P. K. Choudhury

Analysis of the guidance of electromagnetic waves by a deformed planar waveguide with parabolic cylindrical boundaries

An analytical study of a waveguide with two parabolic cylindrical surfaces separating the guiding region of refractive index n1 from two cladding regions of common refractive index n2 with n1≳n2 is presented. The cutoff conditions for modes are derived as equations containing trigonometric functions. The modes show a bunching tendency instead of well‐defined discreteness, and several mode bunches occur because the distance of separation between the two interfaces does not remain constant but continues to increase as one moves away from the region near the vertices of the parabolas. Unlike the planar waveguide, a nonzero cutoff is deduced which may be attributed to the curvature of the boundaries, and the flare.

A Mathematically Rigorous Cutoff Analysis of Parabolic Cylindrical Waveguides

We present a rigorous study of a planar waveguide which, under the influence of unintentional or deliberate effects, takes the form of a parabolic cylindrical waveguide. Extending our earlier study on this problem, where some physically plausible assumptions were made, we now develop the characteristics and the cutoff condition equations for such a waveguide under mathematically rigorous conditions. It is found that results again show a bunching tendency of modal cutoff confirming the validity of our earlier results. Mode-bunching is thus found to be a fundamental characteristic of parabolically bent slab waveguides.

Glass fibers of triangular cross-sections with metal loading on one or more sides: a comparative modal study

Using the point-matching method a comparative study has been made of the propagation characteristics of a triangular-cored glass fiber waveguide which is loaded on one or two sides with a highly conducting substance, the remaining side/sides being adjacent to a dielectric material of lower refractive index than that of the core.

Field cutoffs of three-layer parabolically deformed planar chirowaveguides

The boundary value problem of a parabolically bent planar waveguide that is completely filled with a homogeneous isotropic chiral material is solved and expressions for the field cutoffs are derived considering special cases. Results are expected to have application in the design of composite chiral media as well as in the tolerance study of planar chiroguides with curvature and flare. The presence of chirality in the cladding introduces some attenuated modes in the waveguide.

Weak Guidance in Bent and Unbent Four-Layer Planar Waveguides-A Comparative Study of Dispersion Curves

A comparative study has been made of the dispersion curves of a four-layer bent and an unbent dielectric slab waveguide under the assumption of the weak guidance approximation. It has been noticed that the introduction of a slight bend to a planar waveguide results in mode-proliferation, and lower order modes show no cutoff at all. This anomalous behaviour of the dispersion curves is not seen in the case of an unbent waveguide structure.

Operating Characteristics of an Optical Filter with a Linearly Periodic Refractive Index Pattern in the Filter Material

A design for the fabrication of an IR filter by using a linearly periodic refractive index pattern on the macroscopic scale in a suitable material has been proposed. To predict allowed and forbidden bands of wavelengths for the lower modes, a mathematical analysis is presented. Numerical estimates of allowed and forbidden bands are also made. Several parameters controlling the range of operation and region of allowed wavelength bands have been identified. It is being suggested that the technical knowhow used in the manufacturing of optical fibres and integrated optical devices can successfully be employed for the fabrication of these filters.

On electromagnetic wave propagation in an optical fiber having a core radius with a periodic axial variation

A study is made of the modal properties of electromagnetic waves propagating along a dielectric fiber having a periodically varying core cross-sectional size along the direction of propagation. The maximum variation of the core radius is assumed to be small, and a full cycle of variation occurs over a large change in the axial variable z compared with the core radius. The general characteristic equation and cutoff conditions are derived, assuming that the component of the propagation vector β along the axis varies with the axial coordinate z. In general, β is complex, showing an attenuation or a reinforcement because of alternate flair and constriction. As an illustration, a numerical estimate of ∂β / ∂β as a function of z is made for some modes with simple and special properties where attenuation is negligible, and the axial electric and magnetic fields are in phase. The method can easily be extended to include more general cases by means of straightforward but tedious numerical work.

Partial Guidance of Electromagnetic Waves by a Parabolic Cylindrical Boundary Separating Two Chiral Media

An investigation has been made of the electromagnetic wave propagation in a medium with two regions, having different chiral admittances, separated by a parabolic cylindrical surface. The resulting guide is analogous to a circular chirowaveguide when the circle is degenerated into a parabola. The eigenvalue equation and the cutoff conditions of such a chirowaveguide have been derived for different special cases.

Design of an optical filter as a monochromatic selector from atomic emissions

A bandpass filter with a linearly periodic refractive-index profile is discussed in analogy with the Kronig–Penney model in the band theory of solids. The suggested filter is capable of selecting intense lines from the light emitted by the excitation of neutral and ionized atoms. The particular cases of Ar, Br, and C emissions were considered for the sake of illustration.

Comparative Aspects of a Metal Loaded Triangular Waveguide with Uniform and Non-Uniform Distribution of Goell's Matching Points

Using Goells point matching method, a numerical analysis has been made of a Metal loaded Triangular Waveguide (MTW) with a non-uniform distribution of matching points, some of which are crowded near the corners of the triangle, and the results are compared with those reported earlier for an MTW with the matching points distributed in a uniform manner. An interesting aspect is observed that, as expected, a redistribution of the matching points without changing the total number of such points leads to a significantly different and better approximation to the modal characteristics than those obtained through a uniform distribution of points.