The FBG is a passive and discrete optical component at a specific spot in an optical fibre. An optical fibre consists of a fibre core and a fibre cladding. The Bragg grating is an area in the fibre core with a pre-defined length, Lg and an alternating periodic refractive index change, i.e. n1 ,n2 (Figure 1). An FBG acts as an optical filter or reflection filter and provides a frequency dependent reflection spectrum or stop band to the incident signal over a specific bandwidth. The stop band is centred at the Bragg wavelength, λB, and is given by the well-known Bragg condition , where ΛFBG is the grating period and is the mode index or effective refractive index of the fibre.

Figure 1: Basic FBG principle

The very basic principle of strain sensing of a fibre Bragg grating is shown in Figure 2. When the fibre, having a nominal length, L0, is being elongated to a certain length L, the grating period will be strained and the refractive index of the fibre will change as well. As a consequence, a positive Bragg peak shift is induced from λB0 to λB . In fact, an FBG forms the optical equivalent for a resistive strain gage (RSG). The basic principle of both types of sensor is the same: only one parameter will change when being strained. For an RSG, it is the resistance of the wire which changes as a function of strain and for the optical counterpart, it is the Bragg wavelength which shifts.

Figure 2: Basic FBG strain sensing principle

The main difference between an RSG and an FBG is the fact that the FBG is a passive optical component with absolute sensor properties (i.e. no drift in time). This means that once it is calibrated for a specific temperature region there is no need to re-calibrate it, which is a major advantage once it is in service . Another interesting aspect is the multiplexing ability of fibre optic sensors. One can put  up to more than 20 FBGs in series configuration in one optical fibre (or channel), with each sensor having its unique reflected Bragg wavelength (i.e. “color”). As such a multichannel optical interrogation system can easily monitor hundreds of FBG sensors in one sensing network using a limited number of optical lines. The principle of an FBG-interrogator based on Wavelength Division Multiplexing (WDM) principle with 8 channels is shown in Figure 3. Here only one optical channel is connected to an optical fibre, which can be 100meters in length, with 1 up to N+1 FBGs. The ASE (amplified spontaneous emission) optical source sends light via the optical circulator (passive) and via the optical switch (active) to the optical fibre with one or multiple FBGs, with each FBG reflecting a unique Bragg wavelength. The Bragg wavelength(s) are reflected back via the optical circulator to the Optical Spectrum Analyser (OSA) which reads out the peak wavelength and wavelength shifts. The interrogator is operated using a standard PC or laptop with a Labview based software to record the FBG wavelengths.

Figure 3: FBG-interrogator with 8 channels showing the multiplexing principle