Simple method of measuring propagation properties of integrated optical waveguides: an improvement

In this paper we report a simple method of measuring mode propagation losses of integrated optical waveguides with small scattering. The basic technique, reported previously, uses a microcomputer-assisted video camera; here we have improved our observation scheme by coating the optical waveguide with a thin film of fluorescent dye (Nile Blue A perchlorate) thus permitting observation of the optical fields. The Nile Blue A perchlorate absorbs light centered at 0.63 microm and emits light centered at 0.69 microm. We have measured a mode propagation loss of the order of 0.1 dB/cm of a potassium ion-exchanged glass waveguide and confirmed the damped oscillatory behavior of the attenuation vs a slightly lossy thin-film thickness curve for the ion-exchanged waveguide coating with an indium tin oxide film.     

Quantization of coupled modes propagation in integrated optical waveguides

Abstract

A quantum mechanical analysis of the propagation of coupled modes in integrated optical waveguides is given. The modal orthonormalization property on a cross-section of an optical waveguide, the vector structure of the guided optical modes and the reversal-time symmetry are taken into account to derive the quantum momentum operator and Heisenberg's equations giving a quantum-consistent formulation of the coupled mode propagation as a function of forward and backward creation and annihilation operators.     

Propagation losses in curved integrated optical waveguides based on oxidized porous silicon

The propagation losses are evaluated for the first time in curved integrated (buried) optical waveguides (WGs) based on oxidized porous silicon. In the visible red range, the losses decrease from 20 to 5 dB per 90° bending (for about 0.4 dB/cm loss in the straight WG) when the WG curvature radius increases from 125 to 2500 μm. The main component of the total bending losses in WGs is related to the coupling loss on the passage from straight to curved WG parts. Additional losses are introduced by the regions of incompletely oxidized porous silicon on the walls of curved WGs.     

Propagation losses of copper-ion-exchanged channel optical waveguides

Abstract

We have measured the propagation losses of Cu—Na-ion-exchanged channel glass waveguides using both scattering and interferometric methods. In the first method, the waveguide was coated with a fluorescence film to enhance the scattered optical field along the channel waveguide which was then captured by a charge-coupled device camera. A straight line is adjusted to the intensity profile by the least-squares method, and the slope yields the attenuation coefficient. In the interferometric method, the temperature of the waveguides was changed and the intensity output was, measured. From contrast changes of waveguide's intensity output, the attenuation coefficient is calculated. Also, an analysis of the optical absorption of copper waveguides in the 200–1900 nm range to detect the copper oxidation state is presented.     

Analysis of integrated optical waveguides: variational method and effective-index method with built-in perturbation correction

We show analytically that for the modal analysis of rectangular waveguides, the recently developed effective-index method with built-in perturbation correction is identical to the first-iteration cycle of the variational method that we had developed earlier [Opt. Quantum Electron. 12, 517 (1989)]. We also show that in a number of cases the accuracy improves considerably through iteration beyond the first cycle.     

Strong reduction of propagation losses in LiNbO3 ridge waveguides

We propose optimized processes for the reproducible production of LiNbO₃ ridge waveguides with propagation losses lower than 0.2 dB/cm. The performances are achieved for both Z-propagating and Y-propagating waveguides, TE and TM polarizations, in X-cut LiNbO₃ congruent substrates, and could be easily extrapolated to Z-cut substrates. The fabrication procedure is composed of three steps: titanium deposition, optical grade dicing and diffusion at high temperatures, so that lithography or cleanroom facilities are not required. The reproducible smooth waveguides with low propagation losses result from the diffusion step, which is performed after the production of the ridges. We also show how the losses can be evaluated from the Fabry–Perot effect in the waveguides, without any assumption on the input and output reflection coefficients.     

Planar antiresonant reflecting optical waveguides as integrated optical refractometer

This paper reports on an integrated refractometer sensor, useful for measuring chemical quantities, based on antiresonant reflecting optical waveguides (ARROWs). We show that, by a suitable design, the attenuation in ARROW waveguides can strongly depend on the refractive index of the superstrate layer. We use this property to design and realize an integrated refractometer. The proposed sensor structure is unique in that it consists in the ARROW waveguide itself acting like a vertical interferometer. The device is fabricated using standard silicon technology fully compatible with bipolar and CMOS integrated circuit process. The measurement results show a sensing resolution of /spl Delta/n = 6e - 4 when used in a solution with a refractive index of 1.4600.     

Sharp bends with low losses in dielectric optical waveguides

A new method is proposed for making sharp bends with low radiation losses in dielectric optical waveguides. By modifying the transverse refractive-index profile at curved sections both the pure bend and the transition losses can be minimized. The optimum gradient-index profile requires an inhomogeneous medium. But in practice this can be replaced by a layered medium. By using four homogeneous layers the permitted radius of curvature of a slab waveguide can be reduced, e.g., from 6400 to only 100 wavelengths.     

Plate mode propagation losses in solidly mounted resonators

This paper investigates the acoustic losses of propagating eigenmodes through the acoustic mirror of a solidly mounted resonator (SMR) to clarify how resonator properties are influenced by reflection coefficients for the thickness shear (TS) wave as well as that for the thickness extensional (TE) wave. To this end, we analyze the effective acoustic admittance for several test structures with different mirror properties. Leaky modes are distinguished from plate-like modes and the propagation losses are quantified by calculating mode quality factors. The dependence of the propagation properties of leaky eigenmodes is compared with the mirror properties in terms of bulk wave transmission coefficients obtained by the one-dimensional Mason?s model. It is shown that the TE-like main mode couples with TS-like spurious modes, which then influence the leaky loss of the main mode as well. The coupling strength is strongly frequency-dependent and drastically changes with the mirror design. This result explains previous experimental results reported on SMR design.     

Erbium-doped oxidized porous silicon for integrated optical waveguides

It is shown for the first time that introducing erbium lectrochemically into waveguide structures based on oxidized porous silicon not only preserves their waveguide properties but it also opens up prospects for producing active waveguide devices based on them. It is established that erbium in a waveguide is in an optically active state and light with wavelengths 381 and 523 nm excites the erbium ions most efficiently. Pis’ma Zh. Tekh. Fiz. 25, 69–73 (September 12, 1999)     

Coupling losses between standard single-mode fibers and rectangular waveguides for integrated optics

The butt-coupling loss between different tapered rectangular waveguides and a standard single-mode optical fiber has been calculated. Losses as low as 0.16 dB can be reached for waveguides with a refractive-index contrast in the range of 0.5% to 1.96%. The fabrication tolerances are such that practical devices with coupling losses below 0.25 dB are feasible.     

Non-diffracting Airy beams in planar optical waveguides: a convenient method for visualization

In this paper we present an analysis of non-diffracting Airy beam solutions in planar waveguides. It is proposed that observing fluorescence from dye-doped polymers that can be coated on such planar waveguides can provide a simple and convenient technique for visualization and measurement of the propagation trajectories of such beams as well as studies on other characteristics of such non-diffracting beams.     

Attenuation in thin film optical waveguides due to roughness-induced mode coupling

Marcuse's mode-coupling theory for symmetrical planar optical waveguides was extended to cover the asymmetrical case. Computer programs were written to study the particular case of coupling between the zeroth order guided transverse electric (TE) mode and the radiation modes as a function of interface roughness.It is shown that a simple expression proposed by Tien gives results which correspond to the most pessimistic results obtained by the mode-coupling theory. R.f. sputtered thin film planar optical waveguides were prepared. The zeroth order TE guided mode having a vacuum wavelength of 0.6328 μm was excited in the thin film by means of a prism coupler. The attenuation of this mode was inferred from the reduction in intensity of the radiation modes which was measured with a photomultiplier. A Talystep was used to obtain the film thickness and also profiles of the roughness of the film-substrate and film-superstrate interfaces. The roughness profiles were computer-processed to determine the r.m.s. amplitude and the correlation length of the interface roughness. The refractive index was determined from the film thickness and the synchronous coupling angle using the reduced SYNCANG method. These data were used to compute the attenuation of the zeroth order guided TE mode from the mode-coupling theory. The correlation coefficient between the theoretical and experimental results was 0.96.     

Mixed mode fracture propagation by manifold method

The numerical manifold method combined with the virtual crack extension method is proposed to study the mixed mode fracture propagation. The manifold method is a new numerical method, and it provides a unified framework for solving problems dealing with both continuums and jointed materials. This new method can be considered as a generalized finite element method and discontinuous deformation analysis. One of the most innovative features of the method is that it employs both physical mesh and mathematical mesh to formulate the physical problem. These two meshes are separated and independent. They are inter-related through the application of weighting functions. A local mesh refinement and auto-remeshing schemes previously proposed by the authors are adopted in this study. The proposed model is first verified by comparing the numerical stress intensity factors with the benchmark solutions, and the results show satisfactory accuracy. The maximum tangential stress criterion is adopted and the mixed mode fracture propagation problems are then fully investigated. The numerical solutions by the present method agree well with the experimental results.     

A closed-form solution for wave propagation in optical waveguides of longitudinally invariant structures without using beam propagation algorithm

In this paper, we propose a different method to study wave propagation in longitudinally invariant waveguides with arbitrary index profile. In our method, both the electric field and the refractive index profile are expanded into two Fourier cosine series. With these series substituted into the wave equation, a differential matrix equation can then be obtained. We show here that such a matrix equation can be solved and an explicit expression for the wave field at any longitudinal position along an optical waveguide can be obtained. The solution proposed in this method can thus exclude the use of the beam propagation algorithm. This study demonstrates that our approach yields the same results as those obtained by using commercial softwares in which a beam propagation method with the Padé approximation is used.     

Adaptive Hermite-Gauss decomposition method to analyze optical dielectric waveguides

A novel spectral method with variable transformation, the adaptive Hermite-Gauss decomposition method (A-HGDM), has been developed and applied to the analysis of three-dimensional (3D) dielectric structures. The proposed method includes an optimization strategy to automatically find the quasi-optimum numerical parameters of the variable transformation with low computational effort. The technique has been tested by analyzing two typical 3D dielectric structures: the rectangular step-index waveguide and the rib-waveguide directional coupler. In both cases, the A-HGDM increases the accuracy of the Hermite-Gauss decomposition method (HGDM), especially when the mode is near cutoff, and improves the computational efficiency of previously published optimization strategies (optimized HGDM).     

Measurement of the enhanced evanescent fields of integrated waveguides for optical near-field sensing

The sensitivity of an integrated optical sensing device can be enhanced by coating it with a high refractive index layer, while both incoupled intensity and spatial resolution are maintained. The potential for enhanced sensing is demonstrated using titanium indiffused waveguiding structures in LiNbO(3) coated with a TiO(2) film. To the best of our knowledge, it could be measured for the first time that the outcoupled intensity at the surface was enhanced by a factor of 12-15 while keeping the penetration depth of the evanescent field constant of the order of only a few tens of nanometers. The evanescent fields of the guided modes were measured and characterized with a scanning near-field optical microscope and are in accordance with the numerical simulations.     

Simple method of measuring scattering losses in optical fibers

A simple method of measuring the scattering losses of optical fibers was developed. The method permits the measurement of the scattering-loss spectra by use of photon counting. Measurement is based on right-angle scattering, which is dominated by Rayleigh scattering, a material-intrinsic loss. A reference fiber for which the scattering loss is known is used to cancel out the unknown factors that are dependent on the optical setup. The scattering-loss measurement was demonstrated by use of two different low-loss fibers and was found to agree with predetermined figures to within 10% over the wavelength range 0.44-1.0 mum. Finally, the method was applied to a new high-numerical-aperture optical fiber to find its material scattering loss.     

Modified refraction-interference method for calculating the geometric and optical parameters of optical waveguides

We describe a ray method for analysis of the geometric and optical characteristics of two-layer fiber optical waveguides. We have analyzed the effect of the polarization of the illuminating beam on the overall pattern of the intensity distribution. We consider the characteristic types of rays for a single-layer and two-layer fiber optical waveguide. We have obtained definitive dependences of the beam intensity for each ray individually. We have developed a program for calculation of the scattering pattern in fiber optical waveguides illuminated by a broad beam. Translated from Izmeritel'naya Tekhnika, No. 3, pp. 39–42, March, 1996.