Waveguide (optics)

[1] Optical waveguides can be classified according to their geometry (planar, strip, or fiber waveguides), mode structure (single-mode, multi-mode), refractive index distribution (step or gradient index), and material (glass, polymer, semiconductor).

Similarly, light traveling in the opposite direction (from glass into air) takes the same path, bending away from the normal.

These extra rays correspond to a higher density of states in more-advanced formulations based on the Green's function.

Maxwell's equations can be solved by analytical or numerical methods for a full-field description of a dielectric waveguide.

The slab waveguide consists of three layers of materials with different dielectric constants, extending infinitely in the directions parallel to their interfaces.

For guided modes, the field in domain II in the diagram is propagating and can be treated as a plane wave.

The plane wave in domain II bounces between the top and bottom interfaces at some angle typically specified by the

Because guided modes are trapped in the slab, they cannot be excited by light incident on the top or bottom interfaces.

However, waveguides can also have periodic changes in their cross-section while still allowing lossless transmission of light via so-called Bloch modes.

Configuring the waveguides in 3D space provides integration between electronic components on a chip and optical fibers.

Such waveguides may be designed for a single mode propagation of infrared light at telecommunication wavelengths, and configured to deliver optical signal between input and output locations with very low loss.

An increase in the refractive index of a material may be induced by nonlinear absorption of pulsed laser light.

In order to maximize the increase of the refractive index, a very short (typically femtosecond) laser pulses are used, and focused with a high NA microscope objective.

[10] A variation of this method uses a low NA microscope objective and translates the focal spot along the beam axis.

[11] When transparent material is exposed to an unfocused laser beam of sufficient brightness to initiate photorefractive effect, the waveguides may start forming on their own as a result of an accumulated self-focusing.

Continued exposure results in a buildup of the refractive index towards the centerline of each waveguide, and collapse of the mode field diameter of the propagating light.

Light refracts at a dielectric interface, a. , establishing a correspondence between rays in the two media, b . Some rays in the higher index medium are left out of the pairing (red) and are trapped by total internal reflection . c. This mechanism can be used to trap light in a waveguide . d. This is the basic principle behind fiber optics in which light is guided along a high index glass core in a lower index glass cladding .
A dielectric slab waveguide consists of three dielectric layers with different refractive indices.
Optical waveguides formed in pure silica glass as a result of an accumulated self-focusing effect with 193 nm laser irradiation. Pictured using transmission microscopy with collimated illumination.
The propagation of light through a multi-mode optical fiber.