Optical Waveguides: Basic Geometries

Optical Waveguides: Basic Geometries

The basic element in integrated photonic technology is the optical waveguide. A
waveguide can be defined as an optical structure that allows the confinement of light
within its boundaries by total internal reflection. As we saw in the previous chapter,
in order for total internal reflection to take place it is necessary to surround
a high index medium, where most of the radiation energy is concentrated, by low
refractive index media. A very simple example of light confinement happens in a (planar)
film of glass situated in air. If the refractive index of the glass is n, the rays
inside the film that propagate with an internal angle θ greater than the critical angle
θc = sin−1(1/n) will suffer total reflection at the interfaces, and will remain trapped
inside the film: in these circumstances we say that the film situated in air acts as an
optical waveguide.

This is the basic mechanism that operates in luminescent solar concentrators (LSC)
[1], that consist of a glass film in which some organic luminescent molecules (dyes)
are embedded (Figure 3.1). The dye molecules absorb solar radiation, and then the
radiation is re-emitted isotropically. An important fraction of the emitted radiation
is trapped by total internal reflection at the upper and lower interfaces, and reaches
the sides of the film, where a stack of solar cells are attached; in this way, a high
geometrical collection factor is obtained.
Another type of optical waveguide, that can easily be visualised is a solid cylindrical
glass tube (Figure 3.2): since the refractive index of glass is greater than the outer
medium (air in this case), radiation travelling at angles greater than the critical angle
will be confined in it by total internal reflection: in this case the confinement of the
light extends in two dimensions.