When two optical fibers are connected, because of the difference of the position, shape and structure of the two optical fibers, the energy cannot enter the other optical fiber from one optical fiber 100%, that is to say, the connection loss will occur. In order to minimize the connection loss, the two optical fibers must be precisely aligned. The main function of optical fiber connector is to quickly connect two optical fibers, so that the optical signal can be continuous and form the optical path. And how does the optical fiber connector realize the precise connection of optical fiber?
There are many kinds of optical fiber connectors, but the precise alignment between optical fibers depends on two factors. One is the ceramic insert with precise inner diameter, outer diameter and concentricity, the other is the ceramic sleeve with slit. This ceramic sleeve is a very smart design. From Figure 1, we can see how two optical fibers can be precisely aligned through a ceramic sleeve. The inner diameter of the ceramic sleeve is slightly smaller than the outer diameter of the insert core, because there is a gap on the sleeve, the insert core can be inserted. The expanded sleeve clamps two insert cores to achieve precise alignment.
The core diameter of single-mode fiber SMF is only about 8-10 μ M. in order to ensure low connection loss, the two fibers must be precisely aligned. From Figure 2, we can see the relationship between the connection loss and the amount of transverse dislocation of two optical fibers. The curve is exponential, and a transverse dislocation as small as 2.4 μ m will produce a loss of 1dB. Therefore, for single-mode optical fiber connector, the transverse dislocation between two optical fibers should be less than 0.5 μ M.
Physical contact of fiber end face
However, precision alignment alone is not enough for fiber optic connections. We know that light will reflect on the interface of two different media. The refractive index of quartz fiber at 1.55 μ m is about 1.455, so the reflection echo of fiber end is 3.4%. The backward reflection light will affect the stability of the communication system, and the insertion loss of about 0.15dB will be introduced into each quartz glass air interface. Therefore, each fiber connector will increase the loss by 0.3dB.
People usually plating antireflection film on the end surface to reduce the reflection echo, but the problem of coating is not considered in the optical fiber connector. Firstly, coating antireflection film will increase the cost of connector; secondly, optical fiber connection is not fixed, repeated plugging will destroy the antireflection film. Can antireflection film be plated on the fiber end surface and keep the fiber end surface untouched? From Figure 3, we can see the relationship between the fiber docking loss and the longitudinal distance between the two fibers. The gap as small as 50 μ m will introduce nearly 1dB loss, which is not tolerated in the optical fiber communication system.
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 fiber ends must be discharged, so that the two fiber ends reach physical contact (PC), just like the integrated media. Because the fiber is fixed in the middle of the ceramic insert, any roughness of the ceramic surface will affect the physical contact between the fibers. In order to ensure the physical contact between the optical fibers, the surface of the inserting core is usually ground into a spherical surface, and the end face of the optical fiber is located at the vertex of the spherical surface, which is the second smart design in the optical fiber connector. As shown in Figure 1, the insert is inserted into the sleeve. Under the pressure, the end face of the insert is deformed, which can ensure the physical contact between the optical fibers. Because the physical contact depends on the deformation of the end face, and the ceramic is wear-resistant and elastic to a certain extent, this is the reason why it is chosen as the insert material instead of glass.
Further reduce reflected echo
The physical contact between optical fibers can ensure the low loss of optical fiber connection point, but the return loss RL can only reach 55dB. For some applications requiring higher RL, the end face of optical fiber connector is grinded to a certain angle, which is called bevel physical contact APC. The curve in Figure 4 shows the relationship between the increased RL and the fiber end angle. The fiber end is usually ground to an 8 ° slope, and the RL can increase 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 component in the optical fiber communication system. The basic technical requirements for the optical fiber connector include low insertion loss IL and high return loss RL, that is to say, as low as possible reflection echo br. However, as the most widely used optical passive device, its cost and connection convenience are as important as the technical specifications.