Like coaxial cable transmission system, optical network system also needs to couple, branch and distribute optical signals, which requires fiber optic splitter to realize. Fiber optic splitter is one of the most important passive devices in optical fiber link, and it is a optical fiber junction device with multiple input and output terminals. M × n is commonly used to represent that a splitter has m input terminals and N output terminals. Generally, the fiber optic splitters used in CATV system are 1 × 2, 1 × 3 and 1 × n fiber optic splitters composed of them.
1. Principle of fiber optic splitter
According to the principle, the fiber optic splitter can be divided into two types: optical fiber type and plane waveguide type. The optical fiber fused taper type product is made by side welding two or more optical fibers; the optical wave guide type is a micro optical component type product, which uses photolithography technology to form the optical wave guide on the medium or semiconductor substrate to realize the branch distribution function. The principle of these two types of light splitting is similar. They realize different branching quantities by changing the evanescent field coupling (coupling degree, coupling length) and fiber radius between fibers. Otherwise, they can combine multiple optical signals into one signal called synthesizer. Because of the advantages of simple fabrication method, low price, easy connection with external optical fiber, and resistance to mechanical vibration and temperature change, fused tapered fiber coupler has become the mainstream manufacturing technology in the market.
The fused biconical method is that two (or more than two) fibers without coating layer are disturbed by a certain method, fused under high temperature heating, stretched to both sides at the same time, and finally formed a special waveguide structure in the form of biconical in the heating area. By controlling the twist angle and the stretch length of the fiber, different light splitting ratios can be obtained. Finally, the taper area is solidified on the quartz substrate with a curing adhesive and inserted into the stainless copper tube, which is the fiber optic splitter. Because the thermal expansion coefficient of the curing adhesive is different from that of quartz substrate and stainless steel tube, the degree of thermal expansion and contraction is different when the ambient temperature changes. This kind of situation is easy to cause damage to the fiber optic splitter, especially when the fiber optic splitter is placed in the field, which is the main reason why the fiber optic splitter is easy to be damaged. For the production of splitter with more routes, multiple splitters can be used.
2. Common technical specifications of fiber optic splitter
(1) Insertion loss.
The insertion loss of the fiber optic splitter refers to the DB number of the output light relative to the input light loss of each channel. Its mathematical expression is: AI = - 10lg pouti / pin, where AI refers to the insertion loss of the i-th output port; pouti is the optical power of the i-th output port; pin is the optical power value of the input port.
(2) Additional losses.
The additional loss is defined as the DB of the total optical power of all output ports relative to the input optical power loss. It is worth mentioning that for the optical fiber coupler, the additional loss is the index reflecting the manufacturing process quality of the device, and it reflects the inherent loss in the manufacturing process of the device. The smaller the loss, the better, it is the assessment index of the manufacturing quality. The insertion loss only represents the output power status of each output port, which not only has the inherent loss factor, but also considers the influence of the light splitting ratio. Therefore, the difference of insertion loss between different fiber couplers does not reflect the quality of the device. The additional loss of 1 * n single-mode standard fiber optic splitter is shown in the table below:
(3) Spectral ratio.
The split ratio is defined as the output power ratio of each output port of the fiber optic splitter. In the system application, the split ratio is determined according to the actual optical power required by the optical node of the system, and the appropriate split ratio (except for the average distribution) is determined. The split ratio of the fiber optic splitter is related to the wavelength of the transmitted light, for example, a fiber optic splitter is transmitting 1.31 When the optical ratio of the two output terminals is 50:50 in the case of micron light, it changes to 70:30 in the case of transmitting 1.5-mu; m light (the reason for this is that the fiber optic splitter has a certain bandwidth, that is, the bandwidth of the transmitted light signal when the optical ratio is basically unchanged). Therefore, the wavelength must be indicated when the fiber optic splitter is customized.
(4) Isolation.
Isolation degree refers to the isolation ability of one optical path of the fiber optic splitter to the optical signals in other optical paths. In the above indicators, the isolation degree is more important for the fiber optic splitter. In practical system applications, devices with isolation degree of more than 40dB are often needed, otherwise the performance of the whole system will be affected.
In addition, the stability of the fiber optic splitter is also an important index. The so-called stability refers to that when the external temperature changes and the working state of other devices changes, the fiber optic splitter '. In practical application, I do encounter many fiber optic splitters with poor quality. Not only the performance index is deteriorated rapidly, but also the damage rate is quite high. As an important component of optical fiber trunk line, attention must be paid to the purchase, not only the price, but also the price of fiber optic splitters with low technology level.