Shigeo Nishida

Change of the refractive index in an optical fiber due to external forces.

This paper investigates experimentally the change of the refractive index, due to forces such as pulling and pure bending, in an optical fiber fabricated by the CVD technique. It is found that this phenomenon can be interpreted in terms of a simple model of the fiber, that is, a mechanically homogeneous circular rod. We compare the effect of the refractive-index change and that of a geometrical deformation of the fiber on transmission characteristics. A new method based on photoelasticity is also proposed to measure the curvature distribution of a fiber whose axis is deformed by external forces.

Pulse spreading in a single-mode fiber due to third-order dispersion.

Pulse spreading in a single-mode optical fiber is discussed taking into account the third-order dispersion term of the waveguide when the light source is modulated by a Gaussian pulse. A general expression for the pulse shape is analytically obtained, and an asymptotic approximation is used when the third-order dispersion term is small. It is also shown that the pulse width is no longer proportional to the guide length when the third-order dispersion term becomes large.

Bending losses of dielectric slab optical waveguide with double or multiple claddings: theory.

Bending-loss formulas of a multilayer planar optical waveguide are obtained on the basis of continuation of wave functions without solving the complicated eigenvalue equation. We then consider the bending losses of a doubly clad slab waveguide in which the core has the largest refractive index, and the inner cladding has the lowest. Bending losses are shown to be drastically reduced by inserting low-index inner claddings between the core and the outer cladding.

Design of Nonradiative Dielectric Waveguide Filters (Short Papers)

An efficient design technique of nonradiative dielectric waveguide filters for use at millimeter wavelengths is developed. Filter stuctures considered here are a gap-coupled type and an altenating-width type. According to present theory, 3-pole, 0.1-dB Chebyshev ripple bandpass filters with a 2-percent bandwidth at a center frequency of 49.5 GHz were designed and fabricated with Teflon dielectric. Calculated and measured filter responses agree quite well, and excess insertion losses are found to be as small as 0.3 dB for both types of the fabricated filter circuits.

Nonradiative dielectric waveguide circuit components

The nonradiative dielectric waveguide (NRD-guide) is a newly proposed dielectric waveguide which can substantially suppress radiation at curved sections and discontinuities and is applicable to great advantage for millimeter wave integrated circuits. Basic circuit components such as transitions between a metal waveguide and an NRD-guide, matched terminations, a ring resonator and a bandpass filter were fabricated with polystyrene material and tested successfully at 50 GHz. Measurements showed that each component worked as expected without suffering from undesirable radiation and interferences at all.

An approximate formula for describing dispersion properties of optical dielectric slab and fiber waveguides

A systematic and analytic approach is made to approximate dispersion properties of dielectric three-layered slab and rod waveguides. The explicit formula obtained yields errors less than 1% except very near the cutoff frequencies of guided modes. The errors diminish rapidly as the normalized frequency increases. The formula has the same expression for slab and fiber waveguides.

Bends in Nonradiative Dielectric Waveguides

An experimental study was made of bends in the nonradiative dielectric waveguide (NRD-guide) at 50 GHz. The main cause of the bending losses was found to be due to the reflection at the transitions between the straight and curved waveguides rather than due to the radiation. The width of the dielectric strip was experimentally optimized in order to reduce the reflection, and a bend with a curvature radius as small as one guide wavelength could be realized. The experimental results are examined theoretically. The theory implies that the NRD-guide has a favorable tendency to suppress not only the radiation but also the reflection at the curved sections. It is also shown that the field maximum moves outwards or inwards from the mean path of the curved guide according to whether the dielectric strip is wider or narrower than a certain critical width. This critical width can be adopted as a design criterion for a low loss, very sharp NRD-guide bend.

Pulse spreading in a single-mode optical fiber due to third-order dispersion: effect of optical source bandwidth

Pulse spreading in a single-mode optical fiber is discussed taking into account the third-order dispersion term of the waveguide when an optical source with a finite spectral width is modulated by a Gaussian-shaped pulse. Explicit forms for group velocity and pulse width are obtained taking into account both the optical source bandwidth and modulation bandwidth, and these are compared with those obtained by a simple theory. A condition is also made clear how small the second-order dispersion term has to be before the thirdorder dispersion term becomes important.

Excitation dependent losses and temperature increase in various hollow waveguides at 10.6 μm

Abstract Theoretical and experimental studies have been made on excitation dependent transmission properties of various hollow waveguides at 10.6 μm. A heat problem of the waveguide has been experimentally treated when high powered CO 2 laser light is launched.

Leaky TE modes on an asymmetric three-layered slab waveguide

The characteristics of two types of leaky modes on the asymmetric three-layered dielectric slab waveguide are fully investigated by solving the eigenvalue equation numerically. Loci of propagation constants and field distributions are presented and the effects of asymmetry on the propagation characteristics are discussed. It is found that the characteristics of substrate leaky modes are quite different from those of air leaky modes, and that the attenuation constants of substrate leaky modes become extremely small as the asymmetry becomes small.