TABLE OF CONTENTS Intensity modulated fibre optic sensors have been widely adopted by the industry because of their simplicity in operation. Although they are not as accurate as interferometric sensors they offer sufficient accuracy for several process control applications. As displacement sensors, they can make good velocimeters, accelerometers, force and pressure transducers. Slight modification to these sensors allows their application to vibration, axial motion, proximity, rotation, part gauging, film thickness, eccentricity and liquid level sensing. Alternatively, amplitude modulating sensors may be used for chemical, biochemical and environmental sensing.
As mentioned in Chapter 2, the sensors can be extrinsic (whereby amplitude modulation takes place outside the fibre) or intrinsic (whereby amplitude modulation takes place inside the fibre). Typical examples of the extrinsic type of sensors are the reflection and transmission sensors, whereas typical examples of intrinsic sensors are the microbend and macrobend sensors. For extrinsic type sensors, the amplitude is generally modulated by changing the degree of coupling between the emitting and receiving fibres, so that the absorption, emission or the changes in refractive index in the coupling region may be monitored. Using these basic parameters, fibre optics can be used for measuring pressure and strain, vibration, liquid and solid levels, gas presence, temperature, etc., depending upon the transduction method of choice. However, one of the simplest applications of fibre sensors as sensory devices is in position detection.
In its simplest configuration, a reflectance displacement transducer (Figure 3.1), consists of two...
