Recent achievements

Fibre optic sensors

Prior to the early 1970s the main application for fibre optic waveguides had been for endoscopic instruments used in medical procedures. Optical sensors were well established before the 1970s, but very few of them had been developed specifically for optical fibre based sensors. In 1976 and 1977 respectively the first sensors were presented where the parameter of interest interacts directly with the optical fibre, producing changes in the propagation constants of the guided optical beam. Fibre optic sensors where the measurand directly modulates some physical property of the fibre is termed intrinsic sensor.

In general fibre sensors can be categorised as

 External or extrinsic in which the fibre is only used to transfer the measurand information from a distant location, as for example, a laser Doppler anemometer or others generally expensive sensors,
 Intrinsic in which the measurand (pressure, temperature, etc.) affects some optical property of the fibre resulting in modulation of the light (intensity, phase, frequency/wavelength, polarisation)
 Hybrid sensors, whereby light is transferred over the fibre to be converted to electricity to power a distant conventional sensor head.

Since the first intrinsic sensors the range of measurands detected by fibre optic sensors has increased rapidly, as has the number of transduction mechanisms which have been exploited.

The advantages of fibre optic sensors are:

 Immunity to electromagnetic interference
 Intrinsic safety due to light-based rather than electrical-current-based technology
 Chemical immunity to corrosion in hostile or wet environments
 Biocomaptibility
 Small size and low weight

Fibre Bragg Grating sensors

A fibre Bragg grating is a periodic variation of the index of refraction along the fibre axis. This phase grating acts as a band rejection filter reflecting wavelengths that satisfy the Bragg condition and transmitting the others. A grating is a device that periodically modifies the phase or the intensity of a lightwave reflected on, or transmitted through it.

This refractive index grating acts as a distributed (Bragg) reflector that couples the forward propagating to the backward propagating light beam. The wavelength for which the incident light is reflected with maximum efficiency is called the Bragg resonance wavelength. The equation relating the grating spatial periodicity and the Bragg wavelength depends on the effective index of the transmitting medium and is given by:

The period can be adjusted to obtain Bragg resonance from the visible to the infrared. That means that the FBG is only sensitive to physical impact that changes either Λ or neff, such as temperature change, mechanical strain or even pressure.

Special optical fibres

Various kinds of silica and polymer fibres with different specifications (diameter, coating material etc.) have to be investigated and evaluated. The fibres will be used for incorporation into textiles and for the realisation of three different kinds of fiber sensors - FBG sensor, bending loss sensor and NIRS sensor. A special part will be the fabrication of silica fibre based and polymer fibre based FBGs. For the realisation of the measurement of oxygen saturation in patient’s blood different configurations of side – end emitting fibres have to developed and evaluated.

Side-end emitting fibres

One of the main challenges for the development of a fiber based SpO2 sensor is the realisation of side – end emitting silica or polymer optical fibres. Side – end emitting fibres allow a side illumination at the end of the fiber. During the measurement by reflectance method the optical signal at the fiber tip has to be emitted to the patients tissue und collected back to the system in a very reproducible and stable way. Side – end illuminating fiber bundles can be manufactured in two configurations. The fiber tip itself can be processed to give it side emitting properties. Additionally it is possible to bend the fibres. Both configurations are investigated.