Mode Field Diameter and Penetration depth of the Evanescent field

[Sciencestd]

Source: fobasics.blogspot.com

Source: scirp.org As it is shown in the first pic above that the mode field diameter is defined as the mode field decreases to 1/e (in intensity 1/e^2), if I take the mode field and subtract the core's diameter then I divide it by 2, should I get the penetration depth 'd' of evanescent field as given in the equation? (known that the penetration depth is the distance of the mode portion from core until it get zero in the cladding).Why I ask this question? because when I calculate the portion of mode field in the cladding (as defined) and then I calculate the penetration depth (according to the equation) I got answer that the portion of the MFD is longer than 'd'.. doesn't matter if I plug into the equation angle values close to the critical angle or far from it and the same about the refractive indices... Why this difference? is it right to use the equation here in this case?

 

[Twigg]

I would guess that the portion of the MFD in the cladding, as you defined it, isn't exactly the penetration depth. There is a boundary condition at the core-cladding interface, depending on the ratio of indecies of refraction. If you really wanted to know, you could take a gaussian electric field distribution with the specified MFD and evaluate the boundary condition at the core-cladding interface to solve for the electric field in the cladding. That will give you the penetration depth.

 

[rude man]

Source: fobasics.blogspot.com
Source: scirp.org when I calculate the portion of mode field in the cladding (as defined)

"As defined"? Obviously there is a disconnect between "d" in the formula and that "definition". I haven't looked at your sources; perhaps the answer in contained in them. It's probably a small difference anyway.

 

[Sciencestd]

Actually what I found out that they are not exactly the same.. because we get "d" when the light field go through a changing in the refractive index, for example between two surfaces: a light field hit surface in angle that satisfy the condition of TIR... .But the question now is why fundamentally we have Mode field diameter... I mean if we couple light into a fiber so the TIR condition is satisfied.. then what is the cause of the Mode field diameter inside the cladding.. why we should see Gaussian exceeded the core diameter...?!