The familiar requirement of a sensor system is the measurement of a particular measurand at a particular location, this usually being achieved with a point sensor.This is the way in which most sensors operate, such as those used, for example, in the monitoring of temperature,acceleration, pressure or many chemical parameters. A schematic of the three major sensor schemes — point,distributed and quasi-distributed — is illustrated in Fig. 1.
Fig. 1(a) shows, for example, a point sensor. Many different types of such sensors exist — as examples, they range from liquid level monitors with a prism tip, through chemically-sensitive "dip-in" probes for species monitoring and to resonant structures mounted at the end of the fiber for pressure or acceleration measurement. Alternatively, sen-sor devices may be designed so that they can discriminate in the spatial mode, and in this way, the measurand can be determined along the length of the fiber itself, in a process normally termed distributed sensing, illustrated in Fig.1(b) . This principle has been employed widely in the measurement of temperature using non-linear effects in fibers, such as Brillouin or Raman scattering or in some types of strain sensing. A style of sensor that is somewhat
"in between" these two types of sensors is termed quasi-distributed, as shown schematically in Fig.1(c) , where the measurand information is obtained at particular and pre-determined points along the length of a fiber network. Here,the fiber has been sensitized or special materials have been introduced into the fiber loop to allow the measurement to be taken and this technique has been applied to tempera-ture and chemical sensing, e.g., using different fiber types.