High efficiency second-harmonic generation in KNbO3 crystals

Second-harmonic generation in KNbO₃ crystals has been studied with CW and pulsed Al_xGa_1?xAs and dye lasers. High conversion efficiencies were obtained by noncritical Type I phase-matching at room temperature for the wavelength λ_PM = (860±2) nm using the nonlinear optical coefficient d₃₂ = 21 pm/V. The measured peak power was P_2ω=13.6 μW for a fundamental power P_ω=21 mW with a crystal of l = 8.97 mm in length. The phase-matching wavelength in the orthorhombic phase of KNbO₃ can be temperature tuned between 838 nm (for T = -36°C) and 950 nm (for T = +180°C).     

Type I quasi-phase-matched blue second harmonic generation with different polarizations in periodically poled LiNbO3

First-order type I quasi-phase-matched (QPM) blue second-harmonic generation was demonstrated in periodically poled LiNbO₃ with period of 14.5 μm using d₃₁. 52 μJ of harmonic blue light at 0.473 μm was generated pumped by 114 μJ 35 ps pulse laser at 0.946 μm at 150 °C with a conversion efficiency of 45.6%. The average conversion efficiencies of 41.3% and 19% were also obtained at 150 °C, respectively, in the conventional first- and third-order QPM blue second-harmonic generation at 0.473 μm. The temperature acceptance bandwidths of 20 mm length periodically poled LiNbO₃ with first-order grating periods of 14.5 and 4.5 μm are 2.0 and 0.9 °C, respectively. The larger acceptance bandwidths and grating period for than those for enhance the frequency conversion efficiency, which shows the polarization dependence of quasi-phase matching.     

Thermally induced dephasing of high power second harmonic generation in MgO:LiNbO3 waveguides

High power second harmonic generation(SHG) in MgO-doped LiNbO3 waveguides is investigated using a three-dimensional(3D) coupled thermo-optical model.Simulations performed for a 1111.6-nm fundamental laser show the in?uence of the absorptions and the thermally induced dephasing on the conversion efficiencies of the different waveguides.The onset of the thermally induced dephasing effect for each waveguide is also indicated.As a result of high light intensity in the waveguide,nonlinear absorptions are identified as the possible main factors in efficiency losses in specific cases.     

Second harmonic generation in inhomogeneous MgO:LiNbO3 waveguides

A fibre laser at 1111.6 nm is frequency doubled by two inhomogeneous MgO:LiNbO3 waveguides and the output powers of 85 mW and 49 mW at 555.8 nm have been generated with the conversion efficiencies of 47% and 27% respectively. By analysing the second harmonic generation temperature tuning curves, we investigate the influence of the optical inhomogeneities upon the conversion efficiency. The final result shows that the efficiency difference is mainly affected by the optical inhomogeneities in our case.     

Blue light generation in single-pass frequency doubling of femtosecond pulses in KNbO3

We study the generation of blue light in single-pass frequency doubling of femtosecond pulses in a thick KNbO₃ crystal. Depending on the input IR power, the maximum amount of generated blue light is a function of the position of the nonlinear crystal relative to the focusing lens. At certain focusing distance, the efficiency of blue light generation is substantially degraded. The issue of beam quality of the generated blue light is discussed.     

Thermally induced second harmonic generation in PbO-B2O3 glasses

Second harmonic generation (SHG) was observed in PbO-B₂O₃ glasses after heat treatment at elevated temperature followed by rapid quenching in water at room temperature (about 25 °C). According to the results of thermally stimulated depolarization current (TSDC) measurement and XRD patterns, no depletion layer or crystals was found in the rapidly quenched glass samples. The thermally induced nonlinear layer is located in the regions extended from the surface and 8 μm into the bulk. The possible mechanism responsible for the SHG is that the bond deformation of glass structure induced by stress gradient breaks the inversion symmetry of glass. The influencing factors of the SH intensity are also discussed.     

CW blue light generation at 429 nm by utilizing second harmonic process with KNbO_3

Continuous-wave blue light at 429 nm from a second harmonic process has been investigated at room temperature with Potassium Niobate (KNbO3) crystal. Optimum parameters for nonlinear conversion in our designing single-resonant, external ring cavity pumped by the Ti:sapphire laser are established. 39% of maximum overall conversion efficiency is obtained and factors that limit the conversion efficiency is discussed.     

Annealing effect on planar waveguides in LiNbO3 produced by oxygen ion implantation

We reported on planar waveguides in stoichiometric lithium niobate fabricated by 4.5 MeV oxygen ion implantation with a dose of 6 × 10¹⁴ ions/cm² at room temperature. After ion implantation, these samples were annealed at 240 °C, 260 °C, and 300 °C for 30 min. We investigated annealing effect on the guiding modes and near-field images in the waveguides by prism-coupling method and end-face coupling method respectively. We found that for the extraordinary refractive index a positive alternation occurred in the near-surface region while a negative alternation happened at the end of ion track. Moreover, we measured the transmission spectra for the pure sample and implanted samples before and after annealed at different temperatures, and we observed an absorption peak at ∼480 nm (2.6 eV) in all of these SLN samples.     

On the efficiency of the second harmonic generation in optical waveguides: TM case

In this paper we analyse the second harmonic generation of TM polarisation in a planar waveguide with a non-linear anisotropic substrate using the coupled mode theory under the non-depleted pump approximation. It is an extention of our previous paper in which we treated TE polarisation. Moreover, both guided waves and erenkov radiation are now taken into account simultaneously. It is shown both analytically and numerically that the erenkov efficiency peak follows the L ^3/2 dependence on the interaction length L also for TM polarisation. In an anisotropic KTP/Si₃N₄/SiO₂ waveguide, higher maximum attainable second harmonic generation (SHG) efficiency was calculated for the TM polarisation than for the optimum TE case.     

Analysis of ion implanted waveguides formed on Nd:YVO4 crystals

This work reports, for the first time to our knowledge, the formation of planar waveguides in Nd:YVO₄ by either carbon or proton implantation. The waveguides are characterised using the dark mode method and by spectroscopic studies, indicating that there is a reduction of the refractive index at the end of the ion track. Particularly, carbon implantation generates a considerable variation in the guiding region. It is shown that the waveguiding structure preserves the spectroscopic properties of the Nd³⁺ ions, thus maintaining the crystal quality.     

The ferroelectric mode in orthorhombic KNbO3

Both Raman scattering and infrared reflectivity measurements have been performed to determine the complete optic mode spectrum of orthorhombic KNbO₃ and characterize the ferroelectric modes. The low frequency, k ～ 0 mode is quasitransverse and possesses a spacial anisotropy similar to the TA branch at finite k. Of the original cubic ferroelectric mode, the component along the remaining soft direction is found at 56 cm^-1 and not overdamped (Γ/ω = 1), while the other two components have stiffened equally so that the mode is essentially isotropic in the orthogonal polarizations.     

Phase-matched second-harmonic generation at 440 nm in a LiNbO3 -Gd3Ga5O12 waveguide with a tunable stimulated Raman source

Using a tunable stimulated Raman source, we have observed second-harmonic generation in the blue in a sputtered LiNbO₃ film deposited on Gd₃Ga₅O₁₂. With the choice of Gd₃Ga₅O₁₂ as a substrate, it becomes possible to both phase-match a large range of fundamental wavelengths as well as have an excellent epitaxial quality surface. The SHG efficiency for the TM^ω₀ → TM^2ω₂ phase matched conversion process is estimated to be ～ 10^-4.     

Raman study of phase transitions in KNbO3

The influence of grain size on the phase transitions of ferroelectric KNbO₃ was studied by micro Raman spectroscopy. It was found that the three transitions observed are not sharp for small particles (∼50 μm), indicating that they do not behave like bulk particles. The transition temperatures depend on the size and all particles show hysteresis. From these experiments we have obtained some evidence that in small particles monodomains of the rhombohedral and orthorhombic phases coexist in a range of temperatures.     

Achromatic multibeam coupling in KNbO3: Rb

We examine, both theoretically and experimentally, photorefractive coupling in KNbO₃: Rb with beams at several wavelengths in a dual grating achromatic arrangement. It is shown that, at normal incidence, the coupling gain in the multi-wavelength achromatic configuration is nearly equal to the gain of two beam coupling with a monochromatic source of the same intensity. The dependence of the achromatic gain on the angle between the bisector, of the pump and signal beams, and the normal to the crystal surface is also studied.     

Optical anisotropy of LiNbO3 and KNbO3 in the interband transition region

The reflectivity of single domain crystals of LiNbO₃ and KNbO₃ has been measured using linearly polarized light up to 10 eV. The Kramers-Kronig analysis reveals a large anisotropy of the optical constants in the interband transition region. It is shown that the large electro-optic effects observed in these compounds are due to transitions in a relatively narrow spectral region above the band edge only.     

Broadband and efficient second harmonic generation in LiNbO_3-LiTaO_3 composite ridge waveguides at telecom-band

Broadband nonlinear frequency conversions of optical waves are widely employed in multiple areas of optics and photonics. However, the broadening of conversion bandwidth is often at a cost of reduction in efficiency, which may induce a limitation on practical applications. Here we theoretically propose a novel design of LiNbO₃ ridge waveguides on LiTaO₃ substrates which can be used for efficient and broadband second harmonic generation. Through group velocity engineering of the ridge waveguides, acceptance bandwidth over 20 nm with a high conversion efficiency of > 25%W^-1·cm^-2 is achieved at telecom-band.     

Persistence of tetragonal raman lines in cubic KNbO3

The temperature dependence of the vibrationnal modes in KNbO₃ around the cubic-tetragonal phase transition is studied by Raman scattering measurements. An unexpected intense spectrum is observed above the transition. In addition to broad bands probably due to second order process, it consists of forbidden lines which are characteristic of the tetragonal phase. In particular the presence of the hard component of the soft ferroelectric cubic mode until 30° above T_c can be attributed to the existence of a precursor order near the transition.     

Electron paramagnetic resonance of Fe3+ in orthorhombic KNbO3

EPR spectra of KNbO₃:Fe single crystals are obtained in the - 10–110°C temperature range. The angular dependence of resonant lines is well reproduced by spin Hamiltonian parameters relevant to a Fe³⁺ impurity ion substituted to Nb ion in KNbO₃ crystal. The temperature behaviour of resonant lines is explained by a quadratic dependence of axial parameter D vs polarization P_s of the form D = βP²_s in the orthorhombic phase.     

Tunable wavelength conversion of picosecond pulses based on cascaded sum- and difference-frequency generation in quasi-phase-matched LiNbO3 waveguides

Tunable wavelength conversion for picosecond pulses is proposed and demonstrated exploiting cascaded sum- and difference-frequency generation in quasi-phase-matched LiNbO₃ waveguides. The influences of initial pulse widths and injected pulse powers on the conversion efficiency and converted pulse width are theoretically analyzed. Arbitrarily tunable wavelength conversion is performed for the signal pulse with the temporal width of 1.57 ps and repetition rate of 40 GHz. Approximately −18.9 dB conversion efficiency and 25 nm variable region of the input signal are achieved under the lower launched signal power. The results imply that simultaneous wavelength conversion and pulse compression can be potentially obtained by using the pulsed control wave or designing longer waveguides.     

High conversion efficiency of second harmonic generation in a short nonlinear photonic crystal with distributed Bragg reflector mirrors

Second harmonic generation (SHG) in a short nonlinear photonic bandgap (PBG) structure coated with distributed Bragg reflector (DBR) mirrors is theoretically investigated by means of an iteration approach that fully considers pump depletion. A total conversion efficiency of about 96% can be obtained in an optimized structure with a size scale of about 0.2 mm and at the modest pump intensity of about 0.133 MW/cm², which is four orders of magnitude more than that in a quasi-phase-matching (QPM) structure with the same sample length. Such a high conversion efficiency of SHG will greatly facilitate the success of short frequency conversion devices and all-optical integration in nanostructures.