A single fiber cable consists of a glass core surrounded by a concentric glass cladding; the two glasses having different refractive index so that light is confined to the core by total internal reflection. The protective plastic jacket is color-coded so that individual fibers can be identified in multiple-fiber bundles (tubes). The Below Shown fig Single mode fiber. 
Typical dimensions of the fiber are:

Plastic Jacket : 250micro mt
Glass Cladding : 125micro mt
Glass Core : 62.5micro mt (Single mode)

TYPES OF FIBER
There are categories of fiber as distinguished by their modal and physical properties:
¨ Single mode
¨ Step index (multimode)
¨ Graded index (multimode)
Single-mode fiber is designed such that only one mode is propagated. 
For single-mode operation the maximum core diameter would be 2.6 mm, a very small diameter indeed.
Step-index fiber is characterized by an abrupt change in refractive index, and graded-index fiber is characterized by a continuous and smooth change in refractive index.
Step-index fiber is more economical than graded-index fiber, For step-index fiber the distance-bandwidth product, the measure of dispersion discussed above, is on the order of 10-100 MHz/km. With repeater spacing on the order of 10 km, only a few megahertz of bandwidth is possible.

Graded-index fiber is more expensive than step-index fiber, but it is one alternative for improved distance-bandwidth products. When a laser diode source is used, values of from 400 to 1000 MHz/km are possible. If an LED source is used with its much broader emission spectrum, distance-bandwidth products with grade-index fiber can be achieved up to about 300 MHz/km. 

There are two additional criteria for optical fiber that are important in system design. These are minimum bending radius and fiber strength.

Radiation losses at fiber waveguide bends are usually quite small and may be neglected in system design unless the bending radius is smaller than that specified by the manufacturer. Minimum bending radii vary from about 2 to 10 cm, depending on the cable characteristics, or, as a rule of thumb, around 10 times the cable diameter.

Fiber cable strength is also specified by the manufacturer. For example, one manufacturer for a specific cable type specifies a maximum pulling tension of 1780 Newton's (N) (400 lb) at 20°C, a maximum permissible compression load of 655 N/cm (375 lb/in.) flat plate, and a maximum permissible impact force of 280 N-cm (160lb-in).

Stander Fiber Coding
Fibers inside each fiber optic cable are identified by color. The following list in the industry’s standard fiber color code. Contact the fiber manufacturer for more information. 

1.Blue 2.Orange 3.Green
4.Brown 5.Slate 6.White
7.Red 8.Black 9.Yellow
10.Violet 11.Rose 12.Aqua