VOA (Variable Optical Attenuator)
Optical attenuator is a very important fiber optic passive device. It can attenuate the optical signal energy as expected according to the user's requirements. It is often used to absorb or reflect the optical power margin.
1. Up to now, there have been two types of optical attenuators, fixed and adjustable, in the market. Different types of attenuators use different attenuation principles.
1. Displacement type optical attenuator.
As we all know, when two sections of fiber are connected, a very high alignment accuracy must be achieved in order for the optical signal to pass through with less loss. Conversely, if the alignment accuracy of the fiber is properly adjusted, the attenuation can be controlled. Displacement type optical attenuators are based on this principle, and intentionally make the fiber misalign when it is docked. The light energy is lost, so as to achieve the purpose of controlling the amount of attenuation.
Displacement optical attenuators are divided into two types: lateral displacement optical attenuators and axial displacement optical attenuators. The lateral displacement optical attenuator is a relatively traditional method. Since the magnitude of the lateral displacement parameter is in the order of micrometers, it is generally not used to make a variable attenuator, but only used in the production of a fixed attenuator, and welding or bonding is used. Connection, there is still a large market, its advantage is that the return loss is high, generally greater than 60dB. In the process design of axial displacement optical attenuator, as long as the two optical fibers are pulled apart by a certain distance for centering, the purpose of attenuation can be achieved. This principle is mainly used for the production of fixed optical attenuators and some small variable optical attenuators.
2. Coated optical attenuator
This attenuator is made using the principle that the reflected light intensity of the light on the surface of the metal film is related to the thickness of the film. If the thickness of the metal film deposited on the glass substrate is fixed, a fixed optical attenuator is made. If a series of disc-shaped metal thin wax glass substrates with different thicknesses are inserted obliquely into the optical fiber, so that metal films of different thicknesses are inserted into the optical path, the intensity of the reflected light can be changed, and different attenuations can be obtained Volume, made variable attenuator.
3. Attenuating film type optical attenuator.
Attenuator-type optical attenuator directly fixes the attenuator with absorption characteristics on the end surface of the optical fiber or in the optical path (liquid crystal type) to achieve the purpose of attenuating optical signals. This method can be used not only to make fixed optical attenuators, but also To make a variable optical attenuator.
two. Performance index of optical attenuator.
① Attenuation and insertion loss.
Attenuation and insertion loss are important indicators of the optical attenuator. The attenuation index of the fixed optical attenuator is actually its insertion loss. In addition to the attenuation, the variable attenuator has a separate insertion loss index. The insertion loss of the variable attenuator is below 1.0dB. Generally, the index of the ordinary variable attenuator is less than 2.5dB. When actually choosing an adjustable attenuator, the smaller the insertion loss, the better. But this will inevitably involve prices.
② Attenuation accuracy of optical attenuator.
Attenuation accuracy is an important indicator of optical attenuators. Generally, the attenuation accuracy of the mechanical tunable optical attenuator is ± 0.1 times of its attenuation. Its size depends on the degree of precision machining of mechanical components. The attenuation accuracy of the fixed optical attenuator is very high. Generally, the higher the attenuation accuracy, the higher the price.
③ Return loss.
An important indicator that affects system performance in optical device parameters is return loss. The effect of return light on optical network systems is well known. The return loss of an optical attenuator refers to the ratio of the optical energy incident into the optical attenuator and the optical energy reflected from the incident optical path in the attenuator. The return loss of the high-performance optical attenuator is above 45dB. In fact, due to process and other reasons, the actual return loss of the attenuator is still a certain distance away from the theoretical value. In order not to reduce the return loss of the entire line, high return loss attenuators must be used in the corresponding line, and optical attenuation is also required. The device has a wider temperature range and spectrum range.
three. Application range 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 characteristics are 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 fiber or used in the relay station to reduce the excess optical power to prevent the saturation of the optical receiver; it can also be used to calibrate the optical test instrument. For different line interfaces, different fixed attenuators can be used; if the interface is a pigtail type, a pigtail type optical attenuator can be used to weld between the two optical fibers of the optical path; if there is a connector during system debugging For the interface, it is more convenient to use a converter-type or converter-type fixed attenuator.
In practical applications, an optical attenuator whose attenuation can be changed according to user needs is often required. Therefore, the application range of the variable attenuator is wider. For example, because the design redundancy of EDFA and CATV optical systems is not exactly the same as that of the optical power in actual systems, when evaluating the BER of the system to prevent receiver saturation, a variable optical attenuator must be inserted into the system. In the measurement and calibration of fiber optics (such as optical power meter or OTDR), a variable attenuator will also be used. From the perspective of market demand, on the one hand, optical attenuators are developing toward miniaturization, serialization, and low prices. On the other hand, since ordinary optical attenuators are quite mature, optical attenuators are developing towards high performance, such as intelligent optical attenuators and high return loss optical attenuators.