# Confinement factor for an optical fiber

I was reading the chapter about optical fibers of "Fiber - optic communication systems" and I have read this statement:

The fraction of the power contained in the core is given by the confinement factor. Although nearly 75% of the mode power resides in the core for V = 2, this percentage drops down to 20% for V = 1.

where V is the normalized frequency of an optical fiber.

So, the confinement factor represents how much energy is contained in the core (which is the useful energy) with respect to the total energy, that is higher since part of power is contained inside the cladding:

I have a simple question about this fact. An optical fiber is based on the total internal reflection phenomenon, and so I'd say that all losses through the cladding are parasitic effects due to incidence angles lower than critical angle (for instance determined by fiber bending etc). So, since they are small parasitic effects, it seems too strange to me that the confinement factor may be 20% (if the fiber is rightly coupled to the source, I'd say that only few few rays may be transmitted inside the cladding)! Maybe there is something I'm not considering. Can you help me on understanding that?

• Finally, regarding surprising amount of power which leaks into the cladding for the case of fibers with small $V$, single mode fibers. The point with these fibers is that size of the core is close to the far-field diffraction limit for light. That is, the core is only a few optical wavelengths across. Given that the evanescent penetration is also only a few optical wavelengths it is not surprising that the fraction of power contained in the evanescent field becomes large as the core itself become comparable in size to an optical wavelength. Jul 9, 2020 at 18:37