Visible Optical Fiber Communication
1 Special Issue Visible Optical Fiber Communication
Visible Optical Fiber Communication Review Manabu Kagami
Abstract This paper provides an overview of recent optical fibers, optical circuits and transceiver developments affecting in-vehicle optical modules, are explained. We consider that the use networks. Visible light sources are now being of a wavelength division multiplexing (WDM) used in this field, bringing advantages in both technology is promising for realizing further visibility and workability. In particular, current advances in high-speed communications. We will trends and subjects affecting the key devices used briefly introduce the optical devices for WDM in these networks, such as visible light sources, that have been developed in our research group.
Keywords Automotive network, Optical network, LED, Waveguide, WDM communication
R&D Review of Toyota CRDL Vol. 40 No. 2 2
1. Introduction in actual vehicles. It shows that the rate is steadily increasing by about ten-fold every five years. Thus, In the past decade, broadband-communications improvements in network speed will still continue, technology using optical fibers has spread quickly and advanced features and reduced costs for metal- through Internet lines and cable TV lines to the wired systems are being energetically pursued, so it home. is considered that competition with optical fiber will This is a result of the cheaper cost for optical fiber continue for the time being. At present, for high communication systems that realize the transmission speed applications of more than 10 Mbps, in-vehicle speeds required by those services when compared network standards are mainly being discussed in with copper wire-based systems. Acute competition terms of using optical fiber, as shown in Fig. 2. within the field of electronic communications is still Among these optical network standards, the system continuing in these applications. known as ByteFlight (data rate 10 Mbps)4) has been It is now believed that the amount of information adopted for control systems, and the Digital that will need to be transmitted in the field of Domestic Bus (D2B)(5.6 Mbps) and the Media- automobile technology will increase rapidly in the oriented System Transport (MOST)(22.5 Mbps) near future, not only in information systems, but also standard5) for information systems have been 1) in control systems and safety systems, and an adopted for actual in-vehicle networks. Though increase in network data rates is urgently required. MOST2 (50 Mbps) is under discussion as a next- In several European vehicles, the provision of generation standard to follow the MOST standard, optical networks has already started in luxury grade models,2) and it is considered that a sharp turning point from metal networks to optical networks will 1G arrive in the near future. 100 M Optical
Projected In this review, we introduce trends in transceiver 10 M technology for automobile optical LANs and give a 1M1 brief overview of research activity at the Toyota 100 k Central R&D Labs. Electrical
Data rate (bps) 10 k 2. Optical networks for automobiles 1k1
2. 1 History 0.1 k 1980 1985 1990 1995 2000 2005 2010 The increased use of electronics in automobiles Installation (year) has seen the installation of multiple networks in modern cars, and the expansion of these systems has Fig. 1 Transition in data rates (actually installed and quickly accelerated. There are three principle projected) for in-vehicle networks. networks that are used in cars, known as the information, control, and body systems. The information system deals mainly with entertainment IEEE1394 devices such as in-car navigation and DVDs, while 100M MOSTX MOST2 Optical MOST the control system deals the devices like cruise- 10M ByteFlight D 2 B-opt control and other safety systems. The body system FlexRay 1M includes networks for switches or actuators. In CAN (high) 100k CAN (low) CAN (IDB-C) recent years, increases in the network data rate for Data rate (bps) J1850 IE-BUS Electrical information and control systems are remarkable, and 10k BEAN (AVC-LAN) some European car makers have now adopted an optical fiber network since 1998.3) Figure 1 shows Body Control Information the transition in data rates that have formed the specifications adopted for the network systems used Fig. 2 Network standards for the automotive industry.9)
R&D Review of Toyota CRDL Vol. 40 No. 2 3 there are two working groups using optical or conventional red LEDs in many respects, including a electrical protocols within the MOST consortium, lower loss band in POF (see Fig. 3), lower and competition has also occurred here. IDB13946) deterioration of emitted power in high temperature has also been discussed for next generation in- environments and excellent in high-speed operation, vehicle information network systems since 2000. the large-scale adoption of blue/green devices is This standard features compatibility with IEEE1394 expected in the near future (the details are described (125/250/500 Mbps), which is a wide-spread in this special issue).11) standard for digital household appliances. On the For high-speed serial communication at more than other hand, the standardization organization AMI-C 1 Gbps, the combination of an LD and a graded (established in 2001)7) which was founded by the refractive index-(GI)-type broadband optical fiber is automobile manufacturers, is also carrying out required. However, the wavelength of VCSELs original initiatives. These three high-speed (Vertical Cavity Surface Emitting Laser)10) that have standards are expected to appear successively in the been developed as low-cost LDs for data communi- near future. cation is more than 780 nm, and transmission over In the field of control systems, FlexRay8), which 10 m in conventional PMMA-based POF is imposs- has a maximum data rate of 10 Mbps, is being ible due to high absorption loss in the fiber. discussed as the global standard to replace CAN, and Although both GI-type silica fiber and GI-type the development of a network that promotes X-by- fluororesin-based POF have been identified as wire technology is expected. If FlexRay is put into having some potential for transmitting such wave- practical use, it is believed that the development of lengths over 10 m at 1 Gbps, they have the disadvantage safety and comfort-based driving systems will be of leading to a steep increase in system costs. given a spur forward, especially in relation to on- In terms of installation demands, because optical board camera systems and semi-automatic cruising fibers are typically assembled into the bundle that systems for accident prevention. forms the electrical wiring harness (Fig. 4), a 2. 2 Visible light sources and optical fiber for minimum-bending radius of about 15 mm is required automotive networks in order to minimize the influence of transmission In public networks and premise-based local area loss. In each of the standards under consideration, in networks (LAN), the demand for high-speed and order to suppress the deterioration of transmission long-distance transmission is high, and a combi- power due to bends with a small radius, it has been nation of infrared laser diodes (LD) and silica determined that a large refractive-index difference optical fibers has mainly been used. On the other between the core and the cladding is required. In hand, since in-vehicle networks are only employed POF with a diameter of 1 mm, a numerical aperture for relatively short distances at low speed (<50 Mbps), a combination of red LEDs and plastic optical fiber (POF) has been used.10) The use of 1000 POF has been adopted for the following reasons; . Enables low cost network systems (Physical layer), 100 . POF is low-loss in the red region of the spectrum, . Since the diameter of POF is about 1mm, its coupling efficiency with LEDs (emitting area about 300 µm square) is high. loss (dB/km) Transmission 10 300 400 500 600 700 800 The commercialization of GaN emitting devices Wavelength (nm) has recently started to occur, and blue and green LEDs are now being studied as potential light Fig. 3 Transmission loss of PMMA-based POF sources for optical communication systems. In fact, and the emitting wavelength of several of because LEDs of these colors are superior to the LEDs discussed in this special issue.
R&D Review of Toyota CRDL Vol. 40 No. 2 4 of between 0.5-0.65 is considered as the minimum of optical circuits have already been proposed in the requirement. field of automotive engineering. Figure 5(a) shows 2. 3 Optical circuits and transceivers an example designed for half-duplex operation Optical communication technology is spreading (alternate communication between both transceivers) through many consumer-oriented electronic that was introduced in the BMW 7-Series. In this products, and cost reductions in these systems are in optical system, a red LED chip is mounted on a strong demanded. Many of these devices are quite large-area photo diode (PD) chip using "chip-on- compact, and it is often difficult to find the space to chip technology".4) Since the LED does not emit connect them using large-sized optical connectors. light while the PD is receiving a signal, electrical Moreover, cables that are easy to install (high and optical crosstalk theoretically do not arise. The flexibility of bending) are required. Due to these device can operate with a maximum transmission demands, systems that offer bi-directional communi- rate of 10 Mbps in the ByteFlight14) system, and cation over a single optical fiber have been proposed higher speed devices have also been investigated.4) for many of the standards that operate in these An optical module for full-duplex communication different fields. In these bi-directional systems, that uses the principle of dividing light beams is also since the amount of optical fiber required and the under investigation.15) Because the outgoing beam space needed for the connectors can be reduced by from a VCSEL has strong directivity, the outgoing half, large cost savings can be expected.13) beam from a VCSEL can even couple efficiently Moreover, because this system easily reduces into an optical fiber after reflection at a micro- installation costs and device costs, its use in mirror, as shown in Fig. 5(b). Conversely, about automobile applications has been studied widely. half of the weak-directivity outgoing beam from the Generally speaking, full-duplex single-fiber bi- fiber can be coupled directly into a large PD; the directional communication systems suffer from remainder is lost due to reflections at the mirror. troublesome optical crosstalk between the trans- This device seems to be a simple structure, but it mitted signal and the received signal, which can requires the implementation of precise mounting become a serious problem. Fortunately, since the technology and has issues in terms of both electrical- communication distance is usually short in and optical crosstalk due to proximity arrangement. consumer-oriented electronic products, the signal-to- Especially, the optical crosstalk is strongly noise ratio becomes relatively large and this problem can be solved relatively easily. POF Large area PD In order to realize optical transceivers for single LED fiber bi-directional communications, an optical circuit to separate and to combine the transmitted Chip-on-chip light and the received light is needed. Various types Lead frame Side view Chip front view (a)
VCSEL
PD Embedded fiber
(b) Micro-mirror (a) (b) Fig. 5 Examples of devices constructed using bi- Fig. 4 Photographs of (a) a wiring harness12) and 9) directional optical modules for automotive (b) an assembly plant. applications.4, 15)
R&D Review of Toyota CRDL Vol. 40 No. 2 5 influenced by the device surface arranged in the modification. neighborhood and directivity of the light source. The On the other hand, WDM communication is a optimal arrangement which realizes large tolerance method of simultaneously transmitting light-signals and a small crosstalk is a subject for future studies. using different assigned wavelengths through a 2. 4 Multiplexing communications and future single optical fiber, as shown in Fig. 6(b). Since the developments nature of light prevents the mixing of different Drastic cost reductions cannot be expected by wavelengths, if wavelength-separation can be simply replacing metal cables with optical cables. In achieved using a prism or a color filter at the end of order to promote the conversion to optical cable, it the fiber, then the light can be easily separated into needs to be accompanied by additional merits, such signals of different wavelengths. In a WDM system, as a drastic reduction in the amount of wiring multiple signals simply share the same optical fiber, required and a saving in weight by using optical and there is no necessity to match the data format fiber technology. For this reason, time-division between signals (data rate, coding/encoding, multiplex (TDM) communication and wavelength analogue/digital, etc.). Therefore, a protocol for multiple (WDM) communication systems have been multiplex communication (a different protocol is proposed. assigned and multiplexed for different wavelengths) In TDM, a high-speed optoelectronic device, a by WDM becomes possible without the need for clock-conversion device and a communication complex and costly data-format-matching control device are needed, which counterbalances (arbitration free). However, this system requires the effects of reducing the amount of wiring. For multiple light sources of different wavelengths and example, we will now consider multiplexing the costly multiplexers and de-multiplexers (mux/demux) three different digital signals shown in Fig. 6(a). In for those wavelengths. this case, each bit of data must merge into a pre- Although both of these transmission systems have determined data stream at a data rate that is more already been put into practical use in public than three times higher, after conversion of the data networks, their application to automotive networks into the same format. Also, the use of a buffer and can be difficult in terms of both durability and cost. associated hardware are needed for these clock 3. Brief description of our activities conversion devices, and we are anxious about raising the cost of the electronic components that are As mentioned above, in the field of in-vehicle required. In addition, the reliability of using TDM networks, several network systems involving to combine the signals can be variable, and there are differing specifications have been rapidly expanded. many issues that still need to be solved, such as In order to realize lower costs and to reduce the fixing system specifications and security levels, not amount of wiring required in high-speed networks, to mention the complexity involved in system we believe that sharing of the communication line
Signal A B C Optical fiber Clock E/O O/E Clock conversion conversion Clock Clock multiplexer de-multiplexer TDM
E/O Optical fiber O/E converter Multiplexer De-multiplexer converter Signal A DM B C D WDM
Fig. 6 A comparison of transmission systems using TDM and WDM.
R&D Review of Toyota CRDL Vol. 40 No. 2 6 by several signals that do not interfere with each Drive the Change of Datacom Applications towards other is a promising approach for unifying the POF", Proc. of 13th Int. Plast. Opt. Fibers Conf., (2004), 378, (ICPOF) various communication standards (Fig. 7). We 2) Baierl, W. : "Evolution of Automotive Networks", believe that a system based on WDM is the most Proc. of 10th Int. Plast. Opt. Fibers Conf., (2001), suitable solution, and we consider that this system is 161, (ICPOF) also promising in terms of future expandability 3) Seidl, D., et al. : "Application of POFs in Data Links of Mobile Systems", Proc. of 7th Int. Plast. Opt. (number of wavelengths, data rate). Fibers Conf., (1998), 205, (ICPOF) In general, although LDs are used in high-speed 4) Baur, E. : "Shifting the Borders: POF Transceiver for optical communication systems, the use of LEDs is High Temperature Applications for 200 Mbit/s preferable in automobile applications in respect of Bi-directional Half Duplex Ddata Transmission", Proc. of 11th Int. Plast. Opt. Fibers Conf., (2002), stability in high-temperature environments and low Post Deadline paper, (ICPOF) system costs. However, since high-speed operation 5) Seidl, D. : "Physical Layer Aspects of the New and the range of available wavelengths are limited, Automotive Data Bus MOST," Proc. of Int. POF the practical implementation of WDM using LEDs Tech. Conf., (2001), 138, (Information Gatekeepers) 6) ex. available from
CAN Manabu Kagami Optically multiplexing Research Field : Optical communication data bus Flex ray devices Academic degree : Dr. Eng. Academic society : Inst. Electron., Inf. Commun. Eng., Inst. Electr. Electron. Eng., Opt. Soc. Am., Fig. 7 Basic concept of automotive multi-protocol Jpn. Soc. Appl. Phys., Jpn. Inst. communication using WDM. Electron. Packag.
R&D Review of Toyota CRDL Vol. 40 No. 2