The most important component of a fiber optic transmitter is the light source (usually a semiconductor laser or LED). Both serve the same purpose - the generation of a microscopic light beam that can be introduced into the fiber with high efficiency and modulated (changed in intensity) at a high frequency. Lasers provide higher beam intensities than LEDs and allow higher modulation frequencies; therefore they are often used for long-haul broadband lines such as telecommunications or cable TV. On the other hand, LEDs are cheaper and more durable devices, moreover, they are quite suitable for most systems of short to medium length, and therefore they are widely used in closed TV systems.
In addition to its functional purpose (i.e. what signal it should transmit), a fiber-optic transmitter is characterized by two more important parameters that determine its properties. One is its output power (intensity) of the optical emission, and the other is the wavelength (or color) of the emitted light. Usually these are 850, 1310 or 1550 nm, values selected from the condition of coincidence with the so-called. "Transparency windows" in the transmission characteristic of an optical fiber material.
Fiber optic installation
An optical fiber consists of a high refractive index center (core) surrounded by a low refractive index material cladding, as shown in Fig. 1.2. a fiber is characterized by the diameters of these regions - for example, 50/125 means a fiber with a core diameter of 50 µm and an outer cladding diameter of 125 µm.
Light propagates along the fiber core due to successive total internal reflections at the core-clad interface; its behavior is in many ways similar to that of being caught in a pipe, the walls of which are covered with a mirror layer. However, unlike a conventional mirror, in which reflection is rather inefficient, total internal reflection is essentially close to ideal - this is the fundamental difference between them, which allows light to propagate along the fiber over long distances with minimal loss.
A fiber made in this way (Figure 1.3) is called a stepped refractive index and multimode fiber because there are many possible paths, or modes, for the light to propagate. This set of modes results in dispersion (broadening) of the pulse because each mode travels a different path in the fiber, and therefore different modes have different transmission delays from one end of the fiber to the other. The result of this phenomenon is a limitation of the maximum frequency that can be effectively transmitted for a given fiber length - an increase in either the frequency or the fiber length beyond the limit values essentially leads to the coalescence of successive pulses, making it impossible to distinguish them. For a typical multimode fibers, this limit is approximately 15 MHz · km, which means that the video signal with a bandwidth of, e.g., 5 MHz may be transmitted a maximum distance of 3 km (5 x 3 km MHz = 15 MHz · km). Attempting to transmit the signal over a greater distance will result in progressive loss of high frequencies.
For many applications, this figure is unacceptably high, and a search was made for a fiber design with a wider bandwidth. One way is to reduce the fiber diameter to very small values (8-9 microns), so that only one mode becomes possible. Single-mode, as they are called, fiber (. Figure 1.3, b) is very effective in reducing the variance, and the resulting band - many GHz · km - making it ideal for telephone and telegraph public networks (PTT), and cable television networks. Unfortunately, a fiber of such a small diameter requires the use of a high-power, precision-aligned, and therefore a relatively expensive laser diode emitter, which makes them less attractive for many applications associated with short-range closed-loop TV systems.
Ideally, a fiber with a bandwidth of the same order of magnitude as a single-mode fiber, but with a diameter of the same as a multimode fiber, is required in order to be able to use inexpensive LED transmitters. To some extent, these requirements are satisfied by a multimode fiber with a gradient change in the refractive index (Fig. 1.3, c). It resembles a multimode fiber with a step change in refractive index, which was mentioned above, but the refractive index of its core is inhomogeneous - it smoothly changes from a maximum value in the center to a lower value at the periphery. This has two consequences. First, the light travels along a slightly curving path, and second, and more importantly, the differences in propagation delay of different modes are minimal. This is due to the fact that high fashion, those entering the fiber at a higher angle and traveling a longer path actually begin to propagate at a faster rate as they move away from the center into the region where the refractive index decreases, and generally move faster than the lower-order modes remaining near the axis into fibers, in the region of high refractive index. Increase speedjust compensates for the greater traversable path.
Gradient index multimode fibers are not ideal, but they still exhibit quite good bandwidth. Therefore, in most closed-loop TV surveillance systems of small and medium length, the choice of this type of fibers is preferable. In practice, this means that bandwidth is only rarely a parameter to be considered.
However, this is not the case for fading. The optical signal is attenuated in all fibers at a rate dependent on the wavelength of the transmitter by the light source. As mentioned earlier, there are three wavelengths at which optical fiber attenuation is usually minimal - 850, 1310, and 1550 nm. These are known as transparency windows. For multimode systems, the 850 nm window is the first and most commonly used (lowest cost). At this wavelength, a good quality gradient multimode fiber exhibits an attenuation of about 3 dB / km, which makes it possible to implement communication in a closed-loop TV system at distances over 3 km.
At a wavelength of 1310 nm, the same fiber shows even less attenuation - 0.7 dB / km, thereby allowing a proportional increase in the communication range to about 12 km. 1310 nm is also the first operating window for single-mode fiber optic systems, with an attenuation of about 0.5 dB / km, which in combination with laser diode transmitters allows the creation of communication lines longer than 50 km. The second transparency window - 1550 nm - is used to create even longer communication lines (fiber attenuation is less than 0.2 dB / km).
Fiber Optic Receivers
Fiber optic receivers solve the vital problem of detecting extremely weak optical radiation emitted from the end of the fiber and amplifying the received electrical signal to the required level with minimal distortion and noise. The minimum level of radiation required by the receiver in order to provide acceptable output signal quality is called sensitivity; the difference between the receiver sensitivity and the transmitter output power determines the maximum allowable system loss in dB. For most CCTV surveillance systems with an LED transmitter, the typical figure is 10-15 dB.
Ideally, the receiver should work well when the input signal changes over a wide range, since it is usually impossible to predict in advance exactly what the attenuation will be in the communication line (i.e., line length, number of joints, etc.). Many simple receiver designs use manual gain control during installation to achieve the desired output level. This is undesirable, since changes in the amount of line attenuation caused by aging or temperature changes, etc., are inevitable, which dictates the need to periodically adjust the gain.
All fiber optic receivers use automatic gain control, which monitors the average level of the input optical signal and changes the receiver gain accordingly. No manual adjustment is required either during installation or during operation.
