The simplest way of making physical measurements with optical fiber (OF) is doing what you are usually explicitly being told to not to do - bending the fiber. It makes sense, as for proper fiber optic communication one is aiming at lowest possible loss of signal in OF; but that is not the case with fiber sensing applications. Here, we are looking for some way of modulating the passing light with the measured quantity, usually in form of amplitude, spectral or phase modulation. The easiest way is amplitude modulation (performed by bending the OF), so that's what I've chosen here.
Bend the fiber!
Key parameter of OF is insertion loss - ie. how much of light is attenuated by some amount of length, usually expressed in dB per km. This value is guaranteed by manufacturer, under specific conditions, one of them is bend radius. As bend radius decreases, light in OF core is can't follow the curvature and gets leaked to the fiber cladding, from which it can't enter to the core again and gets attenuated. The smaller bend radius, the more leak to cladding and higher insertion loss. One has to be careful here, as too tight bend can damage or even break the OF.
Provided I can transform the measured quantity into OF bend and measure the insertion loss, I get fiber fiber optic sensor (FOS).
One of possible arrangements is on picture below
OF is placed on compressible material and movable push rod is placed above it. When push rod is pushed into compressible material, it bends the fiber and causes insertion loss. Within some limits, the insertion loss rises as push rod compresses the material; after saturation point (material is compressed, OF can't bend any more) it doesn't change much. The movement of push rod can mean measurement of position or displacement; with help of flexible material it can be transformed to force or weight measurement, or even air/fluid pressure (using flexible bellow) or temperature (with bimetal strip); or whatever else that can be converted to linear motion.
Signal conditioning is not very complicated here.
Led with appropriate wavelength emits light into the fiber, the light gets received with photodiode (PD in this block diagram). The amount of light (=signal from photodiode) is the higher the less attenuation is in path. Electric signal from photodiode is amplified by and ready for processing, in this example is brought to ADC and processed by microcontroller.
As usual, devil is in detail, but that is something for next project log.