Fiber optic cables are used for high-speed data transmission in many industries, especially telecommunications. But what exactly is fiber optic cable? How does he work? How is it constructed? In this article, we will try to provide answers to all these questions.

What are fiber optic cables?

In general, fiber optic cables are not much different from other types of cables. With the exception that they use not energy (electrons), but light (photons) to transfer data. Fiber optic transmission is a general term for the transmission of information in the form of light.

How do fiber optic cables work?

The fiber optic cable is based on a core made of quartz glass or plastic fiber. It is this core that serves as the main light conductor inside the cable. There is another layer between the core of the cable and its sheath, called the “boundary layer”. It serves to reflect light. The refractive index directly affects the transmission rate of the light beam.

Next is the shell of the core itself, which also acts as a conductor of light rays, but has lower index reflections rather thancore ... The envelope is covered by the next layer, called "buffer" (buffer). Its function is to prevent moisture build-up inside the core and sheath.
And finally, the final layer is the outer coating of the cable, which protects the cable from mechanical damage.

How do fiber optic cables transmit light rays?

For data transmission via optical fiber, the incoming electrical signal is converted into a light pulse using a special electro-optical converter. After that, the light beam starts moving along the cables. At the final point of its route, the beam enters the optoelectronic converter, where it is converted into electronic signals.
Different types of fiber optic cables have different core diameters. Larger cores can transmit more rays. Fiber optic cables can be bent, however, make sure the cable is not bent too much, as this could interfere with the transmission of light rays inside the cable.

What are the types of fiber optic cables?

There are several types of fiber optic cables. Let's consider all of them.

Multi-mode fibers with step-index profile (Multimode Stepped Index Cables)

Stepped index multimode cables are the simplest fiber optic cables. They consist of a glass core with a constant reflectance index. This type of cable allows simultaneous transmission of several beams, which are reflected with different intensities and are transmitted along a zigzag path. However, the reflectance index remains constant.
Due to the fact that the beams are repeatedly refracted at different angles, the data transfer rate is reduced. Cables of this type provide bandwidth up to 100 MHz and allow signal transmission over a distance of 1 kilometer.The core diameters of this type of cable are usually: 100, 120 or 400 µm.
Multi-mode fibers with graded index (Gradient index multimode cables).

Like the previous type of cable, this cable allows you to simultaneously transmit many signals, however, the signals inside the optical fiber are refracted not in a zigzag, but along a parabolic path, which can significantly increase the data transfer rate. The disadvantages of these cables include a higher cost. Cables of this type are usually used to build high-speed data networks.
Core diameters: 50 µm, 62.5 µm, 85 µm, 100 µm, 125 µm, 140 µm.

Single-mode fibers (Singlemode cables)

Singlemode fiber optic cables have a very small core diameter and can only transmit one signal at a time. The absence of refractions has a positive effect on the speed and distance of data transmission. Singlemode cables are quite expensive, but provide excellent bandwidth and data transmission distance, up to 100 (Gbit / s) km.

What are the benefits of using fiber optic cables?
Compared to conventional cables, optical fiber provides the following advantages:
Immunity to radio interference and voltage drops
Increased level of strength
High-speed data transmission over long distances
Immunity to electromagnetic interference
Compatible with other cable types

Introduction

Currently, the telecommunications industry is undergoing unprecedented changes associated with the transition from voice-oriented systems to data transmission systems, which is a consequence of the rapid development of Internet technologies and various network applications... Therefore, one of the main requirements for transport networks for data transmission is the ability to quickly increase their bandwidth in accordance with the growth of traffic volumes.

Digital communication via optical cables, which is becoming increasingly important, is one of the main directions of scientific and technological progress.

The advantages of digital streams are in their relatively easy processing by means of a computer, the possibility of increasing the signal-to-noise ratio and increasing the information flow density.

The advantages of optical transmission systems over transmission systems operating over a metal cable are:

Possibilities of obtaining optical fibers with low attenuation and dispersion, which means an increase in the communication range;

Wide bandwidth, i.e. large information capacity;

The optical cable does not have electrical conductivity and inductance, that is, the cables are not exposed to electromagnetic effects;

Negligible crosstalk;

Low material cost of optical cable, its small diameter and weight;

High secrecy of communication;

Opportunities for improving the system while maintaining full compatibility with other transmission systems.

Linear paths of fiber-optic transmission systems are built as two-fiber single-band single cable, single-fiber single-band single-cable, single-fiber multiband single cable (with wavelength division multiplexing).

Taking into account that the share of costs for cable equipment makes up a significant part of the cost of communication, and the prices for optical cable currently remain quite high, the problem arises of increasing the efficiency of using the bandwidth of optical fiber due to the simultaneous transmission of a larger amount of information through it.

The purpose of the work is to consider different ways increasing the bandwidth of optical fiber.

Principles of signal transmission over optical fiber and the main parameters of optical fibers

Principles of signal transmission over optical fiber

The application of optical fiber networks is based on the principle of long-distance propagation of light waves through optical fibers. In this case, electrical signals carrying information are converted into light pulses, which are transmitted with minimal distortion through fiber optical lines communication (FOCL). Such systems have become widespread due to a number of advantages that fiber-optic communication lines have in comparison with transmission systems using copper cables or radio lines as a transmission medium. The advantages of FOCL include a wide bandwidth due to a high carrier frequency - up to 10 14 Hz. Such a band makes it possible to transmit streams of information at a rate of several terabits per second. An important advantage of FOCLs are also factors such as low signal attenuation, which allows, when using modern technologies, to build sections of optical systems of one hundred or more kilometers without repeaters, high noise immunity associated with the low susceptibility of optical fiber to electromagnetic interference, and much more.

Optical fibers are one of the main components of FOCL. They are a combination of materials with different optical and mechanical properties.

The outer part of the fiber is usually made of plastics or epoxy compositions that combine high mechanical strength and high refractive index. This layer provides mechanical protection of the fiber and its resistance to external sources of optical radiation.

The main part of the fiber consists of a core and a sheath. The core material is ultra-pure quartz glass, which is the main medium for transmitting optical signals. The retention of the light pulse occurs due to the fact that the refractive index of the core material is greater than that of the cladding. Thus, with an optimally selected ratio of the refractive indices of materials, a complete reflection of the light beam into the core occurs.

For transmission, light is introduced at a slight angle into the end of the optical fiber. The maximum angle of penetration of a light pulse into the fiber core 6 0 is called the angular aperture of the optical fiber. The sine of the angular aperture is called the numerical aperture NA and is calculated using the formula:

It follows from the above formula that the numerical aperture NA of the fiber depends only on the refractive indices of the core and cladding - n 1 and n 2. In this case, the condition is always fulfilled: n 1\u003e n 2 (Figure 1).

Figure 1 - Propagation of light in an optical fiber. Numerical aperture of the fiber.

If the angle of incidence of light b is greater than b 0, then the light beam is completely refracted and does not fall into the core of the optical fiber (Fig. 2a). If the angle b is less than b 0, then there is a reflection from the boundary of the materials of the core about the shell, and the light beam propagates inside the core (Fig. 2b).

Figure 2 - Conditions of light propagation in an optical fiber

The speed of propagation of light in an optical fiber depends on the refractive index of the fiber core and is defined as:

where C is the speed of light in vacuum, n is the refractive index of the core.

Typical refractive indices of the core material are in the range of 1.45 - 1.55.

In order to transmit light through optical waveguides, a source of strictly coherent light is required. To increase the transmission range, the transmitter spectrum width should be as small as possible. Lasers are particularly suitable for this purpose, which, due to the stimulated emission of light, make it possible to maintain a constant phase difference at the same wavelength. Due to the fact that the diameter of the fiber core is comparable to the wavelength of optical radiation, the phenomenon of interference occurs in the fiber. This can be proved by the fact that light propagates in the glass of the core only at certain angles, namely in the directions in which the introduced light waves are amplified when superimposed. So-called constructive interference occurs. The allowed light waves that can propagate in an optical fiber are called modes (or natural waves). In accordance with the types of propagation of light rays, optical fibers are divided into multimode, that is, using a series of light waves, and single-mode, in which only one light beam propagates. Several basic parameters are used to describe the propagation of light in optical fibers.

Optics opens up wide opportunities where high-speed communications with high bandwidth are required. This is a well-proven, clear and convenient technology. In the Audiovisual field, it opens up new perspectives and provides solutions not available with other methods. Optics penetrated all key areas - surveillance systems, dispatch and situational centers, military and medical facilities, and areas with extreme operating conditions. FOCL provide a high degree of protection of confidential information, allow transferring uncompressed data such as graphics from high resolution and video with pixel precision. New FOCL standards and technologies. Fiber - the future of SCS (structured cabling systems)? We are building an enterprise network.

Fiber optic (aka fiber optic) cable - This is a fundamentally different type of cable compared to the two types of electrical or copper cable considered. Information on it is transmitted not by an electrical signal, but by a light signal. Its main element is a transparent glass fiber, through which light travels over huge distances (up to tens of kilometers) with insignificant attenuation.

The structure of the fiber optic cable is very simpleand is similar to the structure of a coaxial electrical cable (Fig. 1.). Only instead of a central copper wire, thin (about 1-10 microns in diameter) fiberglass is used, and instead of internal insulation, a glass or plastic sheath is used, which does not allow light to go outside the glass fiber. In this case, we are talking about the so-called total internal reflection of light from the interface of two substances with different refractive indices (the refractive index of the glass shell is much lower than that of the central fiber). There is usually no metal braid on the cable, since shielding from external electromagnetic interference is not required here. However, sometimes it is still used for mechanical protection from the environment (such a cable is sometimes called an armored cable, it can combine several fiber-optic cables under one sheath).

Fiber optic cable has exceptional performance on noise immunity and secrecy of transmitted information. In principle, no external electromagnetic interference is capable of distorting the light signal, and the signal itself does not generate external electromagnetic radiation. It is almost impossible to connect to this type of cable for unauthorized wiretapping of the network, since the integrity of the cable is violated. The theoretically possible bandwidth of such a cable reaches 1012 Hz, that is, 1000 GHz, which is incomparably higher than that of electrical cables. The cost of fiber optic cable has been steadily decreasing and is now roughly equal to the cost of thin coaxial cable.

Typical Fiber Optic Attenuation at frequencies used in local area networks, ranges from 5 to 20 dB / km, which roughly corresponds to the performance of electrical cables at low frequencies. But in the case of a fiber-optic cable, with an increase in the frequency of the transmitted signal, the attenuation increases very slightly, and at high frequencies (especially above 200 MHz) its advantages over an electric cable are undeniable, it simply has no competitors.

Fiber-optic communication lines (FOCL) allow transmitting analog and digital signals over long distances, in some cases, over tens of kilometers. They are also used at shorter, more "manageable" distances, such as inside buildings. Examples of solutions for building SCS (structured cabling systems) for building an enterprise network are here: Building an enterprise network: Scheme for building a structured cabling system - Optics horizontally. , We build an enterprise network: SCS construction scheme - Centralized optical cable system. , We build an enterprise network: SCS construction scheme - Zone optical cable system.

The advantages of optics are well known: they are immune to noise and interference, small cable diameter with huge bandwidth, resistance to hacking and information interception, no need for repeaters and amplifiers, etc.
There used to be problems with termination of optical lines, but today they are mostly solved, so that working with this technology has become much easier. There are, however, a number of issues that need to be considered exclusively in the context of the application areas. As with copper or radio transmission, the quality of fiber optic communication depends on how well the output signal of the transmitter and the front end of the receiver are matched. Incorrect signal power specification leads to an increase in the bit error rate during transmission; the power is too high - and the receiver amplifier “oversaturated”, too low - and there is a problem with noise as it starts to interfere with the desired signal. The two most critical parameters of a fiber-optic link are the output power of the transmitter and transmission loss - the attenuation in the optical cable that connects the transmitter and receiver.

There are two different types of fiber optic cable:

* multimode or multimode cable, cheaper, but of lower quality;
* single-mode cable, more expensive, but has better characteristics than the first.

The type of cable will determine the number of propagation modes or "paths" that light travels within the cable.

Multimode cableMost commonly used in small industrial, residential and commercial projects, it has the highest attenuation and only works over short distances. The older type of cable, 62.5 / 125 (these numbers represent the inner / outer diameters of the fiber in microns), often referred to as "OM1", has a limited bandwidth and is used for data transfer rates up to 200 Mbps.
Recently, 50/125 "OM2" and "OM3" cables have been adopted, offering speeds of 1Gbps at distances up to 500m and 10Gbps at distances up to 300m.

Single mode cable used in high-speed connections (above 10 Gbps) or over long distances (up to 30 km). For audio and video transmission, the most expedient is the use of "OM2" cables.
Reiner Steil, vice president of marketing for Extron Europe, says fiber optic lines have become more affordable and more commonly used for indoor networking, driving an increase in the use of optical AV systems. Steil says: “In terms of integration, fiber-optic communication lines already have several key benefits.
Compared to a similar copper-cable infrastructure, optics allows the use of both analog and digital video signals simultaneously, providing a single system solution for working with existing as well as future video formats.
Besides, since the optics offer very high bandwidth, the same cable will work at higher resolutions in the future. FOCL easily adapts to new standards and formats that appear in the process of development of AV technologies. "

Another recognized expert in this field is Jim Hayes, President of the American Fiber Optic Association, formed in 1995, which fosters the growth of professionalism in the field of fiber optics and, among other things, has more than 27,000 qualified optical system installers and implementers in its ranks. He says the following about the growing popularity of fiber-optic communication lines: “The benefit is in the speed of installation and the low cost of components. The use of optics in telecommunications is growing, especially in Fiber-To-The-Home * (FTTH) systems with wireless access, and in the field of security (surveillance cameras).
The FTTH segment appears to be growing faster than other markets in all developed countries. Here in the USA, control networks are built on optics road traffic, municipal services (administration, firefighters, police), educational institutions (schools, libraries).
The number of Internet users is growing - and we are rapidly building new data processing centers (DPCs), for the interconnection of which fiber is used. After all, when transmitting signals at a speed of 10 Gbit / s, the costs are similar to those of "copper" lines, but optics consume much less energy. For years, fiber and copper adherents have fought each other for priority in corporate networks. Wasted time!
Today, WiFi connectivity has become so good that users of netbooks, laptops and iPhones have opted for mobility. And now, in corporate LANs, optics are used for switching with wireless access points. "
Indeed, the fields of application of optics are becoming more and more, mainly due to the above advantages over copper.
Optics penetrated all key areas - surveillance systems, dispatch and situational centers, military and medical facilities, and areas with extreme operating conditions. Reducing the cost of equipment has made it possible to use optical technologies in the traditionally "copper" areas - in conference rooms and stadiums, in retail trade and at transport hubs.
Extron's Rainer Steil comments: “Fiber optic equipment is widely used in healthcare facilities, for example, for switching local video signals in operating rooms. Optical signals have nothing to do with electricity, which is ideal for patient safety. Fiber-optic communication lines are perfect for medical educational institutions, where it is necessary to distribute video signals from several operating rooms to several classrooms, so that students can observe the progress of the operation "live".
Fiber optic technologies are also preferred by the military, since the transmitted data is difficult or even impossible to "read" from the outside.
Fiber-optic communication lines provide a high degree of protection of confidential information, allow transferring uncompressed data such as high-resolution graphics and video with pixel precision.
Long distance transmission capability makes the optics ideal for digital signage systems in large shopping centers, where cable lines can be several kilometers long. If for twisted pair the distance is limited to 450 meters, then for optics and 30 km is not the limit. "
When it comes to the use of fiber in the Audio-Visual Industry, there are two main factors driving progress. First, this is the intensive development of IP-based audio and video transmission systems that rely on high-bandwidth networks - fiber-optic lines are ideal for them.
Second, there is a widespread requirement to transmit HD video and HR computer images over distances greater than 15 meters - which is the limit for HDMI over copper.
There are cases when the video signal simply cannot be “distributed” over a copper cable and it is necessary to use optical fiber - such situations stimulate the development of new products. Byeong Ho Park, VP of Marketing at Opticis, explains, “UXGA, 60Hz bandwidth and 24-bit color require a total speed of 5 Gbps, or 1.65 Gbps per color channel. HDTV has a slightly lower bandwidth. Manufacturers are pushing the market, but the market is also pushing players to use higher quality images. There are specific applications where displays are required that can display 3-5 million pixels or 30-36-bit color depth. In turn, this will require a transfer rate of about 10 Gbps. "
Today, many manufacturers of switching equipment offer versions of video extenders (extenders) for working with optical lines. ATEN International, TRENDnet, Rextron, Gefenand others produce various models for a range of video and computer formats.
At the same time, service data - HDCP ** and EDID *** - can be transmitted using an additional optical line, and in some cases - via a separate copper cable connecting the transmitter and receiver.
As a result of HD becoming the standard for the broadcasting market,other markets — installation, for example — have also begun to use anti-tampering protection for DVI and HDMI content, ”said Jim Jacetta, senior vice president of engineering at Multidyne. - With our HDMI-ONE device, users can send the video signal from a DVD or Blu-ray player to a monitor or display up to 1000 meters away. Previously, no device with multimode lines supported HDCP copy protection. ”

Those who work with FOCL should not forget about specific installation problems - cable termination. In this regard, many manufacturers produce both the actual connectors and assembly kits, which include specialized tools, as well as chemicals.
Meanwhile, any element of the FOCL, be it an extension cord, a connector or a place where cables are connected, must be checked for signal attenuation using an optical meter - this is necessary to assess the total power budget (power budget, the main calculated indicator of the FOCL). Naturally, it is possible to assemble fiber cable connectors manually, "on the knee", but really high quality and reliability is only guaranteed when using off-the-shelf, pre-assembled cables that have undergone rigorous multi-stage testing.
Despite the enormous bandwidth of fiber-optic communication lines, many still have the desire to "cram" more information into one cable.
Here, development is going in two directions - wavelength division multiplexing (optical WDM), when several light beams with different wavelengths are directed into one fiber, and the other is data serialization / deserialization (English SerDes), when parallel code is converted into serial and back.
At the same time, WDM equipment is expensive due to the complex design and use of miniature optical components, but does not increase the transmission speed. The high-speed logic devices used in SerDes equipment also increase the expense of the project.
In addition, today equipment is being produced that allows control data to be multiplexed and demultiplexed from the total light flux - USB or RS232 / 485. In this case, the light fluxes can be sent along the same cable in opposite directions, although the cost of performing these "tricks" of devices usually exceeds the cost of an additional light guide for data return.

Optics opens up wide possibilities where high-speed communications with high bandwidth are required. This is a well-proven, clear and convenient technology. In the Audiovisual field, it opens up new perspectives and provides solutions not available with other methods. At least without significant labor and money costs.

Depending on the main area of \u200b\u200bapplication, fiber optic cables are divided into two main types:

Indoor cable:
When installing fiber-optic communication lines in closed rooms, a fiber-optic cable with a dense buffer (for protection from rodents) is usually used. It is used to build SCS as a trunk or horizontal cable. Supports data transmission over short and medium distances. Ideal for horizontal cabling.

External cable:
Tight buffer fiber optic cable, armored with steel tape, moisture resistant. It is used for external laying when creating a subsystem of external highways and connect individual buildings to each other. Can be laid in cable ducts. Suitable for direct burial.

External self-supporting fiber optic cable:
Self-supporting fiber optic cable with steel cable. It is used for external laying over long distances within telephone networks. Supports cable TV signal transmission as well as data transmission. Suitable for cable ducts and overhead installations.

FOCL advantages:

• Information transmission over FOCL has a number of advantages over copper cable transmission. The rapid introduction of Wols into information networks is a consequence of the advantages arising from the peculiarities of signal propagation in optical fiber.
• Wide bandwidth - due to the extremely high carrier frequency of 1014Hz. This gives the potential for the transmission of information streams of several terabits per second over one optical fiber. High bandwidth is one of the most important advantages of optical fiber over copper or any other media.
• Low attenuation of the light signal in the fiber. Industrial optical fiber currently produced by domestic and foreign manufacturers has an attenuation of 0.2-0.3 dB at a wavelength of 1.55 microns per kilometer. Low attenuation and low dispersion make it possible to build sections of lines without retransmission up to 100 km or more.
• Low noise in fiber optic cable allows to increase the bandwidth by transmitting different signal modulations with low code redundancy.
• High noise immunity. Since the fiber is made of a dielectric material, it is immune to electromagnetic interference from surrounding copper cabling systems and electrical equipment that can induce electromagnetic radiation (power lines, electric motor installations, etc.). Multi-fiber cables also have no cross-talk problem electromagnetic radiationinherent in multi-pair copper cables.
• Light weight and volume. Fiber optic cables (FOCs) are lighter and lighter than copper cables for the same bandwidth. For example, a 900-pair telephone cable with a diameter of 7.5 cm can be replaced with a single fiber with a diameter of 0.1 cm. If the fiber is “dressed” in multiple protective sheaths and covered with steel tape armor, the diameter of such a FOC will be 1.5 cm, which several times less than the telephone cable under consideration.
• High security against unauthorized access. Since the FOC practically does not radiate in the radio range, it is difficult to eavesdrop on the information transmitted over it without disrupting reception and transmission. Monitoring systems (continuous control) of the integrity of the optical communication line, using the properties of high sensitivity of the fiber, can instantly disable the "compromised" communication channel and give an alarm. Sensor systems that use the interference effects of propagated light signals (both along different fibers and with different polarizations) have a very high sensitivity to vibrations and small pressure drops. Such systems are especially necessary when creating communication lines in government, banking and some other special services that impose increased requirements for data protection.
• Galvanic isolation of network elements. This advantage optical fiber lies in its insulating property. Fiber helps avoid electrical ground loops that can occur when two network devices non-insulated computer networks, connected by copper cables, are grounded at different points in the building, for example, on different floors. In this case, a large potential difference can occur, which can damage the network equipment. For fiber, this problem simply does not exist.
• Explosion and fire safety. Due to the absence of sparking, optical fiber increases the safety of the network in chemical, oil refineries, and when servicing high-risk technological processes.
• Profitability of FOCL. The fiber is made of silica based on silica, a widespread and therefore inexpensive material, unlike copper. Currently, the cost of fiber relative to copper pair is 2: 5. At the same time, FOC allows transmitting signals over much longer distances without retransmission. The number of repeaters on long lines is reduced with the use of FOC. With the use of soliton transmission systems, ranges of 4000 km have been achieved without regeneration (that is, only with the use of optical amplifiers at intermediate nodes) at a transmission rate above 10 Gbit / s.
• Long service life. Fiber degrades over time. This means that the attenuation in the laid cable gradually increases. However, due to the perfection of modern technologies for the production of optical fibers, this process is significantly slowed down, and the service life of the FOC is approximately 25 years. During this time, several generations / standards of transceiving systems may change.
• Remote power supply. In some cases, remote power supply of the information network node is required. Optical fiber cannot function as a power cable. However, in these cases it is possible to use a mixed cable, when, along with optical fibers, the cable is equipped with a copper conductive element. This cable is widely used both in Russia and abroad.

However, fiber optic cable also has some disadvantages:

• The most important of them is the high complexity of installation (when installing connectors, micron accuracy is required, the attenuation in the connector greatly depends on the accuracy of the fiberglass chip and the degree of its polishing). To install the connectors, welding or gluing is used using a special gel that has the same refractive index of light as fiberglass. In any case, this requires highly qualified personnel and special tools. Therefore, most often, fiber optic cable is sold in the form of pre-cut pieces of different lengths, on both ends of which the connectors of the required type are already installed. It should be remembered that a poorly installed connector dramatically reduces the allowable cable length, which is determined by attenuation.
• It should also be remembered that the use of fiber-optic cable requires special optical receivers and transmitters that convert light signals into electrical signals and vice versa, which sometimes significantly increases the cost of the network as a whole.
• Fiber optic cables allow signal splitting (for this purpose, special passive couplers are produced for 2-8 channels), but, as a rule, they are used to transfer data in only one direction between one transmitter and one receiver. After all, any branching inevitably weakens the light signal, and if there are many branches, then the light may simply not reach the end of the network. In addition, there is an internal loss in the splitter, so the total signal power at the output is less than the input power.
• Fiber optic cable is less durable and flexible than electrical cable. Typical bending radii are around 10 - 20 cm; at smaller bending radii, the center fiber may break. Poorly tolerates cable and mechanical stretching, as well as crushing effects.
• The fiber-optic cable is also sensitive to ionizing radiation, due to which the transparency of the glass fiber decreases, that is, the signal attenuation increases. Sudden changes in temperature also adversely affect it, fiberglass can crack.
• Fiber optic cable is used only in networks with a star and ring topology. In this case, there are no problems of matching and grounding. The cable provides perfect galvanic isolation of network computers. In the future, this type of cable is likely to supersede electrical cables, or at least strongly suppress them.

FOCL development prospects:

• With the growing demands of new network applications, the use of fiber optic technologies in structured cabling systems is becoming increasingly important. What are the advantages and features of using optical technologies in a horizontal cable subsystem, as well as at user workplaces?
• Having analyzed the changes in network technologies over the past 5 years, it is easy to see that the SCS copper standards lagged behind the "network arms" race. Not having time to install the SCS of the third category, the enterprises had to switch to the fifth, and now to the sixth, and not far off the use of the seventh category.
• Obviously, the development of network technologies will not stop there: gigabit per workplace will soon become a de facto standard, and later de jure, and for LANs (local computer networks) of a large or even medium-sized enterprise, 10 Gbps Etnernet will not be uncommon.
• Therefore, it is very important to use such a cable system that would make it easy to cope with the increasing speeds of network applications for at least 10 years - this is the minimum service life of the SCS defined by international standards.
• Moreover, when changing the standards for LAN protocols, it is necessary to avoid re-laying new cables, which previously caused significant expenses for the operation of SCS and is simply not acceptable in the future.
• Only one transmission medium in SCS meets these requirements - optics. Optical cables have been used in telecommunication networks for over 25 years, in recent times they are also widely used in cable TV and LAN.
• In a LAN, they are mainly used to build trunk cable channels between buildings and in the buildings themselves. , while providing high speed data transfer between segments of these networks. However, the development of modern network technologies actualizes the use of fiber as the main medium for connecting users directly.

New FOCL standards and technologies:

In recent years, several technologies and products have appeared on the market that make it possible to significantly facilitate and reduce the cost of using fiber in a horizontal cable system and connecting it to user workstations.

Among these new solutions, first of all, I would like to highlight optical connectors with a small form factor - SFFC (small-form-factor connectors), plane laser diodes with a vertical cavity - VCSEL (vertical cavity surface-emitting lasers) and optical multimode fibers of a new generation.

It should be noted that the recently approved type of multimode optical fiber OM-3 has a bandwidth of more than 2000 MHz / km at a laser wavelength of 850 nm. This type of fiber provides serial transmission of 10 Gigabit Ethernet data streams over a distance of 300 m. The use of new types of multimode fiber and 850 nm VCSEL lasers provides the lowest cost of implementing 10 Gigabit Ethernet solutions.

The development of new standards for fiber optic connectors made it possible to make fiber optic systems a serious competitor to copper solutions. Traditionally, fiber optic systems have required twice as many connectors and patch cords as copper - telecom sites required a much larger area to house optical equipment, both passive and active.

Small form factor optical connectors, recently introduced by a number of manufacturers, provide twice the port density of previous solutions, since each such connector contains two optical fibers at once, rather than one as before.

At the same time, the sizes of both optical passive elements - crosses, etc., and active network equipment are reduced, which makes it possible to reduce installation costs by four times (compared to traditional optical solutions).

It should be noted that the American standards bodies EIA and TIA in 1998 decided not to regulate the use of any specific type of optical connectors with a small form factor, which led to the appearance on the market of six types of competing solutions in this area at once: MT-RJ, LC, VF-45, Opti-Jack, LX.5 and SCDC. There are also new developments today.

The most popular miniature connector is the MT-RJ type connector, which has one polymer ferrule with two optical fibers inside. Its design was designed by a consortium of companies led by AMP Netconnect based on the MT multi-fiber connector developed in Japan. AMP Netconnect has already presented more than 30 licenses for the production of this type of MT-RJ connector.

Much of the success of the MT-RJ connector is due to its external design, which is similar to that of the 8-pin RJ-45 modular copper connector. Recently, the performance of the MT-RJ connector has improved markedly - AMP Netconnect offers MT-RJ connectors with keys to prevent erroneous or unauthorized connection to the cable system. In addition, a number of companies are developing single-mode versions of the MT-RJ connector.

The company's LC connectors are in high demand in the optical cable solutions market Avaya (http://www.avaya.com). The design of this connector is based on the use of a ceramic ferrule with a diameter reduced to 1.25 mm and a plastic housing with an external lever-type latch to lock into the socket of the connecting socket.

The connector is available in both simplex and duplex versions. The main advantage of the LC connector is its low average loss and its RMS deviation of only 0.1 dB. This value ensures the stable operation of the cable system as a whole. Standard epoxy bonding and polishing procedures are used to install the LC fork. Today, connectors have found their way into 10 Gbps transceiver manufacturers.

Corning Cable Systems (http://www.corning.com/cablesystems) manufactures both LC and MT-RJ connectors at the same time. In her opinion, the SCS industry has made its choice in favor of MT-RJ and LC connectors. The company recently released the first single-mode MT-RJ and UniCam versions of MT-RJ and LC connectors, which feature fast installation times. At the same time, for the installation of UniCam connectors, there is no need to use epoxy glue and poly

The construction of fiber-optic communication lines (FOCL) is based on the principle of transmission of light waves over long distances. In this case, electrical signals (video signals from video cameras, video camera control signals and data) are sent to the transmitter, and then converted into light pulses, transmitting data with minimal distortion.

Fiber-optic lines have become widespread due to a number of advantages that are absent when transmitting signals over copper cables (coaxial and twisted pair) or by radio.

The main advantages of optical fiber (FOCL):

• wide bandwidth
• low signal attenuation
• no electromagnetic interference
• range for tens of kilometers
• service life over 25 years

Fiber types

When building fiber-optic communication lines (FOCL), multi-mode and single-mode fiber are used.

It consists of a core and a shell. The core material is ultrapure quartz glass. The retention of the light pulse occurs due to the fact that the refractive index of the core material (N1) is greater than that of the shell (N2). This is how the light beam is completely reflected inside the fiber core.

Multimode fiber 50/125 nm and 62.5 / 125 nm allow simultaneous transmission of several hundreds of allowed light modes introduced at different angles. All allowed modes have different propagation paths and, accordingly, different propagation times. Therefore, the main disadvantage is the large value of the mode dispersion, which limits the bandwidth, due to which the optical fiber transmitter has a short range. Fiber-optic communication lines (FOCL) transmit data over a distance of no more than 4-5 km.

To reduce mode dispersion and maintain a high bandwidth, in practice, fiber-optic lines with a gradient profile of the refractive index of the cable core are used. Unlike standard multimode fibers, which have a constant refractive profile of the core material, such an optical fiber has a refractive index N, which gradually decreases from the center to the cladding.

Singlemode fiber 9/125 nm is designed in such a way that only one fundamental mode can propagate in the core. That is why such fibers have best performance, and are most actively used in the construction of fiber-optic communication lines. The main advantages are low attenuation of 0.25 db / km, minimal mode dispersion and wide bandwidth - thanks to which uninterrupted transmission of electrical signals is ensured.

It has long been known that copper lines are limited in their capabilities. The kilohertz spectrum of telephone channels can be transmitted over tens of kilometers. The megahertz spectrum of the video signal is hundreds of meters. And this is in optimal conditions, in the absence of interference. And if, say, there is a power plant or a tram park nearby, everything becomes much, much worse. Of course, there are ways to wrestle a little with the laws of nature, but a radical improvement with the current level of technology is possible only with the transition to optical communication lines that are insensitive to interference and noise. Of course, fiber lines also have their limitations, but they are significantly higher than those of copper lines. And certainly an optical cable is in any case completely insensitive to electromagnetic interference. Moreover, there are fully dielectric cables that can be suspended in conjunction with high-voltage line power transmission.

What are the current devices for transmitting video over fiber?

First, the video can be digitized and transmitted over ethernet networks, which also exist at distances of more than 100 m only in fiber-optic form. The disadvantage of this method is significant signal distortion, which significantly complicates the subsequent analysis of the image. The advantage is compatibility and a wide selection of various devices designed to build computer networks.

The second option is to use specialized devices for transmitting video over fiber. Today they provide noticeably better transmission quality. What are the different devices for transmitting video over fiber?

The cheapest and long-known ones use direct transmission of low-frequency video signal over an optical fiber. In this case, the signal at the receiving end is also subject to attenuation, which is non-uniform over the frequency spectrum. Of course, such attenuation begins to affect much later - the worst fiber cable in combination with an incoherent LED emitter provides a bandwidth in the region of 200 MHz per kilometer. This means that one low-frequency video signal can be transmitted over 10-20 km without significant distortion in the frequency domain. True, there is one more parameter that you need to know - just attenuation, which for cheap devices at a wavelength around 900 nm is about 3 dB per kilometer. Unfortunately, the headroom (so called optical budget) of the transmitter / receiver pair is only about 50 dB by itself. Therefore, already at 10 km of the line, the residual signal-to-noise ratio will not exceed 20 dB, which is considered to be the boundary for at least some acceptable signal. Finally, the signal level (attenuation) during direct transmission will inevitably fluctuate depending on the weather, the tension of the connectors, and fatigue (aging) of the fiber. For the cheapest devices that do not even have an AGC in the receiver, this leads to significant fluctuations in the output signal. Of course, most monitors have built-in AGC circuits that themselves will work at least + -6dB, but many devices like digital recorders can be quite capricious.

It is clear that such devices with LF video signal transmission are by definition single-channel (they transmit only one video channel over one fiber). It should be noted that even in this case, the total cost of the system may turn out to be lower than using a copper cable, since fibers, especially if one cable contains many fibers, are much cheaper (and immeasurably more compact) than a copper coaxial cable.

The next type of device for transmitting video over fiber is frequency modulation. Since the transmission is on the carrier, there are multichannel products. Since the bandwidth of the transmitted signal is much wider than that of the video signal (if you fit 4 channels into one fiber, the bandwidth usually takes 150 MHz), then on a cheap cable with a cheap emitter, the permissible range is about 1 km (remember, above I already mentioned that such parameter, such as fiber broadband, can be only 200 MHz * km). Therefore, such products, even for the transmission of one channel, are often performed with narrowband or laser transmitters intended for single-mode fiber.

What are the advantages of FM transmitters? Frequency modulated transmission is significantly less sensitive to line instability, just as radio in the VHF-FM band is much cleaner from interference than in the AM bands. Nevertheless, today these products are almost never produced, they are superseded by digital transmitters.

So, the third type of transmitters, the most common in our time, is digital. Please note that this is not at all the same as all kinds of IP cameras. These devices do not perform digital signal compression, the digitized signal is transmitted directly, despite the fact that it is about 150 Mbit / s. per channel.

The advantage of digital transmitters is the complete absence of interference as long as the signal arrives successfully. However, as soon as the signal begins to compare with the noise, it looks like a terrible mess on the screen, completely hiding the image. Such is the feature digital transmission: As long as the signal is larger than the noise, the transmission is almost perfect. But as soon as the receiver starts to make mistakes in individual bits, it turns out that errors can happen almost equally likely in the least significant bit (it is almost invisible) and in the higher one (which means that the picture will be white instead of black, or vice versa), or, even worse, errors in the overhead synchronization bits will lead to the fact that the bits are randomly mixed up and it will be approximately the same as if you try to receive the Mayak radio station on TV.

The popularity of digital systems is due to the rapid cost reduction of components for computer networks. 100 megabit and gigabit optical networks are so widespread that the components for their production have become significantly cheaper than theoretically simpler, but less common low-frequency emitters.

In addition, for digital transmission it is absolutely not necessary to ensure the linearity of the transmission characteristic of the emitter, it operates in a binary mode: either it is switched on full power, or completely off, which also reduces the requirements for it. That is why digital transmitters now make up the bulk of those offered on the market.

What are the features of their application? First, as you've probably noticed by now, the digital signal itself is very broadband. One video channel takes 150 megabits per second, that is, approximately 70 MHz. The above-mentioned incoherent emitters at a wavelength of 800-900 nm, even one channel can transmit a maximum of 1-2 km. For digital transmission, lasers, such as those found in CD players, are commonly used. However, even lasers struggle to provide efficient transmission over multimode fiber. Especially if they operate at a wavelength of 850 nm. Multimode fiber is not designed to carry broadband signals. Multimode fiber is not designed to work with laser emitters. And although in practice this is possible (now there is even a multimode fiber certified to work with gigabit Ethernet), the transmission distance usually does not exceed 1 km. Manufacturers often indicate that their devices can operate at 2, 5 or even 10 km over multimode fiber. As a rule, this means that high-quality emitters are used - lasers at 1300 nm. However, the quality of the system as a whole in this case will be limited not by the emitter, but by the cable. Worse, since fiber manufacturers do not intend it for such an application, it is almost impossible to get the necessary fiber parameters from them to calculate the design range (the same parameter - megahertz per kilometer, which significantly depends on the composition of the radiation and is determined by the manufacturer for the main emitters for which the fiber intended). You might get lucky and everything will work. Or it may turn out that even a powerful laser emitter will work for only 2-3 km, and then the signal will be disturbed when the weather conditions change (sometimes insignificantly, by tenths of a decibel, the losses in connectors increase from temperature. This is usually insignificant, but if you working to the limit of fiber - and that may be the last straw).

So, if transmission distance is essential to you, you should use single mode transmitters. Moreover, in terms of price, they differ insignificantly from multimode ones (sometimes they do not differ at all in design, although some manufacturers use slightly cheaper emitters in multimode ones, rejected when passing control for standards for single-mode use). By the way, singlemode fiber is cheaper than multimode. This is understandable, because a 9 micron fiber simply contains much less pure glass than a 50 micron fiber.

Why is multimode fiber still in use today? The fact is that it is a little easier to connect it, especially in case of repair. There are quick-fit mechanical connectors that allow you to do without welding, without glue, without polishing. These connectors are relatively expensive ($ 10), so they are not used for mass installations, but in case of repair, such a connector is more than appropriate. Let me remind you that all the problems with the range of digital devices are caused precisely by the bandwidth of the transmitted frequencies, and not at all by the attenuation of the signal in amplitude, and therefore somewhat larger losses on the mechanical connection are insignificant compared to welding.

For single mode fiber, such connectors also exist, but they are even more expensive, require much more careful handling, and introduce even greater attenuation. How do you choose? If you need to transmit for a kilometer or two, you can use multimode devices. If you expect frequent damage and need to be repaired by less qualified personnel, it is best to use multimode fiber, respectively, by designing the system or checking fiber samples before purchasing from the factory. In all other cases, single-mode devices will provide incomparably better performance. For comparison, I will say that if for multimode fiber the broadband is 200-500 MHz * km in the 850 nm range and, in the best case, 2000 MHz * km in the 1300 nm range, then for single-mode fiber the broadband usually takes values \u200b\u200bin the region of 20,000 MHz * km, i.e. a typical 4-channel transmitter works reliably for about 50 km.

What else should you pay attention to when choosing digital transmitter video over fiber. Bit depth. It is often mentioned in advertisements. If not specified, then 8 bits. If 10 or 12 bits, the manufacturer will not hesitate to emphasize this. How important is bit depth? Sometimes it can be important for a color signal. However, no less (and maybe even more) important is the sampling rate, which you are unlikely to find in device descriptions. And quite often the increase in bit depth occurs precisely by lowering the sampling frequency. However, I repeat, this is only important for a color signal. And it is very easy to check the transmission quality. Since the digital signal is either transmitted or not, the quality can be checked even on a meter long piece of fiber, right on the table. Use a standard TV color table or just a striped table different colors, a good video camera and monitor and see how much worse the image with the proposed transmitter is compared to the direct connection of the camera to the monitor. On a real site, the quality will be the same as on a short piece of fiber.

pay attention to temperature Range operation of transmitters. Specifically, transmitters, since they are usually installed near video cameras, on the street, somewhere evenly along the many kilometers perimeter of the object. Make sure you don't have to build a warm hut for the transmitters. By the way, Ethernet transmitters over fiber, as a rule, are designed specifically for warm huts, and rare versions with an industrial temperature range are much more expensive than usual ones. What other features are there?

Not so essential for work, but sometimes making life much easier. For example, the units can be mounted in a 19 ”rack, which is convenient in a crowded central location.

The devices can be powered from an external power supply (this is popular with imported devices) or directly from 220 V. See which is more convenient for you. Remote power supplies are often such that they can only be plugged directly into the sockets, and these are unnecessary detachable connections, which does not increase the reliability of the system.

There are universal devices that can be easily mounted both on a wall and in a rack, which operate on both single-mode and multi-mode fiber, and can operate both from 220 volts and from an external low-voltage power supply. But such versatility is important only for distributors, so as not to keep a large assortment of devices in stock. In each specific project, it is more or less known what exactly is needed, and no one will definitely change the cable during operation.