Fields of planar guided waves are confined in the depth direction (designated as the x direction in this book) to the vicinity of the high index layer which is the core. The mathematical description of the planar waveguide modes has already been discussed in Sections 1.2.3 and 1.2.4. Since the high index layer is often located near the surface, the guided waves are sometimes called surface waves. As the surface contour of the various layers of the waveguide changes gently, the planar guided-waves will follow the contour. Planar guided waves have three distinct properties.

1. (1) The evanescent field of the guided-wave modes extends into the air (or cladding) above the surface. Thus they can be excited or coupled out of the core from the air (or cladding layer) adjacent to the surface.

2. (2) The scattered radiation of the propagating wave is often also visible in the free space above. It can be used to monitor the propagation of the guided wave.

3. (3) Guided waves are free to propagate in any direction in the transverse plane (designated as the yz plane in this book).

Summation of planar guided waves can form divergent, convergent or diffracted waves in the transverse plane. How to analyze the generalized planar guided waves has already been discussed in Section 1.2.5.

A distinct feature of planar waveguide devices is the utilization of the diffraction, focusing and collimation properties in the transverse plane to achieve focusing, switching, deflection, wavelength filtering or other functions.