Single optical fibre transducer for pressure measurement.
In a single optical fibre transducer, light is injected into a fibre through a fibre optic coupler, located at some distance from the distal end of the fibre. Injected light travels toward the front face of the fibre, where it exits in the shape of a cone. then, after reflection from a reflective surface, it is readmitted into the same fibre. The reflected beam of light travels back through the coupler, exits through the distal end of the fibre and illuminates the photosensitive area of a photo diode, where it is converted to an electrical current. Depending on which one of the parameters is allowed to vary the amount of the returning light, one can build a variety of different sensors. Among them are: linear/angular displacement sensors and their derivatives such as pressure or force sensors; transmittance sensors such as turbidity and chemical sensors; reflectance sensors such as temperature sensors, optical encoders and scanners, colourometric and other types of chemical sensors. In the case of the single optic fibre transducer (SOFT) made by Pearl Instruments (Chicago, II, USA), the reflective surface is made of a thin metal foil with 6 microns thickness and 1.25 mm diameter. Optical fibres are pf the multimode type with 55 microns diameter. Their numerical aperture is equal to 0.66. The SOFT head is 1.25 mm in diameter and has an overall length of 50 cm. The disposable part of the system contains the head, fibre, protective sheathing and plug. Its operating range is -15 to +300 mmHg, and +25 to +45 degrees C, and its resolution is +/- 1 mmHg.