, and O. Aytür, “Phase-matched self-doubling optical parametric oscillator,” Opt. Lett. 22, 280-282 (1997). 5. K. G. Köprülü, T. Kartaloğlu, Y. Dikmelik, and O. Aytür, “Single-crystal sum-frequency-generating optical parametric oscillator,” J. Opt. Soc. Am. B 16, 1546-1552 (1999). 6. E. C. Cheung, K. Koch, and G. T. Moore, “Frequency upconversion by phase-matched sum-frequency generation in an optical parametric oscillator,” Opt. Lett. 19, 1967-1969 (1994). 7. R. J. Ellingson and C. L. Tang, “High-power, high-repetition-rate femtosecond pulses tunable in the visible,” Opt. Lett. 18, 438-440 (1993). 8. C. McGowan, D. T. Reid, Z. E. Penman, M. Ebrahimzadeh, W. Sibbett, and D. H. Jundt, “Femtosecond optical parametric oscillator based on periodically poled lithium niobate,” J. Opt. Soc. Am. B 15, 694-701 (1998). 9. X. P. Zhang, J. Hebling, J. Kuhl, W. W. Rühle, and H. Giessen, “Efficient intracavity generation of visible pulses in a femtosecond near-infrared optical parametric oscillator,” Opt. Lett. 26, 2005-2007 (2001). 10. W. R. Bosenberg, J. I. Alexander, L. E. Myers, and R. W. Wallace, “2.5-W, continuous-wave, 629-nm solid-state laser source,” Opt. Lett. 23, 207-209 (1998). 11. K. F. Kashi, A. Arie, P. Urenski, and G. Rosenman, “Multiple nonlinear optical interactions with arbitrary wave vector differences,” Phys. Rev. Lett. 88, 023903 (2002). 12. Z. D. Gao, S.Y. Tu, S.N. Zhu, and A. H. Kung,“Monolithic red-green-blue laser light source based on cascaded wavelength conversion in periodically-poled stoichiometric lithium tantalate,” Appl. Phys. Lett. 89, 181101 (2006). 13. T. Kartaloğlu, Z. G. Figen, and O. Aytür, “Simultaneous phase matching of optical parametric oscillation and second harmonic generation in aperiodically poled lithium niobate,” J. Opt. Soc. Am. B 20, 343-350 (2003) #105197 $15.00 USD Received 10 Dec 2008; revised 10 Jan 2009; accepted 29 Jan 2009; published 3 Mar 2009 (C) 2009 OSA 16 March 2009 / Vol. 17, No. 6 / OPTICS EXPRESS 4289 14. S. N. Zhu, Y. Y. Zhu, N. B. Ming, “Quasi–Phase-Matched Third-Harmonic Generation in a Quasi-Periodic Optical Superlattice,” Science 278, 843-846 (1997).. 15. H. X. Li, Y. X. Fan, P. Xu, S. N. Zhu, P. Lu, Z. D. Gao, H. T. Wang. Y. Y. Zhu, N. B. Ming, and J. L. He, “530-mW quasi-white-light generation using all-solid-state laser technique,” J. Appl. Phys. 96, 7756-7758 (2004). 16. V. Berger, “Nonlinear Photonic Crystals,” Phys. Rev. Lett. 81, 4136-4139 (1998). 17. N. G. R. Broderick, G. W. Ross, H. L. Offerhaus, D. J. Richardson, and D. C. Hanna, “HeXLN: a 2-dimensional nonlinear periodic crystal,” Phys. Rev. Lett. 84, 4345-4348 (2000). 18. N. Fujioka, S. Ashihara, H. Ono, T. Shimura, and K. Kuroda, “Cascaded third-harmonic generation of ultrashort optical pulses in two-dimensional quasi-phase-matching gratings,” J. Opt. Soc. Am. B 24, 2394-2405 (2007). 19. S. N. Zhu, Y. Y. Zhu, Z. Y. Zhang, H. Shu, H. F. Wang, J. F. Hong, C. Z. Ge, and N. B. Ming, “LiTaO3 crystal periodically poled by applying an external pulsed field,” J. Appl. Phys. 77, 5481–5483 (1995). 20. P. Xu, Z. D. Xie, H. Y. Leng, J. S. Zhao, J. F. Wang, X. Q. Yu, Y. Q. Qin, and S. N. Zhu, “Frequency self-doubling optical parametric amplification: noncollinear red–green–blue light source generation based on a hexagonally poled lithium tantalate,” Opt. Lett. 33, 2791-2793 (2008).

In this paper we report the experimental realization of self-doubling optical parametric oscillation (OPO) based on a hexagonally poled lithium tantalate crystal (HexPLT) in which the optical parametric down conversion (PDC) and the second harmonic generation (SHG) are accomplished simultaneously.The high-Q cavity is designed to approach a high gain OPO and a high efficiency noncollinear intracavity SHG.This experiment results indicate another way to extend wavelength such as to obtain the blue laser with very short wavelength based on a single HexPLT crystal.

Experimental setup
The HexPLT sample in the experiment is fabricated by electric field poling technique [19].Figure 1(a) shows its domain structure after slightly etched in acids.The near circularly inverted domains (−χ (2) with radius r) distribute hexagonally in a +χ (2) background with structure parameter a=9.05 μm.We define the reversal parameter r/a=~28% and the corresponding duty cycle is the RLV of the HexPLT with lattice parameter a, and the subscripts m and n are integers, representing the order of RLV. 1 k , 2 k and 3 k represent the interacted three waves.
The experimental setups of the involved several optical tests are rather simple.The pump is incident along x ˆ-axis by a focus lens and the polarization direction of the pump is along the z ˆ-axis of the HexPLT.After the HexPLT we use optical filters to eliminate the pump beam.

Experimental results and discussion
We first examine the detuning characters of QPM-PDC and QPM-SHG processes in this HexPLT.For the PDC process, we apply the pump beam of 10 Hz 532 nm laser with the pulse width of 3.5 ns and linewidth of 0.15 nm.We use f= 150mm lens to focus this z ˆ-polarized pump beam crystal getting the beam waist around 120 μm inside the HexPLT.When the pump beam incident along the direction of . The signal and idler wavelengths vary with the temperature.The experimental data is shown in Fig. 2(a) which consists well with the theoretical calculation.A wide spectrum from 860 nm to 1380 nm is covered when the temperature varying from 20 o C to 200 o C.
For the QPM-SHG process, we employ a tunable laser source (Surelite -10, Continuum) and the wavelength lies within 260 nm-2600 nm.The pump beam is still incident along the direction of 1 , 0 G ( x ˆ-axis) and the HexPLT temperature is set at room temperature (20 o C), multiple noncollinear QPM-SHG processes happen and the momentum conservation is ensured by . The fundamental wavelength required for each SHG and the harmonic beam's output angles are listed in Fig. 2(b).This is consistent well with the theoretical calculation.The wavelength of the second harmonic beam lying between 400 nm-700 nm covers quasi-continuously the most of the visible spectrum.The corresponding fundamental wave is in range of 800 nm ~ 1400 nm.In Fig. 2  G when the duty cycle f=~28%.The temperature for this simultaneous QPM-PDC and QPM-SHG is 178.8 o C.And the wavelengths for signal, idler and idler harmonics are at 873 nm, 1362 nm and 681 nm respectively.The signal and idler are generated collinear with the pump beam while the idler harmonics are of mirror symmetry about the pump beam with external angle of 5 .5 ± o .We put the HexPLT into a flat-flat cavity as shown in the inset in Fig. 3. p ω , s ω , i ω and ih ω represent the pump, the signal, the idler and the idler harmonics, respectively.The cavity mirrors M 1 and M 2 have high-reflection coating for the idler 1362 nm (R=99.8%)and high-transmittance coating for the signal 873 nm (T=98.3%)and the idler harmonics 681 nm (T=92.2%).The crystal end faces have antireflection coating for the idler 1362 nm and the idler's harmonics 681 nm.The residual reflectance is 0.33% and 0.17%, respectively.The pump laser is 10k-Hz 532 nm light which has the time duration of 12.2 ns and the linewidth of 0.15 nm (DS20H-532, Princeton Industries).We use the f=300 mm lens to focus the pump beam into the HexPLT getting the beam waist to be 240 μm.The cavity's length is 60 mm around.In Fig. 3 the output power of the signal and the idler harmonics versus the pump power are given.The signal power obeys the linear relation with the pump power while the idler harmonics follows the quadratic line.The  oscillation threshold is 565 mW corresponding to a peak intensity of 28.3 MW/cm 2 .When pump power reaching 2.37 W, we get the signal 1.04 W and idler harmonics 380 mW after taking all the linear loss mainly by mirror's reflection into account.The efficiency of pump to signal is 44.1% and the corresponding photon number conversion efficiency reaching 72%.The slope efficiency reaches 60%.The conversion efficiency from the idler to its harmonics is about 56.9%.The final conversion efficiency from the pump to the harmonics is around 24%, which is high enough even compared with the femto-second pump case [13].Both the OPO and SHG work in a high-gain regime.
It is worth noted that here this cascaded harmonic generation contains an effective second-order nonlinearity of 2 d eff =0.47d 33 because 0 , 1 G and its mirror symmetry can participate into SHG simultaneously.When the duty cycle f=28%, the effective nonlinearity d eff =0.32d 33 which is close to its maximum.This makes the intracavity SHG very efficient.
Here the HexPLT is proved to be a compact and efficient medium for self-doubling OPO.Furthermore, without an isolator the backward SHG using reciprocal vector 0 , 1 − G and its mirror symmetry in this linear flat-flat cavity occurs at the same time.So in this experiment we observed not only a pair of red spots along the pump propagating direction but also a pair of them along the opposite direction.In the above experiment the power of the idler harmonics only includes the forward two red spots while the backward two spots which have equivalent power are not reckoned in.Additional isolator for the idler or a ring cavity can be used to make SHG flowing unidirectionally.
In Fig. 4 we show the experimental study on the temperature detuning character on the frequency self-doubling OPO.The maximum SHG occurs when temperature set at 178.8 o C and the temperature bandwidth (FWHM) is 2 o C.There is a power dip around 178.8 o C for the signal when the cascaded SHG happens efficiently.This is due to the fact that the effective SHG actually means a loss of the idler and this will make the OPO work less efficient.
Our previous work on the phase-matching condition of this HexPLT gives that there is still a variety of possible SHG cascaded PDC processes [20].For example, when temperature set at around 189 o C, the signal can be doubled through should still possess a high conversion efficiency by proper coating of the cavity mirrors.There should still be other possible self-doubling OPO processes accomplished by other RLVs although these may have lower conversion efficiency.It is very useful to supply a quasi-continuous output of signal or idler doubling in addition to a tunable output from such a compact optical cavity.This could find new applications in medical machine and meet other multi-wave output demands.

Conclusion
In conclusion, we examined the frequency self-doubling OPO based on a single hexagonally poled lithium tantalate.Without any complicated mathematical design, this simple single period structure makes this experiment is feasible to be realized.Both the single resonant OPO and intracavity SHG work at a high-gain regime.This cascaded noncollinear SHG is especially useful for extending wavelength especially approaching the blue laser and even the deep blue.Other possible cascaded SHG processes inside this HexPLT indicate it can serve as a new multi-wave generator.
. The dimension of this sample is 15 mm(x) × 5 mm(y) × 0.5 mm(z).Each reciprocal lattice vector (RLV) has other equivalent 5 ones as shown in Fig.1(b) due to the sixfold degeneration of this structure.The general phase-matching condition for nonlinear interaction in the QPM material is

1 , 0 G
( x ˆ-axis) and setting the crystal temperature during 20 o C ~ 200 o C, a QPM collinear parametric down conversion process happens.The momentum conservation is ensured by

Fig. 3 .
Fig. 3.The measured signal power versus the pump power.The inset is the schematic optical alignment of this OPO cavity.

Fig. 4 .
Fig. 4. The signal power and the idler harmonics power versus temperature.