Multimode Guides Research Articles

Reconstruction of refractive index profiles from multiple wavelength mode indices

A method is presented whereby the refractive index profile of a planar, surface, dielectric optical waveguide may be reconstructed from sets of effective refractive indices, measured at different wavelengths. In order to allow for the dispersive nature of the glass, additional information from highly multi-mode guides, which are produced by the same manufacturing process as the guide under analysis, has been used in the method. The technique provides greater information about the profile than can be obtained from one measured set and is used here to analyse the profiles of potassium ion diffused guides formed in glass.

Fiber-optic sensors with UHF subcarriers

One way of overcoming these difficulties is to use a subcarrier with amplitude and phase measurement, as well-developed in the radio and UHF ranges [2]. It has been shown that a UHF subcarrier with a frequency up to some gigahertz can be transmitted in singlemode and multimode fiber-optic guides with various shapes [3, 4]. Multimode guides should be used for lines with length L < 1 km because they simplify the design of the optical interfacing units.

Polyalkylsilyne photodefined thin-film optical waveguides

Polysilynes, a new class of amorphous alkyl silicon network polymers, undergo a novel photo-oxidative crosslinking reaction associated with up to a 15% decrease in refractive index related to the loss of SiSi bonding. We describe the fabrication and initial measurements of waveguides formed with this index imaging technique in poly(cyclohexylsilyne) films on both SiO2 and poly(methyl methacrylate)-coated silicon wafers. Measurements of multimode guides show typical propagation losses of 0.68 dB/cm at 633 nm. Results indicate the polyalkylsilynes show promise as self-developing, planar optical waveguide media.

Coupled-mode equations for multimode waveguide systems in isotropic or anisotropic media

Coupled-mode theory for parallel waveguides is extended to include systems with multimode guides. The basic set of coupled differential equations is similar to that for single-mode guides, but the matrices are more broadly defined to allow for coupling among all the modes. Cases of embedding medium are also considered. anisotropic dielectric waveguides and/or an anisotropic embedding medium are also considered.

Light coupling measurements in three-guide multimode structures

Waveguiding in symmetrical planar structures of three glass slabs bonded by layers of slightly lower index of refraction is experimentally analyzed. It is shown qualitatively, that for multimode guides the structure acts as a power splitter, with light distributed only among identical layers symmetrically located. This result was observed experimentally even when the cladding between the slabs was not perfect (unintentionally tapered, etc.).

Coupling effects in symmetrical three-guide structures

A directional coupler consisting of three parallel waveguides is analyzed. The radiation is coupled between the two external guides, while the center-guide refraction index controls the coupling. By means of a coupled-mode analysis, a cross-and-bar diagram is drawn for the device, and regions are located in it for near-complete energy transfer between the guides. A rigorous mode analysis is presented for a three-guide layered structure, and the radiation-transfer process is numerically simulated for single-mode as well as for highly multimode guides. Although the analysis is carried out only for slab waveguides, the results are applicable for channel guides as well when the method of effective index approximation is utilized.

Simplified parameter-based analysis of single-mode optical guides

An approach to single- (and low-) mode waveguides is described that enables the important mode characteristics to be derived from much reduced profile data. Uniformity with generalised parameters for multimode guides is demonstrated, and experimental determination discussed.

Propagation of truncated Gaussian beams in multimode fiber guides

The ability to predict the light propagation characteristics in various practical multimode guiding structures is very important in optical fiber communications and in optical image transfer. The usual mode-by-mode analysis is impractical when the guiding structure is capable of supporting hundreds or thousands of modes. In this study a numerical technique is described which is capable of providing useful data on the propagation characteristics of optical multimode guiding structures whose index of refraction variation may be quite arbitrary. As a specific example, the problem of infinite or truncated Gaussian beam propagation in a radially inhomogeneous fiber with parabolic index profile is solved. The numerical results for the infinite Gaussian beam case are compared with exact analytical data and they are in complete agreement.

Holographic formation of gratings in optical waveguiding layers

Thick phase gratings were formed photorefractively (through optically induced refractive index changes) in planar optical waveguides by intersecting short−wavelength coherent guided beams. In multimode guides formed by effusing Li from LiNbO3, maximum diffraction efficiencies were 52% at the 0.488−μm write wavelength and 3.1% at 0.633 μm. In a two−mode LiTa1−xNbxO3 guide formed by diffusing Nb into LiTaO3 at 1100 °C, diffraction efficiencies were 65% and 28% at the same two wavelengths, respectively.

TE modes of graded-index slab waveguides

An analysis of the TE modes of a slab waveguide with graded refractive-index distribution is presented. The refractive index is approximated by a piecewise-linear distribution for which exact solutions of the guided-wave problem can be obtained. For multimode guides, useful approximate solutions can be worked out with the help of the Wentzel-Kramers-Brillouin (WKB) approximation. A formula is given for the total number of guided TE modes supported by the waveguide. The coupling efficiency of a prism coupler is investigated by studying the radiation losses of the guided modes escaping from the waveguide through the coupling prism. It is found that the radiation losses of the modes of the graded-index waveguide are more nearly constant than those of the asymmetric slab waveguide.

Higher-Order Scattering Losses in Dielectric Waveguides

This paper discusses the scattering losses of dielectric slab waveguides that are caused by higher-order grating lobes of the sinusoidally distorted core-cladding interface. The results of this paper are used in a companion paper to evaluate the radiation losses of multimode guides with intentional mode coupling. An exact system of equations is derived for the amplitudes of all grating orders. This system is used to derive first- and second-order approximations that hold for small amplitudes of the sinusoidal interface distortion. The theory is used to derive formulas for the average power loss coefficient for first- and second-order scattering processes.

Material Dispersion in Optical Fiber Waveguides

In weakly guiding dielectric (glass) fiber waveguides the factors governing the envelope delay distortion of optical signals can be separated conveniently into material and waveguide contributions. By making use of the fact that the normal dispersion of the index-of-refraction of glasses accurately fits a single oscillator Sellmeier equation, simple relations of general validity are obtained for the material contribution to the group delay and frequency dependence of the group delay in dielectric fiber waveguides. Using these results expressions are obtained for the material contribution to the information bandwidth of fiber optic waveguides excited by wideband sources (light emitting diodes) and narrow band sources (lasers). It is shown that in single mode guides the bandwidth limitation imposed by material dispersion is negligible, while in multimode guides the difference in group delay of higher order modes imposes the most severe bandwidth limitation.