Evanescent waves

Evanescent waves are formed when sinusoidal waves are (internally) reflected off an interface at an angle greater than the critical angle so that total internal reflection occurs. "Evanescent" means "tending to vanish", which is appropriate because the intensity of evanescent waves decays exponentially with distance from the interface at which they are formed. Any propagating wave is converted into an evanescent wave when hitting a classically forbidden region. In this case, at least one component of the wavevector k becomes imaginary or complex and the wave experiences exponential damping when propagating in this region. In the case of an optical fibre the cladding has less refractive index than the core of the fibre. When light experience total internal reflection at the core-cladding interface, some of the energy of the light waves in the core of the fibre penetrate into the cladding for a very short distance. The energy flow of this evanescent wave is parallel to the surface of the core and in the same direction as the main flow of energy within the core.

In an evanescent wave fibre optic sensor, the cladding of the optical fibre is removed and it is placed in direct contact with the sample. Thus the sample poured in is in direct contact with the evanescent wave. Any materials which absorb the particular wavelength of light being carried by the fibre will take energy out of the evanescent wave. This in turn drains energy from the interior of the fibre and the output at the detector is reduced.