I think that fibre optic sensors have a potential to be used on industial scale in construction engineering. They allow for continuous, real-time monitoring of structural beahviour which finds application in responsible structures, such as bridges. Because of their shape and reltively small dimensions optical fibres can be easily introduced into linear structural elements such as strands and cables. This is happening right now. In future fibre optics are considered to be a major component of the so-called smart structures - structural elements which serve both structural and monitoring functions. Fibre optics glued or woven into the fibrous structure of such a smart element will allow for colection of data about behavior of the structural element (deformations and strains).

You are right. In my opinion, one of the big challenges is related to the management of the temperature effects, that are usually to remove from total strain to get the "mechanical" strain. Another issue is the drift of metrological performance over time, I think there is a lack of reliable data on this topic.

Many thanks for your quick and kind answer.

Fiber optic sensors ( as part of the name optical fiber sensors ) is a fiber optic devices for the detection of certain quantities , usually heat or mechanical stress , but sometimes the same displacement , vibration , pressure, acceleration , rotation (measured by optical gyroscopes based on the Sagnac effect ) , and the concentration of chemical substances. The general principle of such devices is that the light from the laser (usually a single mode fiber laser ) or a superluminescent optical source is transmitted through the optical fiber, testing slight change its parameters in the fiber or in one or more Bragg gratings , and then reaches the detection circuit that evaluates these changes .

         In comparison with other types of sensors , fiber optic sensors have the following advantages :

· They consist of an electrically nonconductive material (not require electrical cables ), which allows to use them , for example in the field of high voltage .

· They can be safely used in an explosive atmosphere , because there is no risk of an electric spark , even in case of breakage.

· They are not subject to electromagnetic interference (EMI), even nearby lightning and by themselves do not electrify other devices .

· These materials can be chemically inert , that is non-polluting and non-corrosive .

· They have a very wide range of operating temperatures (much higher than that of electronic devices ) .

· They can multiplexing ; multiple sensors in a single fiber line can be integrated with one optical source.

Thank you for this relevant answer Leonid

Hello Alexis,

There is an intrinsic OF interferometer, the Fiber Bragg Grating (FBG) sensor. Decoupling thermally induced strains from Fiber Bragg Grating measurements has been sufficiently done in the past due to the discrete contribution of temperature to wavelength shifting ofthe light that is propagating within the OF core.

Therefore decoupling T would not be in the top 3 of challenges for the method. Instead, i would worry a bit more i.e. for the correlation of the measurements with the actual strain distribution within the structure: Indeed you will get different readings when strain distribution changes but identifying the strains within stucture (not along the OF) is a big discussion. The answer depends on the embedding patterns, the structure geometry, the conditions, etc...

Thank you Spiridon

I agree with you about the capacity to measure the actual strain distribution. It is the same, whatever the fiber optics technology, I think (not only for FBG). A colleague of mine, Jean6Marie Henault, have provided some investigations on this topics, e.g. on his RG page: https://www.researchgate.net/publication/267095858_Analysis_of_the_strain_transfer_mechanism_between_a_truly_distributed_optical_fiber_sensor_and_the_surrounding_medium

Best regards

Conference Paper Analysis of the strain transfer mechanism between a truly di...

Indeed, right on the point.

Glad if it could be useful! Good luck.

Fiber optic sensors have interesting characteristics and a large potential as mentioned above. I had some good results (e.g. strain monitoring over a distance of 10 m with a resolution of a few µm at longer terms to check e.g. daily temperature influences or differential thermal expansion due to partial incidence of sunlight).

Practical application in a field environment requires however to consider several technical issues (not pure science - I admit - but in measurement, the devil is in the details - or "c'est le détail qui tue" ;-):

- because optical fibers are so small (thin), and extensive protection is not possible at the measurement part (as mentioned - to maintain the transfer of the quantity monitored from the environment to the fiber), fragility is an issue. Installation (e.g. how to install a string of FBG in a concrete structure being cast) requires utmost attention. Also the connectors - which seem to be developed for lab use or in clean environments such as switchboards - are not optimal in field use (I had to replace already several times the connectors).

- field interventions are less evident: splicing fiber optic lines is far more complex in the field than e.g. soldering cables; feed-through of cables (from a pressurized environment) is also more complex then for traditional cables;

- Brillouin or Rayleigh based devices (single mode distributed measurements) are quite expensive (in constrast with fiber itself) and require some expertise to interprete the results;

- distributed sensing (single mode) sometimes requires the "stress-free" installation of a fiber; but even with a "loose tube", this is not guaranteed (contrary to what a manufacturer told me);

- I recently had to give up a set of interferometric sensors because of watertightness (and the water passing through a 40 m connecting cable up to the connector end...).

This list is just to indicate that sufficient attention has to be put to the field conditions (different from a clean lab), such as installation, harsh conditions,... so it's more than just purchasing stuff of the shelf and installing it.

Fiber optic sensors (FOS) are suitable candidates for SHM of composite materials during operation since they are capable of achieving the goals of diagnostics as well as condition monitoring and being very compact in size and can also be embedded into such structures, acting in many ways as the equivalent of a human nervous system. Previous investigations of FOS

Previous investigations of FOS embedded in composite structures indicate that FOS technology is capable of monitoring stress/strain, temperature, composite cure process, vibration, humidity, delamination and cracking and thus has great potential for condition monitoring of a variety of composite materials applications.