Gigabit in-vehicle communication systems using AGF-harness
Compared to Butt-coupling, it has been found that EBO exhibits higher insertion loss and significant manufacturing variations due to a large optical-coupling area, regardless of the sufficient tolerance against the dust. The next generation MOST system requires the gigabit communication. In the case of EBO, it was revealed that the performance capabilities of the fiber optic transceiver (FOT) are limited and the allowable loss-budget for the optical harness decreases significantly. The butt-coupling method has a proven optical stability and reliability. However, automakers have pointed out the inadequacies of dust tolerance although Butt-coupling meets all the requirements of vehicle environmental testing.
In this study, Sumitomo carried out an experiment to compare the dust tolerance of optical harness connectors on experimental data gathered from several optical coupling methods. The experiment verified the assumption that the feasibility of a high-speed communication system and dust tolerance could be improved. Thus, Sumitomo introduces their approach toward the realization of gigabit communication that remains very stable under the severe environmental conditions of vehicles. The experiment will hopefully contribute to further development of automotive high speed communication systems.
Improvement of environmental tolerance characteristics
Automotive components require high reliability and durability to withstand dense humidity, higher/lower temperatures, contaminants or external forces under severe stress exposure. In a particular use, strict requirements have been established which are intended for testing under the combined environmental stress of vehicles. Because the AGF-based optical harness had never been adopted for automotive use, comprehensive evaluations for environmental tolerance were required. There are no significant differences in methods and requirements for evaluations of the environmental tolerance in different countries although evaluation standards vary from country to country according to the automotive environmental conditions. At the MOST Forum 2013, Sumitomo reported that the AGF solution has been developed by Sumitomo to meet IEEE and European standards.
However, some issues and problems were addressed concerning the use of specialized components called “AGF”. For AGF, thin optical fibers, 125 microns in outer diameter and 50-80 microns in a core diameter, are used as the transmission medium (50, 62.5, and 80 microns are standardized according to the types of AGF). The optical coupling is configured to send optical signals between both fiber cores. The optical coupling, based on plastic optical fiber (POF) 970 microns in a core diameter, may not lead to a significant loss regardless of the entry of dust. As for AGF, the optical signal cannot be sent if dust gets into the fiber cores, and therefore a system failure may result. The EBO introduced by MOSTCO is a technology to expand the diameter of the output beam of the fiber optics based on AGF, and it is expected to use AGF optical cables as the POF cables. As shown in the EBO structure specified in Fig.1, it is inserted into the ferrule. The light beam of the optical signal from the fiber is expanded or concentrated, and thus extremely high accuracy is required for the end-surface of AGF to attach EBO lenses. Moreover, measures should be taken to prevent misalignment of AGF and EBO lenses under the severe conditions found in vehicles. It is difficult to achieve volume production savings for EBO ferrules, which are combined with the optical components, and a lot of difficulties arise in the coupling of optical fibers within the ferrule. Therefore, the Butt-coupling (Fig. 2) has been examined by Sumitomo.
Fig 1 / Fig 2. For high resolution, click here.
The Butt-coupling is among the well-proven LC technologies that have been widely adopted in the optical communication system for consumer use. The structure of optical coupling is simple: AGF-based ferrules are physically in contact with each other. Thus, advanced technologies are required to achieve high dimensional accuracy because of the thin core diameter of AGF. The optical transmission efficiency is stabilized by LC technologies (LC ferrule/sleeve) under the severe conditions of vehicles (Table.1).
Concerning the intrusion of dust into the coupling part, the test item has not been listed in the table above. Accordingly, Sumitomo has newly developed the relative comparison methods. The purpose of connect/disconnect testing of the optical coupling connectors in a dusty environment is to compare the increasing tendency for optical loss in different optical coupling conditions and optical connector structures for a predetermined cycle / amount of time.
Study on relative comparison methods
Arizona road dust (ISO 12103-1_A2/Fine) and Kanto-loam dust (JIS Z 8901_Class8: Japan) are used as a standard for automotive testing (Photo 1, Table 2). The first case indicated SiO2-rich components and the latter case showed that volcanic soils were included in contaminants with components such as SiO2, Al2O3 and Fe2O3. If dust particles were smaller than 20 microns, the weight is expected to be half of the total dust weight. The experimental device is shown in Fig.3. Using this device, dust was mixed using compressed air and dust particles were circulated in the air inside. Optical connectors were then connected and disconnected according to the specified test conditions: (1) Ferrule outer diameter and inner diameter, (2) Connector housing with the protection of the end-surface of ferrules, (3) Connector housing with the shutter were provided as the test samples. Subsequently, POF connectors were evaluated for reference.
Fig.4 shows the comparison result. (1) Dust particles are very likely to intrude when the ferrule outer diameter/sleeve inner diameter becomes larger. (2) The recessed ferrule resists adhesions of dust particles onto the end-surface of the ferrule. (3) Dust particles are less likely to adhere to the covered end with the shutter. (4) Dust particles are much likely to adhere to the surface due to larger sizes of the ferrule outer diameter/sleeve inner diameter of POF connectors. Compared to the other sample group, the slope is less steep, which shows slow increases in optical insertion loss.
We conducted a behavioral analysis of dust particles according to the specified test conditions including dust concentration. When inserting the ferrule into sleeves, dust particles were moved by natural convection of air. However, dust was less likely to remain on the end-surface of ferrules. Therefore, the probability of dust adhesions from the end-surface of ferrule to the core is extremely low. (Table 3)
As a result, Sumitomo observed that there are no concerns about airborne dust when handling the optical connectors because of the extremely low probability of dust inclusions into connectors while mating optical connectors under 20 microns. In this experiment, it is showed that an optical connector with a shutter has the best dust tolerance. This result revealed that “Recessed ferrule: optical connectors with protection of the end-surface of the ferrule”, introduced by Sumitomo, has the sufficient dust tolerance (Fig. 4: Green line of “butt-coupling Recessed end”).
Gigabit communication system designs
To realize FOT designs to perform conversion or inversion of high-speed electrical signals into optical signals, various techniques are necessary. The vertical-cavity surface-emitting lasers (VCSEL) are used as light sources and thus it requires lowering the output level of the optical signals in order to meet eye safety standards (IEC 60826-1_Class-1). For gigabit communications, the sensitivity level of the optical signal receiver element (PD:Photo Diode) cannot be set to higher due to limited allocations of power budgets for the optical wire harness. When further margins are considered to maintain sufficient reliability in a vehicle environment to realize 5-gigabit optical communications (Fig.5), Sumitomo estimated that the power budgets for Butt-coupling would be about 5 dB (TP2-TP3). As for FOT and Butt-coupling, the VCSEL beam is concentrated at the end-surface of AGF where special lenses are faced with each other, and thus the transmitted light beam is expanded through the lenses. The light intensity will not be increased to the level specified in IEC 60826-1 (measurement point is 700mm away from the output port) and the VCSEL output level can be set relatively high. As a result, sufficient power budgets can be allocated for the optical communication systems as shown in Fig. 6.
The EBO loss is estimated to be 2.5 dB for each, because it has 0.7dB as the nominal loss and 1.8 dB as the tolerance. (MOST Forum 2013, TE-Connectivity: report) The power budget would only be 1.7 dB when using EBO, although some margins were narrowed to increase the budgets allocated to the optical wire harness. VCSEL light beams are collimated at the EBO lenses. To reduce the optical output from FOT or the end-surface of AGF ferrule to the level specified in IEC 60826-1, VCSEL output should be intentionally reduced. (Fig.7) The result shows that the optical communication system (Fig.5) cannot be established if the EBO coupling is used.
To consider allocations of power budgets for optical communication systems, physical layers design (include the PHY-IC) specifications should also be examined carefully. Within the limited budget allocations, margins should be decided upon by well consultations. If all the necessary budgets are taken for the optical wire harness (each component), it is not possible to maintain enough of a margin for the optical transceiver. This may lead not only compromising feasibility but also to deterioration in the reliability of the entire system.
Conclusion
In our approach toward advancing development of the gigabit optical communication system using AGF technologies, Sumitomo reviews and categorizes the optical coupling method and optical connector structure by verifying them from the aspects of both FOT development and optical wire harness.
Despite its many benefits for items proposed to develop the next-generation MOST systems, it has not been promoted without consideration of the effect on the system as a whole. That is why Sumitomo came to its conclusions, which include optimizing the AGF, Cables, Connectors, and FOT according to these experimental results. Sumitomo believes that these activities will be able to contribute to promoting further improvement to the gigabit automotive communication systems.
About the authors:
Hayato Yuki
works as Chief Engineer at optical communication project of AutoNetworks Technologies, Ltd.. Since 2000 he joined AMI-C, 1394TA and had been making the physical standard of automotive network.
Takashi Fukuoka
works as Project Manager at Transmission Device R&D Laboratories of Sumitomo Electric Industries, Ltd.. Since 1983 he has been engaged in optical transceiver development.
Satoshi Endo
works as Assistant Senior Manager at Harness Connecting Component R&D Department of AutoNetworks Technologies, Ltd.. Since 1985 he has been engaged in optical connection development.
Kiyoshi Kato
works as Assistant manager at Information network R&D of AutoNetworks technologies, Ltd. Since 2005 he has been engaged optical transceiver development.
Article previously published in Elektronik automotive, MOST Special Edition, May 2014, www.elektroniknet.de