Fiber connector insertion loss

Optical fiber connectors are an indispensable component in optical fiber transmission and optical interconnection systems, and are also the most widely used optical passive components. Currently, there are more than 25 mainstream optical fiber connectors on the market. Since its appearance in the 1970s, fiber optic connectors have been in the mature stage of the industry after thirty or forty years of development. However, with the continuous increase and change of application requirements in high speed, high density and various environments, reducing connection loss has always been one of the focuses of optical fiber connector research.

The optical fiber connector

1. Lateral dislocation and insertion loss

The main factors that cause the insertion loss of optical fiber connectors are lateral dislocation, end face gap, diameter mismatch and inclined connection, etc. Domestic and foreign companies and research units have carried out detailed experiments and quantitative engineering research on this. For example, researchers from AT&T Bell Research Institute in the United States, Optoelectronics Laboratory/Network Laboratory of NTT Corporation in Japan, Alef Photonics Research Center in Canada, Southeast University, and Central South University use finite element analysis, ray tracing, beam propagation simulation, interferometry, etc. A simulation and experimental method was used to study the effects of end face geometric parameters, materials, stress, etc. on the connection loss. Today, the optical performance and repeatability of optical fiber connectors have also been significantly improved, from the initial insertion loss of 0.5-1 dB to the current level of 0.2 dB; after 500 times of insertion and removal, the change in insertion loss can be controlled within 0.1 dB .

In the application process of optical fiber butt engineering, the loss caused by the lateral dislocation of the fiber core is called dislocation loss, which is the main source of insertion loss in optical fiber connections, especially for single-mode fibers. Without considering other factors, the connection loss caused by the lateral dislocation of the fiber can be approximately calculated as follows:

The connection loss caused by the lateral dislocation of the fiber can be approximated

The lateral dislocation of the fiber optic connector core is determined by many factors, such as the concentricity between the inner hole and the outer diameter of the ceramic ferrule, the concentricity between the curing position of the fiber core and the ferrule hole, and the positional deviation in the multi-core arrangement. At present, the concentricity of the inner hole of the ferrule with better processing technology can reach within 0.3 um. Since the inner hole of the ceramic ferrule is slightly larger than the diameter of the optical fiber, it is almost impossible for the optical fiber to be just right when the optical fiber and the ceramic ferrule are fixed by curing glue. Located in the center position, it will also bring a certain amount of eccentricity. The diameter of the inner hole of the ferrule is generally more than 0.5 um larger than the diameter of the optical fiber, so the overall concentricity variation range of 1-1.3 um can be generated, that is, lateral dislocation. It can be seen from Figure 1 that it corresponds to an insertion loss of about 0.2 dB, which is the current mainstream insertion loss range in the industry. If the loss-in loss is to be less than 0.1dB, the lateral dislocation should be controlled within 0.7um.

In order to reduce the optical fiber connection loss, the first step is to reduce its lateral dislocation. There are two main ways in the industry:

1. Through the adjustment process, adjust the eccentric positions of all cores to the connector fixing area.

2. Improve the processing/assembly process and improve the concentricity of the fiber core.

2. Adjustment process to reduce insertion loss

The point adjustment process is for the pre-assembled optical fiber connector, by adjusting the eccentric positions of different fiber cores into one area, to achieve mutual compensation of the eccentric positions, and to reduce the overall lateral dislocation effect. A typical pre-assembled ceramic core consists of a ceramic tube and a tailstock, and there are male and female slots between the tailstock and the sleeve for fixing the ferrule. According to the recommendation of the TIA/EIA standard of the American Electronics Industry Alliance, the four slots on the tailstock are evenly distributed on the circumference, and the eccentricity can be clamped by rotating the ferrule and the specified position (Key key, also called positioning key) The angle is controlled within ±22.5°, that is, when the two connectors are connected, the included angle of the eccentricity is within ±45°.

3. Improve the concentricity of the pre-assembled ferrule

Improving the concentricity of the pre-assembled ferrule in terms of physical size is the most fundamental method to reduce the lateral dislocation, but due to the influence of material processing technology, ferrule manufacturing process, fiber positioning control process, etc. Industry-level products require the use of ultra-precision control machinery and equipment, and the cost is relatively high. Only enterprises with a certain scale will consider it.

3.1 Manufacturing process of ceramic ferrule

At present, there are various manufacturing processes for ceramic ferrules. The typical method is to first use zirconia material to make a ferrule blank, form an inner hole with a diameter of about 120 um by injection molding, and then perform precision machining of the inner hole and outer diameter. In the process of precision machining, the ferrule blank is threaded on a special wire of different thicknesses, and the inner hole is ground by rotating and moving the ferrule until it reaches 125 um or other required values. The outer diameter is repeatedly ground by a rotary device and a grinding wheel to improve concentricity. At present, this process can obtain ceramic ferrules with concentricity below 1 um.

To improve the concentricity of the inner hole and outer diameter of the ferrule, we can start from two aspects. The first is to improve the production accuracy of the ferrule blank, such as using the offset feedback of the non-straight inner hole to trim the mold structure and correct the material channel structure to minimize the offset of the inner hole of the blank ferrule. A company's researchers have tested that the aperture offset in the Z direction of the blank ferrule made after trimming the mold can be controlled within 20 um. The second is to improve the machining accuracy of the outer diameter, such as improving the guide wheel mechanism in the grinding equipment, optimizing the chip flute, and reducing the influence of grinding heat and grinding temperature on the product. The improved grinding equipment can control the overall coaxiality of the inner hole and outer diameter of the ferrule within 0.6 um.

3.2 Fiber core assembly process

This method has only been tested and applied in recent years. Using the method of optical observation (enlarged imaging, machine vision, etc.) as high concentricity as possible. For example, researchers from China Jiliang University have proposed a compact machine vision system, which can quickly detect the concentricity of the ferrule with a specific LED lighting scheme and edge detection algorithm, with a deviation of about 0.01um from the nominal parameter.

Although it has not been applied on a large scale, this method does not need to contact the functional area of ​​the end face of the object during the illumination and imaging process, nor does it affect the optical fiber assembly process, and the result feedback rate is fast. Adjust the position of the fiber core in the ferrule to optimize the concentricity of the pre-assembled fiber ferrule product. This method can be used to manufacture fiber optic connection products with ultra-high concentricity, for example, 0.3 um and below, close to the industrial limit level.