Evanescent field and measurand

[Abu bakar]

Hi, Everyone I have a question that what is evanescent field and measurand? what is the difference between evanescent field and measurand? The fig. is attached below.

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   Fig_03.jpg
   

[Steve Dods]

I’m sorry, v\but I have no idea what is meant by “measurand”. The attached diagram does not help define it.

[Robert Copperwhite]

The Evanescent field is a portion of the light that hits the interface between the core and cladding and is available to interact with anything that is outside the core.

In the “measurand picture”, the measurand could be a liquid or a gas for example.

This can be used as a sensor. In this case, light in launched into the core of the fiber. If the measurand is not absorbing to this light, the signal will be high at the output. If the measurand is absorbing, it will absorb some of the light in the evanescent field and the output signal from the fiber will be decreased.

Hope that makes sense.

[Steve Dods]

Oh, so “measurand” could be taken to mean “that which is to be measured”? That makes sense. The “measurand” is located in the evanescent fields of modes associated with the fibre core. Materials here could indeed affect propagation of the modes. Maybe that is what the figure is intended to suggest.

[Robert Copperwhite]

exactly

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