There are three generic types of fiber as distinguished by their mode characteristics and physical properties: the first fiber type is single mode, also called monomode, fiber, the second type is step-index multimode fiber, and the third type is graded-index multimode fiber. We will talk about them in the following slides.
This figure shows a graphical presentation of a multimode fiber on the left, and a single mode fiber on the right. What we want to stress here is the dimensions difference between single mode and multimode fiber.
We can see that the fiber diameter is the same, no matter single mode or multimode fiber, which is 125 micron meter. However, there is a big difference between the core diameters, 50 micron meter for multimode fiber and between 8.6 and 9.5 micron meter for single mode fiber.
There is another common core diameter for multimode fiber, which is 62.5 micron meter, we will see it in future videos.
This figure shows the fiber construction and refractive index profile for step-index fiber (figure a) and graded-index fiber (figure b).
Step-index fiber has a sharp change in refractive index, and graded-index fiber has a continuous and smooth change in refractive index.
Step-index multimode fiber is cheaper than grade-index multimode fiber. However, step-index multimode fiber offers smaller bandwidth capacity, which is measured by its bandwidth-distance product. It has 10 ~100 MHz*km and has a repeater spacing about 10 km.
Graded-index multimode fiber is a little bit more expensive than step-index multimode fiber, but it has much better bandwidth-distance product, on the order of 400 ~ 1000 MHz*km if a laser diode is used as the light source.
But if an LED light source is used, the bandwidth-distance product with graded-index fiber would be less, on the order of 300 MHz*km. This degradation is caused by material dispersion.
This figure shows refractive index profiles and the resulting modes of propagation for the three types of common silica-based optical fiber.
Single mode or monomode fiber is designed such that only one mode is propagated. To do this, the normalized frequency V has to be less than 2.405. Such a fiber has no modal dispersion because only one mode propagates.
Single mode fiber typically has a core refractive index n1 = 1.48, and a cladding refractive index n2 = 1.46. In this case, if we have a optical light source with a wavelength of 820nm, in order to achieve single mode operation, the maximum core diameter is only about 2.6 micron meter, a very tiny diameter.
In this figure, the (c) figure shows the refractive index profile for a single mode fiber. Single mode fiber has the best bandwidth-distance product of the three fiber types discussed.
As we learned from the previous video, numerical aperture NA is a measure of the light-gathering capability of an optical fiber. The much larger core diameter of a multimode fiber gives a bigger numerical aperture, around 0.22. On the other hand, the much smaller core diameter of a single mode fiber gives a much smaller numerical aperture, only about 0.11.
Since multimode fiber has larger core diameter, so it allows several or many modes to propagate down the fiber. Some of these modes may only travel a short distance and then disappear; others may propagate the full length of the fiber. Multimode propagation is shown in this figure.
However, when these many modes reach the end of the fiber, we will see a major problem called modal dispersion. Let me explain it here.
Let’s consider a light pulse traveling down the fiber some distance. The pulse is represented by light energy of several modes. The lowest-order mode arrives at the receiver first, followed by other light energy delayed some because it or they traveled further.
The arriving pulse, made of light energy components arriving somewhat later, tend to broaden the pulse energy of the initial arriving pulse of the lowest-order mode.
The trouble is that these pulses, or lack of pulses, represent 1s and 0s. Let’s say that a pulse represents a binary 1, and no pulse represents a binary 0. Suppose now we transmit an initial 1 followed by a 0. Dispersed energy from the first pulse spills into the space for the second bit, there is a good chance that the light detector will interpret the second bit as a binary 1 when it was supposed to be a binary 0. A bit error has occurred. This is a rather simplistic description showing the bad result of dispersion causing intersymbol interference.
Under these conditions, as the bit rate is increased, the pulse width gets shorter, and the effects of modal dispersion are more and more worse and the Bit Error Rate on the link approaches a value that is completely unacceptable.
This situation can be mitigated or eliminated by this following three methods:
1.Shortening the fiber link
2.Reducing the bit rate
3.Using single mode fiber