Optical Selectivity

Just as in the RF domain, we can produce wavelength-selective devices that present a
low loss for some wavelengths and a high loss for others by introducing a phase shift
in a portion of the light energy. The options are as follows:
Thin film dielectric coatings. These are substrates with alternative layers of high
and low refractive index dielectric materials. The transmitted wavelengths are a
function of the thickness of the cavity coating. These devices are a very expensive
way of providing multiple channel selectivity.
Fiber Bragg gratings. These are devices in which the refraction index of the fiber
core can be changed in such a way that a narrow wavelength of light will be
reflected and all other wavelengths will be transmitted. The bandwidth is
inversely proportional to the length of the grating.
Optical filtering diffraction gratings. Optical filtering diffraction gratings reflect
light at an angle proportional to the wavelength, creating constructive and
destructive interference. Depending on the wavelength of the incident light,
there is an angle for which the individual light waves will be exactly one wavelength
out of phase, which means they will add constructively. Because insertion
loss is independent of the number of channels, optical filtering diffraction gratings
can support lots of channels.
Optical filtering integrated optical devices. These devices are the optical equivalent
of an integrated electronic circuit. Cores surrounded by cladding are layered
onto a silicon substrate. The cores act as optical waveguides. These devices
are good for optical cross connects and demultiplexers. 328