The paper is intended to understand the Different Types of Dispersions in an opcal fiber medium, it’s construcon, applicaon, technology and upcoming gadgets or tools. The authors have discussed about the Opcal Fiber and its advantages, theory and principles of the fiber opcs, Fiber geometry. Different parameters and characteriscs of fiber are also explained. Dispersion is the spreading of light pulse as it travels down the length of an opcal fiber. It is a negave impact on the result that limits the bandwidth or informaon carrying capacity of a fiber. Opcal Fiber has variety of applicaons and uses in Telecommunicaons Optical Fiber Dispersion, Construcon, Applicaon, Technology, Future 1221 Khalida Sultana Shuravi | Afia Fairooz 1220412643 | 1230428643 T ETE 451 Faculty: MAA North South University
Transcript
The paper is intended to understand the Different Types of Dispersions in an optical fiber medium, it’s construction, application, technology and upcoming gadgets or tools.
The authors have discussed about the Optical Fiber and its advantages, theory and principles of the fiber optics, Fiber geometry. Different parameters and characteristics of fiber are also explained. Dispersion is the
spreading of light pulse as it travels down the length of an optical fiber. It is a negative
impact on the result that limits the bandwidth or information carrying capacity
of a fiber. Optical Fiber has variety of applications and uses in Telecommunications
Department of Electrical and Electronic Engineering, North South University, Bashundhara, Dhaka-1229, Bangladesh
Abstract- The paper is intended to understand the Different Types of Dispersions in an optical fiber medium, it’s construction, technology and upcoming gadgets or tools. The authors have discussed about the Optical Fiber and its advantages, Theory and principles of the fiber optics, Fiber geometry. Different parameters and characteristics of fiber are also explained. Fiber has linear and non-linear characteristics. Linear characteristics are wavelength window, bandwidth, attenuation and dispersion. Non-linear characteristics depend on the fiber manufacturing, geometry etc. Dispersion is the spreading of light pulse as it travels down the length of an optical fiber. It is a negative impact on the result that limits the bandwidth or information carrying capacity of a fiber. Optical Fiber has a variety of uses in Telecommunication and Management.
Index Terms-Optical Fibers, Dispersions, Modal Dispersions, Material Dispersion, Wave Guide Dispersions, Polarization Dispersions, Chromatic Dispersions, Technology, Nanospikes, Faster Internet Speed.
I. INTRODUCTION
Since the invention of the telephone in 1876, copper wires have been used in data transmission. But as the demand increased, the use of copper wires consistently got reduced due to many reasons such as low bandwidth, short transmission length and inefficiency. Optical Fiber is new medium, in which information (voice, Data or Video) is transmitted through a glass or plastic fiber, in the form of light. The field of applied science and engineering concerned with the design and application of optical fibers is known as fiber optics. Optical fibers are widely used in fiber optics, which permits transmission over longer distances and at higher bandwidth (data rates) than other forms of communication. Optical fibers may be connected to each other or can be terminated at the end by means of connectors or splicing techniques. This was the reason for the introduction of optical fiber. Optical fiber ia playing a very important role due to its wide properties like
high bandwidth, long distance transmission, and high level of security.
II. FIBER OPTICS
Optical fiber has the following parts:
●Core - Thin glass center of the fiber where the light travels
●Cladding - Outer optical material surrounding the core that reflects the light back into the core
●Buffer coating - Plastic coating that protects the fiber from damage and moisture.
The reason of introducing optical fibers was for its excellent and important properties. Optical fibers made possible the much efficient long distance data transmission and high level of security.
As light pulses are sent through optical fibers they follow the sequence of transmission and this happens because of total internal reflection. The process of data transmission involves the following steps.
● creating the optical signal involving the use of a transmitter
● relaying the signal along the fiber
● Receiving the optical signal and converting it into an electrical signal
● Electrical signals are decoded into information
III. ADVANTAGES OF FIBER OPTICS
Fiber Optics has the following advantages:
(I) Optical Fibers are non-conductive (Dielectrics).
Optical fibers use the phenomena of “total internal reflection”. Total internal reflection is a phenomenon which occurs when a propagating wave strikes a medium boundary at an angle larger than a particular critical angle with respect to the normal to the surface.
When a light travels from one medium with one lower refractive index (n2) to another medium with a higher refractive index (n1), the light bends or refracts towards the normal. But when a light enters from a medium of higher refractive index to lower, it refracts away from the normal. As the incident angle through n1 becomes greater with respect to the normal line, the refracted light through n2 bends further away from the normal.
At one particular point when the angle reaches 90° (critical angle), the refracted light will not travel into n2, but instead will travel along the surface between the two media since the [critical angle=n2/n1,where n1 and n2 are indices of refraction and n1 is greater than n2]. If the beam through m1 is greater than greater than the critical angle, then the refracted beam will be In an optical fiber, the light travels through the core (n1, high index of refraction) by constantly reflecting from the cladding (n2, lower index of refraction) because the angle of the light is always greater than the critical angle. Light reflects from the cladding no matter what angle the fiber itself gets bent at, even if it's a full circle.
Because the cladding does not absorb any light from the core, the light wave can travel great distances. However, some of the light signal degrades within the fiber, mostly due to impurities in the glass. Signal degradation depends upon the purity of the glass and the wavelength of the transmitted light.
V. FIBER TYPES
The refractive Index profile describes the relation between the indices of the core and cladding. The authors have mentioned that two main relationships exist:
Step Index Graded Index
Fig 1. Types of Optical Fiber.
The step index fiber has a core with uniform index throughout. The profile shows a sharp step at the junction of the core and cladding.
In contrast, the graded index has a non-uniform core. The Index is highest at the center and gradually decreases until it matches with that of the cladding. There is no sharp break in indices between the core and the cladding.
By this classification there are three types of fiber:
Multimode Step Index fiber (Step Index fiber) Multimode graded Index fiber (Graded Index fiber) Single-Mode Step Index fiber (Single Mode Fiber)
VI. MULTIMODE STEP INDEX FIBER
Fig 2. Modes of Propagation in Multimode Step Index Fiber
A multimode step-index fiber has a core of radius and a constant refractive index n1 inside the core with a cladding of slightly lower refractive index n2 surrounds the core. The greatest range (diameter) of core is 50-200 micrometer and the diameter of cladding is 110 micrometer. Since the diameter of the step index of the core is high, more number of light
propagates is possible. Therefore, due to more light with information’s can travel all rays hit the core from the beginning with a certain angle will totally reflect at the core-cladding boundary. Rays which strike the core-cladding boundary with angles greater than the critical angle will be partially reflected and some will go out of the core-cladding boundary. As results with such bounces energy is lost from the fiber. Light rays with information traveling inside the core will arrive at the far end of the fiber at different times due to reflection and total internal reflection of the refractive index. Rays who travel through the central axis reaches the first.
VII. MULTIMODE GRADED INDEX FIBER
In graded index multi-mode fiber there are many alternation of refractive index with larger values towards the centre. When the light travels in the core, it travels faster than from the central axis, as refractive index is higher in the centre. Due to different refractive index, each layer of the core refracts the light and making it bent towards the centre creating helix shape or sinusoidal transverse wave form instead of being straight and sharp. The light traveling near the center of the core has the slowest average velocity.
Fig 3. Modes of Propagation in Multimode Graded Index Fiber
Therefore, all the rays travel at the same time and also reach the end together.
VIII. SINGLE MODE STEP INDEX FIBER
Another way to reduce modal dispersion is to reduce the core's diameter, until the fiber only propagates one mode efficiently. The single mode fiber has an exceedingly small core m. Standard cladding diameter is 125diameter of only 5 to 10 m. Since this fiber carries only one mode, model dispersion does not exists. Single mode fibers easily have a potential bandwidth of 50to 100GHz-km. The core diameter is so small
that the splicing technique and measuring technique are more difficult. High sources must have very narrow spectral width and they must be very small and bright in order to permit efficient coupling into the very small core diameter of these fibers. One advantage of single mode fiber is that once they are installed, the system's capacity can be increased as newer, higher capacity transmission system becomes available. This capability saves the high cost of installing a new transmission medium to obtain increased performance and allows cost effective increases from low capacity system to higher capacity system.
As the wavelength is increased the fiber carries fewer and fewer modes until only one remains. Single mode operation begins when the wavelength approaches the core diameter. At 1300 nm, the fiber permits only one mode, it becomes a single mode fiber. As optical energy in a single mode fiber travels in the cladding as well as in the core, therefore the cladding must be a more efficient carrier of energy. In a multimode fiber cladding modes are not desirable; a cladding with in efficient transmission characteristic can be tolerated. The diameter of the light appearing at the end of the single mode fiber is larger than the core diameter, because some of the optical energy of the mode travels in the cladding. Mode field diameter is the term used to define this diameter of optical energy.
IX. DISPERSION
Dispersion is the spreading of light pulse as its travels down the length of an optical fiber. Dispersion limits the bandwidth or information carrying capacity of a fiber. The bitrates must be low enough to ensure that pulses are farther apart and therefore the greater dispersion can be tolerated.
There are five main types of dispersion in a fiber:
Modal dispersion occurs only in Multimode fibers. It happens due to rays taking different paths through the fiber and eventually ends up at different times at the other end of the fiber. Mode is a concept based on mathematics and physics explaining the propagation electromagnetic waves through a media. In case of fiber, a mode is the route which a light ray takes travelling down a fiber. The number of modes supported by a fiber ranges from 1 to 100000. Thus a fiber happens to provide a path of travels for one or several of light rays depending on its size and properties. Since light reflects at
different angles for different paths or modes, the path lengths of different modes are different.
Hence, different rays take longer or shorter distance to travel to the end of the fiber. The ray that travels straight through the core reaches the other end first, other rays arrive later. Thus the light entering the fiber at same time exit the other end at different time. The light has spread out with time.
The spreading of light is called modal dispersion. The type of dispersion that explains modal dispersion is the ray taking various modal path lengths in the fiber. Typical modal dispersion numbers for the step index fiber are 15 to 30 ns/km. This explains that the ray taking the longer path will reach the other end of 1km long fiber 15 to 30 ns after the ray, following the shortest path.
Dispersion is the main limiting factor on a fiber’s bandwidth. Spreading of pulse, results in a pulse overlapping adjacent pulses as shown in figure. Eventually pulse will merge in a way where one pulse cannot be distinguished from another. The information contained in the pulse is lost. Reducing dispersion increases fiber bandwidth.
Fig 4. Modes of Propagation as ON and OFF signals showing Modal Dispersion
Modal dispersion can be reduced in these ways:
▪ By using a smaller core radius, which allows fewer modes.
▪ By using a graded index fiber so that light rays that takes longer path can also reach other end at faster velocity at nearly same time as the ray that follow shorter path.
▪ By using a single-mode fiber, which allows no modal dispersion.
XI. MATERIAL DISPERSION
Different wavelengths also travel at different velocities through a fiber, in the same mode as
n=c/v
where, n is the refractive index and v is the speed of the same wavelength in material
The value of v in the equation varies for each wavelength. Thus the refractive index varies according to the wavelength. Dispersion from this phenomenon is called material dispersion, since it came up from material properties of the fiber.
XII. WAVEGUIDE DISPERSION
Waveguide dispersion is most significant in a single-mode fiber. It occurs due to optical energy travelling in both the core and cladding, which have slightly different index of refraction. The energy travels differently in the core and cladding due to change in refractive index of the materials. Altering the internal structures of the fiber allows waveguide dispersion to be substantially changed, thus changing the specified overall dispersion of the fiber.
Polarization mode dispersion
Polarization mode dispersion (PMD) is related to the differential group delay (DGD), the time difference in the group delays between two orthogonal polarized modes which causes spreading of pulses in digital system and distortion in analogue system.
The ideal circular symmetric fibers, two polarization modes propagate with the same velocity. However, there is no ideal condition and the real fiber does not have perfect circular shape and it consists of local stress. The propagation light is split into two polarization modes.
Fig 5. Polarization Mode Dispersion
These two local polarization modes travel at different velocities causing a pulse spreading in digital systems. The so induced DGDs vary randomly along the fiber and in time,
leading to a statistical behavior of PMD, both in time and wavelength. At a given time, the DGD values vary randomly with wavelength. The PMD value is the average of the DGD values. While the individual values can shift from one time to another the overall distribution, hence the average is assumed to be fixed.
Chromatic dispersion
Chromatic dispersion is caused by delay differences among the group velocities of the different wavelengths composing the source spectrum. The consequence of the chromatic dispersion is a broadening of the transmitted impulses.
The chromatic dispersion is essentially due to two contributions: material dispersion and waveguide dispersion. The material dispersion occurs because the refractive index changes with the optical frequency. It is generally the dominant contribution, except in the wavelength region in which it vanishes (for silica based material this happens around 1 300 nm). The waveguide dispersion depends on the dispersive properties of the waveguide itself. From a practical point of view, a significant property is that the waveguide dispersion has opposite signs with respect to the material dispersion in the wavelength range above 1300 nm.
Fig 6. Chromatic Dispersion
Spatial channels:
Multiplexed transmission refers to multiplexing multiple optical signals for transmission and then demultiplexing the received optical signals in order to increase the transmission capacity of a single optical fiber. The multiplexed optical signals are considered as mutually independent communication channels. For example, in wavelength multiplexing technology, in which laser wavelengths (colors) are used for multiplexing, each channel is called a wavelength channel. A spatial channel refers to an individual space (core or mode) that allows the transmission of an optical signal independently. The individual cores of a multicore fiber or the individual modes of a multimode fiber constitute separate spatial channels. In the case of a multimode multicore fiber, the number of spatial channels is multiplication of the number of cores and the number of modes.
Petabits:
1 petabits (Pbits) is 1 quadrillion bits, 1 terabits (Tbits) is 1 trillion bits, 1 gigabits (Gbits) is 1 billion bits, and 1 megabits (Mbits) is 1 million bits.
The transmission rate of an ordinary household optical fiber service (FTTH) is about 2 gigabits per second at most, and 1 petabits is about five hundred thousand times greater than that rate.
Spatial coupling device:
In order to utilize a multicore fiber or a multimode fiber in optical communications, the connection with single-mode single-core fibers that are currently in actual use is important. Among various types of connection that have been proposed, a device that optically connects optical fibers of different types with each other via a lens, a prism, etc. by using a laser beam that propagates through free space is called a spatial coupling device. There are other types of connection, other than spatial coupling, for example, employing an optical waveguide, such as a fiber bundle, a 3D waveguide, or a photonic lantern.
Fig 7. Reviewing different sectors of Optical Fiber.
Classification of Dispersion with types of fiber:
Modal dispersion- MMF
Material dispersion- SMF+MMF
Waveguide dispersion- SMF
Polarization mode dispersion- SMF
Chromatic dispersion- SMF+MMF
Fig 8. Types of Fiber.
Why Fibre Optic Connection is better?
Higher data rate and wider bandwidth up to Gigabit-per-second (Gbps).
Lower signal attenuation and line loss eliminates intermittent connection issues associated with legacy network.
Greater resistance to electrical noise, electromagnetic interference (EMI) and radio frequency interference (RFI) result in better quality over a longer distance.
More secure communications, since it is impossible to tap into an optical fibre cable.
Being lighter weight and more compact in size makes it easier to install and maintain.
Optical fibre cables are non-flammable, and immune to lightning.
Fig 9. Different Types of Fiber Connectors
XIII. OPTICAL FIBER TECHNOLOGY
Innovations in optical fiber technology are revolutionizing world communications. Newly developed fiber amplifiers allow for direct transmission of high-speed signals over transcontinental distances without the need for electronic regeneration. Optical fibers find new applications in data processing.
Over the years, Optical Fiber has been improved in the following cases:
Optical Fiber Components New Fiber Materials and Designs Fiber Laser and Amplifiers Fiber Switching, Memoruy and Signal Processing Modulation System for Transmission System Fiber Nonlinearities and Countermeasures Long-haul Transmission System Fiber Local Area Network Fiber Sensors and Instrumations
Application of Optical Fiber:
Military
Fiber optic technology is in high demand in the military today. The military has tested the cables rigorously and decided they were perfect for use in many of their applications. They offer better performance, more bandwidth, and greater security for their signals - all at a lower cost. They're strong, and more importantly lightweight, and can also be used outdoors in harsh environments. Thus, optical cabling is an excellent choice for the military's retrieval and deployment applications.
Missile launchers and radar systems have also begun to utilize these benefits. In many of their control systems, a single pencil-sized optical fiber can replace miles (and pounds) of copper wiring. In 2014, the U.S. Army plans to introduce an Abrams tank that will be almost two tons lighter than the current version, all due to replacing copper wiring with its lighter, faster, more secure counterpart.
Transportation
The fast-paced transportation system has become a growing market for the use of fiber optics. With the increase in traffic and more demand for efficiency, “smart highways” have begun to adopt fiber into things like automated toll booths, traffic signals, and message signs that are changeable.
Beyond that, these cables are being utilized in lots of other technical, complicated ways. One such example can be found in electric trains. Fiber is used as the transmission medium to
control the switching of power semiconductors within the converters that create the right frequency and voltage for the electrical drive motors and electrical systems.
What that basically means is that a transformer has to convert the power grid's electricity to a lower voltage. Since the distances traveled to accomplish such conversions can be quite far, fiber provides a much better solution than copper.
Other Uses of Fiber Optics
Fiber is also used in countless other applications, including decorative lighting for Christmas trees, signs, and art. Showcases displayed in boutiques use optical fibers to illuminate from different angles using a single light source.
Special optical fibers are also used for sensor applications in areas that involve oil-well monitoring and fire or leak detection.
The extra bandwidth offered also enables cable television to transmit signals to their subscribers faster and more efficiently. Fiber also shows up in research institutions, colleges and universities, as well as in the aerospace, biomedical, and chemical industries.
Fiber optic technology is definitely growing rapidly, and is fast becoming an essential part of our everyday lives.
A. New types of Optical Fiber have been constructed:
B. Optical Fiber ‘Nanospikes’ Effectively Trap, Focus Laser Light- ERLANGEN, Germany, March 14, 2016
Using laser light to manipulate a glass optical fiber tapered to a sharp point smaller than a speck of dust, in the middle of an optical fiber with a hollow core, has been demonstrated by a team from the Max Planck Institute for the Science of Light. Optical forces cause the sharp point, or “nanospike,” to self-align at the center of the hollow core, trapping it more and more strongly at the core center as the laser power increases.
The new work could increase applications for hollow-core fibers, a new class of fiber that features a hollow coe rather than one made of glass like traditional optical fibers. Hollow-core fibers are especially good at handling high-power lasers, making them potentially useful for laser machining and other mate cutting of metals, plastics, wood and other materials.
To create the nanospike, the researchers started with an ordinary single-mode glass optical fiber about 100 μm in diameter. The fiber was heate so that it could be stretched to form a tapered portion and then the fiber’s tip was etched with hydrochloric acid to create a nanospike around 100 nm in diameter — smaller than the wavelength of visible light — and less than 1 mm long.
The nanospike was inserted into the hollow core fiber and a high-power 1064-nm laser beam was launched into the single-mode fiber, creating the optical trap. When the laser light entered the tapered portion of the fiber it began to spread out beyond the nanospike into the empty space inside the hollow core fiber. As the taper got smaller and smaller, the light began to sense the boundary of the larger fiber core, causing the light to reflect inward toward the tapered fiber. This reflected light exerted a mechanical force on the nanospike,
New types of optical fiber
Main features
Multimode fiber
Light propagates through a core in multiple modes.Requires a thicker core compared with the single mode.Standardized as a part of 40G/100G Ethernet standards.
Multicore fiber
Optical signals are transmitted via multiple cores.There has been research on 7-core, 10-core, 12-core, and 19-core fibers, etc.
Multimode multicore fiber
Multiple multimode cores are provided.There has been research on 3-mode 7-core fibers, 3-mode 12-core fibers, etc.
forming an optical trap.
“Launching very high power laser light into an optical fiber, especially a hollow-core fiber, can be very difficult and usually requires extensive electronics and optics to maintain alignment,” explained Philip Russell, director at the Max Planck Institute for the Science of Light in Erlangen, Germany, and leader of the research team. “This can be accomplished with our new system by simply pushing the nanospike into the hollow core and then turning up the laser power slowly. Once the nanospike self-stabilizes, you can turn up the laser power and nothing will move or get damaged. “The nanospike is held in place by the light at exactly the right place to perfectly launch the light into the hollow core without any electronics or other systems to keep it in place,” Russell said. “If any of the components move a little, there’s no effect on the laser light because the nanospike self-aligns and self-stabilizes.”
Nearly 90 percent of the laser light was transferred from the nanospike to the hollow-core fiber, researchers said.
“The beauty of the nanospike is that it behaves like a very small particle, but because it is firmly attached to a strong piece of fiber at one end, it isn’t lost if it jumps out of the trap,” said Russell. “This system allows us to measure forces that are almost impossible to measure in other systems, making it feasible to explore of an area of fundamental physics that isn’t very well understood.”
In addition to efficiently coupling high-power laser light to hollow-core fibers, the new system offers an entirely new way to study the mechanical forces exerted by light, or optomechanics, especially at very low pressures. Scientists working to study optomechanical forces under high vacuum conditions have been hampered by the tendency of particles to jump out of optical traps as air pressure is lowered from atmospheric levels. The reasons for this tendency are not fully understood.
C. Optical Feed System Achieves 60-W Power Over 300 M- TOKYO, Jan. 25, 2016
An optical feed system using double-clad fibers has been demonstrated to supply 60-W power over a 300-m optical fiber system. The setup could enable future small-cell mobile communications.
Fig 11. Optical Feed System graph.
Power supply over fiber is limited by power transmission efficiency, which is impeded by the large fraction of power fed into the optical link that is lost as heat during transmission. As such, restrictions on power feed levels are needed to prevent waste heat from damaging optical components in the link.
Researchers from the University of Electro-Communications had previously demonstrated that they could bundle together two multimode fibers for transmitting power with a double-clad fiber for transmitting the data. The bundle was tapered and fused to a double-clad fiber output. However, power was lost in the tapered fiber bundle divider due to the lower cross-sectional area occupied by fiber in the cluster bundle, the researchers said. As a result, the overall power transmission efficiency was only 20 percent, limiting the power that could be fed into the link to just 40 W.
Now they say increasing the number of multimode power-carrying fiber to six optimized the cross-sectional area of fiber in the bundle cluster without introducing other limitations, thereby maximizing the power transmission efficiency.
The researchers tested the cluster of power and data fibers in a bidirectional system consisting of a central station and a radio antenna unit linked by a double-clad fiber.
A laser diode with direct electrical modulation from a signal generator produced test signals at 1550 nm to the standard specifications of the IEEE for the wireless local access network used in Wi-Fi. Commercial laser diodes also fed the optical power.
An erbium-doped fiber amplifier boosted the signal and increased the power level of the data signal for the transmission. The system also included elements to reduce the noise, including bandpass filters and cladding mode strippers.
The multimode and double-clad fibers in the fibers bundle
cluster input were tapered and fused to a 300-m double-clad fibers transmission output. The double-cladding prevents crosstalk, the researchers said, though cost prohibited use of longer-output double-clad fibers in the test system.
The researchers identified the combined cross-sectional area of the two multimode fibers as the limiting factor for power transmission efficiency. The two multimode fibers left empty space in the bundle cluster that was unoccupied by fiber, so that the total fiber cross-section was smaller than that of the double-clad fiber it fused to, making power transfer inefficient.
Using a greater number of narrower multimode fibers increased the filling factor within the bundle and hence their combined cross-sectional area, the researchers said. However, this also meant each fiber was narrower, reducing its power-handling capability.
Ultimately the researchers determined that a bundle of six multimode power-handling fibers gave the optimum compromise between the two limiting factors.
D. Faster internet? Electrical engineers break power and distance barriers for fiber optic communication- University of California, San Diego, June 25 2015
Photonics researchers have increased the maximum power -- and therefore distance -- at which optical signals can be sent through optical fibers. This advance has the potential to increase the data transmission rates for the fiber optic cables that serve as the backbone of the Internet, cable, wireless and landline networks. The new study presents a solution to a long-standing roadblock to increasing data transmission rates in optical fiber.
Fig 12. A wideband frequency comb ensures that the crosstalk between
multiple communication channels within the same optical fiber is reversible.
Electrical engineers have broken key barriers that limit the distance information can travel in fiber optic cables and still be accurately deciphered by a receiver. Photonics researchers
at the University of California, San Diego have increased the maximum power -- and therefore distance -- at which optical signals can be sent through optical fibers. This advance has the potential to increase the data transmission rates for the fiber optic cables that serve as the backbone of the internet, cable, wireless and landline networks. The research is published in the June 26 issue of the journal Science.
The new study presents a solution to a long-standing roadblock to increasing data transmission rates in optical fiber: beyond a threshold power level, additional power increases irreparably distort the information travelling in the fiber optic cable.
"Today's fiber optic systems are a little like quicksand. With quicksand, the more you struggle, the faster you sink. With fiber optics, after a certain point, the more power you add to the signal, the more distortion you get, in effect preventing a longer reach. Our approach removes this power limit, which in turn extends how far signals can travel in optical fiber without needing a repeater," said Nikola Alic, a research scientist from the Qualcomm Institute, the corresponding author on the Science paper and a principal of the experimental effort.
In lab experiments, the researchers at UC San Diego successfully deciphered information after it travelled a record-breaking 12,000 kilometers through fiber optic cables with standard amplifiers and no repeaters, which are electronic regenerators.
The new findings effectively eliminate the need for electronic regenerators placed periodically along the fiber link. These regenerators are effectively supercomputers and must be applied to each channel in the transmission. The electronic regeneration in modern lightwave transmission that carries between 80 to 200 channels also dictates the cost and, more importantly, prevents the construction of a transparent optical network. As a result, eliminating periodic electronic regeneration will drastically change the economy of the network infrastructure, ultimately leading to cheaper and more efficient transmission of information.
The breakthrough in this study relies on wideband "frequency combs" that the researchers developed. The frequency comb described in this paper ensures that the signal distortions -- called the "crosstalk" -- that arises between bundled streams of information travelling long distances through the optical fiber are predictable, and therefore, reversible at the receiving end of the fiber.
"Crosstalk between communication channels within a fiber optic cable obeys fixed physical laws. It's not random. We now have a better understanding of the physics of the crosstalk. In this study, we present a method for leveraging the
crosstalk to remove the power barrier for optical fiber," explained Stojan Radic, a professor in the Department of Electrical and Computer Engineering at UC San Diego and the senior author on the Science paper. "Our approach conditions the information before it is even sent, so the receiver is free of crosstalk caused by the Kerr effect."
The photonics experiments were performed at UC San Diego's Qualcomm Institute by researchers from the Photonics Systems Group led by Radic.
Pitch Perfect Data Transmission: The UC San Diego researchers' approach is akin to a concert master who tunes multiple instruments in an orchestra to the same pitch at the beginning of a concert. In an optical fiber, information is transmitted through multiple communication channels that operate at different frequencies. The electrical engineers used their frequency comb to synchronize the frequency variations of the different streams of optical information, called the "optical carriers" propagating through an optical fiber. This approach compensates in advance for the crosstalk that occurs between the multiple communication channels within the same optical fiber. The frequency comb also ensures that the crosstalk between the communication channels is reversible.
"After increasing the power of the optical signals we sent by 20 fold, we could still restore the original information when we used frequency combs at the outset," said UC San Diego electrical engineering Ph.D. student Eduardo Temprana, the first author on the paper. The frequency comb ensured that the system did not accumulate the random distortions that make it impossible to reassemble the original content at the receiver.
The laboratory experiments involved setups with both three and five optical channels, which interact with each other within the silica fiber optic cables. The researchers note that this approach could be used in systems with far more communication channels. Most of today's fiber optic cables include more than 32 of these channels, which all interact with one another.
In the Science paper, the researchers describe their frequency referencing approach to pre-compensate for nonlinear effects that occur between communication channels within the fiber optic cable. The information is initially pre-distorted in a predictable and reversible way when it is sent through the optical fiber. With the frequency comb, the information can be unscrambled and fully restored at the receiving end of the optical fiber.
"We are pre-empting the distortion effects that will happen in the optical fiber," said Bill Kuo, a research scientist at the Qualcomm Institute, who was responsible for the comb development in the group.
The same research group published a theoretical paper last year outlining the fact that the experimental results they are now publishing were theoretically possible.
E. Scientists achieve a record 57Gbps through fiber optic lines- Milton Feng, March 25, 2016:
Data is key to our modern society, and data transfer has become pivotal for many industries, as well as for our day to day lives. Thankfully, the maximum speed is constantly increasing and while we may not see this in current infrastructure, there are reasons to be optimistic.
Fig 13. Image showing light propagating through optical fiber medium.
University of Illinois researchers report that they’ve set a record for fiber data transmission, delivering 57Gbps of error-free data. This isn’t the fastest speed ever achieved, that record being a whopping 1.125 Tbps, or 1125 Gbps. However, that speed was achieved with an optical communications system that combined multiple transmitter channels and a single receiver. This time, the data transmission was achieved through fiber optic and more importantly, they did that at room temperature. The research team was led by electrical and computer engineering professor Milton Feng.
XIV. CONCLUSION
Fiber optic technology is widely used in all over the world replacing copper wire system due to its efficient transmission of data and supports wide range of bandwidth. Dispersion can be avoided using smaller core radii, which allows fewer modes. And also single mode fiber allows no dispersion. By using a graded index fiber so that light rays that allow longer paths to travel at a faster velocity and there by arrive at the other end of the fiber nearly at the same time. Multicore Fiber has been introduced by constructing several cores in a single
fiber medium. Application of Optical Fiber is immensely increasing in Telecommunication and Business Management Sectors. The impact of fiber materials, devices, and systems on communications in the coming decades will create an abundance of primary literature and the need for up-to-date reviews.
ACKNOWLEDGEMENT
The authors would like to thank their course instructor, Professor Dr. Abdul Awal, Department of Mathematics and Physical Science, North South University for accepting the proposal of choosing this topic.
