FIBER OPTIC 101

Three basic types of fiber optic cable are used in communication systems:
1. Step-index multimode
2, Step-index single mode
3, Graded-index

Step-index multimode fiber has an index of refraction profile that “steps” from low to high to low as measured from cladding to core to cladding. Relatively large core diameter and numerical aperture characterize this fiber. The core/cladding diameter of a typical multimode fiber used for telecommunication is 62.5/125 μm (about the size of a human hair). The term “multimode” refers to the fact that multiple modes or paths through the fiber are possible. Step-index multimode fiber is used in applications that require high bandwidth (< 1 GHz) over relatively short distances (< 3 km) such as a local area network or a campus network backbone.

Step-Index Multimode

The major benefits of multimode fiber are: (1) it is relatively easy to work with; (2) because of its larger core size, light is easily coupled to and from it; (3) it can be used with both lasers and LEDs as sources; and (4) coupling losses are less than those of the single-mode fiber. The drawback is that because many modes are allowed to propagate (a function of core diameter, wavelength, and numerical aperture) it suffers from modal dispersion. The result of modal dispersion is bandwidth limitation, which translates into lower data rates.

Single-mode step-index

Single-mode step-index fiber allows for only one path, or mode, for light to travel within the fiber. In a multimode step-index fiber, the number of modes Mn propagating can be approximated by
Here V is known as the normalized frequency, or the V-number, which relates the fiber size, the refractive index, and the wavelength. The V-number is given by Equation 
or by Equation
In either equation, a is the fiber core radius, λ is the operating wavelength, N.A. is the numerical aperture, n1 is the core index, and Δ is the relative refractive index difference between core and cladding.

The analysis of how the V-number is derived is beyond the scope of this module, but it can be shown that by reducing the diameter of the fiber to a point at which the V-number is less than 2.405, higher-order modes are effectively extinguished and single-mode operation is possible.

The core diameter for a typical single-mode fiber is between 5 μm and 10 μm with a 125-μm cladding. Single-mode fibers are used in applications in which low signal loss and high data rates are required, such as in long spans where repeater/amplifier spacing must be maximized.

Because single-mode fiber allows only one mode or ray to propagate (the lowest-order mode), it does not suffer from modal dispersion like multimode fiber and therefore can be used for higher bandwidth applications. However, even though single-mode fiber is not affected by modal dispersion, at higher data rates chromatic dispersion can limit the performance. This problem can be overcome by several methods. One can transmit at a wavelength in which glass has a fairly constant index of refraction (~1300 nm), use an optical source such as a distributed feedback laser (DFB laser) that has a very narrow output spectrum, use special dispersion compensating fiber, or use a combination of all these methods. In a nutshell, single-mode fiber is used in high-bandwidth, long-distance applications such as long-distance telephone trunk lines, cable TV head-ends, and high-speed local and wide area network (LAN and WAN) backbones. The major drawback of single-mode fiber is that it is relatively difficult to work with (i.e., splicing and termination) because of its small core size. Also, single-mode fiber is typically used only with laser sources because of the high coupling losses associated with LEDs.

Graded-index fiber is a compromise between the large core diameter and N.A. of multimode fiber and the higher bandwidth of single-mode fiber. With creation of a core whose index of refraction decreases parabolically from the core center toward the cladding, light traveling through the center of the fiber experiences a higher index than light traveling in the higher modes. This means that the higher-order modes travel faster than the lower-order modes, which allows them to “catch up” to the lower-order modes, thus decreasing the amount of modal dispersion, which increases the bandwidth of the fiber.

Graded-index fiber