Principles and uses of fiber optic couplers

Optical network systems also need to couple, branch, and distribute optical signals, which requires optical fiber couplers. Optical fiber coupler, also known as optical splitter or optical splitter, is one of the most important passive components in optical fiber links. It is an optical fiber tandem device with multiple input terminals and multiple output terminals. M×N is often used to represent a The splitter has M input terminals and N output terminals. The optical splitters used in fiber optic CATV systems are generally 1×2, 1×3 and 1×N optical splitters composed of them. The principle can be divided into two types: fused tapered type and planar waveguide type. First, the fused tapered type product is made by side-welding two or more optical fibers; the planar waveguide type is a micro-optical component type product that uses photolithography technology to form an optical waveguide on a dielectric or semiconductor substrate to achieve branch distribution functions. . The two types of light splitting principles are similar. They achieve different branch sizes by changing the evanescent field coupling between optical fibers (coupling degree, coupling length) and changing the fiber radius. Conversely, multiple optical signals can be combined into one signal. It's called a synthesizer. Fused cone optical fiber couplers have become the mainstream manufacturing technology in the market due to their simple manufacturing method, low price, easy connection with external optical fibers, and resistance to mechanical vibration and temperature changes.

The fusion tapering method is to twist two (or more) optical fibers with the coating layer removed in a certain way, melt them under high-temperature heating, and stretch them to both sides at the same time, and finally heat them The area forms a special waveguide structure in the form of a double cone. By controlling the twisting angle and stretching length of the optical fiber, different light splitting ratios can be obtained. Finally, the tapered area is solidified on the quartz substrate with curing glue and inserted into the stainless copper tube. This is the optical splitter. This production process is inconsistent with the thermal expansion coefficient of the cured glue and the quartz substrate and stainless steel tube. When the ambient temperature changes, the degree of thermal expansion and contraction is inconsistent. This situation can easily lead to damage to the optical splitter, especially the optical splitter. The situation is even worse when placed in the wild, which is also the main reason why optical splitters are easily damaged. For the production of splitters with more channels, multiple splitters can be used. 2...Commonly used technical indicators (1) Insertion loss. The insertion loss of an optical splitter refers to the dB loss of each output relative to the input light. 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. Additional loss is defined as the DB number of the total optical power of all output ports relative to the input optical power loss. It is worth mentioning that for optical fiber couplers, additional loss is an indicator of the quality of the device manufacturing process. It reflects the inherent loss of the device manufacturing process. The smaller the loss, the better. It is an assessment indicator of the quality of the manufacturing process. Insertion loss only represents the output power status of each output port, not only the inherent loss factor, but also the impact of the splitting ratio. Therefore, the difference in insertion loss between different optical fiber couplers does not reflect the quality of device manufacturing. For the 1*N single-mode standard optical splitter, the additional loss is as shown in the following table:

Number of splitters 2 3 4 5 6 7 8 9 10 11 12 16　Additional loss DB 0.2 0.3 0.4 0.45 0.5 0.55 0.6 0.7 0.8 0.9 1.0 1.2

(3) Split ratio. The splitting ratio is defined as the output power ratio of each output port of the optical splitter. In system applications, the splitting ratio is indeed determined based on the optical power required by the actual system optical node to determine the appropriate splitting ratio (except for even distribution). The splitting ratio of the optical splitter is related to the wavelength of the transmitted light. For example, when an optical splitter transmits 1.31 micron light, the splitting ratio of the two output ends is 50:50; when transmitting 1.5μm light, it becomes 70: 30 (The reason why this happens is that optical splitters have a certain bandwidth, that is, the frequency bandwidth of the transmitted optical signal when the splitting ratio is basically unchanged). Therefore, the wavelength must be specified when ordering an optical splitter.

(4) Isolation. Isolation refers to the ability of a certain optical path of an optical splitter to isolate optical signals from other optical paths. Among the above indicators, isolation is more significant for optical splitters. In actual system applications, devices with an isolation of more than 40dB are often required, otherwise the performance of the entire system will be affected.

In addition, the stability of the optical splitter is also an important indicator. The so-called stability means that when the external temperature changes and the working status of other devices changes, the splitting ratio and other performance indicators of the optical splitter should remain basically unchanged. In fact, the stability of the optical splitter completely depends on the technological level of the manufacturer. The quality of products from different manufacturers varies greatly. In practical applications, I have indeed encountered many optical splitters of poor quality. Not only do their performance indicators deteriorate quickly, but their damage rates are quite high. Important components used in optical fiber trunk lines must be paid attention to when purchasing. Do not just look at them. Price, the price of optical splitting with low craftsmanship is definitely low. In addition, uniformity, return loss, directivity, and PDL all occupy a very important position in the performance indicators of optical splitters.