Fiber Optics

Fiber Optics Table of Contents 1.0 Intro to Fiber Optics 2.0 Fiber Constuction 1.0 Introduction to Fiber Optics Today many communications companies are replacing their copper carrier wires with fiber optic cables. A fiber optic cable is capable of transmitting laser light across thousands of miles and can carry many more messages at the same time than the copper wire of equivalent diameter. With the relentless pursuit of bandwidth, fiber optic cabling is being deployed at an ever increasing rate. This cable, which uses glass to carry light pulses, poses both advantages and challenges. The intent of this paper is to explain the how’s and why’s of fiber optic cabling and to provide a set of solutions to the challenges faced with it’s use and give you an understanding of fiber optic cable technology and its applications. Fiber optic cabling has much to offer, and in most cases, its use will provide benefits which justify the implementation. Since the invention of the telegraph by Samuel Morse in 1838, there has been a constant push to provide data at higher and higher rates.

Today, the push continues. Just as RS-232 attached terminals gave way to 10Mbps Ethernet and 4 and 16 Mbps Token Ring, these are giving way to Fast Ethernet (100Mbps), FDDI (100Mbps), ATM (155Mbps), Fiber Channel (1062Mbps) , Gigabit Ethernet (1000Mbps). With each of these increases in speed, the physical layer of the infrastructure is placed under more stress and more limitations. The cabling installed in many environments today cannot support the demands of Fast Ethernet let alone ATM, Fiber Channel, or Gigabit Ethernet. Fiber Optic cabling provides a viable alternative to copper.

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Unlike its metallic counterpart, fiber cabling does not have the severe speed and distance limitations that plague network administrators wishing to upgrade their networks. Because it is transmitting light, the limitations are on the devices driving it more than on the cable itself. By installing fiber optic cabling, the high cost of labor and the time associated with the cabling plant can be expected to provide service for the projected future. Plastic Optical Fiber (POF) technology is making fiber even more affordable and easier to install. Because the core is plastic instead of glass, terminating the cable is easier. The trade-off for this lower cost and ease of installation is shorter distance capabilities and bandwidth limitations. 2.0 Fiber Construction Fiber optic cabling has the following components (starting in the center and working out): core, cladding, coating, strength member, and jacket.

The design and function of each of these will be defined. The core is in the very center of the cable and is the medium of propagation for the signal. The core is made of silica glass or plastic (in the case of POF) with a high refractive index. The actual core is very small (compared to the wire gauges we are used to). Typical core sizes range from 8 microns (millionth of a meter) for single mode silica glass cores up to 1000 microns for multi mode POF. The cladding is a material of lower index of refraction which surrounds the core.

This difference in index forms a mirror at the boundary of the core and cladding. Because of the lower index, it reflects the light back into the center of the core, forming an optical wave guide. This is the same effect as looking out over a calm lake and noting the reflection, while looking straight down you see through the water. It is this interaction of core and cladding that is at the heart of how optical fiber works. The coating (also referred to as buffer or buffer coating) is a protective layer around the outside of the cladding.

It is typically made of a thermoplastic material for tight buffer construction and a gel material for loose buffer construction. As the name implies, in tight buffer construction, the buffer is extruded directly onto the fiber, tightly surrounding it. Loose buffer construction uses a gel filled tube which is larger than the fiber itself. Loose buffer construction offers a high degree of isolation from external mechanical forces such as vibration. Tight buffer construction on the other hand provides for a smaller bend radius, smaller overall diameter, and crush resistance.

To further protect the fiber from stretching during installation, and to protect it from expansion and contraction due to temperature changes, strength members are added to the cable construction. These members are made from various materials from steel (used in some multi – strand cables) to Kevlar. In single and double fiber cables, the strength members are wrapped around the coating. In some multi-strand cables, the strength member is in the center of the bundle. The jacket is the last item in the construction, and provides the final protection from the environment in which the cable is installed.

Of concern here is the intended placement of the cable. Different jackets provide different solutions for indoor, outdoor, aerial, and buried installations. Fiber Specifications The most common size of multi mode fiber used in networking is 62.5/125 fiber. This fiber has a core of 62.5 microns and a cladding of 125 microns. This is ideally suited for use with 850nm and 1300nm wavelength drivers and receivers.

For single mode networking applications, 8.3/125 is the most common size. It’s smaller core is the key to single mode operation. Numerical aperture and acceptance angles are two different ways of expressing the same thing. For the core / cladding boundary to work as a mirror, the light needs to strike at it a small / shallow angle (referred to as the angle of incidence). This angle is specified as the acceptance angle and is the maximum angle at which light can be accepted by the core.

Acceptance angle can also be specified as Numerical Aperture, which is the sin of the acceptance angle (Numerical Aperture = sin (acceptance angle)). Transmitter Specifications With a basic understanding of fiber construction, explanation of transmitters (the devices that put the pulses of light into the fiber) is in order. From a general level, there are three aspects of transmitters to discuss: 1.type of transmitter 2.wavelength of transmitter 3.power of the transmitter Transmitters can be divided into 2 groups, lasers and LEDs. LEDs are by far the most common as they provide low cost and very efficient solutions. Most multi mode transmitters are of the LED variety. When high power is required for extended distances, lasers are used.

Lasers provide reliable light and the ability to produce a lot of light energy. The drawbacks to lasers are their cost and electrical power consumption. Equipment using high power lasers must provide cooling and access to a primary power source such as 120V AC. Transmitter types can also be broken down into single mode versus multi mode transmitters. Multi mode transmitters are used with larger cable (typically 62.5/125 microns for most data networking applications) and emit multiple rays or modes of light into the fiber. Each one of these rays enters at a different angle and as such has a slightly different path through the cable. This results in the light reaching the far end at slightly different times.

This difference is arrival times are termed modal dispersion and causes signal degradation. Single mode transmitters are used with very small cable (typically 8/125 microns) and emit light in a single ray. Because there is only one mode, all light gets to the far end at the same time, eliminating modal dispersion. The wavelength of the transmitter is the color of the light. The visible light spectrum starts around 750nm and goes to 390nm.

The 850nm transmitters common in multi mode Ethernet can be seen because 850nm is the center of their bandwidth and they emit some visible light in the 750nm range giving them their red c …


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