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Sciencestd
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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?