FBG Principle:

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.


        DTG technology:

Draw Tower Gratings (DTG®s) are fibre Bragg gratings produced using a process that combines the drawing of the optical fiber with the writing of the grating. This process of simultaneously drawing the fiber, writing the grating and coating the fiber directly after the grating inscription, results in high strength grating chains. As such the commonly used stripping and recoating process of standard FBGs is not necessary and the pristine fiber strength is maintained during the DTG® manufacturing process.







       Surface Mounted vs Embedded Sensing:

A clear difference exist between surface mounted and embedded optical fibre sensors.

Surface mounted sensing:

The method to install the optical strain gage is similar to the fixation of electrical strain gauges. The fixation method is applied based on a UV-curable glue and a sensor pad to position the optical fibre (i.e. sensor). First, the UV curable glue is spread along the sensor zone (approximately 30 mm x 8mm) and is cured.

Embedded sensing:

Embedded DTG®s are installed during the production phase of the composite parts. This is feasible thanks to the compatibility of the optical fibre with the composite material. They are light-weight with small diameters and therefore do not distort the composite material. A 100% strain transfer is reached because of the surrounding of the optical fibre.


        Multi-axial Strain Sensing

By embedding an optical fibre in a composite structure, the possibility is created to measure multi-axial strains. This possibility comes from the intrinsic multi-axial strain sensitivity of an optical fibre. Different types of optical fibre sensors can be used to obtain this feature of which the regular DTG®s are still the most simple to use since no extra orientation step of the optical fibre is needed.

This feature is interesting in different applications like production monitoring during which strains through the thickness of the composite part are likely to occur.