O.
OSA/ CLEO 2011
CMR2.pdf
Singly-Resonant Optical Parametric Oscillator in AdhesiveFree-Bonded Periodically Inverted KTiOPO4 Plates: Achieving Oscillations at Dual Wavelengths Pu Zhao, Srinivasa Ragam, and Yujie J. Ding Department of Electrical and Computer Engineering, Lehigh University, Bethlehem, PA 18015 Ph. (610) 758-4582, fax (610) 758-6279,
[email protected]
Ioulia B. Zotova ArkLight, P.O. Box 2, Center Valley, PA 18034
Xiaodong Mu, Huai-Chuan Lee, Stephanie K. Meissner, and Helmuth Meissner Onyx Optics Inc., 6551 Sierra Lane, Dublin, CA 94568
Abstract: Based on adhesive-free-bonded periodically inverted KTiOPO4 plates, we implemented a singly-resonant optical parametric oscillator, which has superior advantages of compensating walk-off and oscillating dual-signal wavelengths based on QPM. ©2011 Optical Society of America OCIS codes: (190.4970) Parametric oscillators and amplifiers; (190.4360) Nonlinear optics, parametric processes.
Nonlinear optics, devices; (190.4410)
1. Introduction Optical parametric oscillator (OPO) is capable of generating signal and idler wavelengths for realizing variety of applications such as laser Doppler measurements, differential absorption Lidar for chemical sensing and detection, fluorescence bio-medical imaging, and terahertz generation. However, spatial walk-off in a nonlinear crystal not only increases threshold for oscillation and limits the conversion efficiency for parametric processes but also sacrifice beam quality. Recently, it was demonstrated that periodically orientated stacks can be used as a novel nonlinear structure to compensate spatial walk-off [1]. Indeed, by adhesive-free-bonding (AFB) 16 KTiOPO4 (KTP) plates, OPO was successfully realized at the output wavelengths of about 2 µm. Due to quasi-phase-matching (QPM), dual signal wavelengths were oscillating at around 2 µm [1]. Here, we report our new results on an OPO implemented based in periodically inverted KTP plates. We have achieved the oscillations at the dual-signal wavelengths of around 1 µm. The structure of the KTP plates studied here is quite different from that in Ref. [1]. First, each KTP plate was cut in such a way that θ = 90.0º and φ = 0º (i.e. x-cut). As a result, at these angles the OPO can be noncritical-phase-matched at the pump wavelength of around 530 nm. Second, all the crystal axes of the adjacent plates are opposite in order to periodically switch all elements of second-order nonlinear susceptibility tensor of KTP to realize QPM based on d24. In comparison, in Ref. [1], z axes for the adjacent KTP plates were bonded in a head-to-tail configuration (i.e. θ = 50.2º) to simultaneously achieve the walk-off compensation and QPM. Third, a singly resonant OPO is inherently more stable than a doubly resonant OPO implemented in Ref. [1]. Fourth, the thickness of each KTP plate, dictated by the frequency separation of the dual signals, is different from that in Ref. [1] (i.e. 2 mm). 2. Experiments, Results, and Discussions AFB KTP Plates z
z
z 1000-1100 nm HR 532 nm HT
532 nm pumping
z z
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z z
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e-wave o-wave
Polarizer
Dual-Wavelength Idlers 1000-1100 nm HR
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0 1020 1030 1040 1050 1060 1070 1080 1090 1100
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Fig. 1 (a) Experimental setup for singly resonant OPO; (b) spectra of signals and idlers at pump wavelength of 532 nm.
OSA/ CLEO 2011
CMR2.pdf
The experimental setup for the singly resonant OPO is shown in Fig. 1(a). The KTP sample was fabricated by Onyx Optics. It consists of 20 x-cut KTP plates, each of which has a thickness of 1.19±0.01 mm, with their z axes being periodically switched as shown by Fig. 1(a); they were adhesive-free-bonded together as a composite for realizing QPM at the dual signal and dual idler wavelengths. As a result, the QPM condition is given by Δkl = ±π, where l is the thickness of each KTP plate. Consequently, a pair of the signals having slightly different wavelengths satisfies the QPM condition above. In contrast, only a single signal wavelength would satisfy the birefringence-phasematching condition (i.e. Δkl = 0) between the new oscillating signal wavelengths if a single KTP crystal were used in the experiment. The two corresponding idler wavelengths satisfy the QPM condition. The composite has a clear aperture of 5.6 mm×10.5 mm and a total interaction length of 23.8 mm. Both the input and output facets are hightransmission (HT)-coated at the wavelengths of around 532 nm and 1000-1100 nm. To demonstrate the tunability of our OPO based on the KTP composite, we tuned the pump wavelength around 532 nm. Based on our experimental results, we have concluded that a commercially available and low-cost pulsed pump source at 532 nm is sufficient to achieve stable oscillations in the KTP composite. The configuration for the OPO cavity is shown by Fig. 1(a). Two flat mirrors with high reflection (HR) coating in the range of 1000-1100 nm are designed as the cavity mirrors. To ensure the oscillations only at the signal wavelengths, a cubic polarizer was placed inside the cavity. It allows the opolarized idler beams to go through (i.e. T 97%) to reflect almost all of the e-polarized signal beams (R > 99%). Since the polarization of each signal is perpendicular to that of the corresponding idler, we were able to just oscillate the dual signals inside the cavity. Almost all the powers of the dual-idler beams were transmitted through the polarizer. Fig. 2 illustrates the spectra of the dual signal and dual idlers generated by the singly resonant OPO at the pump wavelength of 532.12 nm. The dual signals appearing in the spectra were caused by their very weak transmissions through the polarizer. The wavelengths of the dual signals oscillating in the OPO cavity were measured to be 1034.8 nm and 1044.2 nm, whereas the wavelengths of the dual idlers were 1084.1 nm and 1093.9 nm, respectively. These measured wavelengths coincide with the calculated values, see Fig. 1(b). 8 (a)
(b)
1096
Idler Wavelength (nm)
Output Power (mW)
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1094 1092 1090 1088 1086 1084
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531.0
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Fig. 2. (a) Average output power of dual-idler beams is plotted vs. pump power at 532 nm. Solid line stands for linear fit. (b) Output wavelengths of dual-idler beams are plotted vs. pump wavelength. Solid lines correspond to theoretical results.
When a pump beam at 532 nm with repetition rate of 10 Hz and pulse width of 5 ns was loosely focused onto the KTP composite (a beam diameter of about 2 mm), we measured the total output power from the OPO vs. the pump power, see Fig. 2(a). By linearly fitting the data points in Fig. 2(a), we deduced the threshold power for the single resonant OPO to be 21 mW. Such a threshold power corresponds to the threshold intensity of 13.4 MW/cm2. At the pump power of 48 mW, the OPO output power reached 7 mW, corresponding to a conversion efficiency and slope efficiency of 14.6% and 25.3%, respectively. In Fig. 2(b), we plotted the output wavelengths of the dual-idler beams as a function of the pump wavelength. According to Fig. 2(b), our measured idler wavelengths agree quite well with the theoretical values. Such a single-resonant OPO can be used to realize a number of novel applications. For example, it can be used to investigate the feasibility for five-photon entanglement among pump and nearlydegenerate dual signals and dual-idlers, which has not been demonstrated, instead of tripartite [2]. The dual-idler beams can be used as the mixing beams for THz generation based on difference-frequency generation [3]. 3. References [1] X. Mu, H. Meissner, and H.-C. Lee, “Optical parametric oscillations of 2 μm in multiple-layer bonded walk-off compensated KTP stacks,” Opt. Lett. 35, 387-389 (2010). [2] A. S. Villar, M. Martinelli, C. Fabre, and Nussenzveig, “Direct production of tripartite pump-signal-idler entanglement in the above-threshold optical parametric oscillator,” Phys. Rev. Lett. 97, 140504/1-4 (2006). [3] Y. Jiang, Y. J. Ding, and I. B. Zotova, “Power scaling of widely-tunable monochromatic terahertz radiation by stacking high-resistivity GaP plates,” Appl. Phys. Lett. 96, 031101/1-3 (2010).
