Fiber Optic Harnesses

01 Technical Overview of Fiber Optic Harnesses

Fiber optic harnesses use light waves as the carrier and optical fibers as the transmission medium and possess advantages such as high speed, high reliability, low loss, and electromagnetic interference resistance. Their transmission rate is much higher than that of traditional copper wires or coaxial cables, and they can meet the communication requirements of in-vehicle systems for large amounts of data and high real-time performance. The optical fiber communication link mainly uses optical connectors and optical fiber carriers to achieve high-speed signal transmission.

1.1 Optical Fiber Connectors

Optical fiber connectors are optical passive devices that realize the movable connection between optical fibers. They are mainly composed of parts such as optical fiber reinforcement, optical fiber alignment, elastic docking, head and seat locking, optical cable fixing, pin anti-rotation, and optical cable buffering (see Figure 1). Optical fiber connectors usually use ceramic pins and ceramic sleeves for optical fiber alignment, and ensure the elastic docking of the pin ends through springs. The spring is in a pre-compressed state before docking, so that the pin will not move under the locking force. During docking, secondary compression will occur due to the retraction of the pin, and the elastic force will be fed back to the pin so that the two docked pins are always in a state of force contact and compression during the docking process.

1.2 Optical Fiber Conductors

Optical fibers are cylindrical and mainly composed of the core, cladding, and coating layers (see Figure 2). The transmission principle of optical fibers utilizes the phenomenon of total internal reflection of light, that is, when light is incident from an optically denser medium (with a relatively higher refractive index) to an optically rarer medium (with a relatively lower refractive index), if the incident angle is greater than the critical angle of total internal reflection, the light will no longer be refracted but will be completely reflected bato the original medium, ensuring that the optical signal can be transmitted in the optical fiber without leakage.

Optical fibers have the following characteristics:

(1) Fast transmission speed, long distance, and large amount of content: Optical fibers have a very fast transmission speed, can transmit data over long distances, and can simultaneously transmit a large amount of data.

(2) Immune to electromagnetic interference: Optical fibers transmit optical signals and are not affected by electromagnetic waves, so the signal quality is more stable.

(3) Wide bandwidth: Optical fibers have a very wide bandwidth and can support high-speed data transmission.

(4) Low loss: The transmission loss of optical fibers is very low. Theoretically, it can transmit hundreds of kilometers or even farther without losing signal quality.

(5) High security: Optical fiber transmission of signals does not generate electromagnetic radiation and cannot be eavesdropped on by external electromagnetic waves, so it is more suitable for scenarios with high requirements for data security.

(6) Small size and lightweight: Compared with traditional copper cables, optical fibers are smaller in size and lighter in weight, making them convenient for deployment and maintenance. This advantage is more obvious, especially in large-scale communication networks.

02 Advantages of Fiber Optic Harnesses in the Application of Automobile Intelligence

When the transmission rate of copper wires reaches above 10 GB/s, thicker copper wires will be required to meet the rate requirements. However, in the vehicle layout environment, the thickening of copper wires will increase the overall weight of the vehicle and the cost of the car, and cannot meet the requirements of the vehicle's high-speed and low-weight layout environment. The fiber optic harness technology can significantly improve communication quality and reliability, reduce electromagnetic interference, and at the same time, not add additional weight, improving the safety and stability of the vehicle.

(1) The transmission rate of fiber optic harnesses is much higher than that of traditional copper wires or coaxial cables and can transmit millions of megabytes of data per second, which can meet the communication requirements of modern cars for large amounts of data and high real-time performance. The maximum rate of the traditional copper wire network can only reach 10 GB/s, which makes it difficult to meet the requirements of high-speed transmission.

(2) The transmission loss of optical fibers is extremely low, and the loss per kilometer is usually less than 0.0035 dB/m, which can ensure that the signal still maintains high quality during long-distance transmission. In contrast, the transmission loss of the traditional copper wire network is 0.5 dB/m, and the loss is large during long-distance transmission.

(3) The optical fiber communication technology uses light waves to transmit signals in optical fibers. Compared with traditional copper wire transmission, it has significant immunity to electromagnetic interference.

(4) Compared with traditional metal wires, plastic optical fibers (POF), as a type of automotive optical fiber, can significantly reduce the vehicle's mass and improve the vehicle's economy.

03 Application Scenarios of Fiber Optic Harnesses in Automobiles

Fiber optic harnesses are currently widely used in fields such as the medical field, communication field, Internet field, medical field, and industry. However, their application in the in-vehicle field faces various challenges such as the lack of basic theories, lack of technical specification standards, unclear test standards, and lack of in-vehicle application practices.

3.1 Application of Fiber Optic Harnesses in Layout Areas

The use of fiber optic harnesses in automobiles is different from their use in other fields, and the environmental characteristics of the use area need to be fully considered. For example, areas such as the engine and chassis involve vibration requirements; areas such as the engine compartment and the lower vehicle body need to consider waterproof performance; and high-temperature areas need to consider the high-temperature resistance requirements of optical fiber conductors and connectors. According to the different use environments of the automobile, the layout areas of the entire vehicle can be roughly divided into the wet area, the potential wet area, and the dry area.

(1) The wet area refers to the areas where the wires and connectors are certain to have the possibility of contacting liquids in the normal use scenario, such as the wire harness connectors in the areas outside the passenger compartment such as the chassis and the engine compartment. In rainy and snowy weather, whether during driving or parking, these areas will more or less come into contact with various liquids.

(2) The potential wet area refers to the areas where the wire harness connectors may come into contact with liquids in some daily use scenarios, such as when it rains and the door is opened, a water cup is spilled, frozen items melt, or condensed water drips. For example, the floor of the passenger compartment, the door armrest, the surface of the seat, etc.

(3) The absolute dry area refers to the areas where the possibility of the wire harness connectors coming into contact with liquids is very small in the normal vehicle use scenario, such as the inside of the instrument panel and the inside of the roof. It should be noted that the waterproof and sealing requirements of the wire harness connectors decrease sequentially from the wet area, the potential wet area to the dry area.

3.2 Application Solutions for Fiber Optic Harnesses 

In addition to meeting the electrical performance requirements, the application of fiber optic harnesses in automobiles often also needs to meet the mechanical performance requirements, and it is necessary to consider the temperature grade, vibration requirements, and waterproof requirements of the optical fiber wires. The solutions are as follows:

(1) Optical Fiber Cables: First, select high-temperature resistant materials and optimize the vehicle layout scheme for heat dissipation design. For example, select wire materials that can withstand high temperatures, such as silicone wires and XLPE wires, and these materials can maintain insulation in a high-temperature environment. Second, adopt special processes, such as using double-layer coating and ultraviolet curing technology. Finally, optimize the layout scheme in the vehicle. For example, avoid the path of the engine exhaust pipe and the high-temperature vortex area during the wire harness layout scheme, and obtain a more optimized layout scheme through thermal management. At the same time, high-temperature-resistant and heat-insulating cladding can also be selected for the vehicle wire layout. For example, wrapping the outside of the fiber optic harness with a high-temperature resistant aluminum foil fiberglass tube can effectively ensure the use of the optical fiber cable in the high-temperature environment of the vehicle and improve the aging resistance performance. At the same time, to ensure that the optical fiber cable can be used in the wet area environment, the optical cable can also be designed to have a multi-layer protection structure for effective waterproofing.

Specifically, the outermost layer of the optical cable is usually a plastic sheath, which not only provides mechanical protection but also has a certain waterproof function. Inside the plastic sheath is a metal sheath, which further enhances the pressure resistance and waterproof ability of the optical cable. Inside the metal sheath, there is also a water-swelling water-blocking layer, which is a key barrier to prevent water penetration. Once water invades, the water-blocking layer will expand rapidly to seal the invasion path and prevent the further spread of water. The core part of the optical cable also takes waterproof measures. The optical fiber is tightly wrapped with an ointment and bonded to other components in the core. This ointment not only plays a lubricating role, but more importantly, it can absorb and lock the trace amount of water in the core to prevent the water from damaging the optical fiber. Through this series of multi-layer protection structures, the optical cable can keep the optical fiber dry and stable in various harsh environments and ensure the smooth transmission of communication signals. For example, the in-vehicle optical wiring harness solution of quartz multimode optical fiber released by Longfei Optic Fiber meets the automotive standards in terms of bending (radius 10 mm), tensile strength (150 N), temperature characteristics (-40 °C to 125 °C), aging (125 °C, 3000 h), vibration (V3), etc.

(2) Optical Fiber Connectors: The main methods to improve the temperature resistance of optical fiber connectors include designing a heat dissipation structure and selecting high-temperature resistant materials. Through reasonable heat dissipation design and the use of high-temperature resistant materials, the performance and service life of optical fiber connectors in high-temperature environments can be effectively improved.

First, designing a heat dissipation structure is the key to improving the temperature resistance of optical fiber connectors. Optical fiber connectors will generate heat during operation, and a reasonable heat dissipation design can help reduce the temperature of the connectors and ensure their stable operation. The following are specific methods.

① Reserve metal parts to transfer the heat through the reserved metal parts to help with heat dissipation.

② Design heat dissipation fins: Design heat dissipation fins on the outer shell of the connector to make it better contact with the air and dissipate heat through a fan or natural wind.

③ Use a metal outer shell: Use a metal material with better heat dissipation performance to make the outer shell improve the heat dissipation efficiency.

Second, selecting high-temperature resistant materials is also an important means to improve the temperature resistance of optical fiber connectors. In a high-temperature environment, ordinary materials are prone to aging or deformation, while high-temperature-resistant materials can better resist the influence of high temperatures.

① High-quality engineering plastics: Select high-quality engineering plastics with medium and low-temperature resistance to make the outer shell and internal structure of the connector improve the overall temperature resistance.

② Special alloy materials: Use special alloy materials to make the key components of the connector to improve its high-temperature resistance and anti-deformation ability. For example, Letas Optics has launched a non-contact optical connector, which has the characteristics of high plugging and unplugging repeatability, long plugging and unplugging life, low insertion loss, and low dust sensitivity compared with traditional optical connectors.

③ In the waterproof treatment, heat-shrinkable tubes can be used: Put the heat-shrinkable tube on the connection part and use a heat-shrinkable tube heater to heat and shrink the heat-shrinkable tube. The function of the heat-shrinkable tube is to protect the optical fiber and prevent water and pollutants from entering. Ensure that the heat-shrinkable tube fits tightly on the joint and the optical fiber without gaps. Apply an appropriate amount of sealant on the outside of the heat-shrinkable tube to enhance the waterproof performance of the joint. Ensure that the sealant is evenly applied and fills all possible gaps and pores, and wait for the sealant to dry and harden for sealing treatment.

(3) High-speed connection is achieved through optical fibers, and the electrical connection port is responsible for powering the sensor.

In conclusion, optical fiber communication technology has broad application prospects and great potential in the automotive communication system. In the future, optical fiber communication technology will continue to play its unique advantages and provide strong support for the development of in-vehicle electronic communication systems. At the same time, with the continuous progress and innovation of technology and the continuous improvement of scale, the cost of optical fiber communication in in-vehicle applications will be further reduced, the solutions will be more mature, and it will become the preferred choice to solve the high-speed communication requirements in the development of automobile intelligence.