Waveguide - Definition and Applications

A waveguide is an optical structure that directs the electromagnetic waves of an optical spectrum. Some of the most common types of waveguides include rectangular waveguides and optical fibers.

In general, waveguides consist of a region of increased refractive index with respect to the surrounding medium, known as cladding. Electromagnetic wave guidance can also be achieved using reflections at metallic interfaces. Certain waveguides also involve plasmonic effects in metals.

Most of the waveguides feature two-dimensional guidance, which limits the extension of directed light in two directions, and allows light propagation in only one direction. Optical fibers are the most common type of two-dimensional waveguides. One-dimensional waveguides are also available, e.g. planar waveguides.

Types of Waveguides

Waveguides are classified into two major types based on their structure:

• Planar waveguides – These waveguides have an optical confinement in one transverse direction. The core of these waveguides is sandwiched between cladding layers in one direction, with an index profile. The upper and lower cladding layers are known as the cover and substrates respectively, while the core is also called the film
• Non-planar waveguides – The optical confinement of non-planar waveguides is in two-directions. The core of these waveguides is surrounded by a cladding in all transverse directions, e.g. channel waveguides

Modes of Waveguides

Waveguides have a finite number of guided propagation modes, with finite intensity distributions around the waveguide core. The number of waveguide modes, propagation constants of modes, and transverse amplitude profiles are dependant on the optical frequency and the waveguide structure.

Another type of waveguide mode is known as the cladding mode, with intensity distributions filled in the cladding and core regions. Optical fibers include a large number of cladding modes that feature substantial propagation losses at an outer interface of the cladding.

Different modes of fiber and other waveguides can be determined using the mode solvers. These mode solvers are included in fiber simulation software, and they can numerically calculate the modes based on the symmetric profiles and guidance of the waveguides.

Applications

Waveguides are widely used in the following:

• Optical fiber communication
• Photonic integrated circuits
• Maintaining high optical intensities in non-linear devices
• As mode cleaners
• Optical interferometers

References

• Waveguide Modes – RP Photonics
• Optical waveguides – The Hong Kong University of Science & Technology