To Study the Basic Structure and Types of the Optical Fiber

Nov 30 -0001

Objective / Aim of the Experiment

To study the basic structure and types of the optical fiber.

Theory

An optical fiber (or fibre) is a glass or plastic solid rode that carries light along its length with the help of the total internal reflection. It consists of core and cladding. The refractive index of the core is greater then the cladding. They can be either single mode or multi-mode fibers.

Multi-mode Fiber

Fiber with large core diameter (greater than 10 micrometers) may be analyzed by geometric optics. Such fiber is called multi-mode fiber, from the electromagnetic analysis.

Step-index Multi-mode Fiber

In a step-index multi-mode fiber, rays of light are guided along the fiber core by total internal reflection. Rays that meet the core-cladding boundary at a high angle (measured relative to a line normal to the boundary), greater than the critical angle for this boundary, are completely reflected. The critical angle (minimum angle for total internal reflection) is determined by the difference in index of refraction between the core and cladding materials.

Rays that meet the boundary at a low angle are refracted from the core into the cladding, and do not convey light and hence information along the fiber. The critical angle determines the acceptance angle of the fiber, often reported as a numerical aperture. A high numerical aperture allows light to propagate down the fiber in rays both close to the axis and at various angles, allowing efficient coupling of light into the fiber. owever, this high numerical aperture increases the amount of dispersion as rays at different angles have different path lengths and therefore take different times to traverse the fiber. A low numerical aperture may therefore be desirable.

Graded-index Fiber

In graded-index fiber, the index of refraction in the core decreases continuously between the axis and the cladding. This causes light rays to bend smoothly as they approach the cladding, rather than reflecting abruptly from the core-cladding boundary. The resulting curved paths reduce multi- path dispersion because high angle rays pass more through the lower-index periphery of the core, rather than the high-index center. The index profile is chosen to minimize the difference in axial propagation speeds of the various rays in the fiber. This ideal index profile is very close to a parabolic relationship between the index and the distance from the axis.

Single mode Fiber

The most common type of single-mode fiber has a core diameter of 8?10 micrometers and is designed for use in the near infrared. The mode structure depends on the wavelength of the light used, so that this fiber actually supports a small number of additional modes at visible wavelengths. Multi-mode fiber, by comparison, is manufactured with core diameters as small as 50 micrometers and as large as hundreds of micrometres. The normalized frequency V for this fiber should be less than the first zero of the Bessel function J_o approximately 2.405). Single mode fiber has the least dispersion and hence are used for longer distances.