Specialty Optical Fibers Handbook

the optical rays do not reflect exactly on the boundary between two optical

media. In fact, the rays (electromagnetic field) break into the adjacent medium

and do not suddenly drop to zero at the core–cladding boundary. They decay

instead exponentially within the cladding. The penetration depth d

p

depends on

the difference between the respective refractive indices of core and cladding

material (the higher the difference, the lower d

p

), the angle « of incidence (the

steeper « the higher d

p

) and the wavelength l of the incident light (the higher l,

the higher d

p

). In ray optics, this phenomenon is known as ‘‘Goos-Ha

¨

nchen-

Shift,’’ indicating that the ...