The ridge waveguide fabrication with periodically poled MgO-doped lithium niobate for green laser

Ridge waveguides were fabricated using an external field, a precision lapping machine and neutron loop discharge (NLD) in magnesium-oxide-doped lithium niobate. The measured quasi-phase-matching (QPM) wavelength of the second-harmonic generation (SHG) in a 30 mm long periodically poled magnesium-doped lithium niobate (PPMgLN) ridge waveguide which has a domain period of 6.8 μm is about 532 nm. A fabricated periodically poled magnesium-doped lithium niobate ridge waveguide was duty cycle of 51.9 ± 2.83% and demonstrated second-harmonic generation. By using this periodically poled magnesium-doped lithium niobate ridge waveguide, highly effective, low-cost optical devices with high power or short wavelength can be achieved.     

Modelling the formation of optical waveguides produced in LiNbO3 by laser induced thermal diffusion of lithium ions

Analytical and numerical modelling of the fabrication of optical waveguides in lithium niobate (LiNbO₃) through direct writing with a continuous wave ultraviolet (UV) laser has been performed. It is proposed that the UV illumination induces heating of the surface and subsequent thermal diffusion of lithium which alters the refractive index in accordance with the lithium concentration dependence of the Sellmeier equations. PACS 61.80.Ba; 66.30.Dn; 77.84.Dy     

Odd waveguide mode quasi-phase matching with angled and staggered gratings

Angled and staggered gratings are used for quasi-phase matching of antisymmetric TM(10) modes in periodically poled lithium niobate waveguides with high efficiency. Control of the symmetry of the nonlinear coefficient d adds a new degree of freedom in the choice of which waveguide modes will interact in a quasi-phase-matched device.     

Laser-assisted fabrication of new slab-coupled lithium niobate optical waveguide

Recently a new type of lithium niobate waveguide was suggested for potential nonlinear optic applications. The waveguide consists of a uniform large core and a leaky coupled slab for realizing a lateral optical confinement to support the fundamental spatial mode propagation. Inside the waveguide, the slab layer is required to have a refractive index slightly lower than that of the core, but higher than that of the substrate. Lithium niobate doped with magnesium oxides shows an increased refractive index that is dependent on the dopant's concentration. Therefore, in order to fabricate such waveguides, the pulsed laser deposition approach was used to study the growth of such composition-modified lithium niobate as the slab layer. The as-grown films were characterized on its expitaxy, structure, and optical performance, via X-ray diffraction analysis, optical guiding experiment, etc.     

Laser-induced-forward-transfer: a rapid prototyping tool for fabrication of photonic devices

We propose a rapid prototyping method for the fabrication of optical waveguides based on the direct laser-printing method of ultrafast Laser-Induced Forward Transfer (LIFT) followed by further processing. The method was implemented for the fabrication of titanium in-diffused lithium niobate channel waveguides and X-couplers by LIFT-depositing titanium metal followed by diffusion. Propagation loss as low as 0.8 dB/cm was measured in preliminary experiments.     

Frequency dependence of acoustooptic interaction efficiency in anisotropic graded-index waveguide

An analysis of the dependence of the Bragg acoustooptical interaction efficiency of TE modes in a graded-index optical waveguide on the acoustic surface wave (ASW) frequency has been made. It is shown that two waveguides having different optical parameters but the same number of modes are described practically by the same frequency dependence. This conclusion is confirmed by the calculation for two four-mode waveguides on the Y-cut lithium niobate. The frequency dependence for various processes in the range 50÷550 MHz has been experimentally measured.     

Oscillateur paramétrique optique intégré : revue des réalisations sur niobate de lithium polarisé périodiquement

To realize an IOPO, one need a good nonlinear substrate in which it is possible to realize low-loss waveguides and fulfill the phase-matching conditions. In this paper, I will present the problems researchers have faced, the techniques they have used and the results they have obtained by using either proton exchange or titanium indiffusion to create waveguides in periodically poled lithium niobate.     

Photorefractive LiNbO3 waveguides fabricated by combining He-implantation and copper exchange

We report on the fabrication and properties of He-implanted waveguides in lithium niobate, which are additionally doped with copper by using an ion-exchange process. We show that the photorefractive sensitivity in the waveguides is increased by a factor of 3000. Received: 23 November 1998 / Revised version: 15 February 1999 / Published online: 12 April 1999     

Dependences of the refractive indices on the proton concentration in H:LiNbO3 waveguides

Proton-exchanged optical waveguides in lithium niobate crystals exhibit a rich variety of structures and phases. It is established that seven H_xLi_1−x NbO₃ crystalline phases with a lithium niobate structure may form under various conditions of proton exchange and post-exchange annealing. A method is proposed to determine the proton concentration in the various phases identified. Relationships are established between the structural parameters, the proton concentration, and the ordinary and extraordinary refractive indices of various H_xLi_1−x NbO₃ crystalline phases. The results can explain various optical phenomena observed in proton-exchanged waveguides and permit prediction of the characteristics of light-guide structures. Zh. Tekh. Fiz. 69, 47–57 (March 1999)     

Nonlinear optical waveguides generated in lithium niobate by swift-ion irradiation at ultralow fluences

A novel method to produce optical waveguides is demonstrated for lithium niobate (LiNbO(3)). It is based on electronic excitation damage by swift ions, i.e., with energies at approximately 1 MeV/amu or above. The new technique uses high-energy medium-mass ions, such as Cl, with electronic stopping powers above the threshold value for amorphization (5-6 keV/nm), reaching the maximum value a few micrometers inside the crystal. At the ultralow fluence regime (10(12)-10(13) cm(-2)) an effective nanostructured medium is obtained that behaves as an optical waveguide where light propagates transversally to the amorphous nanotracks created by every single impact. The method implies a reduction of 4 orders of magnitude with respect to He implantation. The optical waveguides present reasonable losses (~10 dB/cm) and significant second-harmonic generation (SHG) and electro-optic (EO) responses (>50% bulk) for the lowest fluences.     

Fabrication of ridge waveguides in zinc-substituted lithium niobate by means of ion-beam enhanced etching

We present results on the fabrication and characterization of ridge waveguides in zinc-substituted lithium niobate. High-quality waveguides were fabricated by a combination of liquid-phase epitaxy and multiple applications of ion-beam enhanced etching. The two major demands on ridge waveguides, a very low side-wall roughness and a rectangle shape with side-wall angles close to 90 degrees , were realized simultaneously by using this technique. Single-mode waveguiding at a wavelength of 1064 nm was demonstrated with attenuation values of 0.9 dB/cm.     

A novel Y-branch waveguide for power dividing and mode splitting

A novel Y-branch waveguide for variable-ratio power dividing and transverse electrical-transverse magnetic (TE-TM) mode splitting depending on the applied voltage is presented. The Y-branch waveguide is formed by two closely coupled waveguides tabricated by nickel indiffusion (NI) and by magnesium-oxide induced lithium outdiffusion (MILO) in a y-cut lithium niobate (LiNbO₃) substrate. The TE component of the randomly polarized light is tuned between these two waveguides, such that the device can be either a power divider or a mode splitter depending on the applied voltage. The measured TE mode extinction ratio is about 20 dB.     

Estimation of the performance of a 3-dB Y-junction optical coupler with a channel profile of proton-exchanged lithium niobate

A 3-dB optical coupler (power splitter) based on a Y-junction waveguide with a channel profile of proton-exchanged lithium niobate has been modeled. Finite difference beam propagation method has been used to investigate light wave propagation in axially varying waveguides. It is found that the structure splits the input power equally at the Y-junction with an efficiency of 99% and an average excess loss ?0.04 dB. The novelty of the coupling structure proposed is its capability to function as a wavelength-independent 3-dB coupler while maintaining very low-power imbalance for widespread optical communication wavelengths of 1.3 and 1.55 µm.     

Analysis and Determination of Refractive Index Profiles of O^2＋ Ion-Implanted LiNbO3 Planar Waveguide Using Etching and Ellipsometry Techniques

The refractive index profiles of 3 MeV O^2＋ ion-implanted planar waveguides in lithium niobate are reconstructed based on etching and ellipsometry techniques. SRIM2003 code is used to simulate the damage distribution in waveguide. It is demonstrated that the index profile of this kind of waveguide, extending to several micrometres in depth, can be determined by etching in combination with following ellipsometric measurements. A good agreement is found between the simulated damage distributions in waveguide and the index profiles based on experimental data, and the width of refractive index barrier is wider than the result of SRIM2003.     

Direct ultraviolet writing of channel waveguides in congruent lithium niobate single crystals

We report the fabrication of optical channel waveguides in congruent lithium niobate single crystals by direct writing with continuous-wave ultraviolet frequency-doubled Ar+ laser radiation (244 nm). The properties and performance of such waveguides are investigated, and first results are presented.     

Quasi-one-dimensional photonic lattices and superlattices in lithium niobate: Linear and nonlinear discrete diffraction of light

The effects of linear and nonlinear light propagation in quasi-one-dimensional systems of coupled optical waveguides (photonic lattices and superlattices) obtained by projection optical induction in photorefractive lithium niobate samples have been experimentally studied and numerically simulated.     

Theoretical analysis of electromagnetic field distribution and Cerenkov second harmonic generation conversion efficiency based on lithium niobate ion-implanted channel waveguide

Cerenkov type second harmonic generation (CSHG) is proposed in ion-implanted Z-cut lithium niobate (LiNbO₃) channel waveguides. We have obtained exact analytical solutions for the field distribution and the power of second-harmonic waves. Conditions for attaining higher conversion efficiency are discussed on the basis of numerical calculations.     

Interactions between one-dimensional quadratic solitons

The interaction between two one-dimensional quadratic solitons has been investigated experimentally in lithium niobate planar waveguides for both parallel- and crossing-launched solitons.     

Mixed data rate and format transmission (40-Gbit/s non-return-to-zero, 40-Gbit/s duobinary, and 10-Gbit/s non-return-to-zero) by mid-link spectral inversion

A polarization-diverse subsystem based on periodically poled lithium niobate waveguides is used as an optical phase conjugator for compensation for linear and nonlinear distortion. We show successful transmission formats of 13 x 40 Gbit/s non-return-to-zero mixed with 6 x 10 Gbit/s non-return-to-zero and 40-Gbit/s duobinary over 8 x 100 km of standard single-mode fiber. A single phase conjugator is used to conjugate all data formats, including the alternative duobinary format, simultaneously.     

Aging effect in proton-exchanged optical waveguides based on lithium niobate crystals

The optical characteristics of planar waveguides produced by proton exchange in lithium niobate crystals in different acids (with and without additives) are unstable with time (“aging” effect). The aging effects in LiNbO₃ optical waveguides are investigated at room and elevated (50, 60, or 70°C) temperatures. In all cases, variations in the refractive index of the waveguide layer with time, i.e., for two or three years at room temperature and for several weeks at elevated temperatures, are determined for waveguides prepared in different media. The dependences of the optical characteristics of waveguides on the nature of proton sources and the aging conditions are obtained experimentally.