How many types of Fiber Optics?
The refractive Index profile describes the relation between the indices of the core and cladding. Two main relationship exists :
(I) Step Index
(II) Graded Index
The step index fiber has a core with uniform index throughout. The profile shows a sharp step at the junction of the core and cladding. In contrast, the graded index has a non-uniform core. The Index is highest at the center and gradually decreases until it matches with that of the cladding. There is no sharp break in indices between the core and the cladding.
By this classification there are three types of fibers :
(I) Multimode Step Index fiber (Step Index fiber)
(II) Multimode graded Index fiber (Graded Index fiber)
(III) Single- Mode Step Index fiber (Single Mode Fiber)
This fiber is called “Step Index” because the refractive index changes abruptly from cladding to core. The cladding has a refractive index somewhat lower than the refractive index of the core glass. As a result, all rays within a certain angle will be totally reflected at the core-cladding boundary. Rays striking the boundary at angles grater than the critical angle will be partially reflected and partially transmitted out through the boundary. After many such bounces the energy in these rays will be lost from the fiber.
The paths along which the rays (modes) of this step index fiber travel differ, depending on their angles relative to the axis. As a result, the different modes in a pulse will arrive at the far end of the fiber at different times, resulting in pulse spreading which limits the bit-rate of a digital signal which can be transmitted.
The maximum number of modes (N) depends on the core diameter (d), wavelength and numerical aperture (NA)
This types of fiber results in considerable model dispersion, which results the fiber’s band width.
(2) GRADED INDEX MULTI-MODE FIBER
This fiber is called graded index because there are many changes in the refractive index with larger values towards the center. As light travels faster in a lower index of refraction. So, the farther the light is from the center axis, the grater is its speed. Each layer of the core refracts the light. Instead of being sharply reflected as it is in a step index fiber, the light is now bent or continuously refracted in an almost sinusoidal pattern. Those rays that follow the longest path by travelling near the outside of the core, have a faster average velocity. The light travelling near the center of the core, has the slowest average velocity.
As a result all rays tend to reach the end of the fiber at the same time. That causes the end travel time of different rays to be nearly equal, even though they travel different paths.
The graded index reduces model dispersing to 1ns/km or less.
Graded index fibers have core diameter of 50, 62.5 or 85 mm and a cladding diameter of 125 mm. The fiber is used in applications requiring a wide bandwidth a low model dispersion. The number of modes in the fiber is about half that of step index fiber having the same diameter & NA.
(3) SINGLE MODE FIBER
Another way to reduce model dispersion is to reduce the core’s diameter, until the fiber only propagates one mode efficiently. The single mode fiber has an exceedingly small core diameter of only 5 to 10 m m. Standard cladding diameter is 125 mm. Since this fiber carries only one mode, model dispersion does not exists. Single mode fibers easily have a potential bandwidth of 50to 100GHz-km.
The core diameter is so small that the splicing technique and measuring technique are more difficult. High sources must have very narrow spectral width and they must be very small and bright in order to permit efficient coupling into the very small core dia of these fibers.
One advantage of single mode fiber is that once they are installed, the system’s capacity can be increased as newer, higher capacity transmission system becomes available. This capability saves the high cost of installing a new transmission medium to obtain increased performance and allows cost effective increases from low capacity system to higher capacity system.
As the wavelength is increased the fiber carries fewer and fewer modes until only one remains. Single mode operation begins when the wavelength approaches the core diameter. At 1300 nm, the fiber permits only one mode, it becomes a single mode fiber.
As optical energy in a single mode fiber travels in the cladding as well as in the core, therefore the cladding must be a more efficient carrier of energy. In a multimode fiber cladding modes are not desirable, a cladding with in efficient transmission characteristic can be tolerated. The diameter of the light appearing at the end of the single mode fiber is larger than the core diameter, because some of the optical energy of the mode travels in the cladding. Mode field diameter is the term used to define this diameter of optical energy.
Optical fiber systems have the following parameters.
It is a characterstic of light that is emitted from the light source and is measures in nanometers (nm). In the visible spectrum, wavelength can be described as the colour of the light.
For example, Red Light has longer wavelength than Blue Light, Typical wavelength for fiber use are 850nm, 1300nm and 1550nm all of which are invisible.
It is number of pulse per second emitted from a light source. Frequency is measured in units of hertz (Hz). In terms of optical pulse 1Hz = 1 pulse/ sec.
A narrow window is defined as the range of wavelengths at which a fiber best operates. Typical windows are given below :
|800nm – 900nm||850nm|
|1250nm – 1350nm||1300nm|
|1500nm – 1600nm||1550nm|