1. Introduction
Fiber optic connectors are essential components in optical fiber communication systems, enabling the connection of optical fibers to transmit light signals. The quality of these connectors is vital for maintaining the integrity and efficiency of the transmitted signals. One of the key factors determining the quality of fiber connectors is the polishing method used during their manufacturing process. Different polishing methods can result in varying levels of insertion loss and return loss, which are critical parameters for evaluating the performance of fiber connectors.
2. Overview of Fiber Connector Polishing Methods
2.1 Four-Step Polishing Method
The four-step polishing method is a widely used approach in the manufacturing of fiber connectors. It consists of the following stages:
Deburring: This initial step removes the protective coating or "glue package" from the fiber connector. For connectors with ceramic sleeves, such as FC, SC, ST, and LC types, carbon silicon carbide grinding sheets (e.g., SC30/15) are typically used for deburring.
Coarse Grinding: The purpose of coarse grinding is to quickly remove a significant amount of material from the connector's end face. Different grit sizes of diamond grinding sheets are used depending on the specific requirements. For example, D9, D6, or D3 diamond grinding sheets can be employed for this stage.
Semi-Fine Grinding: This step further refines the end face of the connector, reducing surface roughness and preparing it for the final polishing. D1 diamond grinding sheets are commonly used for semi-fine grinding.
Fine Grinding and Polishing: Fine grinding with finer grit diamond grinding sheets (e.g., D0.5) is followed by polishing using specific polishing pads and polishing liquids. For APC ceramic sleeve connectors, a large-grit diamond grinding sheet is first used to create an 8-degree angle on the end face, and then the D9-D1-ADS polishing sequence is applied. For plastic sleeve connectors like MT-RJ types, a different set of grinding sheets (e.g., SC30/15-SC9-SC6-SC3-SC1) and polishing materials (black leather + cerium oxide grinding liquid with a glass polishing pad) are used.
2.2 Importance of Polishing Medium and Parameters
Water is commonly used as the polishing medium in the fiber connector polishing process. The selection of abrasives is crucial as it directly affects the polishing effect. The general principle is that the grinding sheet should be harder than the workpiece, while the polishing pad should be softer. For example, in the polishing of APC connectors, ADS/cerium oxide polishing film + SiO2 polishing liquid is often used, and a rubber polishing pad is employed.
3. Comparison of APC and UPC Connectors
3.1 End Face Structure
APC Connectors: APC connectors feature an 8-degree angled end face. This angled design is achieved through precise grinding and polishing processes. The 8-degree angle allows the reflected light to be directed into the cladding of the optical fiber rather than being reflected back to the light source, which significantly reduces the return loss.
UPC Connectors: UPC connectors have an end face with a slight curvature, forming a more rounded shape compared to PC (Physical Contact) connectors. This design aims to achieve a more precise alignment of the optical fibers, resulting in better optical performance.
3.2 Optical Performance
Return Loss: APC connectors typically offer a return loss of ≥60 dB, which is higher than the return loss of UPC connectors (≥50 dB). A higher return loss indicates less light reflection, which is beneficial for maintaining the stability and integrity of the transmitted signals, especially in high-performance optical communication systems.
Insertion Loss: Both APC and UPC connectors can achieve low insertion loss, typically less than 0.3 dB (and often even lower, around 0.2 dB in some cases). However, due to the smaller air gap in UPC/PC connectors, they may be more prone to achieving lower insertion loss in certain situations. But it's important to note that insertion loss can also be affected by factors such as dust particles on the connector end faces.
3.3 Application Scenarios
APC Connectors: APC connectors are commonly used in high-wavelength range optical RF applications, such as CATV (Cable Television) systems. The 8-degree angled end face design helps to improve the quality of television signals, which are often based on analog optical modulation. In these applications, the reduced reflection from the APC connectors prevents interference with the transmission signals and damage to the laser sources.
UPC Connectors: UPC connectors are widely used in basic networks, television signal transmission, and telephone systems. Their better surface finish and lower return loss compared to PC connectors make them suitable for a variety of general-purpose optical communication applications.
4. Selection of Abrasives in Polishing
4.1 Factors Influencing Abrasive Selection
Material of the Connector: The material of the fiber connector sleeve (e.g., ceramic or plastic) plays a significant role in determining the appropriate abrasives. For ceramic sleeve connectors, diamond grinding sheets are commonly used due to their hardness and ability to achieve precise grinding. For plastic sleeve connectors, different types of abrasives and polishing materials are required to avoid damaging the plastic material.
Desired Surface Finish: The level of surface finish required for the connector end face also influences the choice of abrasives. Finer grit abrasives are used for achieving a smoother surface finish, which is essential for reducing insertion loss and improving return loss.
Polishing Process Parameters: The polishing parameters, such as polishing pressure, speed, and time, need to be considered when selecting abrasives. Different abrasives may require different polishing conditions to achieve the best results.
4.2 Common Abrasives and Their Characteristics
Diamond Grinding Sheets: Diamond grinding sheets are widely used in the coarse and fine grinding stages of fiber connector polishing. They are available in various grit sizes, ranging from coarse (e.g., D9) to fine (e.g., D0.5). Diamond is an extremely hard material, making it suitable for removing material quickly and efficiently from the connector end face.
Cerium Oxide Polishing Film: Cerium oxide polishing film is often used in the final polishing stage to achieve a high-quality surface finish. It has good polishing properties and can effectively remove any remaining surface defects and (modified layer) on the connector end face. The modified layer is a thin layer of material that is formed during the grinding process and can affect the optical performance of the connector. By using cerium oxide polishing film, the thickness and refractive index of the modified layer can be reduced, thereby improving the return loss.
5. Impact of Polishing Quality on Connector Performance
5.1 Evaluation Parameters
The quality of fiber connector polishing is evaluated based on several key parameters, including curvature radius, apex offset, and fiber core depression. These parameters need to be within specific ranges to ensure good physical contact between the two fiber end faces. Additionally, the presence of scratches or other contaminants on the connector end face should be minimized, and the connector should meet the requirements for low insertion loss and high return loss.
5.2 Experimental Studies
Experimental studies have shown that the choice of polishing method and abrasives can significantly affect the optical performance of fiber connectors. For example, using fine-grit diamond grinding sheets and cerium oxide polishing film in the polishing process can result in connectors with lower insertion loss and higher return loss. The thickness and refractive index of the modified layer can also be reduced through proper polishing, further improving the connector's performance.
6. Conclusion
In conclusion, the science behind low-loss fiber connectors is closely related to the polishing methods used during their manufacturing. The four-step polishing method, along with the careful selection of abrasives, plays a crucial role in achieving the desired optical performance of fiber connectors. APC and UPC connectors, with their different end face structures and optical characteristics, are suitable for various applications in the field of optical fiber communication. By understanding the principles of fiber connector polishing and the factors influencing the selection of abrasives, manufacturers can produce high-quality fiber connectors that meet the increasing demands for efficient and reliable optical signal transmission. Future research in this area may focus on further improving the polishing techniques and developing new abrasives to achieve even better optical performance and reduce manufacturing costs.
