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Strain sensing with optical phase interrogation technique

Posted: 31 Jul 2015 ?? ?Print Version ?Bookmark and Share

Keywords:optical phase interrogation? OPI? polymer optical fibre? POF?

This technical note is intended to give integrators an introduction to optical phase interrogation (OPI) technology, and a basic understanding of how a sensing solution can be used together with polymer optical fibre (POF).

Principle
If we couple a modulated signal into a POF and subject the POF to strain, the signal will experience a phase shift (figure 1). The amount of phase shift scales linearly with the amount of strain (figure 3).

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Figure 1: Signal phase shift due to POF elongation.

Figure 2: Measuring phase shift with phase comparator.

Figure 3: Degree of phase shift depending on strain.

By splitting the modulated signal into a measuring fibre subjected to strain and a reference fibre not subjected to strain and subsequently comparing the 2 signals over a phase comparator, we can determine the amount of strain the sensing POF is experiencing.

The optimal workpoint is at 90 phase difference between the reference and sensing fibre where the phase comparator is working in a linear region (figure 4).

Figure 4: Working point of the phase comparator.

POF characteristics
Made out of polymer, POF offers a comparably high elasticity. Most POF can be stretched more than 5% and remain within the elastic range (figure 6).

This translates into a wide potential dynamic range of the technology. The large dimension of the fibre makes it easy to handle (figure 5). POF is used in a variety of industrial and automotive applications where it has proven its robustness. In addition to its robust characteristics, POF also offers complete EMI/ EMC immunity.

Figure 5: Dimension of POF compared to glass optical fibres.

Figure 6: Fibre attenuation (transmission) vs strain.


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