Fibre Optic Intensity Modulated Sensors

Abstract

Appendix 1 gives examples of the types of modulating signals used for measurement purposes and most of these have been applied to modulate light in fibre optic sensor systems. However radiant signals embrace a wider field than that of optics so there are a few examples of certain forms of radiant energy being capable of modulating optical systems eg the Cherenkov effect. In order to examine modulating techniques in a logical way it is usual to classify them as five separate effects resulting in change of intensity, wavelength, phase, polarization or time. This classification can be arbitrary depending on how one frames the rules. For example, an interferometric sensor can be regarded as a device measuring either a time delay or a phase lag. Again a photoelastic force sensor modulates polarised light but the measurement can be in the form of an intensity signal. In spite of the ambivalence of the classification system it is generally the simplest and the preferred method of analysing modulating techniques. The characteristics of optical fibres are important parameters in the modulating process. A variety of characteristics and materials can be found in the various types of fibre, - monomode, multimode, birefringent and coated.