Why Using Fiber for transmission Why Using Fiber for transmission Optical fibers are widely used in fiber-optic communications, where they permit transmission over long distances and at very high bandwidths. Advantages over copper wiring The advantages of optical fiber communication with respect to copper wire systems are: Broad bandwidth Immunity to electromagnetic interference Light transmission through optical fibers is unaffected by other electromagnetic radiation nearby, so information traveling inside the optical fiber is immune to electromagnetic interference. Low attenuation loss over long distances Attenuation loss can be as low as 0.2 db/km in optical fiber cables, allowing transmission over long distances without the need for repeaters. Electrical insulator Optical fibers do not conduct electricity, preventing problems with ground loops and conduction of lightning. 2
How Optical Fiber Works As a light ray passes from one transparent medium to another, it changes direction; this phenomenon is called refraction of light. How much that light ray changes its direction depends on the refractive index of the mediums. Refractive index measures how much a material refracts light. When a light ray crosses an interface into a medium with a higher refractive index, it bends towards the normal. Conversely, light traveling cross an interface from a higher refractive index medium to a lower refractive index medium will bend away from the normal. At some point, it will not pass through to the second medium at all. Instead, all of it will be reflected back into the first medium, a process known as total internal reflection. 3
How Optical Fiber Works Optical fibers are based entirely on the principle of total internal reflection. Optical fiber is a long, thin strand of very pure glass about the diameter of a human hair. Optical fibers are arranged in bundles called optical cables and used to transmit light signals over long distances. 4
Structure of an Optical Fiber Typical optical fibers are composed of core, cladding and buffer coating. The core is the inner part of the fiber, which guides light. The cladding surrounds the core completely. The refractive index of the core is higher than that of the cladding, so light in the core that strikes the boundary with the cladding at an angle shallower than critical angle will be reflected back into the core by total internal reflection. The buffer coating is usually made from soft or hard plastic such as acrylic, nylon. Buffer coating provides mechanical protection and bending flexibility for the fiber. 5
Optical Fiber Modes An optical fiber guides light waves in distinct patterns called modes. Mode describes the distribution of light energy across the fiber. Light rays enter the fiber at a range of angles, and rays at different angles can all stably travel down the length of the fiber as long as they hit the core-cladding interface at an angle larger than critical angle. These rays are different modes. Fibers that carry more than one mode at a specific light wavelength are called multimode fibers. Some fibers have very small diameter core that they can carry only one mode which travels as a straight line at the center of the core. These fibers are single mode fibers. 6
About Single-Mode and Multi-Mode Multimode fibers Manufactured using larger glass fibers that are measured with either a 50 or 62.5 micron diameter The larger core diameter allows for less precise tolerances than those found in singlemode fibers and are often used with lower cost light sources. Because of the high dispersion and attenuation rate with this type of fiber, the quality of the signal is reduced over long distances, and the usable bandwidth is more limited than with Single-mode. Typically, LAN networks, security systems and other low speed fiber applications have been using multimode fiber optic cables. Single-mode fibers Manufactured using a small, 9 micron core fiber. The small core diameter needs extremely precise connectors. Single-mode connectors are thus usually more expensive. Single-Mode enables higher bandwidth and lower attenuation, allowing light to travel much longer distances. This combination of characteristics make singlemode cables the ideal choice for high bandwidth/long distance Telecommunication and CATV networks. 7
Basics of WDM Wavelength-division multiplexing (WDM) is a technology which multiplexes a number of optical carrier signals onto a single optical fiber by using different wavelengths (i.e., colors) of laser light. This technique enables bidirectional communications over one strand of fiber, as well as multiplication of capacity. Wavelength of a sinusoidal wave is the spatial period of the wave the distance over which the wave s shape repeats, and the inverse of the spatial frequency. WDM systems became popular as they allowed to expand the capacity of a network without laying more fiber. By using WDM, it is possible to accommodate several generations of technology development in an optical infrastructure without having to overhaul the backbone network. Capacity of a given link can be expanded simply by upgrading the multiplexers and demultiplexers at each end. 8
Need For WDM technologies Early WDM systems were using two IR channels per fiber, allowing to increase capacity or to provide dual-link transmissions over a single fiber. Typically, Wavelengths of 1310nm and 1550nm were used as they are providing the less attenuation over long distances. Note: Optical receivers, in contrast to laser sources, are usually wideband devices. Therefore the demultiplexer must provide the wavelength selectivity of the receiver in the WDM system. 9
Need For WDM technologies In order to add more capacity in a single link, CWDM and DWDM technologies became popular. By multiplexing a high amount of channels into a single fiber, the total bandwith that could be used over a single fiber can expand tremendously, typically above hundred gigabits per second (Gbps). WDM systems are divided into different wavelength patterns, mostly Coarse Wavelength Division Multiplexing (CWDM) and Dense Wavelength Division Multiplexing (DWDM). 10
About CWDM Coarse wavelength division multiplexing (CWDM) transmit multiple channels over a fiber, by using a 20nm separation between channels. In contrast to DWDM, CWDM uses increased channel spacing to allow less sophisticated and thus cheaper transceiver designs. 20nm separation 16(18) Channels Low / Medium cost Typical applica)ons Bi- direc)onal Mul)ple channels, mul)ple formats Long distance 11
About CWDM The channel spacing grid for use with CWDM has been standardized by the ITU (ITU-T G.694.2), using the wavelengths from 1270 nm through 1610 nm with a channel spacing of 20 nm. 12
About DWDM DWDM systems use a much smaller channel separation (typically 0.8nm) allowing to transmit much more channels over a single link. In order to do this, DWDM have to maintain more stable wavelength or frequency than those needed for CWDM because of the closer spacing of the wavelengths. Precision temperature control of laser transmitter is required in DWDM systems to prevent "drift" off a very narrow frequency window of the order of a few GHz. This usually results into higher costs for transmission equipments. 0.8nm separation (typically) 88++ channels Higher cost Typical applica)ons Bi- direc)onal Mul)ple channels, mul)ple formats Long distance 13
About SFP Transceivers The Small Form-factor Pluggable (SFP) is a compact, hotpluggable transceiver used for both telecommunication and data communications applications. SFPs interfaces a network device to a fiber optic or copper networking cable. The form factor and electrical interface are specified by a multi-source agreement (MSA). SFP transceivers are designed to support SONET, gigabit Ethernet, Fiber Channel, as well as many other communications standards including HD-SDI, HDMI,... For fiber optic transmissions, SFP transceivers are available with a variety of transmitter and receiver types, allowing users to select the appropriate transceiver for each link(single-mode/multimode/wavelength type, distance, ). 14
About SFP Transceivers The SFP MSA defines a 256-byte memory map into an EEPROM describing the transceiver's capabilities, standard interfaces, manufacturer, and other information. Because SFPs are hot-swappable, there s no need to shut down an equipment to swap out a module it s easy to change interfaces on the fly for upgrades and maintenance. An important characteristic of SFP transceivers is that they re OSI Layer 1 devices they re transparent to data and do not examine or alter data in any way. 15
Fiber optic connector types Since the introduction of fiber optic technology, a very high amount of connector styles have been introduced. Each design was trying to achieve better performances (less light loss and reflectance) together with easier and reliable connection. In the various connectors type, the 4 below are found especially often. SC connectors is a is a snap-in connector who offer excellent packing density, and have been widely used in the industry. LC connectors due to their smaller size, are perfect for high-density applications. Because they are often found on SFPs transceivers, they became a major connection type in the industry. FC connectors allows to screw on to the device firmly. It has been one of the most popular connector for single-mode, but are now often replaced by LC connectors. ST connectors have a key which prevents rotation, and a bayonet lock similar to a BNC shell. It has been one of the most popular connector for multi-mode networks. 16
Insertion Loss Measurement Typical SFP transceivers are usually characterized by their maximum bandwidth, their wavelength, as well as an indicative distance that can be covered. The distance information provided is mainly informative, and in order to make sure that the transmission link will be established properly, it is required to measure the total amount of loss in the fiber optic link. The Power of the Transmitter reduced by the total loss over the transmission link should stay above the sensitivity of the receiver. Example of a SFP specifications (single mode, 3G, 40km type) Tx Pwr: 0dBm Rx sensitivity: -20dBm 18
Optical Link Budget Estimation To be able to judge whether a fiber optic cable link is good, one does a insertion loss test with a light source and power meter and compares that to an estimate of what is a reasonable loss for that cable link. The estimate is calculated using typical component losses for each part of the cable link. The three main source of losses are: Connector loss Splice loss Fiber Loss If the measured loss exceed the calculated loss by a significant amount, the system should be tested segment-by-segment to determine the cause of high loss. 19
Optical Link Budget Estimation Typical Connector or Splice Loss: 0.5dBm Typical Fiber Loss (Single-Mode): 0.3~0.5dBm/km 20
21 Connector and Splice Losses Connector and splice loss is caused by a number of factors. Loss is minimized when: the two fiber cores are identical and perfectly aligned. the connectors or splices are properly finished no dirt is present. Only the light that is coupled into the receiving fiber's core will propagate, so all the rest of the light becomes the connector or splice loss. A critical factor in a fiber optic connector or splice is alignment. The ideal connection will perfectly align the fibers, especially the lightcarrying cores, so that the joint is transparent with no loss of optical energy. Unfortunately, both the fiber and connector are subject to manufacturing tolerances that create less than perfect alignment. 21
22 Connector and Splice Losses The end of the fiber must be properly polished and clean to minimize loss. A rough surface will scatter light and dirt can scatter and absorb light. Since the optical fiber is so small, typical airborne dirt can be a major source of loss. Whenever connectors are not terminated, they should be covered with dust caps provided by the manufacturer to protect the end of the ferrule from dirt. Before connection and testing, it is advisable to clean connectors with special dry fiber cleaners. 22
Fiber Loss Fiber loss will vary depending on the fiber type (Single-Mode or Multi-Mode) as well as the wavelength used. For multimode fiber, the loss is about 3 db per km for 850 nm sources, 1 db per km for 1300 nm. For single-mode fiber, the loss is about 0.5 db per km for 1310 nm sources, 0.4 db per km for 1550 nm. A phenomenon known as Water Peak Loss induce a much higher loss for wavelengths close to 1400nm. For this reason, it is usually advised to avoid wavelengths of 1390nm and 1410nm when possible. Water Peak Loss is dependent of the fiber used, and some fiber types allows to greatly reduce the Water Peak Loss effects. 23
Budget Calculation Example Assumptions: Connector loss : ~0.5dBm Fiber Loss ~0.3dBm/km 32ch DWDM Mux loss: ~6dBm MAX 120km of Fiber TX Power : ~+3dBm (see SFP specs) Rx Sensitivity: ~-30dBm (see SFP specs) 24