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Comprehensive Analysis of the Working Principle of Optical Fiber Connectors

Views:128     Author:Site Editor     Publish Time: 2020-12-28      Origin:Site

The optical fiber connector is an indispensable device for connecting various devices in optical fiber communication systems, and it is also the most used optical fiber device at present. Due to the gradual optical fiberization of local communication networks and the increasing demand for metropolitan area networks and user access networks, the total demand for global optical fiber connector markets has continued to expand in recent years, and the annual growth rate in the next ten years is expected to be around 20%.

 

Although there are more than 70 kinds of optical connectors in the world, and new varieties are still emerging, the mainstream varieties in the market (especially the Chinese market) are still precision φ2.5mm fiber optic connectors (such as FC, SC, ST, etc.) composed of ceramics ferrules and ceramic tubes. In addition, the demand for small connectors with φ1.25mm ceramic cores (such as LC, MU, etc.) and multi-core connectors (such as MTP, etc.) based on ribbon fiber connectors are gradually increasing.

 

Principle


Generally, the main optical characteristic indexes to measure the quality of optical fiber connector products are Insert loss and Return loss. In addition, physical characteristics such as the geometric parameters of the ferrule end face that affect product quality and reliability are increasingly being valued by system manufacturers or high-end customers. Starting from the working principle of the optical fiber connector, the following briefly introduces the insertion and return loss of the connector:

 

The optical fiber connector cannot be used alone. It must be inter-matched with other connectors of the same type to form the optical path connection. At present, the most popular way of assembling and docking optical fiber connectors is to use an epoxy resin thermosetting agent to fix the optical fibers in the high-precision ceramic pinholes. Then, under the action of external force, the two pins are positioned through the adapter sleeve to realize the butt connection between the optical fibers (as shown in Figure 1).

 

The principle of optical fiber connector docking

Figure 1 The principle of optical fiber connector docking


It can be seen from Figure 1 that ensuring that the two butted fiber cores are in a straight line when they are in contact is the key to ensuring the excellent connection quality of the connector. It mainly depends on the physical properties of the optical fiber itself, the manufacturing accuracy of the connector pins, and the assembly processing accuracy of the connector. At the same time, the optical performance index of the optical fiber and the polishing quality of the pin end face also have a direct impact on the optical performance and reliability of the connector.

 

Insertion loss

Insertion loss refers to the optical power attenuation caused by the connected connector to the system (that is, the relative decrease in the output power of the optical connector relative to the input power). Insertion loss is mainly caused by the lateral deviation between two consecutive optical fibers. As shown in Figure 1, if two optical fibers are arranged in a straight line and the lateral deviation is zero, the insertion loss caused by it is minimal. But in the actual docking process of the connector, this is unlikely to be achieved. Because the core and fiber cladding are not centered, the fiber cladding is not centered with the pinhole, and the concentricity error between the pinhole and the outer diameter, etc., will cause the lateral deviation between the fibers.

 

At the same time, the longitudinal gap in the optical fiber connector and the quality of the end face are also one of the factors that cause the insertion loss. The UPC plug contact method commonly used in recent years has better solved the longitudinal gap problem. In this way, the pin and the fiber end face are polished with a spherical surface, so that the two opposite pins are meshed together under the action of external force, so that the apex of the meshed fiber is deformed and flattened to form a sufficient butt of the fiber which can reduce the number of longitudinal gap of fiber connectors. 

 

Return loss

Return loss is a parameter used to measure the size of the back-reflected light on the connector end face. The essence of the echo is the reflection of light. According to the principle of Fresnel reflection, Fresnel reflection occurs when light encounters two interfaces with different refractive indexes during transmission, causing signal superimposition or interference in the optical path. In a single-mode fiber system with a high transmission rate, especially a cable television system (CATV), the reflection phenomenon will cause a time lag in the transmission signal. This delays the time it takes for the signal to reach the user end, causing ghosting and degraded image clarity.

 

The UPC contact method of the connector joint reduces the gap between the connection end faces, in addition to reducing the insertion loss, it also reduces the reflection of the connection end face and improves the return loss. For users such as CATV systems, the APC-type contact method is a better choice. Because the spherical normal of the end face of the ceramic ferrule of the APC type connector has an angle (usually 8°) with the axis of the fiber, the light reflected from the end face leaks out without returning to the core, which greatly improves the echo of the connector loss.

 

optical fiber connector


Factors affecting the quality of optical fiber connectors

 

For an excellent fiber connector manufacturer, in order to ensure the high quality of the product, three factors are very important. They are high-quality tight-buffered fiber optic cables, high-precision ceramic pins, and excellent assembly processing technology. 

 

Optical fiber

As far as optical fiber is concerned, the impact of optical fiber on connector performance is mainly reflected in the attenuation coefficient of the optical fiber itself and the manufacturing tolerance of the optical fiber and cable (especially the core/cladding concentricity error). For a long jumper connector, the excessive attenuation coefficient of the fiber itself will cause the congenital deficiency of the connector jumper and increase the energy loss in the optical path. Larger core/cladding concentricity errors can easily cause the lateral deviation of the core. Therefore, high-quality optical fiber is essential for the product's low insertion loss.

 

Ceramic pin and high-precision assemble parts

As for the ceramic pin, the smaller concentricity error and the good match between the inner hole diameter and the outer diameter of the fiber cladding (ie, small gap) can also reduce the lateral deviation of the core and reduce the insertion loss. At the same time, the high-precision assembly of parts can ensure that the product is in full docking and good stress state during the connection, which directly affects the return loss.

 

Excellent assembly processing technology

The excellent product manufacturing technology includes complete process control, sophisticated grinding and testing equipment, and matching grinding and polishing processes and quality monitoring. These enable the product to meet the high-quality end face and optical characteristics index, while adjusting the end face geometric parameters of the ferrule within the corresponding standard requirements according to the different requirements of customers, so as to improve the system connection and use reliability.

 

Using advanced grinding and testing equipment, Dowell’s mature production technology, as well as a complete quality assurance system, supplemented by modern enterprise management application software SAP (R/3) system for material management and energy consumption in the production process monitoring and optimization enable Dowell products to fully ensure that the insertion loss and return loss of the connector products are better than the existing industry standards. The typical values of product testing are far higher than the standard requirements, and product performance and quality stability are at the leading domestic level.


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