Analog devices for receiving and transmitting signals via fiber-optic communication lines.
Methods for transmitting various types of signals, data and control commands via fiber-optic communication lines began to be actively implemented in the last decade of the past century.
However, for quite a long time they could not seriously compete (at least in the TSB segment) with coaxial cable and twisted pair.
Despite such disadvantages as high resistance and capacity, which significantly limits the range of signal transmission, coaxial cable and twisted pair prevailed in security systems.
Today the situation is beginning to change, and I would venture to say that these changes are radical.
No, in small systems where video and control signals need to be transmitted over short distances, coaxial cable and twisted pair are still irreplaceable.
In large and especially distributed systems, fiber optics have virtually no alternative.
The fact is that fiber optic equipment has become much more affordable today and the trend towards further reduction in price is quite stable.
So fiber optics currently makes it possible to offer the customer of security systems not only a reliable, but also an economically advantageous solution.
Using a light beam to transmit a signal, a wide bandwidth allows transmitting a high-quality signal over significant distances without the use of amplifiers and repeaters.
The main advantages of using fiber optics are known to be:
— wider bandwidth (up to several gigahertz) than copper cable (up to 20 MHz);
— immunity to electrical interference, no «ground loops»;
— low signal transmission losses, signal attenuation is about 0.2-2.5 dB/km (for RG59 coaxial cable — 30 dB/km for a 10 MHz signal);
— does not cause interference in adjacent «copper» or other fiber optic cables;
— long transmission range;
— increased data transmission security;
— good quality of the transmitted signal;
fiber optic cable is miniature and lightweight.
How does a fiber optic line work
Fiber optics is a technology that uses light as an information carrier, and it does not matter what type of information we are talking about: analog or digital. Usually, infrared light is used, and the transmission medium is glass fiber.
Fiber optic equipment can be used to transmit analog or digital signals of various types.
In its simplest form, a fiber-optic communication line consists of three components:
— a fiber-optic transmitter for converting the input electrical signal from a source (e.g., a video camera) into a modulated light signal;
— a fiber-optic line through which the light signal is transmitted to the receiver;
— a fiber-optic receiver that converts the signal into an electrical signal that is practically identical to the source signal.
The source of light propagated through optical cables is a light-emitting diode (LED) (or semiconductor laser – LD).
At the other end of the cable, a receiving detector converts light signals into electrical signals.
Fiber optics rely on a special effect – refraction at the maximum angle of incidence, when total reflection occurs.
This phenomenon occurs when a beam of light leaves a dense medium and enters a less dense medium at a certain angle.
The inner core (thread) of a fiber optic cable has a higher refractive index than the cladding.
Therefore, a beam of light, passing through the inner core, cannot go beyond it due to the effect of total reflection (Fig. 1).
Thus, the transported signal goes inside a closed medium, making its way from the signal source to its receiver.
The remaining elements of the cable only protect the fragile fiber from damage by external environments of varying aggressiveness.
Fig. 1 Fiber optics is based on the effect of total reflection
Physical parameters of optical fibers
All common types of fibers are characterized by two most important parameters: attenuation and dispersion.
A distinction is made between modal and material dispersions – signal distortions caused by the peculiarities of light wave propagation in the medium.
Material dispersion is caused by the fact that waves of different lengths propagate at different speeds, which is associated with the peculiarities of the physical structure of the fiber. This effect is especially noticeable when using single-mode fiber.
Reducing the bandwidth of the source and choosing the optimal wavelength leads to a decrease in material dispersion.
Mode dispersion occurs in multimode fiber due to the difference in path lengths traveled by beams of different modes. It can be reduced by decreasing the fiber core diameter, reducing the number of modes, and using fiber with a gradient profile.
Signal attenuation in fiber optic cable depends on the properties of the material and on external influences.
Attenuation characterizes the loss of transmitted signal power over a given distance and is measured in dB/km, where decibel is the logarithmic expression of the ratio of the power leaving the source P1 to the power entering the receiver P2, dB = 10*log(P1/P2). A loss of 3 dB means that half the power is lost.
A loss of 10 dB means that only 1/10 of the source power reaches the receiver, a loss of 90%.
Fiber optic lines are typically capable of functioning normally with a loss of 30 dB (receiving only 1/1000 of the power).
There are two fundamentally different physical mechanisms that cause this effect. Absorption losses. Associated with the transformation of one type of energy into another. An electromagnetic wave of a certain length causes a change in the orbits of electrons in some chemical elements, which in turn leads to heating of the fiber. Naturally, the process of wave absorption is less, the shorter its length and the cleaner the fiber material.
Scattering losses.
The reason for the decrease in signal power in this case means that part of the light flux leaves the waveguide.
This is due to the inhomogeneities of the refractive index of materials.
And with a decrease in wavelength, scattering losses increase.
Fig. 2 Transparency windows of optical fibers
In theory, the best overall attenuation values can be achieved at the intersection of the absorption and scattering curves.
The reality is somewhat more complex and is related to the chemical composition of the medium.
In quartz fibers (SiO2), silicon and oxygen are active at a certain wavelength and significantly impair the transparency of the material in two neighborhoods.
As a result, three transparency windows are formed (Fig. 2), within which the attenuation has the lowest value.
The most common wavelength values are:
0.85 µm;
1.3 µm;
1.55 µm.
In analog transmission, wavelengths of 850 and 1310 µm are most often used.
It is for these ranges that special heterolasers have been developed, on which modern fiber-optic communication systems (FOCL) are based.
Currently, optical fiber with such a characteristic is already considered obsolete. The production of optical fiber of the AllWave ZWP (zero water peak) type, in which hydroxyl ions in the quartz glass composition are eliminated, has been mastered for quite a long time. Such glass no longer has a window, but an opening in the range from 1300 to 1600 nm.
All transparency windows are in the infrared range, i.e. the light transmitted through the fiber-optic communication line is not visible to the eye.
It is worth noting that visible radiation can also be introduced into a standard optical fiber. For this, either small blocks present in some reflectometers or even a slightly modified Chinese laser pointer are used.
With the help of such devices, you can find breaks in the cords. Where the optical fiber is broken, a bright glow will be visible.
Such light quickly fades in the fiber, so it can only be used at short distances (no more than 1 km).
Analog transmission
The simplest video signal transmitters use amplitude modulation (AM): the intensity of the emitted light changes proportionally to the change in the amplitude of the video signal. To obtain a more stable result, increase the distance of signal transmission, and achieve a better signal-to-noise ratio, frequency modulation (FM) is used.
Amplitude modulation (AM) is a type of modulation in which the variable parameter of the carrier signal is its amplitude. The intensity of the emitted light changes proportionally to the change in the amplitude of the video signal. Since it is quite difficult to control the radiation intensity at a high level, even small changes in it introduce significant distortions into the transmitted signal.
Frequency modulation (FM) is a type of analog modulation in which the information signal controls the frequency of light pulses. Compared to amplitude modulation, the amplitude remains constant.
The analog method is used to transmit video and audio signals, control signals, 10/100M Ethernet, and contact status monitoring.
It should be noted that analog devices are not the best choice for transmitting video or audio information.
Transmitting and receiving it via fiber-optic communication lines using analog equipment can be quite difficult. In addition, the price differences between analog and similar digital equipment are insignificant.
Equipment of this type is present in the range of many market players; readers will be able to familiarize themselves with some models in the review part of the article.
S732DV (GE Security, Fiber Option)
The set of analog transceivers is designed to transmit video and data over 1 single-mode or multi-mode fiber at a distance of up to 60 km.
Distinctive features of the device are a wide range of operating temperatures (from -40 C to +75 C), Plug-and-Play technology, CWDM, SMARTSä diagnostics, allowing you to test the system in real time. The equipment is guaranteed for 5 years.
DE7400 (GE Security, EtherNAVä IFS Series)
The 2-port transceiver series is designed to transmit and receive data at 10/100/1000 Mbps over multimode, single-mode fiber or Cat 5 electrical cable. The DE7400 features enhanced environmental protection for operation at extreme temperatures (-40 C to +85 C).
A standard feature is contact actuation to initiate a remote alarm when the optical link is lost.
The RJ-45 connector has LED indicators for power status and data transfer rate.
It also supports RSTP, QoS/CoS, IGMP, VLAN, and SNMP protocols. It supports IEEE 802.3 standards, which makes it possible to connect any local area network devices.
The equipment comes with a lifetime warranty.
The IFS equipment line includes equipment with various port configurations.
Receiver/transmitter OVT/OVR-1 («BIK-Inform»)
The OVT/OVR-1 series equipment (receiver/transmitter) is designed to transmit analog video signals in real time in video surveillance systems at industrial and extended facilities.
The device allows transmitting high-quality color and b/w video signals via multimode optical fiber over a distance of up to 5 km in the frequency band of 25 Hz — 10 MHz with a signal-to-noise ratio of at least 5 dB.
The equipment is highly noise-immune. It has a built-in test signal generator, AGC systems (automatic level control based on the sync signal level), low current consumption — no more than 85 mA for the transmitter and 75 mA for the receiver.
Compact dimensions allow the devices to be placed both in DIN rail mounting cabinets and in small junction boxes. The equipment does not require additional settings and can be operated in the temperature range from -40 °C to +50 °C.
SFS10-100/W-80 (SF&T)
A set consisting of two analog transceivers is designed to organize 1 Ethernet 10/100M data channel over 1 single-mode fiber. This device, the latest in the SFS10-100/W-xx series, allows you to increase the signal transmission distance to 80 km. Operating modes: duplex and half-duplex.
Thanks to the support of IEEE 802.3 10 Base-T/100Base-Tx/100Base-Fx standards, it is possible to connect most IP devices used to organize local networks, as well as to build video surveillance systems.
A wide range of operating temperatures (from -10 to +70 °C), Plug-and-play support, no need for additional settings and attenuators, and compact dimensions (165 x 144 x 33 mm) make installation of the devices as quick and convenient as possible. The modular design allows using the SFS10-100/W-80 as separate modules and installing them in a rack.
All SF&T equipment comes with a 3-year warranty.
SVP-11T/12R
SVP-13T/14R («Spetsvideoproekt»)
The devices are designed to transmit signals in television surveillance systems over distances of up to 6–12 km. The transmitter and receiver kits provide transmission of one composite video signal over a multimode optical cable at a wavelength of 850 and 1310 nm.
Video signal resolution is 570 TVL, signal/noise ratio at maximum range is not worse than 50 dB, frequency band: 50 Hz – 8 MHz.
The automatic gain control system constantly maintains a video signal amplitude of 1 V at the output.
The light signaling shows the presence or absence of a video signal. The devices have small dimensions, low power consumption, and are equipped with wall mounting elements.
The devices are protected against power supply reversal – they do not fail if turned on incorrectly.
They operate in plug and play mode – no setup or adjustment is required during installation.
The signal receivers are also available in a housing designed for installation in standard 19” racks.
SVP-21T
SVP-22T («Special Video Project»)
The SVP-21T and SVP-22T fiber optic video signal transmitters are designed to work with outdoor television surveillance cameras. The sealed casing is equipped with cable glands and has an IP66 weather protection rating. Operating temperature from -35 to +50 °C. The signal is transmitted over long distances: up to 6–12 km.
The SVP-21T and SVP-22T transmitters complete with the SVP-12R, SVP-14R, SVP-12-2Rack, SVP-14-2Rack receivers provide transmission of one composite video signal via a multimode optical cable at a wavelength of 850 and 1310 nm. The devices are available with AC power supply of 220 V or 24 V.
They operate in plug and play mode – no setup or adjustment is required during installation. The automatic gain control system in the receivers constantly maintains a video signal amplitude of 1 V at the output.
The hermetic housing has free space for crossing the cable of other equipment. Overall dimensions: 200 x 150 x 55 mm.