Quasi-phase-matched second harmonic generation in a LiTaO3 waveguide

The authors report third-order quasi-matched (QPM) second harmonic generation (SHG) in a LiTaO₃ channel waveguide. A deep domain-inverted region is first fabricated by a proton-exchange and heat treatment technique. Then a uniform and low-pass channel waveguide is fabricated by pyrophosphoric acid proton exchange. Consequently, 12 mW of blue light is obtained at 424 nm wavelength with a conversion efficiency of 6%. The observed FWHM (full-width half maximum) temperature acceptance width for SHG power is 3.2°C and FWHM wavelength acceptance bandwidth for that is 0.2 nm. It is also shown that diffraction limited focusing of the generated blue light may be obtained     

Electrooptic coefficient measurements in LiTaO3 andLiNbO3 waveguides

The magnitudes of linear electrooptic coefficients r₁₃ and r₃₃ in Zn:LiTaO₃ repoled channel waveguides are reported. The measurements were made at 0.633-μm wavelength using a Fabry-Perot interferometer. The waveguides were produced by diffusion from the vapor phase at a temperature above the Curie temperature. For full recovery of the Pockels effect, an electric field of 200 V/cm is needed during repoling. The measured values of r₁₃ and r₃₃ at 32-MHz modulation frequency are 7.2 and 30.3 pm/V, respectively. The difference between unclamped and clamped coefficients is comparable to that from bulk crystals. Measurements were also made on Ti:LiNbO₃ waveguides that did not require repoling, and good agreement with bulk crystal values was obtained     

Noncritical quasi-phase-matched second harmonic generation in anannealed proton-exchanged LiNbO3 waveguide

We report the demonstration of dimensional noncritical phase matching, a phase-matched interaction length exceeding 10 mm, and an internal conversion efficiency of 204%/W for second harmonic generation of 976 nm radiation in a periodically poled, annealed proton-exchanged LiNbO₃ waveguide. Using models for the linear and nonlinear optical properties of annealed proton-exchanged LiNbO₃ waveguides and the observed ferroelectric domain grating, the phase-matching wavelength was predicted to within several nm and the conversion efficiency to within ≈20% of the measured values. Optimization of waveguide second harmonic generation devices is discussed     

Fabrication of LiTaO3 optical waveguides by Ni diffusionbelow the Curie temperature

Using Ni diffusion into LiTaO₃ below the Curie temperature for the fabrication of optical waveguides was presented for the first lime. Ordinary and extraordinary polarization waveguide modes were obtained. The index distribution profiles of both modes were measured by a prism coupler. Loss in planar waveguides at 0.6328 μm is 0.7±0.1 dB/cm for the ordinary mode and 1.3±0.2 dB/cm for the extraordinary mode. Moreover, a Mach-Zehnder interferometer was fabricated by this method for electrooptic characterization. The measured half-wave voltage is 5.5 V and the extinction ratio is greater than 29 dB     

Polarization-independent LiTaO3 guided-wave electroopticswitches

Polarization-independent LiTaO₃ optical switches that utilize the off-diagonal electrooptic coefficient r₅₁ are discussed. The waveguides are formed by Zn vapor diffusion. Interguide transfer efficiencies of 84 and 88% for TM and TE polarizations, respectively, have been obtained. Modulation efficiency of 99.8% for TM and 95% for TE have been demonstrated at nearly equal voltages by thermally tuning to 20°C. The relatively high voltage-length product (30-35 V-cm) attained in these initial devices at a 0.633-μm wavelength is attributed to a weak overlap between optical and electrical fields in the periodic electrode structure     

Photorefractive effects in proton exchanged LiTaO3optical waveguides

A circuit model is proposed to describe photorefractive effects in LiNbO₃/LiTaO₃ channel waveguides at any intensity level. Capacitive charge storage at the waveguide boundaries is assumed to be provided by trapping states associated with photoconductivity. A consequence of this model is that photoconductive transients are independent of optical intensity at low intensity levels. Photovoltaic and photoconductive effects in proton exchange LiTaO₃ channel waveguides were experimentally investigated. Dark conductivities of 2×10^-15 to 2 ×10^-14(Ω-cm)^-1 were extrapolated from photoconductivities up to 2×10^-13 (Ω-cm)^-1 for power levels of 0.1 to 3 mW. Large DC voltage dependent effects on the conductivity were observed. Straight channel waveguides were observed to be free of photovoltaic effects for output power levels below 35-75 mW     

Fabrication of LiNbO3 channel waveguides using water

A channel waveguide formed in LiNbO₃ was fabricated by using distilled water instead of acids for proton exchange and a self-aligned SiO₂ cap for diffusion. It was demonstrated that the water reaction is an important reaction to cause the index change in proton exchange     

Interferometric polarization-independent modulator in LiTaO3

Polarization-independent Mach-Zehnder modulators that utilize the off-diagonal r₅₁ electrooptic coefficient are produced in LiTaO₃. The waveguides are formed by Zn vapor diffusion. Extinction values of 85% at 0.633-μm wavelength are demonstrated for both transverse electric and transverse magnetic polarization using a single control voltage and thermal tuning. The relatively large voltage-length product values (VL⩾15 V-cm) attained in these devices are attributed to a weak overlap between optical and electrical field profiles. Electrode alignment and dielectric loading are critical factors for the optimization of device operation     

Second-harmonic generation efficiency in periodically poled LiNbO3 waveguides

Quasi-phase-matched second-harmonic generation efficiency in periodically poled LiNbO₃ using titanium indiffusion, has been theoretically investigated. This permits clarifying how the involved guided modes convert the triangle-shaped form of the ferroelectric polarization in the crystal, via the overlap integral between this form and these modes, into an effective periodic nonlinear coefficient. As a consequence, the periodic function that really permits the quasi-phase-matching operation is the modulation of this effective nonlinear coefficient, the shape of which can be far from a triangular shape, as shown using different cases of mode profiles     

Extended range of second harmonic generation in β-BaB2O4

Cooling a β-barium metaborate crystal to 89 K extends second harmonic generation to 203.1 nm and greatly increases it efficiency     

基于钽酸锂晶片的太赫兹热释电探测器

针对自由空间和波导传输太赫兹辐射功率兼容测试的需求,开展了光敏面直径为10 mm的多功能太赫兹热释电探测器的相关研究。通过有限元分析及热电耦合仿真设计,建立了敏感元件由100 μm 厚的钽酸锂（LiTaO₃）晶片和碳纳米管吸收层组成的太赫兹热释电探测器模型;采用优化的精确减薄抛光和剥离等关键工艺,重点攻克了采用大晶片多阵列方式制作LiTaO₃基太赫兹热释电探测器敏感元件的工艺难题,并完成了太赫兹热释电探测器的研制。在设定条件下,该探测器的响应度为371.8 V/W,噪声等效功率为0.34 nW/Hz^1/2。实验结果表明,设计并制作的太赫兹热释电探测器的集成度高、响应度良好、噪声等效功率低,能够有效解决大光斑太赫兹光束功率测试问题。     

Increased acceptance bandwidth for quasi-phasematched secondharmonic generation in LiNbO3 waveguides

The authors demonstrate waveguide second harmonic generation using a variable-spaced phase-reversed quasi-phasematched grating that has an acceptance bandwidth 15 times greater than a uniformly quasi-phasematched interaction of the same length     

Some optical properties of KTP, LiIO3, and LiNbO3

Measurements of the absorption coefficient for KTP, LiIO₃ , and LiNbO₃ are discussed. The variation of the refractive index with temperature has been measured for KTP and LiIO₃. It is necessary to know both the absorption coefficient β and the variation in the indexes of refraction with temperature change dn/dT to determine the average power limit of a nonlinear interaction. With the dn/dT information, it is also possible to estimate the temperature half width of any nonlinear interaction by calculating the variation of the phase-matching condition with temperature     

切伦科夫倍频用质子交换MgO∶LiNbO_3波导性能研究

本文研究了用质子交换(PE)法生长的MgO∶LiNbO_3波导的性能,分析了切伦科夫倍频条件并实现了由1.06μm到0.53μm的切伦科夫倍频转换(CSHG),转换效率接近1％。     

Blue-light generation by frequency doubling of a laser diode in aperiodically domain-inverted LiTaO3 waveguide

The authors report quasi-phase matched second-harmonic generation by frequency doubling of a laser diode in LiTaO₃ having a first-order periodically domain-inverted region and proton-exchanged channel waveguide. A deep domain-inverted region and a low-loss channel waveguide with strong confinement are formed by using proton-exchange and quick heat treatment techniques. Utilizing this structure, a high normalized conversion efficiency of 157%/W is obtained with a Ti:Al₂O₃ laser. Using a temperature-controlled laser diode and AR coating on the input and output facet of the waveguide, the laser diode maintains single-mode oscillation without any mode hopping. Consequently, 1.1 mW of blue light is obtained at a wavelength of 436.5 nm     

Electrooptic lens stacks on LiTaO3 by domain inversion

The design and analysis of one-dimensional (1-D) electrooptic lens stacks is presented. Experimental results of a seven-element stack fabricated on Z-cut LiTaO₃ substrates using domain inversion support the analysis. The optical focusing power of these bulk stacks is shown to be voltage-controlled     

Electro-optic effect in proton exchanged LiNbO3 andLiTaO3 waveguides

Electro-optic properties of proton-exchanged (PE) waveguide layers in LiTaO₃ and LiNbO₃ are studied and related to their optical characteristics. The proton-exchange process induces a degradation of the electro-optic activity in both types of waveguides, PE LiNbO₃ and PE LiTaO₃. The measured electro-optic effect is close to the detection sensitivity even when the exchange regime is performed at low temperatures for short periods of time. The PE samples have been annealed (APE waveguides) and the changes of their r₃₃ electro-optic coefficient has been followed at successively higher temperatures and periods of time. Subjected to annealing at temperatures between 265-420°C, the LiTaO₃ layers show a partially recovered r₃₃ coefficient, the recovering being different for quick and slow cooling of the samples. In thin APE LiNbO₃ waveguiding layers a restoration of r₃₃ up to 75% of the bulk value is observed due to the annealing at temperatures between 200-340°C     

Characterization of vapor diffused Zn:LiTaO3 opticalwaveguides

Diffusion and optical properties of planar waveguides produced in LiTaO₃ by a recently developed technique that utilizes Zn diffusion from vapor phase are presented. The waveguides are obtained by 6-h diffusion at temperatures as low as 800°C. The results indicate that Zn is a fast diffusant in LiTaO₃ and has activation energies that are slightly less than those for Ti diffusion into LiTaO ₃. The diffusion temperatures for Zn are much lower than the 1150-1200°C require for Ti metal indiffusion. The diffusion coefficient of Zn at 800°C, is comparable to that of Ti at 1200°C. The resulting waveguides support both ordinary and extraordinary modes of polarization. The low temperature diffusion slows out diffusion and has been shown to be advantageous for making low-loss optical waveguides in LiTaO₃     

Enhancement of domain inversion in LiTaO3 using heattreatment with metal-oxide mask

The domain inversion depth is enhanced in proton-exchanged LiTaO ₃ using heat treatment with a metal-oxide mask. Since this method can lower the proton exchange rate to get the same domain inversion depth, the degradation of the crystal quality in the domain-inverted layer is reduced     

Characterization of LiTaO3 thin films fabricated by sol-gel technique

Lithium tantalite (LiTaO₃) thin films have been fabricated by sol-gel technique and crystallized by RTA process. The effect of heating temperature on the structural properties of LiTaO₃ is investigated. The thin films are characterized by means of X-ray diffraction, Raman spectroscopy and thermal analysis (DCA/DTA). After the optimization of the growth parameters of LiTaO₃ prepared by sol-gel processing, the pyroelectric property of LiTaO₃ thin films deposited on Si (1 0 0) substrates have been also investigated.