2. Some configurations of fiber-optic gas sensors

2.1 Extrinsic sensors: operating principle

In the case of extrinsic optical sensors, the optical fiber is used only to transmit light to and from the sensing element, as shown in figure 2 . This element is usually an enclosure into which the gas to be measured is injected. The dimensions of the enclosure are chosen so that the light leaving the input fiber can recombine, with moderate attenuation, in the output fiber. They are therefore usually centimetric. In addition, a set of lenses is generally used to ensure good light coupling. The working wavelength is that which corresponds to an absorption band of the gas to be detected, so that it affects the transmitted optical power. This...