Selecting fiber optic video transmitters.

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 a video signal — over hundreds of meters. And this is under optimal conditions, in the absence of interference. But if there is, say, a power plant or a tram depot nearby, everything becomes much, much worse. Of course, there are ways to fight the laws of nature a little, but a radical improvement at the current level of technology is only possible by switching 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 an optical cable is certainly completely insensitive to electromagnetic interference in any case. Moreover, there are completely dielectric cables that can be suspended together with a high-voltage power line.What devices currently exist for transmitting video signals via fiber?
Firstly, video can be digitized and transmitted via Ethernet networks, which also currently exist only in fiber optic form for distances over 100 m. The disadvantage of this method is significant signal distortion, which significantly complicates subsequent image analysis. 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 significantly higher transmission quality. What types of devices are there for transmitting video over fiber?
The cheapest and longest known ones use direct transmission of low-frequency video signal via optical fiber. In this case, the signal at the receiving end is also subject to attenuation, and unevenly across 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 of about 200 MHz per kilometer. This means that one low-frequency video signal can be transmitted over 10-20 km without significant distortions in the frequency domain. True, there is one more parameter that you need to know — simply attenuation, which for cheap devices at a wavelength of about 900 nm is about 3 dB per kilometer. Unfortunately, the reserve (the so-called optical budget) of the transmitter/receiver pair itself is only about 50 dB. Therefore, already at 10 km of the line, the residual signal-to-noise ratio will be no more than 20 dB, which is considered the limit for at least somewhat acceptable signal. Finally, the signal level (attenuation) during direct transmission will inevitably fluctuate depending on the weather, connector tension, and fiber fatigue (aging). In the cheapest devices, which do not even have an AGC in the receiver, this leads to significant fluctuations in the signal at the output. Of course, most monitors have built-in AGC circuits that will work out at least +-6 dB, but many devices like digital recorders can be very capricious.
It is clear that such devices, with the transmission of low-frequency video signals, are by definition single-channel (they transmit only one video channel over one fiber). It is worth noting that even in this case, the total cost of the system may be lower than with the use of copper cable, because fibers, especially if one cable contains many fibers, are significantly cheaper (and incomparably more compact) than copper coaxial cable.
The next type of devices for transmitting video over fiber is with frequency modulation. Since the transmission is on the carrier, there are multi-channel products. Since the band of the transmitted signal is much wider than that of the video signal (if 4 channels are placed in one fiber, the band usually occupies 150 MHz), then on a cheap cable with a cheap emitter, the permissible range is approximately 1 km (remember, I already mentioned above that such a parameter as fiber bandwidth can be only 200 MHz * km). Therefore, such products, even for transmitting one channel, are often made with narrow-band or laser transmitters designed for single-mode fiber.
What are the advantages of FM transmitters? Frequency modulation transmission is much less sensitive to line instability, just as VHF-FM radio is much cleaner from interference than AM radio. However, these products are almost never manufactured today, having been replaced by digital transmitters.
So, the third type of transmitter, the most common in our time, is digital. Please note that this is not the same as all sorts 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/sec. per channel.
The advantage of digital transmitters is the complete absence of interference as long as the signal is transmitted 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. This is the peculiarity of digital transmission: as long as the signal is greater than the noise, the transmission is almost perfect. But as soon as the receiver begins to make mistakes in individual bits, it turns out that errors are almost equally likely to occur in the least significant bit (it is almost invisible), and in the most significant (which means that the picture will be white instead of black, or vice versa), or, even worse, errors in the service synchronization bits will lead to the bits being accidentally mixed up, and it will be about the same as trying to receive the Mayak radio station on TV.

Digital systems owe their popularity to the rapid reduction in the cost of computer network components. 100-megabit and gigabit optical networks are so widespread that the components for their production have become significantly cheaper than the theoretically simpler but less common low-frequency emitters.
In addition, for digital transmission it is not at all necessary to ensure the linearity of the emitter's transfer characteristic, it operates in binary mode: either on at 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? Firstly, as you have probably already noticed, the digital signal itself is very broadband. One video channel takes up 150 megabits per second, i.e. approximately 70 MHz. The above-mentioned incoherent emitters at a wavelength of 800-900 nm can transmit even one channel at a maximum of 1-2 km. Lasers similar to those in CD players are usually used for digital transmission. However, even lasers can hardly ensure effective transmission over multimode fiber. Especially if they operate at a wavelength of 850 nm. Multimode fiber is not designed to transmit broadband signals. Multimode fiber is not designed to work with laser emitters. And although in practice this is possible (nowadays they even produce multimode fiber certified for work with gigabit Ethernet), the transmission range usually does not exceed 1 km.
Manufacturers often state 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 – 1300 nm lasers. However, the quality of the system as a whole in this case will be limited not by the emitter, but by the cable. What’s worse, since fiber manufacturers do not intend it for such use, it is almost impossible to obtain the necessary fiber parameters from them to calculate the design range (that 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 is intended). You may be lucky, and everything will work. Or it may turn out that even a powerful laser emitter will only work at 2–3 km, and even then the signal will be disrupted by changing weather conditions (temperature sometimes increases losses in connectors slightly, by tenths of a decibel. This is usually insignificant, but if you are working at the limit of the fiber’s capabilities – this may be the last straw).
So, if the transmission range is important for you, you should use single-mode transmitters. Moreover, their price is not significantly different from multi-mode ones (sometimes they do not differ at all in design, although some manufacturers use slightly cheaper emitters in multi-mode ones, rejected during the inspection for the standards for single-mode use). By the way, single-mode fiber cable is cheaper than multi-mode. This is understandable, because a fiber with a diameter of 9 microns simply contains much less pure glass than a fiber with a diameter of 50 microns.
Why is multimode fiber still used at all? The fact is that it is slightly easier to connect, especially in case of repair. There are quick-mount mechanical connectors that allow you to do without welding, without glue, without polishing. These connectors are relatively expensive (about 10 dollars), so they are not used for mass installation, 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 by the band of transmitted frequencies, and not by the attenuation of the signal in amplitude, and therefore slightly greater losses on a mechanical connection compared to welding are insignificant.
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 to choose? If you need to transmit over a kilometer or two, you can use multimode devices. If you expect frequent damage and need to carry out repairs by not very qualified personnel, it is better to use multimode fiber, respectively, having designed the system or tested fiber samples before purchasing at 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 bandwidth is 200-500 MHz * km in the 850 nm range and at best 2000 MHz * km in the 1300 nm range, then for single-mode fiber the bandwidth, as a rule, takes values ​​in the region of 20,000 MHz * km, i.e. a typical 4-channel transmitter confidently operates at about 50 km.
What else should you pay attention to when choosing a digital fiber video transmitter? Bit depth. It is often specified in advertising. If not specified, then 8 bits. If 10 or 12 bits, the manufacturer will not fail to emphasize this. How important is bit depth? For a color signal, it can sometimes be important. However, no less (and perhaps even more) important is the sampling frequency, which you are unlikely to find in the descriptions of the devices. And often, an increase in bit depth occurs precisely at the expense of a decrease in the sampling frequency. However, I repeat, this is important only for a color signal. And it is very easy to check the transmission quality. Since a 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 television color table or just a striped table of different colors, a good video camera and monitor and see how much worse the image is with the proposed transmitter compared to a direct connection of the camera to the monitor. On a real object, the quality will be the same as on a short piece of fiber.

Pay attention to the operating temperature range of the transmitters. Specifically transmitters, since they are usually installed near video cameras, outdoors, somewhere evenly along the multi-kilometer perimeter of the object. Make sure that you do not have to build a warm hut for the transmitters. By the way, Ethernet over fiber transmitters are usually designed specifically for warm huts, and rare versions with an industrial temperature range are significantly more expensive than usual. What other features are there?
Not so essential for work, but sometimes significantly facilitating life. For example, devices can be mounted in a 19” rack, which is convenient in a crowded central point.
Devices can be powered from an external power supply (this is popular with imported devices) or directly from 220 V. See what is more convenient for you. External power supplies are often such that they can only be plugged directly into sockets, and these are unnecessary detachable connections, which does not increase the reliability of the system.
There are universal devices that are easy to mount both on a wall and in a rack, which work both on single-mode and multi-mode fiber, can work both from 220 volts and from an external low-voltage power supply. But such versatility is important only to distributors, so as not to store a large assortment of devices in a warehouse. In each specific project, it is more or less known what exactly is needed, and no one will definitely change the cable during operation.