Fibres wake up to sensors

Fibres wake up to sensorsSteve Bush
Optical fibres are affected by their environment.
This can be a problem if you are shipping data long distances, but exceedingly useful if you are a sensor maker. Temperature, force and pressure can all now be measured by their effect on fibres.
By making use of force sensitivity, reports the IEE’s Electronic Letters (vol 34, no 21, page 1991), fibres are now being used to measure physiological parameters.  
The sensor concerned is the size and shape of a wrist watch and has been developed by a multi-discipline academic team from Lorient in France to indicate the phases of human sleep.
It measures the heartbeat, blood pressure variations, breathing frequency and breathing amplitude.
In the sensor, a fibre is trapped between two metal plates, one of which is ridged. Forcing the plates together causes the ridges to deform the fibre which changes its attenuation through variations in refractive index at the contact points.
An 1mW laser diode and a photodiode allow the attenuation to be sensed.
An ordinary elastic wrist watch strap holds the assembly against the inner wrist and, being attached to the outer plate, sets a bias force on the fibre.
The signal from the photodiode is processed using a Texas Instruments MPS 430, transmitted over an RF link, then further processed to extract the required parameters.
Although the first application will be in sleep analysis, the developers see it finding use in other medical areas and industry. For instance, it could be the basis of a fatigue detector for workers operating dangerous machinery.

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