Views: 162 Author: Site Editor Publish Time: 2020-09-18 Origin: Site
When two optical fibers are connected, due to the difference in position, shape, structure of the two optical fibers, the energy cannot be 100% from one fiber to another fiber, and connection loss will occur. In order to minimize the connection loss, the two fibers must be precisely aligned. The main function of the optical fiber connector is to quickly connect two optical fibers, so that the optical signal can be continuous to form an optical path. And how does the optical fiber connector realize the accurate connection of the optical fiber?
There are many types of optical fiber connectors, but the precise alignment between optical fibers depends on two factors. One is a ceramic ferrule with a precise inner diameter, outer diameter, and concentricity. The other is a ceramic sleeve with a slit. The inner diameter of the ceramic sleeve is slightly smaller than the outer diameter of the ceramic ferrule. Because the ceramic sleeve has a slit, the ferrule can be inserted. The expanded ceramic sleeve tightens the two ceramic ferrules to achieve precise alignment.
However, precision alignment alone is not enough for optical fiber connections. We know that light will reflect back at the interface between two different media. The refractive index of quartz fiber at 1.55μm is about 1.455, so the reflected echo BR at the end face of the optical fiber is 3.4%. The backward reflected light will affect the stability of the communication system. At the same time, the insertion loss of approximately 0.15dB is also introduced at each quartz glass to air interface. Therefore, each optical fiber connector should increase the loss of 0.3dB.
People usually apply anti-reflection coating on the end surface to reduce the reflected echo, however, the coating problem is not considered in the optical fiber connector. First, the anti-reflection coating will increase the cost of the connector. Second, the optical fiber connection is not fixed, and repeated insertion and removal will damage the anti-reflection coating. The relationship between optical fiber butt loss and the longitudinal separation of the two optical fibers. A gap as small as 50 μm will introduce a loss of nearly 1 dB, which is intolerable in fiber optic communication systems.
Therefore, we have a consensus that two optical fibers must contact each other and the end face of the optical fiber cannot be coated. The reflection echo occurs on the interface of two different media, and the air between the optical fiber ends must be discharged, so that the two optical fiber end faces reach physical contact. Since the optical fiber is fixed in the middle of the ceramic ferrule, any roughness on the ceramic surface will affect the physical contact between the optical fibers. In order to ensure the physical contact between the optical fibers, the ceramic ferrule surface is usually ground into a spherical surface, and the end face of the optical fiber is located at the vertex of the spherical surface. Under pressure, the ceramic ferrule is inserted into the ceramic sleeve, the end face of ceramic ferrule deforms, which can ensure physical contact between the optical fibers. The physical contact depends on the deformation of the end face. The reason why ceramics are chosen to replace glass as insert is that ceramics are wear-resisting and have certain elasticity.
The physical contact between the optical fibers can ensure low loss of the fiber connection point, but the return loss (RL) can only reach 55dB. For some applications that require higher RL, the end face of the optical fiber connector is ground to a certain angle, which is called APC. The fiber end face is usually ground to an 8° slope, and the RL can be increased by an additional 36dB, so the total RL of the APC connector is usually greater than 65dB.
The optical fiber connector is the most basic optical passive device in the optical fiber communication system. The basic technical requirements of the system for the optical fiber connector include low insertion loss (IL) and high return loss (RL), that is, the lowest reflected echo. However, as the most widely used optical passive device, its cost and connection convenience are as important as the technical indicators.