Fundamentals of Light Propagation in Light Guides

Fiber Optic light guides are media whose transverse dimension (diameter, thickness) can be very small, typically 10mm to 1 mm. They are very flexible and can be produced in virtually any desired length. The material is usually glass, quartz or plastic. For special applications, other exotic materials such as liquid light guides, sapphire, fluoride or calcogenide may be used. There are some unavoidable requirements for good light transmission, such as pure glass materials for the core and cladding and high transparency for the spectrum of interest. Minimal optical dispersion is also desired. Process parameters such as glass transformation temperature, viscosity, inclusions and chemical affinity dictate the economics and quality of the fiber product. Light launched into a fiber will after a given length reach the core material boundary and pass to another medium (glass, air, etc.). Depending on the incident angle, some of the energy will be refracted outward (leaky modes) and some will reflect back into the core material (Figure 2).

Fig 2.: Light transmission in Medium with n1 > n2

Maximum light can only be transmitted through the light guide if total internal reflection occurs at the core clad interface. In this case, b > bMin, where b Min is the angle of incidence for which b’ = 90°.

The light guide has a cladding material n2 ; n .. Index of refraction for the coupling medium (usually air; n = 1)

Propagation Mode

A fiber core is fairly thick relative to a wavelength of light and allows the light to enter it many difference angles. There are a finite number of angles at which the rays reflect and propagate the length of the fiber. Each angle defines a path or a mode.

There are basically two modes of a transmission in a fiber. A multimode fiber has a number of paths in which light ray may travel. A singlemode has a light ray in one direction only

Fiber are further classified by the refractive index profile of their core. They can be either step index or graded index. Three main types of fibers are multimode grades index and single mode fiber.