Self-gated two-photon photorefractive effect induced by light upconversion in Er-Doped LiTaO3.

It is demonstrated that Er doping increases the photorefractive effect in LiTaO(3). Raman and fluorescence spectra are used to establish that the upconverted green emission of Er(3+) clusters acts as a gating source, significantly increasing the two-photon photorefractive effect of a monochromatic red or near-infrared pump source.

Proton-exchanged periodically segmented waveguides in LiNbo(3).

We report on the characterization of single-mode and multimode proton-exchanged periodically segmented waveguides in lithium niobate. Variation of effective indices of different modes for different annealing times as a function of duty cycle and period of segmentation is presented. It is shown that the proton-exchanged periodically segmented waveguide can be modeled by an equivalent z-invariant depth-independent graded-index waveguide with a Gaussian index distribution. Although the depth of the equivalent waveguide is independent of duty cycle, the peak refractive-index change increases almost linearly with the duty cycle and saturates at large duty cycles. The results should benefit the design of linear and nonlinear devices that are based on segmented waveguides.

Comparison of photorefractive index change in proton-exchanged and Ti-diffused LiNbO(3) waveguides.

A quantitative comparison of the photorefractive effect in proton-exchanged (PE), annealed-proton-exchanged (APE), and Ti-diffused LiNbO(3) waveguides in the visible region is reported. In the low-intensity region, the optically induced index change in PE/APE waveguides is almost 3 orders of magnitude smaller than that of Ti-diffused waveguides for a given intensity, primarily as a result of the increased dark conductivity owing to proton exchange. However, the photorefractive index change at higher intensities is almost the same for all the waveguides. Our results indicate that the optically induced space-charge field is relatively independent of the concentration or the valence state of iron impurities and the waveguide fabrication process.

Efficient quasi-phase-matched blue second-harmonic generation in LiNbO(3) channel waveguides by a second-order grating.

We report what is to our knowledge the first efficient blue second-harmonic generation in a second-order quasi-phase-matching LiNbO(3) waveguide. Blue-light generation with a conversion efficiency of 10%/W was demonstrated. A procedure for optimizing the waveguide and the periodic domain-inversion grating has also been developed for various-order quasi-phase-matching gratings in a LiNbO(3) waveguide.

Efficient direct diode-laser frequency doubling in quasi-phase-matched LiNbO(3) waveguides.

We report efficient (22%/W) blue-light generation by direct diode-laser frequency doubling in second-order quasi-phase-matched LiNbO(3) channel waveguides. We address several critical issues of direct diode-laser doubling, which include input coupling, phase-matching bandwidth, and, in particular, the effect of the spectral mode structure of a high-power diode laser on the frequency-conversion efficiency.

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.

Effect of annealing on diffused channel waveguides.

Mode size measurements of the Ag(+)-Na(+) ion-exchanged channel waveguides in BK7 glass show a square root dependence on the annealing time in contrast to the linear variation predicted in the literature.

Simulation of tapered transitions in ion-exchanged channel waveguides.

Tapered transitions in the ion-exchanged glass waveguide are modeled to predict the index profile, propagation constant, and modal field along the taper. Results are in excellent agreement with the experimental data in Ag(+)-Na(+) exchanged tapers fabricated by a two-step process in BK7 glass.

Adiabatic invariance in GRIN channel waveguides and its use in 3-dB cross couplers.

We have extended the local normal mode approach of Burns and Milton to derive the condition for adiabatic invariance in graded-index channel waveguides. The results show that in spite of the index-grading and 2-D confinement, the inequality describing the adiabatic condition is basically the same as that for step-index slab waveguides. To verify the predictions of the theory, we have fabricated, using ion exchange in glass, a cross coupler with two asymmetric input channel waveguides and two symmetric output guides. The experimental results show equal output power (3.0 +/- 0.1 dB) from the two symmetric guides when either of the two unequal guides is excited, demonstrating excellent agreement with the theory.

Process optimization of buried Ag(+)-Na(+) ion-exchanged waveguides: theory and experiment.

We report optimization of the processing conditions for fabrication of single-mode Ag(+)-Na(+) ion-exchanged waveguides in a soda-lime glass. The surface-index change, the mobility and diffusion coefficient of Ag(+) have been measured at low melt concentrations and the results used for calculation of the index profile using a new simple numerical method for solving the diffusion equation in a two-step process where the index profile is buried by applying an electric field in the second step. The calculated profile is in agreement with that measured. This correlation allows solution of the Helmholtz equation for given process parameters. Single-mode waveguides with desired mode-field distribution have been designed and fabricated with optimized performance and fiber compatibility at 1.3 microm.

Characterization of highly elliptical submicron core polarization preserving fibers: theory and experiment.

Propagation characteristics of highly elliptical submicron core single-mode fibers have been calculated and compared with the measured beat length, cutoff wavelength of the first higher-order mode, and the far-field intensity distribution. The results show excellent agreement in the case of beat length and only reasonable agreement in the case of cutoff and mode field diameter. These discrepancies are discussed and explained.

Guided waves in graded-index planar waveguides with nonlinear cover medium.

Dispersion relations for TE modes in a planar exponentially graded-index waveguide with self-focusing nonlinear cover material have been solved numerically. It is shown that the threshold power required to pull the field maximum out of the film region into the cover is lower compared with that for the step-index waveguide and agrees well with the experimental results. Empirical relations to calculate the corresponding minimum film thickness and the minimum threshold power are given for the lowest-order mode.