Pranabendu Ganguly

Modal profiles in Ti:LiNbO3 two-waveguide and three-waveguide couplers by effective-index-based matrix method

Abstract Graded-index optical channel waveguides like Ti-indiffused LiNbO 3 waveguides have been modelled by effective refractive index profile along the lateral direction, discretising the profile and then using the matrix method. This effective-index-based matrix method, which was earlier used to compute the modal propagation constants of Ti:LiNbO 3 channel waveguides and directional couplers [Fib. Integrat. Opt. 17 (1998) 139] and the bending loss [J. Opt. Commun. 155 (1998) 125], has been established in this paper as a computationally fast, analytical method to compute electric field and modal intensity profiles in single and coupled waveguides. In order to handle coupled waveguides consisting of three waveguides, the concept of overlap integral has been used to analyse power coupling to the middle waveguide for applications in power splitting. Some of the computed intensity profiles have been compared with experimental data obtained by the authors. Although computation has been performed only for TE and TM polarisations, the method has the inherent capability of handling arbitrary polarisation.

Modelling of titanium indiffused lithium niobate channel waveguide bends: a matrix approach

Abstract An analytical model for computation of bending loss of Ti:LiNbO 3 channel waveguide bends has been presented. The analytical steps involved are as follows. The 2D refractive index profile over the cross-section of Ti:LiNbO 3 waveguide is first transformed to 1D effective-index profile along the lateral direction. A conformal mapping technique is then used to transform the effective-index profile of the waveguide bend to that of an equivalent straight waveguide. A stair-case type step-index profile is generated from the equivalent effective-index profile in lateral direction by partitioning the latter into a large number of thin sections of varying refractive indices. The overall transfer matrix of the step-index layered structure so obtained may be computed by the progressive multiplication of individual 2×2 transfer matrices relating the field components in adjacent layers. The excitation efficiency of the wave in the guiding layer shows a resonance peak around the mode propagation constant. The full-width-half-maximum (FWHM) of this peak determines the power attenuation coefficient of the bent waveguide. The losses due to the discontinuity of the curvature are also computed. The computed results for different bends including S-bends are in good agreement with the published experimental data. The computation using the model is quite fast and versatile to consider arbitrary waveguide dimensions, Ti-film thickness, diffusion parameters and wavelength of light for both TE and TM polarizations. The model, in principle, is not limited to Ti:LiNbO 3 channel waveguides only but is valid for any arbitrary graded-index channel waveguide bends provided that the refractive index profile and the wavelength dependence of the refractive index are known.

Poling-inhibited ridge waveguides in lithium niobate crystals

Ultraviolet laser irradiation of a lithium niobate +z polar surface enables the production of ridge waveguides. Ultraviolet laser induced inhibition of poling is used to define an inverted domain pattern which transforms into a ridge structure by differential etching in hydrofluoric acid. The laser irradiation step also induces a refractive index change that provides the vertical confinement within the ridge structure. Furthermore, it was observed that poling-inhibition results in a significant enhancement of the refractive index contrast between the bulk crystal and the ultraviolet irradiated tracks.

Simulation of refractive index profiles for titanium indiffused lithium niobate channel waveguides

Abstract A generally applicable method to simulate the two-dimensional profiles of Ti-concentration and refractive index of Ti:LiNbO3, waveguides is presented. The influence of the fabricational parameters on the refractive index profiles has been studied in detail. The model parameters are extracted from the available experimental data. The computed values of surface refractive index change agree closely with the experimental results. The model is also extended for coupled waveguides.

Determination of Refractive Index Profile and Mode Index from the Measured Mode Profile of Single-Mode LiNbO3-Diffused Waveguides

Abstract A refractive index profile and a mode index of single-mode LiNbO3 channel waveguides are simultaneously estimated from the measured near-field intensity distributions. A third-order low-pass Butterworth filter is used to filter the measured amplitude and its double derivative to remove high-frequency spatial noise and impulses. The mode index or effective index of the propagating mode is extracted by matching the index change profile equal to zero at the waveguide boundary. The proposed technique is applicable to any slowly varying, single-mode, integrated-optic waveguide for TE and TM modes and for any operating wavelength within the single-mode regime.

Integrated optical waveguides in LiNbO3: modeling and experimental analysis

Graded-index optical channel waveguides rare fabricated by diffusion of Ti in LiNbO3. A model has been developed for calculating the mode profiles of the same, by first finding the effective refractive index profile along the lateral direction, and then discretizing the profile and applying the matrix method. This effective-index-based matrix method has been presented in this paper and established as a computationally fast, analytical method to computer modal electric field and intensity profiles in single and coupled graded-index channel waveguides. The method can be used in coupled systems consisting of three or more waveguides where the input power may be launched to any of the waveguides. Some of the computed intensity profiles have been compared with experimental data. Although computation and measurements have been performed only for TE and TM polarizations, the method has the inherent capability of handling arbitrary polarization.

A three-waveguide polarization independent power splitter on lithium niobate substrate

Abstract A three-waveguide polarization independent power splitter on LiNbO 3 substrate is proposed and demonstrated. Splitting ratio between the output ports and the excess loss of the device are found independent of the polarization states of the input light. The measured imbalance of the output power levels and the average excess loss of a 1×2 polarization independent power splitter are 0.2 dB and 0.58 dB, respectively. The device can be cascaded in tree-structure to fabricate 1×2 N power splitters which are important in optical communication networks.

Enhanced electro-optic response in domain-engineered LiNbO3 channel waveguides

Substantial enhancement (36.7%) of the intrinsic electro-optic coefficient (r_33) has been observed in lithium niobate channel waveguides, which are made to overlap with a pole-inhibited ferroelectric domain. The waveguide and the overlapping ferroelectric domain are both produced by a single UV irradiation process and are thus self-aligning. The enhancement of the electro-optic coefficient effect is attributed to strain, which is associated with the ferroelectric domain boundaries that contain the channel waveguide.

Semi-Analytical Simulation of Titanium-Indiffused Lithium Niobate–Integrated Optic Directional Couplers Consisting of Curved Waveguides

Integrated optic directional couplers consisting of curved waveguides are simulated analytically by solving the Riccati equation. The coupling coefficient between the curved waveguides with a parabolically varying gap and the condition of total power transfer between the waveguides are derived. In order to compute the overall coupling coefficient and hence the power distribution along the waveguides for Ti:LiNbO 3 curved waveguide directional couplers, the coupling coefficient for straight waveguide couplers is computed for different gaps using the effective-index-based matrix method (EIMM). Finally, the power distribution in the curved waveguides along the length is computed. The method is mostly analytical except the effective-index method and is computationally simple.

Comparison of calculated and measured refractive index profiles of continuous wave ultravoilet written waveguides in LiNbO3 and its analysis by effective index based matrix method

The refractive index profiles of the continuous wave (CW) UV-written waveguides in congruent LiNbO3 crystal are theoretically computed and compared with the evaluated profiles from the measured mode intensity distributions. It has been observed that change in activation energy of Li-indiffusion affects the computed refractive index profiles to a large extent, and the computed and measured profiles match fairly well for 305 nm and 275 nm written waveguides with an activation energy equal to ∼1.3 eV. The calculated refractive index profiles are used in effective index based matrix method (EIMM) to compute the guided mode propagation constants of these waveguides at 632.8 nm transmitting wavelength. For both the writing wavelengths, the waveguides are single mode in nature, which tally with experimental results. The bending losses of the bent waveguides for different bending radii are also computed using a conformal mapping technique along with EIMM. Finally, the behavior of CW UV-written waveguides under an...