Novel waveguide configuration for convenient and sensitive fluorescence and Raman measurements of liquids over optical fibers

Fluorescence has been measured from a waveguide formed by a PTFE tube with an internal coating of a low-refractive-index amorphous fluoropolymer. The configuration is suited to taking measurements from liquids having a refractive index down to 1.32, including, in particular, aqueous solutions. The parameters which determine the optical collection efficiency have been mathematically modelled. We have produced waveguides up to 1m long, and with 0.955 mm and 0.445 mm internal radii, and measured a (fluorescence) collection enhancement factor of 3 from a 140 mm long, 0.955 mm internal radius waveguide. Work is continuing to increase the enhancement factor

Gas sensors using correlation spectroscopy compatible with fibre-optic operation

This paper reviews methods for gas detection using real-time correlation spectroscopy. These methods involve using a gas sample in a reference cell as a matched optical filter to detect, preferentially, similar absorption spectra in a measurement cell. The methods all have the advantage of excellent selectivity, even using broadband sources, and are suitable for remote detection over optical-fibre leads. Our previously published methods using pressure and Stark modulation are reviewed. In addition, our recent theoretical treatment of the methods is presented. Finally, we describe new atmospheric-pressure measurements of gases using the method of phase-modulation spectroscopy

New multiplexing scheme for monitoring fiber optic Bragg grating sensors in the coherence domain

A new multiplexing scheme for monitoring fiber optic Bragg gratings in the coherence domain has been developed. Grating pairs with different grating distances are distributed along a fiber line, and interference between their reflections is monitored with a scanning Michelson interferometer. The Bragg wavelength of the individual sensor elements is determined from the interference signal frequency

Factors affecting the embedding of optical fibre sensors in advanced composite structures

Composite materials offer tremendous benefits for engineering applications and arc now specified for use in several safety critical studies. However, despite extensive materials research and development, they do have a number of areas where their behaviour is still not fully understood. This is particularly so with the more complex mechanical parameters in larger structures. Current structural design attempts to allow for these unknowns by overdesigning, extensive testing and frequent inspection. Embedded optical fibre sensors offer the potential to monitor many of these parameters, and are additionally of a similar physical and mechanical nature to the reinforcement fibre used in advanced composites

Model to predict the response of correlation spectroscopy gas detection systems for CH₄

We present a comprehensive model of a correlation spectroscopy gas sensor for CH4. Response and performance are predicted for typical fibre optic-coupled systems

Propagation model for multimode optical-fibre waveguide

Pulse dispersions of 5 ps/m have been measured in cladded-glass and liquid-core multimode fibres. A theoretical model is proposed which gives excellent agreement with measured propagation delay and pulse dispersion. In the fibres used, there is little light scattering either in the core or at the core-cladding interface

Progress with optical gas sensors using correlation spectroscopy

The paper reviews recent progress on gas detection using real-time correlation spectroscopy. The general method relies on using a gas sample in a reference cell as a matched optical filter, to preferentially detect similar absorption spectra in a measurement cell. All variations of the method have the advantage of excellent selectivity for gases with narrow line spectra, even when using broadband sources for illumination. They are also suitable for remote detection over optical fibre leads. The recent progress is in two main areas. Firstly, the earlier reported Stark modulation method has been extended to investigate a novel hygrometer. Secondly, we have developed a novel multi-line light source, by combining a broadband optical source with a Michelson interferometer, where the latter contains a gas in one arm

Finite element analysis of embedded optical fibre sensors

Micromechanical Finite Element (FE) analysis was used to model stress and strain fields in and around embedded optical fibres EOF'S) in flexural test coupons of carbon fibre composite. The coupons were used in studies of EOF effects on macroscopic laminate properties. The FE method allows complex material inhomogeneities, laminate boundaries and load conditions to be modelled

Review of methods of optical gas detection by direct optical spectroscopy with emphasis on correlation spectroscopy

Gas sensors are of importance for a variety of environmental, industrial, medical, scientific and even domestic applications. The gas may be, for example, hazardous to human health, an atmospheric pollutant, or important, in terms of its concentration, for an industrial or medical process. Apart from systems merely providing an alarm signal, it is frequently required to obtain accurate real-time measurements of the concentration of a particular target gas, often in a mixture of other gases

A multiplexed CW Brillouin system for precise interrogation of a sensor array made from short discrete sections of optical fibre

A novel configuration for precise temperature sensing in an array of optical fibre sections is reported. The sections are chosen to have different Brillouin shifts, to enable separation of scattered light signals in the frequency domain, rather than the traditional time domain, allowing lower-noise CW operation and greater precision due to the 100% duty cycle