2024-04-07
Heat-Resistant Optical Fiber Cable
1. Temperature Limitations of Optical Fiber Cable
Conventional optical fiber consists of a core, cladding, and shielding. The core and cladding determine its optical characteristics and are typically made by drawing molten quartz in an environment of 2000°C, providing excellent heat resistance naturally. During the process of drawing quartz glass, tiny cracks are inevitably left on the surface. These cracks can rapidly expand or even cause fiber failure under various environmental stresses during use. Therefore, as soon as the bare fiber is produced, it is coated with a protective layer called the coating, which significantly improves its mechanical properties, making it more resistant to bending and pulling.
The sheath material consists mainly of organosilicon or acrylic resin, which is attached to the bare fiber using processes such as thermal setting or UV curing. However, whether it is an organosilicone or an acrylic resin, the temperature of use is below 180 degrees. If beyond this temperature, materials will decompose. Industries such as petrochemical, aerospace, and laser industries place higher demands on the high-temperature performance of optical fibers. Therefore, breaking the temperature constraints of the sheath can greatly expand the application scenarios of optical fiber cables.
The significance of heat-resistant optical fiber cable lies in its ability to maintain stable transmission ability in extremely high-temperature environments, which can solve the problem of easy failure of ordinary optical fiber cables in high-temperature environments. The advent of this kind of fiber has greatly expanded the applications of fiber optic communication, especially in industries such as petrochemicals, energy, metallurgy, automotive, aerospace, and others that require long-term operation in high-temperature environments.
According to international understanding, the application scenarios of heat-resistant optical fibers are quite extensive. For example, in oil and gas production, optical cables for high-quality temperature measurement must withstand underground environments with high temperature and high pressure, which requires the use of heat-resistant optical fiber cables. In the thermal power industry, real-time monitoring of boiler temperature and pressure also requires the stable transmission of heat-resistant optical cables.
In addition, in the automotive industry, heat-resistant fiber optic cables are used in on-board communication and entertainment systems to ensure stable information transmission in high-temperature environments such as engines and exhaust systems. In the aerospace field, there is a high demand for the high-temperature resistance of communication equipment, and the use of heat-resistant optical fibers can improve the reliability and stability of communication equipment in high-temperature environments.
2. High-Temperature Optical Fiber - Polyimide
Polyimide (PI), with its outstanding temperature range of -190°C to +385°C, has permeated all aspects of our lives since its commercialization by DuPont in 1961. For instance, flexible printed circuits (FPC) commonly used in electronic products are made of polyimide as their substrate because they need to be involved in lead-free soldering at 280°C. Additionally, polyimide is spun into fibers and woven into fabrics, which can be found in the gear of firefighters, astronauts, and race car drivers.
The key to achieving high-temperature resistance in polyimide lies in its unique molecular structure. Polyimide molecules contain multiple benzene rings and conjugated bonds, which make the molecular structure relatively rigid. At the same time, the covalent bonds between the acyl groups and nitrogen atoms in the molecule are very strong, giving polyimide excellent thermal stability.
Polyimide has a high thermal decomposition temperature. Some specific types of polyimide, such as biphenyl tetracarboxylic dianhydride-p-phenylenediamine (BPDA-PDA), can have thermal decomposition temperatures exceeding 600°C. This high thermal stability makes polyimide an ideal coating material for manufacturing heat-resistant optical fiber cables, significantly expanding the fiber's temperature range of application. Optical fiber cables made with this material are often referred to as PI fibers.
Mass production of PI fibers is not an easy task. Typically, fiber coating requires both inner and outer layers: the inner layer has a low modulus for buffering, while the outer layer has a high modulus for protection. Polyimide does not seem to possess these characteristics. Common practices either sacrifice their mechanical properties and use polyimide for a single coating, or use traditional acrylic resin for the inner layer and polyimide for the outer layer to withstand immediate high and low temperatures. Additionally, the curing process of polyimide is not as mature as traditional coatings, so it cannot adhere uniformly and firmly. Therefore, only a few manufacturers worldwide can provide polyimide, and the prices are generally higher.
The process of depositing polyimide on the surface of optical fiber typically involves shielding technology. One common method is the dip shielding process. In this process, the bare fiber is slowly immersed in a polyimide solution, ensuring full contact between the fiber and the solution. Then, the fiber is pulled out from the solution at a controlled speed to control the thickness of the coating. The surface tension and viscosity of the polyimide solution are carefully adjusted to achieve a smooth shield. After shielding, the fiber is cured at elevated temperatures to cross-link the polyimide molecules and enhance the shield's mechanical properties.
3. Advantages and Obstacles of Heat-Resistant Optical Cables
The development of heat-resistant optical fibers has provided new opportunities for various industries that require reliable communication in high-temperature environments. These fibers have a number of advantages over conventional optical fiber cables:
(1) High Temperature Resistance: Heat-resistant optical fibers can withstand much higher temperatures without significant deterioration or failure. This allows them to work in environments where conventional fibers are not suitable.
(2) Reliable transmission: The optical performance of heat-resistant fibers remains stable even at high temperatures. They can maintain low signal loss and high data rate, providing reliable communication in extreme environments.
(3) Extended Application Range: Heat-resistant fibers expand the applications of fiber optic communication, allowing them to be used in industries such as petrochemicals, power generation, metallurgy, automotive, aerospace, and more. They facilitate real-time monitoring, data transfer, and communication in high-temperature environments.
Despite the benefits, heat-resistant optical fibers also face obstacles:
(1) Manufacturing complexity: The production of heat-resistant fibers requires special protection processes and materials. The deposition of materials such as polyimide on optical fiber cables is challenging and requires precise control of shielding thickness, uniformity and adhesion.
(2) Limited availability: Currently, only a few manufacturers worldwide are able to supply heat-resistant optical fiber cables. The production volume is relatively low, which leads to higher prices compared to conventional fibers. Increased demand and improved production methods can help increase availability and reduce costs in the future.
(3) Mechanical properties: Heat-resistant optic fiber cables may have lower mechanical strength compared to conventional fibers due to the challenges involved in achieving a strong and uniform shielding. Providing flexibility and protection in coating applications remains a technical obstacle.
Coping with these obstacles and further improving the performance and availability of heat-resistant fiber optic cables will contribute to further research and development in this area. As technology advances, we can expect more reliable and cost-effective solutions to satisfy the growing demand for reliable high-temperature communications.
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