Fiber Optics Transmission Systems

Fibers that carry more than one mode are called multi-mode fibers. There are two types of multi-mode fibers. One type is step-index multi-mode fiber and the other type is graded-index multi-mode fiber.

The following illustration shows the differences between these two types of multimode fibers on refractive index profile and how they guide light.

Fiber Optic Transmission in a Step-Index Multimode Fiber and a Graded-Index Multimode Fiber

Step-index multimode fibers are mostly used for imaging and illumination. Graded-index multimode fibers are used for data communications and networks carrying signals moderate distances – typically no more than a couple of kilometers

Modal-Dispersion and Limit on Step-Index Multimode Fibers’ Bandwidth

Take a look at the illustration for a step-index multimode fiber. Rays of light enter the fiber with different angles to the fiber axis, up to the fiber’s acceptance angle (numerical aperture). Rays that enter with a shallower angle travel by a more direct path, and arrive sooner than those enter at steeper angles (which reflect many more times off the core/cladding boundaries as they travel the length of the fiber). The arrival of different modes of the light at different times is called Modal Dispersion.

Index Profile Difference Between Step-Index Multimode Fiber and Graded-Index Multimode Fiber

Light Transmission in a Step-Index Multimode Fiber and a Graded-Index Multimode Fiber

Step-index multimode fibers are mostly used for imaging and illumination. Graded-index multimode fibers are used for data communications and networks carrying signals moderate distances – typically no more than a couple of kilometers

Modal-Dispersion and Limit on Step-Index Multimode Fibers’ Bandwidth

Take a look at the illustration for a step-index multimode fiber. Rays of light enter the fiber with different angles to the fiber axis, up to the fiber’s acceptance angle (numerical aperture). Rays that enter with a shallower angle travel by a more direct path, and arrive sooner than those enter at steeper angles (which reflect many more times off the core/cladding boundaries as they travel the length of the fiber). The arrival of different modes of the light at different times is called Modal Dispersion.

Modal Dispersion is also called modal distortion, multimode dispersion, intermodal distortion, intermodal dispersion, and intermodal delay distortion.

Digital communications use light pulse to transmit signal down the length of the fiber, as we explain in the fiber optic network tutorial. Modal dispersion causes pulses to spread out as they travel along the fiber, the more modes the fiber transmits, the more pulses spread out. This significantly limits the bandwidth of step-index multimode fibers.

For example, a typical step-index multimode fiber with a 50 µm core would be limited to approximately 20 MHz for a one kilometer length, in other words, a bandwidth of 20 MHz·km.

Graded-Index Multimode Fibers Solves the Problem of Modal Dispersion

Graded-index fiber’s refractive index decreases gradually away from its center, finally dropping to the same value as the cladding at the edge of the core. The change in refractive index causes refraction, instead of total internal reflection, which bends light rays back toward the fiber axis as they pass through layers with lower refractive index. No total internal reflection happens because refraction bends light rays back into the fiber axis before they reach the cladding boundary.

Different light modes in a graded-index multimode fiber still follow different lengths along the fiber, as in step-index multimode fiber.  However their speeds differ because the speed of guided light changes with fiber core’s refractive index.

So the farther the light goes from the center of the fiber, the faster its speed. So the speed difference compensate for the longer paths followed by the light rays that go farthest from the center of the fiber. This equalizing of transit times of different modes greatly reduces modal dispersion.

The bandwidth of a typical off-the-shelf graded-index multimode fiber with a 50 µm core may approach 1 GHz·km or more. Multimode graded-index fibers having bandwidths approaching 3 GHz·km have been produced.

But please note that modal dispersion may be considerably reduced, but never completely eliminated.

Laser Optimized Multimode (Multi Mode) Optical Fibers from Corning

As the core of the fiber is so small that one ray of light at an angle of 0 ° incident can stably pass through the fiber length, without much loss, this type of fiber called single-mode optical fiber. The basic requirement for single-mode fiber is that the basic limit small enough to transfer to a mode of singing. This lowest order mode can propagate in all fibers with small cores (as long as light can be physically fibers).

The most common type of single mode fiber has a base diameter of 8-10 mu m and is designed for use in the near infrared (the most common are designed 1550nm and 1310nm). Please note that the structure of method depends on the wavelength of light used, so that these fibers actually a few extra modes to wavelengths visible support. multi-mode fiber, compared with core diameters as small as 50um multimode and as large as a hundred microns.

The image below shows the structure of a single mode optical fiber.

What Are the Conditions for Single Mode Transmission?

To calculate the number of modes N_m in a step-index fiber, N_m can be simplified as:

Where
D is core diameter of the fiber
λ is the operating wavelength
n_f is refractive index of the fiber core
n_c is refractive index of the fiber cladding

Reducing the core diameter sufficiently can limit transmission to a single mode. The following formula defines the maximum core diameter, D, which limits transmission to a single mode at a particular wavelength, λ :
If the core is any larger, the fiber can carry two modes.

Mode field diameter (MFD)

The core diameter typical communication single-mode fibers from 8 ~ 10um for the operation of 1.31um 1.5um wavelength. Fibre with a base diameter of less than about ten times the wavelength of light propagation can be modeled using geometric optics, as in explaining the multimode fiber with step index have. Instead, they must be analyzed, reduced by solving Maxwell’s equations of the electromagnetic wave equation as an electromagnetic structure.

Thus, although the fiber cladding, the light confined in the heart of the fiber, the light penetrates into the mantle, despite the fact that it undergoes total internal reflection nominally. This is done both in mono mode and multimode fibers, but the phenomenon is important in single mode optical fiber.

be used for a Gaussian distribution (laser in the Gaussian distribution of communication) in a single mode fiber, the mode field diameter (MFD) as the point where the intensities of electric and magnetic fields up to 1 / e is defined reduce their peak values, the diameter of the power 1/e2 (0135) of the peak power (because the force is proportional to the square of the field strength) is reduced. For singlemode fibers, the peak in the middle of the nucleus.

Mode field diameter is slightly larger than the base diameter, as shown in the following figure.

References on how to measure mode field diameter for a single mode fiber

• EIA/TIA-455-191 (FOTP-191), Measurement of Mode-Field Diameter of Single-Mode Optical Fiber.
• http://www.corning.com/docs/opticalfiber/mm16_08-01.pdf Corning’s paper on how to measure mode field diameter for a single mode fiber
• Measurement of Mode-Field Diameter of Single-Mode Optical Fiber, Fiberoptic Test Procedure FOTP-191, Telecommunications Industry Association, Standards and Technology Department, 2500 Wilson Blvd., Suite 300, Arlington, VA, 22201 (1998).
• Measurement of the Effective Area of Single-Mode Optical Fiber, Fiberoptic Test Procedure FOTP-132, Telecommunications Industry Association, Standards and Technology Department 2500 Wilson Blvd., Suite 300, Arlington, VA, 22201 (1998).

Advantages of single-mode fiber

dispersion single mode fiber modal, modal noise and other effects that come with multiple gears, single-mode fiber capable of carrying signals at much higher rates than multimode fiber. You are span standard option for high data rates and long distance (over a few kilometers), the laser diode based telecommunications transmission technology used fiber.

Disadvantages of singlemode fiber

Since the heart of the single-mode fiber is much smaller than the multimode fiber optic core, the light coupling single mode optical fiber requires much tighter tolerances than the light coupling into the veins over multimode fiber. However, these tolerances were more available.

singlemode fiber optic components and devices are also more expensive than their counterparts multimode fiber to multi-mode is widely used in systems where low cost connections and distances and transmission speeds must be taken are used modestly.

Single Mode Optical Fibers from Corning

Applications such as Voice over IP, streaming video and teleconferencing rates push the communication of data to 10 Gigabit Ethernet and beyond the enterprise.

These higher speeds could lead to believe that the system designer who has a singlemode fiber is increasingly on edge multimode fiber in space applications. However, no high-speed Ethernet does not mean single-mode fiber was the right choice.

Although single-mode fiber has advantages in terms of bandwidth and range for long distances (> 1 km at 1 Gb / s), multimode fiber supports most easily lines for local networks and businesses is necessary. In fact, the multimode fiber, to support transmission at 10 Gbit / s up to 550 meters long, and even short-Campus Backbone runs.

The optoelectronics with multimode fibers are generally less expensive than for a single-mode is necessary. And multimode fiber is easier to install and terminate in the region – important aspects in enterprise environments with frequent moves, adds and changes.

Multimode and Single-Mode: What’s the Difference?

The two fiber types get their names from the way they transmit light. Generally designed for systems of moderate to long distance (e.g., metro, access, and long-haul networks), single-mode fibers have a small core size (< 10 μm) that permits only one mode or ray of light to be transmitted. This tiny core requires precision alignment to inject light from the transceiver into the core, significantly driving up transceiver costs.

By comparison, multimode fibers have larger cores that guide many modes simultaneously. The larger core makes it much easier to capture light from a transceiver, allowing source costs to be kept down. Similarly, multimode connectors cost less than single-mode connectors due to the more stringent alignment requirements of single-mode fiber. Single-mode connections require more care and skill to terminate, which is why components are often pre-terminated at the factory. Multimode connections, on the other hand, can be easily performed in the field, offering installation flexibility, cost savings, and peace-of-mind.

Multimode fiber continues to be the most cost-effective choice for enterprise applications up to 550 meters.

Enterprise environments present particular network challenges, including limited spaces and tight bends, high connection density, and components that get handled frequently. Multimode fibers are ideally suited for these conditions. And since distances within a premises system rarely approach 550 meters, multimode fiber should be the choice for these applications.

Beyond 550 meters at 10 Gb/s (or 1 km at 1 Gb/s), it is necessary to utilize single-mode fiber. There are new choices for single-mode fiber today, so be sure to consider your options; a bend insensitive full spectrum singlemode fiber provides more transceiver options, more bandwidth, and is less sensitive to handling of the cables and patch cords than conventional single-mode fiber.

The network designer or end user who specifies multimode fiber for short reach systems still must choose from two types – 50 μm or 62.5 μm. 50 μm multimode fibers were first deployed in the 1970s for both short and long reach applications. 62.5 μm multimode fiber, introduced in 1985, supported campus applications up to 2 kilometers at 10 Mb/s. The mid- 1990s, with the introduction of the VCSEL laser light source, saw a shift back to 50 μm fiber. Today, 50 μm laser-optimized multimode (OM3) fiber offers significant bandwidth and reach advantages for most building applications, while preserving the low system cost advantages of 850 nm-based multi-mode fiber.

Planning for the Future

Since optoelectronics is a large percentage of the total system cost, the most economical solution for transmitting 10 Gb / s in the company to 50 OM3 fibers, which are designed or manufactured for use with low-cost VCSELs. This advantage has become reality at speeds higher, because the transceiver developed future should have to take advantage of 10Gb / s technology companies.

Looking to listen in the future, broadband Study Group (HSSG) of IEEE in November 2006 to support 100 Gb / s Ethernet that the next speed. In addition, the HSSG suggest that higher transmission rates with OM3 fiber arrays at low cost parallel optical transceiver, or a combination of optics and parallel multi-wavelength (CWDM Coarse Wavelength Division Multiplexing or support).

For example, the fibers are ten OM3 operating at 10 Gb / s can in a system of 100 Gb / s (10 x 10 array) activities. Or five OM3 fiber can transmit two wavelengths operating at 10 Gb / s (5 x 2 x 10 array).

For single-mode fiber is the HSSG recommend the use of optical CWDM or DWDM. In this case, multiple wavelengths, each operating at 10, 20 or 25 Gb / s, would be transferred by
a single fiber. Why not single-mode fiber with a single laser (serial transfer) operating at 100 Gb / s? Such a laser is simply not available in stores and probably will not for long. It is very difficult to develop and produce such a laser at a low cost. Therefore achieve higher speeds on singlemode fiber or DWDM optical CWDM lasers require several more wavelengths Drive. But now that you use the transceiver and the connector can lead very direction challenges, the cost of these items when they face used with single mode fiber.

In general, then, continue to multimode fiber, the best choice for enterprise applications can be up to 550 meters. single-mode fiber is preferred for distances over 550 meters.

If the network transmission distances, the use of single mode optical fibers show consider the words of the recent “Bend-insensitive Zero Water Peak (Full Spectrum) fibers. These fibers are designed to provide long-term reliability in applications with tight turns and small parcels.

The history of Fiber Optic Cable

In 1870, John Tyndall demonstrated that light follows the curve drizzle of water from a container, it is this simple principle, that the investigation and development of applications has led to this phenomenon. John Logie Baird patented a method of transmitting light in a glass rod for use in a color television at first, but the losses in optical materials at the time of use impractical. In the 1950s, more research and development in the transmission of visible images through optical fibers have led to some success in the medical world as it began with them in the remote lighting and visualization tools. In 1966, Charles Kao and George Hockham proposed the transmission of information on fiber, and they also knew that a practical proposition, much smaller losses in the cables were essential. This was the driving force behind the development of losses in optical fiber production improved and today optical losses are significantly lower than the initial target by Charles Kao and George Hockham together.

The advantages of using fiber optics

Because of the low loss, high bandwidth properties of fiber cable can be used over greater distances than copper cables, data networks, it can become so only two miles without the use of repeaters . Their low weight and small size make it ideal for applications where copper cables running would be impractical and the use of a fiber optic multiplexer could replace hundreds of copper cables. It’s pretty impressive for a small glass filament, but the real benefits in the industry is given the immunity to electromagnetic interference (EMI), and the fact that the glass is not an electrical conductor. Because the fiber is not conductive, it can be used where electrical insulation is required, for example, between buildings where copper cables would require cross bonding to eliminate differences in ground potential. Fibers, no threat in dangerous environments such as chemical plants where a spark triggered an explosion. Last but not least, the safety aspect, it is very, very difficult to tap into a fiber optic cable to read the data signals.

The structure of Fiber Optical Cable
There are many types of fiber optic cable, but for the purposes of this declaration, we will deal with one of the most common species, 62.5/125 micron loose tube. The numbers represent the diameter of the fiber and heart of the mantle, they are in microns, millionths of a meter measuring. Loose tube cable is inside or outside, or both, external cables usually have the tube filled with gel as a vapor barrier that prevents penetration of water law. The number of cores in a cable can be anywhere 4-144 produced over the years a variety of sizes were based, but now there are only three main sizes that are used in data communications, these are 50/125, 62.5/125 and 8.3/125. The 62.5/125 micron multimode 50/125 and are the most widely used in wired networks, but it has recently become the first choice of 62.5. It is very unfortunate, because is 50/125, the best option for Gigabit Ethernet applications.

The Micron is a 8.3/125 singlemode fiber, which, until now rarely used in data networks, this is due to the high cost of equipment in mono. Things begin to change because the size limit for Gigabit Ethernet over fiber 62.5/125 about 220m, and now with 8.3/125 which could reduce choice for some campus size networks. Hopefully this change in single-mode begins to lower costs.