Transmission of video signal over fiber-optic network: very important nuances.
The advantages of using fiber-optic communication lines (FOCL) are obvious. Of course, it is advisable to transmit the signal from video surveillance cameras, especially megapixel ones with IP output within integrated security systems as the most voluminous, via FOCL. However, there are many subtleties in the actual design and construction of FOCL. Designers of video surveillance systems know that due to losses during the signal transmission over the network, the resolution of the system will deteriorate from block to block. Therefore, you need to choose cameras with a resolution higher or the same as that of the video recorder (in no case lower!). On the contrary, it is recommended to choose a monitor with a resolution no higher than that of the video recorder.
Losses in the transmission path are also reduced by the contrast of the video image, which is especially important when working in low light conditions (contrast is the number of gradations of brightness (halftones) with which the object will be drawn).
In this case, if we are talking about a remote observer located several kilometers or tens of kilometers from the video cameras and the observation point with the installed video recorder or server, then the influence of the transmission path on the signal can be significant. This article is devoted to the discussion of a number of current and previously little-discussed issues of constructing optical transmission networks.
General description of fiber-optic communication lines
Fiber optic technologies are based on the principle of using light as the main source of information. Light is much easier to transmit over long distances with less loss than electric current. In addition, it is much less susceptible to the effects of electromagnetic fields and is capable of transmitting orders of magnitude more information. Optical lines themselves are not sources of electrical noise.
Optical fiber transmits electromagnetic radiation in the optical wavelength range corresponding to frequencies of 1014-1015 Hz, which ensures very high throughput and speed.
High noise immunity of fiber-optic communication lines is due to the fact that in nature and industry there are virtually no sources of electric and magnetic fields of intensity that can change the conditions for the propagation of a light pulse in an optical fiber. In addition, optical cables most often do not contain metal elements, so there are no problems associated with the potential difference between floors and buildings, with stray currents in the soil, etc. Fiber-optic systems have almost complete electrical insulation, are not afraid of high humidity, and do not require equipment to protect them from leaks, breakdowns, and short circuits. Semiconductor receivers and transmitters of light have a fairly high stability.
It is known that light propagates through air and glass with the least loss. For example, modern fibers have an attenuation of 0.2 dB/km, which gives an attenuation of 0.02 dB over a length of 100 m. Over the same length, a modern high-quality symmetrical electric cable has an attenuation of about 20 dB, i.e. 1000 times greater.
Initially, fiber-optic communication lines were used in the military-industrial complex. All modern successes of fiber-optic communication lines are due to the active development of weapons in the middle and at the end of the last century. The immunity of fiber-optic communication lines to electromagnetic interference and the high speed of information transfer determined their use in communication systems between control and measuring and command complexes, which included computers. According to foreign data from the 80s, about 5,000 individual fiber-optic communication lines with a total length of 150 km between computers were used as part of the command complex of the MX missile system, providing information transfer at a speed of 3.2 Mbit/s [1].
Of course, it is impossible to transmit power voltage for devices operating on PoE technology via fiber-optic communication lines; complex and expensive active optoelectronic equipment is used for fiber-optic communication lines; the technology for producing optical cables and transceivers is more complex and expensive; working with fiber-optic communication lines places increased demands on the qualifications and culture of production personnel. When working with fiber-optic communication lines, it is necessary to take into account the aging of the optical fiber under the influence of moisture and hard gamma radiation.
The structural diagram of a fiber-optic communication line is shown in Fig. 1.
The optical transmitter and optical receiver are shown by the dotted line. At point 1, the light signal appears, at point 2, the light signal disappears.
In this case, the transmitter and receiver are structurally combined into one device — a transceiver, which has two optical adapters for connecting two optical fibers. That is why the computer network card has an adapter for two optical fibers at the output: light enters the card through one, and leaves it through the other.
The diagram shows:
AI — information source
PC — code converter
I — light emitter
SU — matching device (optical)
K — optical connector
FOC — fiber optic cable
OM — optical cable coupling
PD — photodiode
RS — signal regenerator
PI — information receiver
The sender converts information into a light wave, and the recipient, receiving the latter, in turn, interprets the light as information.
The electrical signal enters the input of the optical transmitter and modulates the intensity of the output signal of the emitter.
The optical signal propagates along the fiber optic cable and enters the input of the optical receiver, which demodulates it and restores the original electrical signal.
To ensure normal operation, the optical transmitter and receiver are equipped with optical connector sockets.
The distances between transceivers on the optical line are inversely proportional to the data transfer rate (Fig. 2).
The figure shows the dependence of the distance over which information is transmitted on the transmission speed when using a cable with a loss of 2.7 dB/km and a light-emitting diode with an emission wavelength of 0.84 μm and a spectral line width of 0.03 μm:
1 — light guide with a stepped profile
2 — light guide with a gradient profile
Reducing attenuation in the cable and improving the input system leads to an increase in the transmission length. It is necessary to correctly connect the photodetector to the light guide, reducing reflection losses to a minimum, for example, by using an «antireflective» layer between the end of the LED and the photodetector window.
Optoelectronic devices for fiber-optic communication lines
The construction of fiber-optic communication lines in their modern form became possible thanks to a colossal breakthrough in the development of semiconductor optoelectronics starting in the 60s of the last century.
Speaking about light sources, it is necessary to note the creation of semiconductor surface-emitting lasers with a vertical cavity resonator. These are long-wave VCSEL lasers (Vertical Cavity Surface Emitting Laser), which are an excellent alternative to more expensive traditional edge-emitting Fabry-Perot lasers and distributed feed back (DFB) lasers.
DFB and Fabry-Perot lasers often require special optical elements (coupling) to input the beam they form with an elliptical profile and a wide divergence into the fiber, which complicates the assembly of optical systems and increases their cost.
VCSEL semiconductor lasers produce a beam with a narrow directional pattern (low divergence) and a symmetrical profile.
The VCSEL laser has a resonator located perpendicular to the plane of the substrate, which facilitates testing of lasers during production, and as a result, their assembly costs are reduced. Currently, its cost is about .
In the 1980s, Tomsk Research Institute of Optical Devices developed a series of LEDs for the range of 0.85 µm and 1.3 µm in housings that represent a socket of a unified optical connector. These diodes are characterized by a response time of 8–15 ns, forward currents of up to 50 mA, high linearity of the watt-ampere characteristic, the diameter of the emitting area is 200 µm, and the power introduced into the optical fiber is 0.02–0.5 W.
The Ioffe Physicotechnical Institute has developed single-mode and multi-mode lasers with a wavelength of 1.3 μm with high power at low pump currents and a narrow spectral envelope. For single-mode lasers, the power input into the optical fiber was 0.5–10 mW at pump currents of 30–250 mA and a spectral envelope width of 10–15 nm. Multi-mode lasers with a wavelength of 1.3 μm provided an input radiation power of up to 50 mW at pump currents of up to 600 mA.
High-speed picosecond lasers with a wavelength of 0.8 μm and 1.3 μm have been developed for use in reflectometers, providing a radiation pulse duration of 6–30 ps at a radiation power of up to 500 mW. A single-mode laser with a wavelength of 1.3 μm created at the Lebedev Physical Institute provided a radiation power input into the optical fiber module of 1.5 mW at an operating current of 80 mA. The module had a flat design, small dimensions, and provided radiation output through a section of single-mode optical fiber [2].
The most commonly used semiconductor material for light sources for fiber-optic communication lines is the solid solution Ga 1-x Al x As, covering the emission range of 0.63–0.94 μm. Semiconductor light-emitting diodes based on Ga 1-x Al x As generally have a low emission power, allowing optical power of up to 1 mW to be introduced into the fiber and, depending on the composition, have an emission line width of 2540 nm and a service life of 105–106 hours. Semiconductor lasers with a double heterostructure based on gallium arsenide generate radiation in the wavelength range of 0.83–0.94 μm. The band shift is necessary so that the laser emission wavelength does not coincide with the absorption band of the hydroxyl group OH of the fiber [3].
A serious disadvantage of semiconductor lasers (and not only domestic ones) is the low mean time between failures (durability) and degradation of characteristics — a decrease in power during operation.
If we talk about optical radiation receivers — photodiodes, then the technology of their production was also honed within the framework of the military-industrial complex. The leading institute for the development of photodetectors was the Research Institute of Applied Physics (NIIPF). Epitaxial structures for photodetectors were grown at the Giredmet of the Ministry of Non-Ferrous Metallurgy [2] by order of NIIPF.
Avalanche and p-i-n photodiodes are usually used as photodetectors in fiber-optic communication lines. Photodetectors for fiber-optic communication lines must be broadband, have a high value of the product of the bandwidth by the avalanche multiplication factor, have a low level of excess noise (for LDs), be fast-acting, have a small capacity, low dark current, be stable to external influences, have maximum sensitivity at the wavelength of the emitter and a long service life, and also provide the ability to match with the subsequent amplifier cascade.
Meeting these conflicting requirements made it possible to create a series of photodiodes in packages convenient for connection to fiber-optic communication lines with the following parameters:
silicon p-i-n photodiodes – current sensitivity of 0.4–0.5 A/W, wavelengths of 0.85 μm, response time of 1–10 ns, dark current of 2–10 nA at an operating voltage of 5 V (24 V);
p-i-n photodiodes based on InGaAsP/InP heterostructures with operating wavelengths of 1.3 and 1.55 μm, current sensitivity of 0.6–0.9 A/W, response time of 0.07–0.3 ns, dark current of 0.1–5 nA at an operating voltage of 5–10 V;
germanium avalanche photodiodes with operating wavelengths of 1.3 μm and 1.55 μm with a current sensitivity of 6014 A/W, a noise current density of (5–10) 10-12 A/Hz -1/2 , a response time of 0.1–0.6 ns, a capacitance of 0.6-2 pF at an operating voltage of 30–100 V;
photodetectors with p-i-n FET with operating wavelengths of 1.3 and 1.55 μm, a bandwidth of 170–700 MHz, and a sensitivity from -36 to 43 dBm.
Thus, by the beginning of the 90s, the necessary optoelectronic element base for the creation of fiber-optic communication lines had been created in our country. The developments were introduced into serial production and found application in the production of domestic equipment for fiber-optic communication lines, local networks, cable television and other communication lines [2].
Fiber optics
Light guides must provide low attenuation, not distort the shape, spectral composition and mode distribution of the transmitted light signal, their parameters must not depend on temperature, humidity, mechanical stress and chemical corrosion, and radiation losses must not depend on the bending radius of the light guide and must be minimal. Light guides are made of silicate glasses with additives TiO2, Al2 O3, GeO2, P2 O5 with a coating of their surface with a polymer film (sometimes plastic light guides are used) to protect against mechanical damage, chemical effects and moisture.
The numerical aperture and the maximum angle of radiation input are related by the dependence NA = sin The numerical aperture of the fiber is determined by the refractive index profile, the type and concentration of dopants, and for fibers with a stepwise change in the refractive index from the periphery to the axis is equal to:
NA = (nc2 – no2) 1/2
Increasing the difference nc – no improves the conditions for input of radiation into the fiber, but at the same time increases the dispersion, which leads to pulse broadening.
Dispersion is manifested in the blurring of the optical signal over time. Dispersion is the main factor limiting the bandwidth of a light guide. Dispersion can be intermode, arising from the superposition of the fronts of two adjacent pulses on each other, and chromatic, arising from the fact that shorter waves propagate faster in the waveguide, and from the unevenness of the core density, as well as from the spectral properties of the radiation source.
Intermode dispersion is most significant in multimode light guides, chromatic dispersion comes to the fore in single-mode light guides. At the same time, intermode and chromatic dispersion, estimated by the broadband coefficient, determine different dimensions of these coefficients for multimode and single-mode light guides.
For multimode optical fibers, the bandwidth is measured in MHz*km. From the definition of the bandwidth, it is clear that dispersion imposes restrictions on the transmission range and the upper frequency of transmitted signals. At the same time, the physical meaning of the bandwidth is the maximum modulation frequency of the transmitted signal with a line length of 1 km. That is, if the graph shows the bandwidth coefficient for a specific type of multimode cable > 200, this means that a frequency of no more than 200 MHz is fully transmitted over a distance of 1 km. If the dispersion increases linearly with increasing distance, then the bandwidth is inversely proportional to the distance.
For single-mode light guides, the dispersion parameter takes into account its dependence on the spectral properties of the radiation source, so its dimension is specified in ps/nm x km (differential delay (ps), per central wavelength of the source (nm) and per length of the measured section (km). Since waveguide dispersion depends on the refractive index profile, then, by varying this parameter, for single-mode light guides, it is possible to obtain zero or close to zero dispersion at a predetermined wavelength or in a certain spectral band.
Attenuation in optical fibers depends on the presence of impurities (especially transition metals and OH ions), non-uniformity of the boundary between the core and the cladding, fluctuations in composition and density.
According to the international standard ISO/IEC 11801:2008 (E), the optical transmission channel is divided into several classes:
OF-300 class: from 300 meters
OF-500 class: from 500 meters
OF-2000 class: from 2 km
The numerical value specified in the class name defines the minimum channel length in meters over which a channel of this class is guaranteed to support the corresponding application if the channel is created in accordance with the requirements of the specified standard.
Class OF-500 supports the Gigabit Ethernet 1000Base-LX application over multimode OM1, OM2, and OM3 fiber for up to 500 meters with an attenuation of 2.35 dB at a wavelength of 1300 nm.
The highest class OF-2000 supports applications, including the Gigabit Ethernet 1000Base-LX protocol over single-mode OS1 fiber for up to 2000 meters with an attenuation of 4.56 dB at a wavelength of 1310 nm.
Example calculation for OF-500 application implemented on multimode fiber with an emitter operating at 850 nm:
1.5 dB (3 joints for connecting equipment) + 500 m x 3.5 dB/1000 m = 3.25 dB
A properly designed structured cabling system is capable of providing guaranteed operation of any active equipment, including switches.
The ISO/IEC 11801:2008 (E) standard is based on the average characteristics of products manufactured by the modern industry.
However, in practical implementation of structured cable systems, it often turns out that the quality of cables is high and the lengths of lines and channels do not reach the maximum. Which, unlike symmetrical cables, leads to a paradoxical result — switches transmitting video signals do not work normally (attenuation is negligible). The absence of attenuation can lead to exposure of the photodetector, and in the worst case, to its complete failure. In this case, a passive attenuator with the required attenuation coefficient must be included in the channel. The actual loss values are not known until the end of the optical path installation, since they are determined by the quality of its installation.
Radiation wavelengths at which the signal is transmitted in fiber-optic communication lines
Fig. 3 shows the spectral dependence of the attenuation coefficient of real optical fibers taking into account all losses. It is clear that work on fiber-optic cables is effective not at all wavelengths, but only in certain sections of the spectrum where minimal losses are achieved.
The areas of minimum losses are called transparency windows. Three transparency windows are of practical interest for quartz light guides. Most often, these are three lengths: 850 nm, 1300 nm, and 1500 nm. Attenuation for industrial optical fibers is: from 2 to 3 dB/km (850 nm); from 0.4 to 1 dB/km (1300 nm); from 0.2 to 0.3 dB/km (1500 nm). The characteristics of semiconductor emitters and photodetectors are optimized for operation in these windows.
Main types of modern light guides
Single-mode fiber with shifted dispersion DSF (Disperrsion – Shifted Fiber) is characterized by a wavelength at which the dispersion turns to 0, equal to 1550 nm.
This wavelength ( = 1550 nm) is called the zero dispersion wavelength. This effect is achieved by specially selecting the refractive index by the diameter of the core, since dispersion depends on the refractive index profile.
This shift is achieved by a special profile of the refractive index of the fiber. As a result, the best characteristics of both minimum dispersion and minimum loss are realized in the dispersion-shifted fiber. The operating wavelength is taken close to 1550 nm. The need for DSF was caused by the development of lasers with a wavelength of 1550 nm. However, with the advent of broadband amplifiers and wave multiplexing, the chromatic dispersion of these fibers began to introduce undesirable effects into the integrity of multi-wave pulses.
Single-mode fiber with non-zero dispersion shifted fiber NZDSF (Non-Zero Dispersion Shifted Fiber), unlike DSF, is optimized for transmitting not one, but several wavelengths at once (multiplexed wave signal) and can be most effectively used in the construction of all-optical network trunk lines — networks at the nodes of which there is no optoelectronic conversion during the propagation of an optical signal. NZDSF fiber maintains a limited chromatic dispersion coefficient in the optical range of 1530–1625 nm.
Classification of fiber-optic networks
Since fiber-optic systems are a relatively young branch of technology, and their widest distribution has been achieved in the last few years, there are several standards describing them. Let us say a few words about some features of the existing standardization.
The international standard ISO/IEC 11801:2008 (E) considers fiber-optic communication lines as a structured cabling system and assumes that it uses cables, connecting equipment and cords that meet this standard. Compliance with the recommendations of this standard guarantees that the fiber-optic communication line itself will also comply with this standard, transmitting applications with certain characteristics.
Local area networks used for transmitting video signals, in particular via fiber-optic communication lines, are described by Ethernet technology. Ethernet is mainly described by IEEE standards of the 802.3 group. The IEEE 802.3ae standard is defined for such a progressive optical medium of the future as supporting a data transfer rate of 10 Gbit/sec for optical fibers of various types. This new standard offers an easy way to upgrade Gigabit Ethernet backbones and provides a connection between LAN and WAN. The IEEE 802.3ae standard was finally adopted on June 27, 2002. Thus, the 10 Gigabit Ethernet technology and standard have received unconditional approval from experts and wide industrial recognition, in particular, for video surveillance systems, for intensive exchange with memory devices, especially taking into account the improvement of equipment and its reduction in cost for 10G Ethernet. In addition, the use of 10G Ethernet equipment with reduced power consumption can significantly reduce energy costs [4].The ITU-T section of the International Telecommunications Union is devoted to recommendations in the field of fiber-optic communications, it regulates the parameters of optical fibers, including the most modern ones.
Thus, the fiber-optic networks that are best suited for transmitting video signals have many features. Consequently, the choice of transmitting equipment, for example, active switches, for transmitting a video signal must be made either taking into account the existing SCS at the facility, or taking into account the territorial extent of the facility and the rules for designing a structured cabling system on fiber-optic communication lines.
REFERENCES 1. Aviation Week and Space Technology, 1980, No. 19 Fiber-optic technology: current status and prospects. Edited by Dmitriev S. A., Slepov N. N. M.: «Fiber-optic technology», 2005. 576 p. Radiation resistance in optoelectronics. Ed. Sredina V. G. Voenizdat, 1987, 166 p. Networks and communication systems, No. 8 (170). 08/27/2008, p. 34.