Technology

Fibre optic sensing is now a fully mature and cost-effective technology that offers major advantages over conventional measurement methods. In particular the use of Fibre Bragg Gratings (FBGs) for measuring strain and temperature is now widespread throughout many industries and in many demanding applications.

Fibre Bragg Grating Illustration
Key Features

Fibre Optics Sensors

  • Long-term accuracy and stability,
    no re-calibration required
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  • Multiple sensors can be multiplexed on a single optical fibre
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  • Excellent compatibility with composite structures
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  • Exceptional resistance to mechanical fatigue
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  • Immune to electro-magnetic interference
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  • Passive, non-electrical sensors are suitable for use in flammable and explosion hazard applications

Fibre Bragg Grating Sensors



FBGs are produced by using a UV laser to modify the refractive index of the glass in the core of the optical fibre to form an interference pattern. This can be pictured as a series of stripes over a length of typically 5 to 10mm of the optical fibre core with around 1300 “stripes” per mm. The interference pattern will allow most wavelengths of light to travel freely down the optical fibre whilst strongly reflecting one particular wavelength. This wavelength is equivalent to twice the pitch of the interference pattern so that any strain induced in the optical fibre will increase the pitch of the interference pattern and therefore the wavelength of the reflected light. In this way, the FBG provides a direct and virtually ideal means of measuring strain.
transmission spectrum reflection spectrum
Unlike other strain measurement methods, FBGs offer a wide measurement range and excellent linearity and repeatability over many millions of load cycles. The interference pattern is permanently “written” into the glass, therefore, the FBG can provide an absolute reference for strain measurements over the entire operating lifetime of a structure without the need for recalibration.

Other benefits include immunity from Electro-Magnetic Interference (EMI) and also the capability of multiplexing up to 100 sensors on a single optical fibre, thus minimising cabling and installation costs and improving reliability.

Temperature Response In addition to being sensitive to axial strain, FBGs are also sensitive to temperature. For many strain sensing applications it is therefore necessary to compensate for this temperature effect. There are a number of ways of doing this effectively, the simplest of which is to install a second FBG that is isolated from the strain in the structure but subjected to the same temperature as the strain sensing FBG. The temperature response of the FBG can also be used as the basis of a temperature sensor either by using an un-strained FBG or by bonding it to a carrier with a high co-efficient of thermal expansion in order to provide an enhanced response.

 
Interrogation Systems
A practical fibre optic sensing system must also include a suitable opto-electronic unit to interrogate the FBGs. The interrogation unit sends broadband light down the optical fibre and accurately measures the wavelengths of the light reflected back from the FBGs. Where there are many FBGs written into a single optical fibre the interrogation unit must be able to determine which reflection came from which FBG. There are principally two ways in which this can be done. The advantages and disadvantages of both are discussed below.
 
 
Time Domain Multiplexing & Wavelength Division Multiplexing Comparison
 
Time Domain Multiplexing (TDM)
For TDM systems the individual FBGs are identified according to the “time of flight” of a light pulse to travel from the interrogation unit, to the FBG and back. FBGs located at different distances along the optical fibre will therefore exhibit a different “time of flight”.

Advantages of TDM:

  • A large number of FBGs (100 or more) can be interrogated in a single optical fibre
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  • All sensors can have the same initial wavelength, meaning sensors are interchangeable, simplifies system design, reduces lead times and inventory costs
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  • Every sensor can use the full measurement range of the interrogator
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  • No risk of mis-identification of sensors even during extreme unanticipated load conditions

Disadvantages of TDM:

  • In order to be able to reliably resolve differences in the “time of flight” of the reflections from the FBGs there is a minimum allowable spacing between FBGs along the optical fibre. This is typically about 2m
However, in practice this constraint can often be avoided. For example, when a closer density of FBGs is required, several optical fibres can be used with the location of the FBGs staggered in order to achieve the desired spacing. The optical fibres can then be interconnected to form a single array if necessary.
Wavelength Division Multiplexing (WDM)
In WDM each FBG on a single optical fibre must be written at a slightly different wavelength from all its neighbours. Each FBG is allocated a wavelength band within the available bandwidth of the interrogation instrument. Any operational strain or temperature change must not take the FBG outside its allotted wavelength band or it may be wrongly identified by the interrogation system.

Advantages of WDM:

  • There is no restriction on how closely the FBGs can be spaced along the fibre making it easy to locate several sensors in a single small component

 

Disadvantages of WDM:

  • Although many WDM systems are marketed as being capable of interrogating a large number of FBGs on a single optical fibre, in practice this number can be severely restricted by the need to share out the operating bandwidth of the instrument between the number of FBGs
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  • Because they require different wavelengths, individual sensors are not interchangeable, making installation and maintenance more complicated and increasing inventories
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  • The maximum expected strain and temperature variations for each sensor location must be determined at the system design stage in order to make the best use of the available bandwidth of the instrument
In the worst case, an unexpectedly large strain may cause sensors to exceed their allotted wave band and therefore be wrongly identified. This may be precisely the time when structural engineers are most interested in the data, particularly if this is associated with a structural failure or overload event.
 
Design and implementation of complete fibre optic sensing solutions