We report on the waveguiding properties of a new type of low-loss optical waveguide. The photonic crystal fiber can be engineered to support only the fundamental guided mode at every wavelength within the transparency window of silica. Experimentally, a robust single mode has been observed over a wavelength range from 337nm to beyond 1550nm (restricted only by available wavelength sources). By studying the number of guided modes for fibers with different parameters and the use of an effective index model we are able to quantify the requirements for monomode operation. The requirements are independent of the scale of the fiber for sufficiently short wavelengths. Further support for the predictions of the effective index model is given by the variation of the spot size with wavelength,

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Properties of photonic crystal fiber and the effective index model

Consideration is given to a new optical fiber technique for the measurement of the spatial distribution of physical fields (e.g., magnetic field, electric field, temperature, mechanical stress): polarization—optical time domain reflectometry (POTDR). The technique relies upon the time resolution of light backscattered from a pulse propagating in a monomode optical fiber to measure the spatial distribution of the fiber’s polarization properties. These properties are modified by the field under investigation. The technique appears feasible and could form the basis for a new measurement technology.

Polarization-optical time domain reflectometry: a technique for the measurement of field distributions.

Photonics has long been used to study non-solid materials such as liquids, gases and plasmas, but these fluidic media have traditionally not comprised a functional part of the photonic device or system. The emerging field of optofluidics seeks to create new ways of uniting solid and non-solid materials in a single photonic system whose optical properties are typically defined by the fluidic component. This Review summarizes the current state of optofluidics from a photonics perspective. First, we describe a new class of photonic elements based on the combination of fluidic media and integrated optical structures. We then discuss the applications of optofluidic principles to particle sensing and manipulation in fluids, and finally assess current challenges and potential directions for future developments.

The photonic integration of non-solid media using optofluidics

The material dispersion of optical fibres having cores of silica or phosphosilicate glass falls to zero at a wavelength between 1.2 and 1.3µm. A considerable increase in bandwidth can be obtained, especially with an l.e.d. source, by operation in this region.

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Zero material dispersion in optical fibres

Optical signals transmitted through cladded glass fibers are subject to delay distortion because of (1) dispersion in the material, (2) dispersion caused by the waveguide characteristic, and (3) delay differences between modes. We isolate these effects and evaluate their significance for cases of practical interest. These concern fibers in which the refractive index of the cladding is only slightly lower than that of the core.

Dispersion in Weakly Guiding Fibers

The propagation of electromagnetic waves along a radially-inhomogeneous single-mode optical fibre with small index difference between core and cladding is studied. As in the case of the step-index fibre the propagation modes are weakly guided and their transverse fields are essentially polarized in one direction. The simple field descriptions and characteristic equation obtained here enable mode spot size and bending loss to be determined and are useful for practical design work of radially-inhomogeneous single-mode optical fibres.

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Propagation in radially-inhomogeneous single-mode fibre

The combined effects of composite material dispersion, profile and waveguide dispersion can be made zero at a wavelength which depends on core diameter. refractive-index profile and composition. Wide bandwidths over large repeater spacings are therefore possible although close fabrication tolerances may be required.

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Zero total dispersion in graded-index single-mode fibres

A study of propagation in curved single-mode fibres shows that the transmission loss increases sharply below a critical bend radius. However the radiation does not leak away uniformly with distance but in a series of discrete well-defined rays.

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Radiation from curved single-mode fibres

A method is described for determining unambiguously, from the far-field pattern of single-mode fibres, the core diameter and the refractive-index difference between core and cladding. It involves measurements only of the half-power width of the main lobe and the width of the first minimum. Universal curves are presented that can be used with single-mode fibres or any fibre operating in the single-mode regime. Independent determinations of core diameter over the range 4-8µm by an etching technique are in excellent agreement with those obtained from the far-field pattern.

Determination of core diameter and refractive-index difference of single-mode fibres by observation of the far-field pattern

A new type of silica-based optical fibre has been made from relatively cheap and abundant materials. The attenuation is very low over the entire range from the near ultraviolet to the gallium-arsenide-laser wavelength. The minimum loss of 2.7 dB/km occurs at 0.83µm.

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W.A. Gambling

Papers

Propagation in radially-inhomogeneous single-mode fibre

Zero total dispersion in graded-index single-mode fibres

Radiation from curved single-mode fibres

Determination of core diameter and refractive-index difference of single-mode fibres by observation of the far-field pattern

New silica-based low-loss optical fibre