1、Performance index of optical attenuator
① Attenuation and insertion loss
Attenuation and insertion loss are essential indexes of optical attenuators. The attenuation index of fixed optical attenuators is the insertion loss. In addition to attenuation, variable attenuators have separate insertion loss indexes. The insertion loss of high-quality variable attenuators is less than 1.0db. Generally, the index of ordinary variable attenuators is minor than 2.5dB. The smaller the insertion loss, the better. But it's bound to involve prices.
② Attenuation accuracy of optical attenuator
Attenuation accuracy is an essential index of optical attenuators. Generally, the attenuation precision of the mechanical adjustable optical attenuator is less than half of its attenuation ± 1 time. Its size depends on the degree of precision machining of mechanical components. The fixed optical attenuator has high attenuation accuracy. Generally, the higher the attenuation accuracy, the higher the price.
③ Return loss
The return loss is one of the most critical parameters that affect the performance of optical devices. The influence of return light on optical network systems is well known. The return loss of an optical attenuator is the ratio of the light energy incident into the optical attenuator to the light energy reflected along with the incident optical path in the attenuator. The return loss of a high-performance optical attenuator is more than 45dB. In fact, due to the process and other reasons, the actual return loss of the attenuator still has a particular gap from the theoretical value. It is necessary to use a high return loss attenuator in the corresponding line. At the same time, the optical attenuator should have a more comprehensive temperature range and spectrum range.
2、 Application scope of optical attenuator
The fixed optical attenuator is mainly used to attenuate the light energy in the optical path by a fixed amount, and its temperature characteristic is excellent. In the debugging of the system, it is often used to simulate the corresponding attenuation of the optical signal after passing through a section of optical fiber or to reduce the surplus optical power in the relay station to prevent the saturation of the optical receiver; It can also be used for the calibration of optical testing instruments. Different Fixed Attenuators can be used for other line interfaces; If the interface is pigtail type, the optical attenuator of pigtail type can be welded between two sections of optical fiber; If there is a connector interface in the process of system debugging, it is more convenient to use converter type or converter type fixed attenuator. In practical applications, optical attenuators with variable attenuation are often needed. So variable attenuator is widely used. For example, because the designed redundancy of EDFA and CATV optical systems is not the same as that of the optical power in the actual procedure, variable optical attenuators must be inserted when BER evaluation is carried out prevent receiver saturation. In addition, variable optical attenuators will also be used to measure and calibrate fiber optics (such as optical power meters or OTDR). On the one hand, optical attenuators are developing towards miniaturization, serialization, and low price from the perspective of market demand. On the other hand, as the standard optical attenuators are pretty mature, the optical attenuators develop in the direction of high performance, such as intelligent optical attenuators and high return loss optical attenuators.
3、 Optical isolator
An optical isolator is a kind of nonreciprocal optical element, which only allows the light beam to pass along one direction and has a strong blocking effect on the reflected light. In the CATV optical transmission system, due to optical fiber movable connector, optical fiber fusion joint, visual elements, and Rayleigh scattering of the optical fiber itself, there is always reflected light wave, which hurts system performance. Therefore, an optical isolator must be used to eliminate the influence of reflected waves. An optical isolator is installed in optical reflector and optical amplifier,
The isolator consists of a polarizer, polarizer, and polarizer. A polarizer is a kind of optical device. When the beam incident on it, its output beam becomes a linear polarized light in a particular direction, which is the polarization axis of the polarizer. When the polarization direction of the incident light is perpendicular to the polarization axis of the polarizer, the light can not pass through so that the polarizer can be used as a polarizer. The rotator is composed of a rotatory material and a permanent magnet. With the help of the magneto-optical effect, the polarization direction of the light passing through it can be rotated to a certain extent.
The working principle of an optical isolator is: the polarization axis difference between polarizer and polarizer is 45o. When the incident light passes through the polarizer, it becomes linearly polarized light. Then through a polarizer, its polarization plane is rotated 45o, just in line with the polarization direction of the polarizer. Hence, the optical signal smoothly passes through the optical isolator and enters the optical path. If there is reflected light, the polarization direction of the reflected light passing through the polarizer and polarizer is orthogonal to the polarization direction of the polarizer. It cannot pass through the polarizer to achieve the purpose of isolating the reflected light. The loss of the reflected light of each stage of the optical isolator is as high as 35dB.
The requirements of optical isolator in CATV system are low forward loss, high reverse isolation, high return loss, small size, and good environmental performance. Because the optical isolator is more expensive, so it is generally used in light sources, not invisible fiber lines, so it is not unnecessary but from the cost consideration. If the optical isolator is cheap and the insertion loss is slight, it can improve the system performance.
4、 Optical switch
The optical switch is a kind of optical path control device, which plays the switching optical path. In optical fiber transmission network and various optical switching systems, it can be controlled by a microcomputer to realize the optical switching and realize the intelligent information distribution and exchange between terminals and between terminals and centers;
In the ordinary optical transmission system, it can be used to switch the main and standby optical path, and also can be used in the testing of optical fiber and optical devices and optical fiber sensing network, so that the optical fiber transmission system, measuring instrument or sensing system can work stably and reliably, and it is convenient to use.
In CATV optical network, to ensure the uninterrupted operation of the CATV system, a backup optical transmitter should be equipped. When the working optical transmitter fails, the backup optical transmitter can be connected to the system in a short time (less than 1ms) by using the optical switch to ensure its regular operation.
According to its working principle, an optical switch can be divided into mechanical and non-mechanical types. The automatic visual control changes the optical path by the movement of optical fiber or optical elements. At present, the optical switch on the market is generally mechanical, which has the advantage of low insertion loss, usually less than 1.5dB; High isolation, generally more than 45dB, not affected by polarization and wavelength. The non-mechanical optical switch depends on the electro-optic effect, magneto-optic effect, acousto-optic effect, and thermo optic effect to change the refractive index of the waveguide and change the optical path is also a new technology. The advantages of this kind of switch are short switching time, small size, ease for visual integration, or optoelectronic integration; The disadvantages are significant insertion loss and low isolation.
5、 Wavelength division multiplexer
Transmitting several different wavelengths of optical signals in one optical fiber at the same time can be used as wavelength division multiplexing (WDM). Using WDM technology, as long as a small amount of combiner and demultiplexer are added at the transmitter and receiver, the transmission capacity of optical fiber can be significantly increased. The economic benefit can be improved. The WDM technology can also realize multi-channel transmission for the laid optical cable, which can reduce the cost and expand the capacity. Wavelength division multiplexer (WDM) plays the role of combination and division in the optical path. It gathers (combines) optical signals of different wavelengths into an optical fiber, transmits them to the receiving end, and separates (divides) the multiplexed optical signals transmitted by the optical fiber. According to the different principles of light splitting, WDM can be divided into three types: Branch mirror type, interference model type, and diffraction grating type. At present, most products on the market are diffraction grating types. The main specifications of WDM include insertion loss, crosstalk loss, wavelength spacing, and multiplexing channels. Insertion loss refers to the optical power loss caused by WDM, which is generally about 1-5db. Crosstalk loss represents the degree of wavelength separation of WDM. The greater the crosstalk loss is, the better. It should be greater than 20dB.
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