Single mode cable is a single stand (most applications use 2 fibers) of glass fiber with a diameter of 8.3 to 10 microns that has one mode of transmission. Single mode fiber with a relatively narrow diameter, through which only one mode will propagate typically 1310 or 1550nm. Carries higher bandwidth than multimode fiber, but requires a light source with a narrow spectral width.
Single mode fiber is used in many applications where data is sent at multi-frequency (WDM Wave-Division-Multiplexing) so only one cable is needed - (single-mode on one single fiber).
Single-mode fiber gives us a higher transmission rate and up to 50 times more distance than multimode, but it also costs more. Single-mode fiber has a much smaller core than multimode. The small core and single light-wave virtually eliminate any distortion that could result from overlapping light pulses, providing the least signal attenuation and the highest transmission speeds of any fiber cable type.
Single-mode optical fiber is an optical fiber in which only the lowest order bound mode can propagate at the wavelength of interest typically 1300 to 1320nm.
Multimode fiber optic cable is the another commonly used cables. Multi mode cable diameter is a little big, with a common diameters in the 50-to-100 micron range for the light carry component (in the United States, the most common size is 62.5um). In most applications, the use of multimode optical fiber, two fibers (WDM, usually without the use of multimode fiber). POF is a relatively new based on the plastic of the cable, the cable's commitment is similar to that of the performance on the glass cable very short run, but at a lower cost.
Multimode fiber gives us a high bandwidth, high speed (10 to 100MBS -Gigabit to 275m to 2 kilometers), from the medium. Light waves are scattered into countless path or patterns, because they through the cable core is usually 850 or 850nm. Typical of the multimode optical fiber in the fiber in the fiber core diameter is 50, 62.5, and 100 microns. However, in the long cable (greater than 3000 feet 914.4m), the light of the multiple paths may lead to distortion of the signal at the receiving end, resulting in unclear, incomplete data transmission, so the designers now called for a new application using single mode fiber optic gigabit and beyond.
Step-index multimode fiber has a large core, up to 100 microns in diameter. As a result, some of the light rays that make up the digital pulse may travel a direct route, whereas others zigzag as they bounce off the cladding. These alternative pathways cause the different groupings of light rays, referred to as modes, to arrive separately at a receiving point. The pulse, an aggregate of different modes, begins to spread out, losing its well-defined shape. The need to leave spacing between pulses to prevent overlapping limits bandwidth that is, the amount of information that can be sent. Consequently, this type of fiber is best suited for transmission over short distances, in an endoscope, for instance.
Graded-index multimode fiber contains a core in which the refractive index diminishes gradually from the center axis out toward the cladding. The higher refractive index at the center makes the light rays moving down the axis advance more slowly than those near the cladding. Also, rather than zigzagging off the cladding, light in the core curves helically because of the graded index, reducing its travel distance. The shortened path and the higher speed allow light at the periphery to arrive at a receiver at about the same time as the slow but straight rays in the core axis. The result: a digital pulse suffers less dispersion.
Singel-mode has a narrow core (eight microns or less), and the index of refraction between the core and the cladding changes less than it does for multimode fibers. Light thus travels parallel to the axis, creating little pulse dispersion. Telephone and cable television networks install millions of kilometers of this fiber every year.
One of the major ways of specifying optical fibre cables is by the diameters of the inner core and the external cladding. As may be expected there are industry standards for these and this helps in reducing the variety of fittings needed for connectors, splices and the tools needed for fitting.
The standard for most optical fibres is 125 microns (um) for the cladding and 245 microns (um) for the outer protective coating. Multimode optical fibres have core sizes of either 50 or 62.5 microns whereas the standards for single mode fibres is approximately 8 to 10 microns.
When specifying optical fibre cables, the diameters usually form the major part of the cable specification. A multimode fibre with a core diameter of 50 microns and a cladding diameter of 125 microns would be referred to as a 50/125 fibre.
Currently we have six optical fibre “types” or “Categories” specified in the generic cabling standards. OM1, OM2, OM3 , OS1 and OS2(Generic cabling for industrial premises). In addition, OM4 fiber has been on the market since 2005, sold as premium OM3 or OM3+ fiber. the designation OM1, OM2, OM3/OM4, OS1 and OS2 relate to cable transmission performance
The 62.5/125 µm (OM1) has been the most popular multimode fiber choice throughout the 80's, 90's and into the early 2000's and was the most common multimode fiber used and yet it has the lowest data carrying capacity and shortest distance limitations as compared with other Multimode fiber types. OM1 (62.5 µm) fiber has reached its performance limit.
The 50/125 µm core size comes in three different classifications (OM2, OM3 and OM4). Please note that OM3 is usually just referred to as 10GIG since it is generally the best choice for 10 Gigabit Ethernet over Multimode fiber and was designed specifically for that purpose (unless you need the extra distance provided by OM4). 50 µm fiber offers as much as ten times the bandwidth of 62.5 µm fiber.
OM3 was the first standard to emerge, codifying laser optimization of multimode fiber. This technology was the first to allow designs of laser transmission systems utilizing multimode optical fiber without the use of mode conditioning cables. This new fiber when paired with new low cost Vertical-cavity surface-emitting laser technology allowed for 10 Gig transmission.
OM4 fiber has been on the market since 2005, sold as premium OM3 or OM3+ fiber. The OM4 designation standardizes the nomenclature across all manufacturers so that the customer has a clearer idea of the product that they are buying. OM4 is completely backwards compatible with OM3 fiber and shares the same distinctive aqua jacket. OM4 was developed specifically for VSCEL laser transmission and allows 10 Gig / second link distances of up to 550 Meters (compared to 300M with OM3).
The effective modal bandwidth for OM4 is more than double that of OM3 (4700 MHz.km for OM4 v/s 2000 MHz.km for OM3).
While OM3 fiber will still be future proof in most applications, allowing speeds of 10GB/s up to 100GB/s, OM4 fiber offers users longer length distances and more wiggle room in optical budgets
Table- Mutlimode fibre optic cable specifications
|Minimum modal bandwidth|
|Overfilled launch||“Laser” launch|
Singlemode fiber(OS1/ OS2), because of the more expensive electronics required for it is usually used for much greater distances. So for reasons of practicality, most Local Area Networks (LANs) will typically use one form or another of Multimode Cable.
Although fibre optic cables offer a far superior performance to that which can be achieved with other forms of cable, they nevertheless suffer from some levels of attenuation. This is caused by several effects:Loss associated with the impurities
There will always be some level of impurity in the core of the optical fibre. This will cause some absorption of the light within the fibre. One major impurity is water that remains in the fibre.Loss associated with the cladding
When light reflects off the interface between the cladding and the core, the light will actually travel into the core a small distance before being reflected back. This process causes a small but significant level of loss and is one of the main contributors to the overall attenuation of a signal along an fibre optic cable.Loss associated with the wavelength
It is found that the level of signal attenuation in the optical fibre depends the wavelength used. The level increases at certain wavelengths as a result of certain impurities
Despite the fact that attenuation is an issue, it is nevertheless possible to transmit data along single mode fibres for considerable distances. Lines carrying data rates up to 50 Gbps are able to cover distances of 100 km without the need for amplification.
There are two main types of material used for optical fibres. These are glass and plastic. They offer widely different characteristics and therefore fibres made from the two different substances find uses in very different applications.
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