Optical Fibers
Fig. 1 shows the cross-section of a typical single mode optical fiber. The core is a strand of ultra pure silica doped with selective elements. The purpose of dopant is to adjust the refractive index of silica thereby controlling its light propagation characteristics.
Fig. 1. Cross-sectional view of an optical fiber cable.
The core is covered with cladding, a silica layer of different composition with refractive index lower than core. Although the cladding does not carry light, it is nevertheless an essential part of the fiber. The cladding keeps the value of the critical angle constant throughout the whole length of the fiber. Optical Fibers are optical waveguides (carrier of light). So optical fibers can be used to make light bend round corners. Methods of producing optical fibers are reviewed at Fabrication of Optical Fiber. The propagation characteristics of light in silica fiber depend on the chemical composition and the cross-sectional dimensions of core and cladding. Based on these factors Fibers can be divided in to Single Mode and Multimode Fibers. See Fiber modes below. Click here for a history of optical fiber.
Propagation of light in a fiber
Fig. 2. Total internal reflection from core-cladding interface causes light to propagate through the fiber.
The angle qA in the Fig. 2 is called the acceptance angle. Any light entering the fiber at less than this angle will meet the cladding at an angle greater than qC. If light meets the inner surface of the cladding (the core-cladding interface) at greater than or equal to qC then total internal reflection occurs; all the energy in the light is reflected back into the core. In real applications, the light which enters the fiber is a focused beam. The beam zigzags along the core of the fiber, crossing over each other, and filling up the core with light.
Modes of Optical Fibers
Fig. 3. Profile of refractive index of an optical fiber. a) Core and clad, b) step profile, c) graded profile.
The profile of refractive index of an optical fiber determines how the fiber transports the light through itself. Propagation characteristics of step profile and graded profile fibers are illustrated in Fig. 4 a & b, respectively.
Fig. 4. a) Propagation characteristics through step profile and graded profile fibers.
The geometries D and d (see Fig. 3 a) determine the mode of the fiber. Typically core and cladding together have a diameter (D) of 125 µm. However, fiber with a core diameter (d) of 50 µm is known as multimode fiber, whereas, single mode fiber's core diameter is much smaller, varies between 8.6 to 9.5 µm (see ref. 1). Optical fiber transmission takes place through guided modes. Modes are labeled as TEMN or TMMN, depending on the value of the transverse electric field (EZ=0) or transverse magnetic field (HZ=0) at the surface of the fiber core in the transverse direction. Fibers, based on their dielectric constant and dimensions, support the fundamental mode TE11 (also known as HE11) or higher modes. A single mode fiber supports only one mode along its longitudinal axis (HE11). A multimode fiber supports many modes.
Fig. 5. Loss spectrum of optical fiber.
Modes may be thought of as specific path eigen-directions. The number of modes, M, of a multimode fiber with a step index profile can be computed from
.
Here, l is the wavelength and d is core diameter. Multimode and single mode fibers are manufactured by different processes. They have different refractive index profiles and different dimensions (see above). Consequently, they have different transmission characteristics, therefore, different applications.
Fig. 6.
© 1999 Anis Rahman