The success of light treatment is largely dependent on the light-delivery system used. Generally, a light guide, which delivers light energy to a target to illuminate it, consists of an assembly of optical fibers that are bundled but not ordered. A fiber is an optical waveguide that uses the phenomenon of total internal reflection for light transportation with low losses and is made from transparent glass, quartz, polymer, or crystal, usually with a circular cross section. It consists of at least two parts, an inner part or core, with a higher refractive index and through which light propagates, and an outer part or cladding, with a lower refractive index and which provides a totally reflecting interface between the core and the cladding.
A multimode fiber is a single fiber that allows the excitation (direction) of many modes (rays); for example, for a fiber with a core diameter of 50 pm, numerical aperture, NA = 0.2, and an excitation wavelength of 633 nm, the number of excited modes is equal to 1250.
A single-mode fiber is a fiber in which only a single mode can be excited; for a fiber with a numerical aperture NA = 0.1 and wavelength 633 nm the single mode can be excited if the core diameter is less than 4.8 pm.
The numerical aperture is characteristic of the light-gathering power of an objective or optical fiber; it is proportional to the sine of the acceptance angle, a higher NA more light is collected by a fiber.
A fiber bundle is a flexible bundle of individual optical fibers arranged in an ordered or disordered manner and correspondingly named regular and irregular bundles. The irregular fiber bundles are used for illumination and collecting light from a tissue, as well as a regular bundle provides transportation of tissue image.
Fiber-optics is a well-developed industrial field, where various fiber instrumentation is available, such as connectors, couplers, GRIN – or selfoc-lenses, focons, fiber multiplexes (dividers), as well as fiber-optic catheters—a flexible single fiber or a fiber bundle used to move light into body cavities and back. For example, GRIN (gradient index) lens focuses light through a precisely controlled radial variation of the lens material’s index of refraction from the optical axis to the edge of the lens; this allows a GRIN lens with flat or angle – polished surfaces to collimate light emitted from an optical fiber or to focus an incident beam into an optical fiber; end faces can be provided with an antireflection coating to avoid unwanted back reflection.