Optical fibers theory?

Question

Does anybody knows to explain me (qualitatively) why electromagnetic field exists in cladding layer even if the total internal reflection condition is satisfied?
Does it mean that for example: laser ray contains electromagnetic field beyond its visible track or what? It doesn't seems logical to me.

Answer 1

It's called an evanscent wave. Basically, it is leakage of the propogating EM field into the cladding because the materials, while support the condition for internal reflection, aren't perfectly reflecting. They have a loss which results in leakage.

A laser, if it is diffraction limited coming out of the laser cavity does have light spread over more than just the central "beam". There are higher order diffraction modes at different angles off the center line of the beam, but the energy in them is very small compared to the main beam.

Answer 2

The first answerer got it mostly right in identifying this as an evanescent field. However, it is NOT due to any imperfection in the material, but is an essential part of the physics of total internal reflection. It comes directly from solving the wave equations at the point of reflection. The evanescent field is the reason why contamination on the outside of a prism can affect its reflectivity. In fiber optics, it means that flaws in the cladding can absorb energy, and has also been used to tap the optical signal without breaking the fiber.

Answer 3

Light is wave.
The wave has the right to penetrate to distance λ/2π inside the places where it particles cannot.

Lasers do not produce sharp cylindrical beams, the beams always decay exponentially at best, but often much worse as 1/(φ² + 1).

Lasers do not have visible tracks in vacuum.
What you see is scattering of laser beam in air and dust.
If you can see the track, there is something to scatter in that place. If there is imperfection inside the cladding which can scatter light, the light will scatter, and signal inside the fiber will be weakened. It means that penetration is real.

Answer 4

when light hits the core and the cladding layer only the light is reflected... not the radiation part of it.. and since the radius of the core is too small the radiation will exist in the cladding also..