
16.7.5 Variable Attenuators
An in-fiber variable attenuator can be built on a D-shape fiber as a type of the
evanescent field device. If a bulky external material, whose refractive index is
greater than the mode effective index, replaces a part of the evanescent field
reachable cladding, the mode can become leaky and some of the optical power
can be radiated [119]. If the index of the external material can be changed with a
controllable mean, through the effects such as thermo-optic, electro-optic, or
acoustic-optic, a device with controllable attenuation is achievable.
The combination of the fiber and the external bulk overlay has been modeled
as