Fiber Optics: How does it work? What is it used for? And more

Telecommunications, or long-distance communication, is possible thanks to the various transmission lines available. Among these, one of the most reliable is fiber optics. Learn how fiber optics work in this article, enabling you to use the internet and enjoy many other benefits at home and in the office.

Optical fiber

Optical fiber is a flexible filament generally made of silicon, although there are also optical fibers made of plastic. It is a very delicate material, with a diameter almost like that of a hair strand. It is transparent, to be able to transmit light without the frequency of one color distorting the others.

Fiber optics emerged as a potential substitute for coaxial cable. Like copper cable, fiber is used to transmit data between the two ends of it, using light as a means of transport.

Today, fiber optics is one of the most widely used communication mechanisms. This is because the information can travel long distances without needing many, or perhaps no amplifiers so that the signal is not lost. It also has a bandwidth or data speed higher than copper cables.

Among other reasons why fiber optics is used to replace copper cable, is that there is less attenuation of the signal per kilometer of travel. And another of its benefits is that fiber optics is immune to electromagnetic interference, so it is practically impossible to violate security.

Likewise, it is common to find fiber optics for lighting. This is due to the fact that at the moment it is through the fiber that it has been possible to provide the highest intensity light of all the forms of providing artificial light.

The structure of an optical fiber is made up of a core, which is the fiber itself through which the information travels. The core is covered with a coating, which is characterized by having a lower reflection index than the core. This is done with the intention of preventing the beam of light traveling in the nucleus from leaving it and losing the information, that is, it is a way of confining the information in the nucleus using the phenomenon of reflection.

Subsequently, the lining is surrounded by a mantle, and this mantle of Kevlar. And finally, a hood, usually yellow or orange, that identifies the type of fiber. Both serve to give the fiber protection and mechanical rigidity, since otherwise the fiber would be extremely brittle, even more than it already is.

Within the optical fiber, more than one beam of light can be sent, because there are different routes or possible ways to do it. This type of optical fiber is called multimode, and its name comes from what it has multiple modes to send the light beams simultaneously. Instead, single-mode fiber uses a single linear path to transmit the light beam. Physically they tend to be differentiated by the color of their hood, generally, being orange for multimode and yellow for singlemode.

When splicing the optical fiber it is important to take into account that its alignment between the cores must be extremely precise, since otherwise there would be losses due to false coupling. This implies that the joints between optical fibers are more complex than those used in conventional copper cables.

Fiber Optic History

In fiber optics there are two fundamental elements that presented advances to achieve the technology we know today in terms of data transmission through light. Those elements are the optical fiber and the light that travels in it. In this segment, we will travel through the history of fiber optics and how it evolved over time.

In ancient Greece, sunlight reflected off mirrors was used to send messages, or to obstruct the vision of your enemies. This same system was applied by Claude Chappe in 1792 to apply optical telegraphy using towers and mirrors that were distributed over 200 km. He managed to transmit a message in record time for the time of 16 minutes.

In 1910 Demetrius Hondros and Peter Debye were the first to implement light capture inside cables made of glass. It is curious that this experiment took so long, because by the year 1820 the equations that established this phenomenon already existed.

This principle is called as the confinement of light by refraction. Remember that refraction is a phenomenon that causes a beam of light to change direction when it changes from one transmission line to another that have different refractive indices because they have different densities.

In the 1840s, Jean-Daniel Collado and Jacques Babinet were able to prove this principle. Similarly, in 1870 John Tyndall observed that light can travel in water, and that light is refracted within this medium. These first steps allowed the studies to be carried out that would confirm the potential of crystal as a material par excellence for the transmission of light over long distances. At first its application was for the lighting of water fountains.

Later, John Logie, a Scottish engineer, patented an electromechanical system for color television, which used rods made of glass that transmitted light. This system was not very successful due to the attenuation it had due to the materials and techniques it applied, so it was not possible for light to travel great distances. Also, their system did not have optical couplers.

In 1952 thanks to the previous studies carried out by John Tyndall, the physicist Narinder Singh Kapany was able to make the invention of the optical fiber. It should be noted that it was in the 1950s when research on fiber optics was deepened. In fact, in 1957 Basil Hirschovitz manufactured a semi-flexible endoscope using fiber optics. He managed to send images and this endoscope is a more comfortable tool for surgeries.

The University of Michigan proposes in its version of a semi-flexible endoscope the use of a material with a lower refractive index than the core, instead of using oils or wax as was previously used. In addition, much thinner fiber optic strands were also manufactured, as much as the thickness of a hair. However, the light was attenuated or lost after 9 meters of travel with this optical fiber. It was Charles K. Kao, who presented that the maximum theoretical attenuation that optical fibers should have was 20 decibels for its application to be feasible, in his doctoral thesis

Again, Charles K. Kao together with George Hockham stated that it was possible to manufacture fibers with a higher percentage of transparency. Likewise, they were the ones who proposed the use of fiber optics for the transmission of telephone messages instead of using the ones convinced is copper and electricity cables.

It was necessary to improve the optical fiber, which at the moment had an attenuation of 100 dB / km, small bandwidth and great mechanical fragility. To achieve this, incessant and in-depth studies and investigations had to be carried out, which made it possible to determine that the cause for which there was this level of loss in light was the intrinsic impurities that existed in the silicon or glass.

It was thanks to this discovery that fibers began to be manufactured with an attenuation reduction of up to 20 dB / km and with a greater bandwidth. In addition, the cores were 100 µm thick, which were covered with Naylon wire to respect the refractive index foundation, but which could also provide greater mechanical rigidity, making it impossible to break the fiber with the hands.

The work of Kao and Hockman served as the foundation for the research carried out by Robert Maurer, Donald Keck, Peter Schultz and Frank Zimar who proposed and manufactured the first optical fiber with impurities of Titanium. These impurities were intentionally incorporated into the silicon to increase refraction in the fiber. This fiber allowed light to travel in the optical fiber with only 17 dB / km of attenuation. In that same decade of 1970, it was possible to manufacture optical fibers with only 0.5 dB / km of losses.

Another important advance that was made in the technology of transmitting data by means of light, was made thanks to the physicists Morton B. Panish and Izuo Hayashi who developed a semiconductor laser that could operate continuously without increasing its temperature. Together John MacChesney and other collaborators generated methods of fiber preparation.

The first telephone transmission using the transmission line was made on April 22, 1977 by the North American company General Telephone and Electronics, and reached a speed of 6 Mbit / s.

By 1980 the fibers had such transparency that they could send signals through an optical fiber for a path of up to two hundred and forty kilometers before they were completely lost. These fibers arose when researchers realized that pure silicon without any metal could only be made using tools and components that used steam. This prevented inherent contaminants from arising in the manufacturing process.

AT&T submitted its project for a fiber optic system approximately 1980 miles apart to the Federal Communications Commission in XNUMX. This system would run through and connect the cities of Boston and Washington DC. It was after four years of its presentation of this project, that the system began to become operational. This cable was ten inches in diameter and was capable of providing up to eighty thousand voice channels for simultaneous telephone conversations.

The first optical fiber to be installed transoceanically became operational in 1988, the transparency of its core was so impeccable that it was only necessary to place optical amplifiers every forty miles. Subsequently, more connections of this type were made and more extensive tours were made between cities and continents.

All these advances allowed fiber optics to continue improving its transparency and could be applied for communications in the market and not only at an experimental level. In fact, General Telephone and Electronics on April 22, 1977 managed to make the first successful telephone transmission with a speed of 6 Mbits / s, using fiber optics as the transmission line.

Fabrication process

It was at Bell Laboratories that independent methods for manufacturing fiber optics were developed. From there, there are four processes for manufacturing fiber optics

MCVD (Modified Chemical Vapor Deposition)

This method was originally developed by Corning Glass and adapted by Bell Laboratories for industrial application. They consist of a pure quartz tube, in which silicon dioxide is mixed with other elements to dope the preparation. This tube is then placed on a rotating lathe.

Subsequently, it is brought to temperatures of up to 1600 degrees Celsius with a hydrogen and oxygen burner. This is done by rotating the lathe while the quartz tube is heated over its entire length. At this point the additives are added, which are those that will contribute to a better refractive index in the core, at one end of the tube.

The subsequent layers are also incorporated, due to the continuous subjection of the burner. This technique allows the nucleus to be synthesized. Then, the burner is brought to a temperature of 1800 degrees Celsius, which is the temperature that allows the quartz to soften and thus obtain the preform.

The preform refers to the glass rod or tube that is used to create fiber optics. That is, it is the solid tube that is obtained after this method. Usually its dimensions are a little over 1 meter long and 1 centimeter in diameter.

VAD (Vapor Axial Deposition)

This technique is based on that developed by Nippon Telephone and Telegraph. It is widely used in Japan in those companies engaged in the manufacture of fiber optics. It uses the same basic materials as in the MCVD method. However, they differ in the fact that the latter, only the core was placed. Now, the lining is also placed in addition

That is why this method is a bit more delicate at the time of doping, since Germanium dioxide must be incorporated in a greater proportion in the core than in the coating. For this manufacture, a software is used as an essential assistant where the parameters are established.

Taking with an auxiliary glass rod or tube, the preform process begins. This auxiliary tube serves as its support. It begins by incorporating the different materials in an orderly manner from the end of the cylinder, obtaining the porous preform, which as it grows, becomes detached from the auxiliary glass tube.

Subsequently, the collapse process is carried out, which consists of raising the temperature up to 1700 degrees Celsius in order to achieve the softening of the quartz. This is done to go from an internally hollow porous preform to a solid, transparent cylinder.

If we compare this method with the previous one, the VAD method has the advantage that preforms of greater length and greater diameter are obtained, and also reducing the energy input. However, its disadvantage is that it requires much more sophisticated manufacturing equipment.

OVD (Outside Vapor Deposition)

This method was developed by Corning Glass Work. In this case, your raw material starts with a ceramic substrate cylinder and the burner. The vaporous chlorides are placed on the burner fire, and the fire heats the rod. At this point, the synthesis of the preform is carried out. This procedure consists of drying the rod by means of gaseous chlorine, to later carry out the collapsing process in the same way as those that will be carried out with the previous method. This is how the core and its cladding are synthesized, obtaining the preform.

Some of the advantages of this method is that it is possible to manufacture optical fibers with very low attenuation and good quality, thanks to the optimization of the drying process. This optimization allows to obtain smooth profiles without an important annular structure.

PCVD (Plasma Chemical Vapor Deposition)

This method is developed by the Philips company in the Netherlands. Likewise, its characteristic is its smooth profiles and without recognizable annular structure. The principle of this foundation is based on the oxidation of Silicon chloride and Germanium chloride. By rusting these chlorides, a plasma state is achieved, followed by the doping process of the interior.

Preform stretching stage

Regardless of the type of method used, it is common among all of these that the preform stretching process is carried out. To achieve stretching of the preform it is necessary to have an open tube furnace. Inside this oven, the preform is placed and subjected to temperatures of up to 2000 degrees Celsius, this in order to soften the preform and be able to manipulate it.

In this process, the diameter of the optical fiber is achieved, and it is of utmost importance to maintain a constant tension so that the diameter throughout the length of the optical fiber does not vary. The way to ensure that the core does not have such variations in its diameter is to maintain a constant uniform tension. In addition, the absence of convection currents in the oven must be guaranteed.

Likewise, it is extremely important that when the preform softens again, the entry of agents that can contaminate or generate microcracks is avoided, which would cause loss of its attenuation and even breakage in the optical fiber.

During this process, synthetic material is also added to the figure, generally it is a viscous polymer. The importance of this polymer is that it allows the optical fiber to be stretched at higher speeds. This creates a uniform, impurity-free layer around the fiber. Finally, this protection is dried and hardened, by means of thermal processes or chemical reactions using ultraviolet radiation.

Fiber Optic Applications

The fiber has an impressive versatility, so it can be applied in digital communications, jewelry, sensors, lighting, decorations, among others. Here are some of the most common applications of fiber optics and additional ones.

Fiber optic communications

The largest scale or magnitude use of fiber optics is for telecommunications. Due to its flexibility, it is possible to group several wires to form fiber optic cables. Usually, the fibers used for this are made of plastic or glass, and sometimes even of both materials.

Fiber optic sensors

Fiber optic sensors can be distinguished between intrinsic sensors and extrinsic sensors. Intrinsic sensors refer to the figure itself as the sensor. On the other hand, in extrinsic sensors, the fiber is the means for transmitting the signals that a sensor emits to a system that processes said signals.

Because there is no circulation of electrical current in optical fibers, they have an advantage compared to electrical sensors. Even the fiber yarn is by itself an excellent sensor to measure deformations, temperature, atmospheric pressure, humidity, electric fields, magnetic fields, gases, vibrations, among others.

Another application of fiber optics in the application of aquatic microphones for the detection of earthquakes or wave applications generated by sonar. For this, more than a thousand sensors made with fiber optics have been used to create hydrophonic systems. This type of system is used mainly by the oil industry and defense organizations and some countries. Likewise, the Sennheiser company from Germany created a microphone that operates with laser light and fiber optics.

In this same sense, fiber optic sensors that measure temperature and atmospheric pressure are used in oil wells. These types of sensors are capable of withstanding more extreme conditions compared to sensors made with semiconductors.

In aviation there is a gyroscope made of fiber optics, as well as hydrogen microsensors.

These photonic sensors made of fiber optics generally contain four fundamental parts, which are:

The sensor: is the transducer
The interrogator: who emits and receives the signal coming from the optical fiber.
The optical cable: it is the optical fiber
Optical couplers, multiplexers, amplifiers or switches: the elements that help the optical and electrical system to be coupled without losing the signal, and are capable of handling different signals from different sources.

The operation or functioning of this system begins with the generation of an optical signal created by the interrogator, this in order to request information from the receiver. This information travels through the optical fiber of the sensor. When it proceeds to measure environmental conditions such as gases, atmospheric pressure, temperature and other factors, there is a variation in the intensity of the light, or its wavelength is affected and, therefore, there is a change in it. .

This change variation, either in the wavelength or in the intensity of the light is returned again through the optical fiber to the interrogator. Then, the percentage of variation of these changes is estimated. By applying different algorithms and tools such as optoelectronic couplers, it is possible to convert optical signals into electronic signals, so that the electrical systems at the ends can interpret the information, such as a control or display system. real-time data.

Likewise, depending on the amount of data traffic such as those passing through an Internet network, there may be optical multiplexers, optical switches, optical amplifiers, or different optical couplers in the same way.

Similarly, fiber optic sensor systems can be classified as point or distributed.

Optical point sensor systems

This type of system uses distributed sensors defined positions within a sensor network that allow the parameters to be monitored individually. Because of this, point systems allow the measurement of more parameters simultaneously. Unlike distributed systems, point system monitoring can span up to 250 km.

Distributed Optical Sensor Systems

In this case, the measurements and detections of variation of the optical parameter that the interrogator receives come from the data obtained along the entire optical fiber. This presents an advantage since it is a fiber optic strand zone that is used as the translator of the system. The distributed optical system can span up to a range of 120 km in length.

Automotive Lighting

The first applications that fiber optics had were precisely the lighting of spaces. Even today, this application continues to exist for fiber optics. This is due to the fact that the optical fiber allows to illuminate areas without generating heat and without risk of short circuits, since the optical fiber is designed for the transmission of light beams.

It is even possible, by modifying the frequency, to change the color of the lighting. This is very useful for example if lighting is used in a lamp, since it is possible to change the color without having to change the lamp.

Likewise, it is possible to expand the illumination areas since it is possible to position different optical fibers in multiple places, using a single light source.

More uses of fiber optics

It is used as a wave guide for the beams of light emitted by medical or industrial equipment that require illuminating areas where the line of sight is not directly or easily accessible.

We can use as an example the semi-flexible endoscope used in medicine, which uses fiber optics in conjunction with lenses to be able to visualize the interior of the organs without the need for highly invasive surgeries. In the case of industries, to inspect equipment such as turbines.

In fact, fiber optics are currently used as decorative elements, as is the case with Christmas trees that have optical fibers in their branches that illuminate the tree, and it is also possible to vary their color.

Another example of the application in fiber optics is the one applied in certain buildings, which capture natural light from their rooftops and thanks to fiber optics this light can travel to the internal spaces of the building.

And finally, today there is a mixture between concrete and fiber optics that results in a translucent concrete. This material was created by the architect Ron Losonczi, and the amazing thing about this concrete is that it can still have the strength of concrete and, in addition, the quality of fiber optics to transmit light.

Fiber optic characteristics

Fiber optics is a dielectric transmission line that operates within the electromagnetic spectrum in the optical band. In this optical band is where we can find the colors, but there is also the near infrared band and the infrared band. In optical fiber, part of these frequencies is usually used.

Each fiber optic strand has a core in the center made of either plastic or glass, that is, Silicon and Germanium oxide. This core has a high refractive index and is covered by a coating with a lower refractive index. This allows the light to only travel through the core and not escape to the outside. It is common for this coating layer to be made of a polymer or plastic.

This difference between refractive indices that must exist between the core and its cladding is due to the principles of refraction of light. This principle states that when a surface with a certain refractive index borders another surface with a lower refractive index, the light is reflected and, the greater the difference between these indices, the greater the angle of incidence, so there will be a total internal reflection.

In optical fiber, the light is bouncing or is reflected inside the nucleus, and these angles of reflection are very wide, so it can practically be assumed that the light travels in a straight line through its center, allowing it to travel at great distances without fading.

Fiber optic performance

The laws of optical geometry are those that establish the operation and basic principles of the operation of optical fiber. The optical fiber is mainly regulated by the law of refraction is to go by the principle of total internal reflection.

The light beams are transmitted by the core of the optical fiber, I have given the difference in the refractive indices this beam cannot pass through the cladding but is actually reflected on it and continues to propagate through the core.

Next, we will present the advantages and disadvantages of fiber optic technology.

Advantages

It has a very large bandwidth, allowing very fast transmission speeds
It is a minimalist technology, that is, it takes up very little space.
It is light, since it only weighs a few grams per kilometer. Unlike the electrical cable that can even weigh 9 times more than fiber optics.
It is completely immune to electromagnetic contamination. So it has a transmission quality superior to conventional lines, since it is not disturbed for example by external short circuits or electrical storms.
Precisely because it is immune to interference, fiber optics guarantees a high level of information security. This is because the only way to get inside the fiber optic transmission system is by weakening it and even interrupting it, making it easily detectable.
It does not generate interference to other systems.
It is not affected by parasitic signals, so in systems such as the subway where there are systems that can easily disturb communications, fiber optics becomes the alternative par excellence.
Its attenuation is significantly small compared to conventional cables, so it is possible to travel long distances without the need to include active elements such as amplifiers to maintain the signal.
Depending on the materials with which the mantle and hood are manufactured, the optical fiber can have good mechanical resistance.
It is resistant to corrosion.
It has a system called optical reflectrometry that allows you to easily detect points of weakness or fiber cut along the route.

Disadvantages

Since we present all the advantages of fiber optics, we will proceed to present the disadvantages of this technology compared to other transmission lines.

High fiber fragility.
It requires more expensive transmission and reception equipment.
Splices made with fiber optics are more complex to do, especially in the field, so repairs are more difficult.
Since it cannot transmit electricity, it is not directly compatible with end systems, which are generally electronic.
It cannot transmit very high powers.
It cannot store information optically.
It is affected by high or low temperatures, so the jacket and coating must be temperature resistant materials.
Vibrations can affect data transmission correctly.

Fiber Optic Types

Within the core of the optical fiber there are different paths that the light beam can follow. Each of these paths is called a mode of propagation. Optical fiber can be classified as multimode or singlemode fiber.

Multimode fiber

Multimode fiber refers to one in which light can travel through more than one path or mode. A single strand of multimode fiber can have up to 1000 modes of propagation of the light beams. This implies that the light beams do not arrive simultaneously. This type of fiber is generally used for short distances, approximately distances less than 2 kilometers.

The refractive index of the core of a multimode fiber is slightly higher than the refractive index of the cladding. In addition, the thickness of the core of a multimode fiber is greater than that of a singlemode fiber, this allows it to be easier to connect because it does not require such exact precision.

Multimode fiber can in turn be classified into two forms depending on the type of refractive index of its core, which are:

Step index : In this case, the refractive index is constant along the entire length of the core, resulting in high modal dispersion.

Graded index : In this case, the core is made up of different materials, so the refractive index is not constant along the entire length of the fiber and, therefore, it has less modal dispersion.

Likewise, the standard that is established in ISO 11801, indicates the classification of multimode optical fiber according to the bandwidth and the light source to be used, to say if it is multimode on laser, or multimode on led light.

OM1: Fiber 62.5 / 125 µm, 1 Gigabit (1 Gbit / s), LED.
OM2: Fiber 50 / 125 µm, 1 Gigabit (1 Gbit / s), LED.
OM3: Fiber 50/125 µm, 10 Gigabit (300 m), Laser.

Singlemode fiber

As we explained previously, the term mode is applied to indicate the number of trajectories that the light beams can have. In single-mode fiber, there is only one mode that light can travel through. This then means that the diameter of the core is smaller. Likewise, light theoretically travels through the center of the fiber unlike multimode that bounces off the walls of the core. This type of fiber is used mainly for long distance routes.

Loose structure cable

It is also possible to classify optical fiber according to its design, and there are also two types of optical fiber according to this classification.

This type of fiber can be applied outdoors and indoors and consists of several fiber strands that are divided into groups that are introduced into tubes that surround a central reinforcement, and these in turn are covered by a protective jacket.

The term loose structure comes from the fact that the fiber optic strands are loosely within the tubes through which they are guided. This tube can be hollow or have a hydrophobic material inside, so that it serves as protection for the optical fiber against humidity.

In addition, it had to be loose, it allows the optical fiber to be isolated from the external mechanical forces that are exerted on the cable.

The center brace is generally flexible and provides strength to the cable. It can be made of metal or dielectric material.

Tight structure cable

This cable has mainly applications for the interior of buildings since it is more flexible and allows smaller bend radii than loose structure cables.

This cable consists of the union of several fiber optic strands that individually have a mantle and jacket. These threads surround a central piece and this whole set in turn is protected by an outer layer. Its name comes from the fact that all fiber threads are very tight, which provides good physical support.

Fiber Optic Components

In a fiber optic communication system there are some components that are necessary for the transmission to be successful. These components include optical transmitters, optical detectors, fiber optic connectors or terminals, among others.

Optical transmitters

These are the elements in charge of transforming the information or data that come from an electronic source to optical data or light beams. To achieve this, the transmitter uses electrons at a certain frequency to be excited within materials such as silicon, often generating beams of light, called photons, in the form of energy. Photons are the elementary quantum particle of light. The transmitter internally has a modulator that fulfills the function of transforming electronic energy into optical energy.

Beam emitters

In optical transmitters there are two types of emitters that emit optical signals, which are:

LEDs.

It is a diode that emits light, or Light Emitting Diode. This type of light emitter is mainly used in multimode fiber due to its ease of use and life time. Although it is important to note that this type of light is not capable of traveling long distances, so it is used for short distances because they fulfill the function and reduce costs.

Lasers

It is the Amplified Stimulated Spontaneous Radiation of Light. It emits highly coherent light, and uses semiconductors for light emission. laser light can be used in multimode fibers and in single-mode fibers, although they are generally only used in single-mode fibers since their circuitry is more complex and therefore more expensive. The life time of the laser, although long, is usually below the average of the LEDs.

Electric light-current converters

In fiber optic fiber transmissions it is necessary to have an element that detects the presence of photons. Usually, it is a photodiode that is responsible for the conversion of optical signals to electronic signals. It does this by translating the presence or absence of light into signals with highs and lows or ones and zeros.

And also, they are applied for the reverse process, that is, to convert electrical signals into optical signals. Even though it is possible to transform light into electrical signals, and these emit a certain power, it is not enough to power the terminal equipment. These terminal equipment are usually electrical, so an alternative power source is almost always required.

As we mentioned before, usually these opto-electrical converters consist of a photodiode or a semiconductor. To guarantee the correct operation of these semiconductors, certain conditions must exist, which are:

When there is no light, the reverse current must not be very high in order to be able to detect very weak optical signals.
It must have a large bandwidth, to be able to provide speed of response.
The noise levels generated by these semiconductors should be minimal.

In turn, there are two types of detectors, PIN photodiodes and APD avalanche photodiodes.

PIN detectors

This type of detector consists of a semiconductor composed of three layers, the two outer layers are one of the P type and one of the N type and the one in the middle is an Intrinsic semiconductor. This is where its PIN name comes from. This intrinsic material in practice is usually placed as an extension of material P or material N.

APD detectors

These are avalanche semiconductors. These photodiodes, when a reverse voltage is applied to it, generate a current gain. The operation of these semiconductor avalanche detectors consists of making an electron travel and it meets an atom so that it can release another electron. The reason avalanche semiconductors are used is because this electron being sent is required to handle a sufficient amount of energy.

APD detectors can be classified into three types, according to the material it is made of:

Silicon detectors

These types of detectors have a high performance and generate low noise levels. The supply of this type of detectors is within the range of 200 V to 300 V

Germanium detectors

Generally, it works with wavelengths within the range of 1000 and 1300 nm, although a little lower performance.

Detectors other materials

These detectors are composed of materials or chemicals that are located in groups III and V of the periodic table.

Types of fiber optic polishing

The types of polishing will depend on the connectors that are located at the ends of the optical fiber, they can be classified according to their type of polishing. This type of polishing will vary according to the way it is connected.

Flat : this polishing leaves the ends of the fiber smooth and perpendicular to its axis, that is, completely flat.

PC (Physical Contact) : The fibers are terminated in a convex shape, bringing the nuclei of both fibers into contact.

SPC (Super PC) : It is similar to PSP, the edges are slightly filed so that it becomes a triangular shape without a point in the center, but rather it remains flat.

UPC (Ultra PC): It is the same as SPC but the edges are further called so that only the center of the fiber is the flat part.

Enhanced UPC: This is a more refined version than the previous one, so the contact must be extremely precise.

APC (Angled PC): This type of polishing consists of creating a profile with a certain angle. This angle allows for a more accurate physical contact between the cores of both parts.

Fiber Optic Connectors

Connectors are the components that allow fiber optic cables to be connected to terminal equipment. This terminal equipment has computer communication ports for fiber optic connections. Depending on the type of port, a specific type of connector is used to connect the fiber. This is similar to how it works with conventional cables; for example, coaxial cable has different types of connectors, each with a specific function.

In short, the types of connectors for fiber optics are:

FC
FDDI
LC and MT-Array
SC and SC-Duplex
ST or BFOC

The connectors that are commonly used in fiber optics, specifically for local area networks are the ST, LC, FC and SC connectors.

Optical fibre wires

A fiber optic cable consists of the group of several optical fibers through which different signals are seen. Each fiber can be sending large amounts of data from different sources, so a fiber optic cable can be sending information from different services at the same time.

Fiber optic cables are the most feasible alternative for the replacement of coaxial cables in the telecommunications industry and the electronics industry. Even a cable with 8 optical fibers is still considerably smaller than conventional cables. A fiber optic cable has the capacity to send the information equivalent to that sent by 60 copper cables of 1623 pairs, or 4 coaxial cables of 8 tubes. Additionally, fiber optics can send information over greater distances without the need to place as many repeaters or amplifiers as would happen in the case of using copper cables.

Also, it is important to highlight the difference in weight that exists between the fiber optic cable and the copper cable. For example, an 8-fiber fiber optic cable can weigh as little as 30 kg per kilometer, while coaxial cable can weigh up to 45 kg per kilometer. And likewise, fiber optics allows for a single run from 2 to 4 kilometers apart. In the case of coaxial cable, it only allows for runs of 250 to 300 meters.

However, it is also true that optical fiber requires an additional coating and other elements that provide reinforcement in its installation. This is done in order not to put the laying at risk and, in the future, breakages may occur due to their fragility.

Cable Features

These fiber optic cables have different functions. In the first place, we can mention that it acts as an element that protects the internal optical fibers so that they do not suffer damages or breaks that may occur at the time of the cable installation or during its useful life, which is generally 20 years. .

Second, fiber optic cables provide mechanical rigidity to the internal fiber optic so that it can withstand tensile compression torsional conditions and the environmental factors to which it may be exposed. That is why in addition to the cable, other elements are also incorporated to reinforce and isolate the optical fiber from these external agents and forces to which it is subjected.

These cables can have an underground, submarine or transoceanic or aerial installation. One of the most critical points of a system with fiber optic cables is at the time of installation, and that is why certain elements are used to protect the fiber from damage.

Fiber optic cable design and elements

The role that a fiber optic cable will play will determine its structure. However, even when they can be applied for different functions, all fiber optic cables have many elements in common, which are the secondary coating, the internal fibers, the elements that contribute to the reinforcement and structure of the cable, the jacket that groups together all fiber yarns and moisture-insulating materials. Secondary coatings can be divided into three types:

Snug liner

This liner is generally a solid annular crown that is made of nylon or polyester that covers the primary liner. Therefore this secondary coating increases the final diameter of the optical fiber. The function of this coating is to provide protection against microbends that may exist in the optical fiber. However, even though this coating protects against these bends, it is important to be vigilant when installing the optical fiber, as they can still occur at the time of installation.

Hollow loose liner

This liner has an oversized space, which consists of a hollow tube that is made of metal and combined with plastic. This makes this tube a hard material, but at the same time flexible. The purpose of making an oversized cladding is that it protects the optical fiber from vibrations, temperatures and mechanical forces.

Loose lining with padding

It is the same coating mentioned above, but inside a material capable of insulating moisture is introduced. By introducing a hydrophobic material inside, it prevents water from reaching the optical fiber. In addition to providing protection against vibrations and other environmental agents, it is also capable of withstanding certain temperatures. It is common that materials derived from petroleum or silicone are used.

Structural elements

These elements are the structures that serve as a central guide for the path that the optical fiber must follow. Optical fiber works well whether it is distributed along this structure or braided around it. Generally, these structures have channels or grooves that serve as an extra guide for the optical fiber.

Reinforcing elements

As their name indicates, their mission is to provide additional reinforcement to the fiber optic cable in order to isolate, as far as possible, the tensile forces to which the fibers may be subjected and, in addition, to there is no significant elongation that could cause core ruptures. In addition to protection for elongation, it also protects fiber optic cables against kinks and vibrations. The most common materials used for reinforcement structures are Kevlar fiberglass and steel since they are flexible materials but also have a solidity.

Funda

All fiber optic cable has a jacket that is commonly made of plastic. This sheath is the outer covering of the fiber optic cable and its function is to provide protection to the core from external agents, forces and phenomena, such as humidity, temperature, vibrations, among others.

The materials that will make up our covers will vary according to their installation and application, for example the optical cables that are interoceanic must provide protection against humidity, atmospheric pressure and even against shark bites. If it is a fiber optic cable that will be installed aerially, then the sheath must protect the core against vibrations and kinks generated by the wind, and also against temperatures and humidity. Or if finally, its installation will be underground, then the cover must be a little heavier to withstand shocks and pressures given, for example, vehicular traffic.

Splicing techniques

It is common that in very large runs, up to more than 120 kilometers, it is necessary to make splices between fibers, since there is hardly a continuous fiber that has this length. And even in the event of a breakage, it is necessary to carry out this type of repair.

The different types of splices that exist are the following:

Mechanical splice

This type of splice consists of a kind of sleeve where the two fibers are introduced and a mechanical twist is made to join the two cores. These splices are generally used on a temporary basis or when fusion splicing is deemed unnecessary. The losses related to this splice time are in the order of 0,5 dB.

Splicing with glues

In this case, a special transparent glue is applied that allows the two ends of the fiber to be joined and this Union is protected with some type of external reinforcement. It has losses of 0.2 dB, but it is usually not very reliable since the glue can tend to peel off again.

Fusion splicing

A tool called a fusion splicer is used where more delicate and precision work is done. In this work, the operator must previously prepare the fiber before introducing the ends into this splicer. This tool has the ability to visualize if the ends have any contaminating agent or if a finer polishing is required, or if there should be a better alignment between the fibers. If all these requirements are met, then it proceeds to heat only that area, melting the fiber and thus joining its cores. The losses of this splice are 0.02 dB.

Attenuation in fiber optic cables

The term attenuation means the power losses that occur in the transmission line. Its unit of measurement is the decibel (dB). In fiber optics there are different reasons why attenuation occurs in the cable. There are two types of losses, which are intrinsic loss or loss and extrinsic loss.

Intrinsic attenuations are those that are generated by chemical composition and other factors of its manufacture. That is, those causes that are part of the very composition of Silicon and Germanium and of the thread manufacturing processes. As much as the processes can continue to be improved, it will not be possible to reach a fiber yarn without attenuation.

On the other hand, extrinsic attenuations are those that are generated by external factors, such as impurities, bad connections, incorrect polishing of their profiles, joints, among others. This attenuation is or losses can be classified in turn as follows:

Absorption losses

This type of attenuation occurs when there is the presence of impurities within the fiber. These impurities absorb or interrupt the passage of light. This absorption usually transforms light into heat energy, generating losses from 1 to 1000 dB / km.

Loss of Rayleigh

When the optical fiber is being manufactured there is a moment of its cooling that the fiber is not in a liquid and solid state, and it is possible that there is an incorrect application of tension when it is stretched, this can generate microscopic irregularities. These irregularities cause the diffraction of the light beams when they pass through them.

Dispersions

Dispersion occurs when there is a variation in the refractive index and therefore the light is refracted in a different way than expected, this happens due to micro-cracks within the fiber, pollutants or intrinsic reasons of the fiber.

Intermodal dispersion

This type of dispersion occurs when there is a difference in the propagation time of the light when they take different routes within the core of the fiber. It can also be known by the name of modal dispersion. This type of dispersion only happens in multimode fibers.

Chromatic dispersion of the material: this is the result of the different wavelengths of light that propagate at different speeds through a given medium.

Chromatic dispersion of the waveguide: It is a function of the bandwidth of the information signal and the configuration of the guide is generally smaller than the previous dispersion and therefore can be neglected.

Radiation losses

These losses are generated by kinks or bends in the fiber optic cable. This generally occurs at the time of installation or when bends occur within the path that the fiber optic travels.

Coupling losses

When splicing or at terminal points where connectors are required, attenuation will always exist. These attenuations are usually low, but not negligible. As when there is an incorrect alignment between the nuclei, however this must be corrected to avoid the return of waves.

Fiber Optic Working Windows

These work windows allow us to take advantage of part of the infrared light band of the electromagnetic spectrum. In this case they are windows in which the wavelengths are in the order of nanometers. It has been shown on different occasions that when operating in these work windows there is less attenuation. Specifically, there are three windows:

1st working window: the wavelength is in the order of nine hundred and eighty nanometers.
2nd work window: in this case the wavelength is XNUMX nanometers.
3rd working window: the wavelength is in the order of one thousand five hundred and fifty nanometers. This last window is divided into the S band, C band and L band.

Fiber Optic Connections

There are two forms of fiber optic connections in a network system. These topologies are point-to-point networks and point-to-multipoint networks.

Point-to-point networks are those where a node is generated from the source of the information directly to the companies, home or users who require the service. In other words, there is no intermediary or other node on the network between the user and the service.

Point-to-multipoint networks are those that require a splitter or optical separators, similar to those used for television that allow us to connect different televisions even when the cable company only delivers a coaxial cable. Then, from the emitter an optical fiber emerges that divides the signal through the optical splitters between two, four, six and up to eight users. In very wide networks, one of these eight divisions is taken to incorporate another optical splitter in the network that can feed another 8 users. However, these divisions have a limit and require amplifiers in their path.

Optical amplifiers

They still consist of optical fibers but in the manufacturing process they are doped with different chemical components, especially rare earths. One of the most widely used fiber optic amplifiers is erbium doping.

This amplifier is commonly found as EDFA for its abbreviation in English. This amplifier operates in the third working window, specifically in the C band and L band. However, it can also operate in the S band, but it requires other agents or additional chemical components.

The gain it provides to the optical signal can be from fifteen to forty decibels. It usually consists of an optical fiber stored in a rectangular housing and can have ten to sixty meters in length of doped fiber.

Summary

Fiber optics is a transmission medium that sends data through beams of light. The principles on which the optical fiber is based are in the laws of optical geometry, especially in the law of refraction.

In the beginning, different studies were carried out that complement each other until the optical fiber that is known today is developed. In these studies, it was observed that the transparency of the fiber or core thread was essential to reduce attenuations and achieve the minimum losses that today of 0.02dB / Km.

Common components of optical fiber are silicon oxide and germanium. The cladding components that cover the core are generally some type of plastic. Then comes a mantle that provides mechanical rigidity that can be made of Nylon or Kevlar, and finally a plastic jacket that protects the entire cable and isolates the fiber from external agents.

There are different manufacturing methods which are:

MCVD (Modified Chemical Vapor Deposition)
VAD (Vapor Axial Deposition)
OVD (Outside Vapor Deposition)
PCVD (Plasma Chemical Vapor Deposition)

Regardless of the type of method used, it is common among all of these that the preform stretching process is carried out.

The applications of fiber optics are diverse. In communications, they became one of the means or transmission lines par excellence, due to their great bandwidth and the transmission speed that it can reach, and the reliability or security of the information that this system provides. In sensors that allow detecting conditions or parameters such as: temperature, humidity, atmospheric pressure, and even for sonar systems.

Also, it is used for illumination, such as the flexible endoscope that uses fiber optics as a means of guiding the light in order to illuminate organs and be able to perform less invasive or more precise surgeries in a more comfortable way. Likewise, for decorative effects such as Christmas trees.

It is possible to change the colors that an optical fiber displays or reflects by varying the wavelength or frequency that travels through it.

Some of the most important advantages of optical fiber and its uses are its bandwidth, the transmission speed that the data can reach, electromagnetic immunity, the reduction of occupied space and weight, and high information security.

Although, on the other hand, it is a more advanced technology, and therefore more expensive, the installation and maintenance are more complex than conventional systems, they are extremely fragile and not very tolerant to temperatures, humidity, vibrations and elongation.

There are two types of optics according to the amount of noise or modes they can transmit.

Multimode fibers are those that can send different wavelengths simultaneously through different modes, hence their name. Compared to single-mode fibers, multimode fibers have a larger core and the refractive index between the core and the cladding differ, but only slightly. So the waves travel in the nucleus bouncing off the walls of the nucleus. They are used for short distance routes or networks. It is usually identified because the outer jacket is usually orange.

Single-mode fibers are those that have only one transmission path and their core is smaller compared to multi-mode fibers. The mode of transmission is usually the central axis of the core, since it bounces at very large angles. They are usually different from multimode because they use a yellow outer jacket.

Now, according to its design, there are also two types. Loose-structured optical fibers are those in which the fiber optic strands are loosely within the tubes through which they are guided. This tube can be hollow or have a hydrophobic material inside, so that it serves as protection for the optical fiber against humidity.

In contrast, tight structure cables consist of the joining of several fiber optic strands that are individually covered with a mantle and jacket. These threads surround a central piece and this whole set in turn is protected by an external layer.

In a fiber optic communication system there are some components that are necessary for the transmission to be successful. Among these components are optical transmitters such as LEDs or Lasers, opto-electrical converters that are responsible for transforming electrical signals into optical to be sent through the fiber and later to convert the optical signals that are received into electrical ones again. .

Also, there are the optical detectors that are:

PIN
APD
Silicon
Germanium
Other materials

The types of polishing will depend on the connectors that are located at the ends of the optical fiber, they can be classified according to their type of polishing.

Plano
PC (Physical Contact)
SPC (SuperPC)
CPU (UltraPC)
Enhanced UPC
APC (Angled PC)

The connectors are the elements that allow the fiber optic to be connected to the terminal equipment. The connectors that are commonly used in fiber optics, specifically for local area networks are the ST, LC, FC and SC connectors.

Mechanical splice: this type of splice consists of a kind of sleeve where the two fibers are introduced and a mechanical twist is made to join the two cores. The losses related to this splice time are in the order of 0,5 dB.

Splicing with glues: in this case a special transparent glue is applied that allows the two ends of the fiber to be joined and this joint is protected with some type of external reinforcement. It has losses of 0.2 dB, but it is usually not very reliable since the glue can tend to peel off again.

Fusion splicing: The splicer has the ability to visualize if the ends have any contaminating agent or if a finer polishing is required, or if there should be a better alignment between the fibers. Then, it proceeds to heat only that area, melting the fiber and thus joining its cores. The losses of this splice are 0.02 dB.

There are elements that provide support in the structure and reinforcement of the fiber optic cable. The structural elements which serve as a central guide for the path that the optical fiber must follow. Optical fiber is either distributed along this structure or braided around it. Generally, these structures have channels or grooves that serve as an extra guide for the optical fiber.

The reinforcing elements provide additional reinforcement to the optical fiber cable in order to isolate it from the traction forces to which it may be subjected and, in addition, there is no significant elongation that could generate ruptures in the cores.

In extrinsic attenuations there are several causes, which are:

Absorption losses
Loss of Rayleigh
Dispersions
Radiation losses
Coupling losses

Conclusions

Optical fiber is a transmission line that has allowed greater speed and efficiency in the transmission of data today. Even though it is a technology that must go a long way to be able to replace conventional systems, it is still sometimes the best option for communications.

However, fiber optics is an expensive type of technology compared to conventional systems because it requires equipment and optical tools that tend to be more expensive. It also requires appropriate training to be able to work with fiber optics. In addition, its installation is usually a very delicate process, and the presence of breaks in the fiber could generate considerable losses if it is not treated in time. Additionally, certain equipment still needs to be developed so that the technology and the system are completely optical, since, for example, there are still no optical memories.

It is common for fiber optic networks to have backup systems such as ring or double ring, which allows that in case of these incidents the information can travel in another direction, in order to avoid the interruption of the service for a long period of time while the situation is resolved.

Fiber optic systems are very reliable because it is practically impossible to breach the network without being detected or without interrupting data transmission. That is why it is common to see optical fibers installed underwater between allied countries, where they transmit sensitive and secret information.

What distinguishes fiber optics from other media in data rate and transmission capacity. In addition, it guarantees minimal loss of information due to the fact that it has very little attenuation in its main element, making it unnecessary to install so many restoration and amplification equipment within the system. Since information travels at the speed of light, it has been possible to see a considerable migration of large companies to this type of technology.