Definition: waveguide structures guiding light only in one dimension
Alternative term: slab waveguides
Planar waveguides, also called slab waveguides, are waveguides with a planar geometry, which guide light only in one dimension. They are often fabricated in the form of a thin transparent film with increased refractive index on some substrate, or possibly embedded between two substrate layers. For example, a thin neodymium-doped YAG layer can be embedded in undoped YAG with slightly lower refractive index (see Figure 1). Advantages of that more symmetric design are a symmetric shape of the waveguide mode and the easier realization of single-mode waveguiding.
Figure 1: A planar waveguide made on a crystal or glass piece (left side), or embedded between two layers (right side).
A waveguide function can also be achieved by diffusion of an index-raising agent into a surface, often achieved by treatment in an oven. The resulting refractive index profile is then usually not a step-index profile, but one with a smooth shape.
Active planar waveguides are sometimes used e.g. for optical amplifiers with high gain (compared with that of bulk amplifiers), relatively high output power (at least multiple watts) and high beam quality at least in one dimension. Some of these devices can be side pumped with a proximity-coupled laser diode, not requiring any pump optics.
There are also planar waveguide lasers, using reflecting end surfaces, e.g. using the Fresnel reflections.
Bibliography
[1]
A. W. Snyder and J. D. Love, Optical Waveguide Theory, Chapman and Hall, London (1983)
