Meaning
Optical phenomena occur when light traveling through a high refractive index medium strikes a boundary with a lower index medium at an angle greater than the critical angle. Total internal reflection guides light through fiber optic cables and glass light guides with zero transmission loss. Engineers use this effect to design optical sensors and communication networks that carry high data rates.
Misaligning the input beam or contaminating the boundary surface causes light leakage and ruins the signal.
Critical Angle
The minimum angle of incidence for complete reflection depends on the ratio of the two refractive indices. Below this angle, a portion of the light refracts through the boundary and escapes the waveguide. Spectrometers measure the angle at which transmission drops to zero to verify the material properties of the cladding.
Small index variations can shift the critical angle and cause signal loss in the system.
Evanescent Wave
An electromagnetic field extends into the lower index medium to a depth of a few hundred nanometers during total internal reflection. This field enables high sensitivity sensing of surface coatings and biomolecules in optical instruments. Sensor chips measure changes in the reflected light intensity to detect the presence of target molecules.
Any dust on the reflecting surface scatters light and introduces noise into the measurement.
Refractive Index
Difference in optical density between the core and the cladding determines the guiding efficiency of the waveguide. To achieve total internal reflection, the cladding material must possess a lower refractive index than the core material.
