Ramu V. Ramaswamy

Ion-exchanged glass waveguides: a review

Passive glass waveguides made by ion-exchange technique are potential candidates for integrated optics (IO) applications. The authors review the advantages in the technology as applied to fabrication of useful single-mode structures. Progress in process, fabrication, modeling, and waveguide performance using different monovalent cation systems and glass compositions is described with emphasis on fiber-compatible single-mode structures prepared from molten baths. It is shown how a systematic study allows a correlation between the process parameters and the waveguide characteristics so desirable in assuring reproducible characteristics of the devices. >

A 1.3 mu m directional coupler polarization splitter by ion exchange

A glass waveguide polarization splitter for operation in the 1.3 mu m wavelength region is reported. The device, which has a symmetric directional coupler configuration, exploits the stress-induced birefringence in K/sup +/-Na/sup +/, ion-exchanged waveguides, giving rise to an adequate difference in the coupling lengths for the two polarizations. Starting from the measured potassium concentration (refractive index) profile of the structure and utilizing a combination of the multilayer stack theory and the effective-index method, the normal mode propagation constants and mode field profiles are calculated to determine the polarization splitting length and the extinction ratio, and the results are compared with the experimental data. It is shown that in a given coupler, the splitting occurs at several wavelengths in the 1.0-1.45 mu m range. A 25 mm-long coupler, fabricated by thermal diffusion of K/sup +/ ions in BK7 glass, exhibits an extinction ratio of 18.2 dB at 1.32 mu m, in excellent agreement with the simulation results. >

Modeling of graded-index channel waveguides using nonuniform finite difference method

A finite-difference method (FDM) with nonuniform discretization for the analysis of channel waveguides is presented. Application of the boundary conditions for either the quasi-TE or quasi-TM mode is illustrated. Flexible discretization of the grid structures minimizes memory size, resulting in much smaller computing time without sacrificing the accuracy of the solution. This nonuniform discretization FDM technique is used to model the well-guided small-mode-size Ti:LiNbO/sub 3/ waveguides. The model treats both finite and infinite source diffusion cases. Quasi-TM mode profiles and the corresponding eigenvalues are rigorously evaluated and the theoretical results agree very well with the experimental results. >

Overlap integral factors in integrated optic modulators and switches

Coplanar strip and complementary coplanar strip electrodes are often used in integrated optical switches and modulators using the optical directional coupler. The field analysis of these electrodes is performed to determine the optimum overlap integral between the optical field and the modulating electromagnetic field. The optimal placement of the electrodes with respect to the optical channel waveguides is presented as a function of electrode width and spacing. >

Fiber-compatible K/sup +/-Na/sup +/ ion-exchanged channel waveguides: fabrication and characterization

A systematic study of waveguides fabricated by K/sup +/-Na/sup +/ exchange in soda-lime silicate and BK7 glasses is presented. The measured K/sup +/ concentration profile, the refractive index profile, and the diffusion profile obtained by solving the one-dimensional diffusion equation are correlated to explain the differences in the index profiles in the two glasses. The mobility of the potassium ions was measured by fabricating waveguides using electromigration. Surface waveguides formed by diffusion from a molten KNO/sub 3/ salt bath were buried by applying an electric field. Single-mode channel waveguides for operation at a wavelength of 1.3 mu m that exhibit excellent mode matching with conventional optical fibers, achieving a fiber-waveguide insertion loss of less than 1 dB for a 20-mm-long waveguide, have been obtained. >

Analysis and design of high-speed high-efficiency GaAs-AlGaAs double-heterostructure waveguide phase modulator

A P-p-i-n-N, GaAs-AlGaAs, TE/TM mode phase modulator, which has both the high phase shift efficiency of a p-n homojunction modulator and the high speed associated with a P-i-N modulator, is considered by incorporating p- and n-GaAs buffer layers and utilizing the higher order effects in these layers. The device structure is analyzed by considering the individual contributions of both the electrooptic (linear electrooptic (LEO) and quadratic electrooptic (QEO)) effects and the free carrier (plasma (PL) and bandgap shift (BS)) effects. These effects are studied in detail as a function of the reverse bias, operating wavelength, doping concentration, and intrinsic layer thickness. The results are in excellent agreement with the theoretical predictions. >

Highly linear Y-fed directional coupler modulator with low intermodulation distortion

In this paper, we explore the feasibility of linearizing Y-fed directional couplers. We demonstrate that the linearity of their performance can be enhanced significantly by the appropriate selection of the interaction lengths with respect to the coupling length. The proposed linearization technique brings about the advantage of highly linear performance combined with a simple device design. The single- and two-section embodiments of the Y-fed coupler are compared both in terms of the linearity of the transfer curve and the level of the third-order intermodulation distortion. We show the two-section Y-fed coupler to be the device of choice since it can exhibit high linearity and improved tolerance to fabrication imperfections. We have determined that if the lengths of the two sections are set to 4.1 and 1.05 times the coupling length, respectively, the transfer curve is linear within 1% of the maximum output power over 98% of the entire interval of modulation. Advantageously, a 100% optical modulation depth is achieved at the same time. On the other hand, if these relative lengths are set to 2.3 and 1.05, the third-order intermodulation distortion in the optical output can be significantly suppressed (by more than 100 dB below the carrier). While comparable performance is possible with other linear modulator schemes of more complex designs, practical applications demand simple design with few or no controls. In this regard, the Y-fed modulator is known to benefit from the built-in quadrature point and nearly equal time-averaged power levels in the arms. The former feature can minimize or even eliminate the necessity of using bias voltages, while the latter minimizes potential impact of optical damage on device performance. Furthermore, we also describe a preferred scheme of a two-section Y-fed coupler with a domain reversal in a ferroelectric material. In this case, only a simple uniform electrode structure is utilized which is fully compatible to a single driving source and hence, is convenient for high-speed applications.

Characterization of annealed proton exchanged LiNbO/sub 3/ waveguides for nonlinear frequency conversion

A thorough and detailed characterization of annealed proton-exchanged (APE) waveguides in Z-cut LiNbO/sub 3/ is described. The mode index measurements in planar waveguides as a function of wavelength and annealing time are reported, including useful analytical relations for the refractive index change, its dispersion, and the depth profile as a function of annealing parameters. Analytical expressions for the mode propagation characteristics are presented and experimentally verified with reasonable accuracy. It is shown that the planar waveguide characterization results can be used to model the channel waveguide characteristics accurately. The model provides closed-form expressions for the mode index and the mode field profile, and the theoretical results are in excellent agreement with the measured data. The technique is used to accurately predict the phase mismatch between the fundamental and second harmonic modes in frequency-doubling experiments using APE channel waveguides. An optimum waveguide geometry for which the phase mismatch is relatively insensitive to the waveguide nonuniformity was predicted and verified experimentally. >

Recovery of second-order optical nonlinearity in annealed proton-exchanged LiNbO/sub x/

The second-order nonlinear coefficients measured in proton-exchanged LiNbO/sub 3/ as a function of annealing time are discussed. Measurements of reflected second-harmonic power indicate that the second-order nonlinear coefficient d/sub 33/ is reduced to 60% of the bulk value as a result of proton exchange in pure benzoic acid. It is also shown that annealing restores the d-coefficients to almost the original value of the virgin crystal. For example, recovery to approximately 90% of the bulk value was obtained for a sample with a 0.3- mu m-bulk proton-exchanged layer, annealed for 10 h at 350 degrees C.<<ETX>>

Modeling of the index change in K + –Na + ion-exchanged glass

We have measured the surface index change and birefringence in K(+)-Na(+) ion-exchanged waveguides and compared the results with theory. The contribution to the index change caused by the polarizability/volume changes (Deltan(p)) is calculated using two theoretical models which use empirical relations based on the glass composition. In both cases, we encounter large discrepancies between the predicted and measured values which are attributed to the inherent deficiency in the models, which assume free expansion of the glass in calculating the volume changes. Recognizing that the net volume change is much smaller, we accurately measure its value and show that both models can be used to predict Deltan(p) with the same accuracy, provided that the correct volume change is used. We show that the limitation in accuracy is dictated by measurement errors and uncertainties in the values of the ionic radii and polarizabilities. We also present a unique and systematic method for determining the compressive stress generated in the glass resulting from ion exchange.