High-Bandwidth, Rugged Fiber Finds Novel Automotive Applications

Robert Dauphinais, OFS Specialty Photonics Div.

In the rapidly developing field of fiber optics, the life span of any product is often measured in years, not decades. Yet hard-clad silica fiber has been around for nearly 30 years and is an industry standard among large-core, step-index optical glass fiber. In fact, although hard-clad silica fiber is OFS’ proprietary design patented and manufactured since 1979, in the past year many other manufacturers have started churning out silica clad in hard plastic to meet the growing needs.

Figure 1. A cross section of hard-clad silica fiber shows the glass core, the fiber cladding and a protective buffer layer of ETFE.

In hard-clad silica fiber, the polymer that surrounds the core acts not only to guide light, but also to enhance the strength of the fiber. The polymeric material forms “bridge bonds” that heal small flaws on the surface of the silica.

The cladding also significantly retards static fatigue and makes possible tighter bend radii. Hard-clad silica fiber can withstand a bend of less than an inch for 20 years.

The combination of flexibility and ruggedness has led to the use of these fibers in the medical, industrial, environmental and commercial fields, including such unusual and diverse applications as wind turbines and casinos. Most intriguingly, they are being adopted into use in novel automotive applications that adhere to the Media Oriented System Transport (MOST) protocol, which uses both hardware and software to enable the transmission of a variety of data from diverse vehicle systems.

Founded in 1998, the MOST Cooperation is a partnership of about 20 automakers and 60 component suppliers who together seek to define and adopt a common multimedia network protocol. The organization uses its set of standards to encompass new classes of multimedia applications in automobiles, such as advanced sensors, global positioning system navigation with intelligent route finding, integrated mobile communications and multimedia entertainment.

These systems, now seen primarily in higher-end vehicles, are expected to become standard across many lines in just a few years. The result will be an exponential increase in the demand for cost-effective, high-performance data transfer that exceeds the capacity of conventional automotive wiring systems. Instead, these applications will be made possible by using optical fibers that transmit signals at a rate of 25 Mb/s.

Figure 2. Hard-clad silica fibers manufactured in a low-bioburden room under sterile conditions are used in the human body for a variety of medical procedures — for example, to deliver light and laser power, and to cut, coagulate or vaporize tissue.

Step-index hard-clad silica fibers, which exhibit overall data-transmission performance that falls between that of plastic optical fiber and that of graded-index fibers, are well suited for applications supported by MOST. As with all optical fibers, they are unaffected by the presence of electromagnetic and radio-frequency disturbances; however, it is the harsh automotive environment — with its extremes of temperature and vibration — that demonstrate their advantages over plastic optical fibers.

The mechanical durability of hard-clad silica fibers, including bend radius, tensile strength, crush strength and cyclic flex, are superior to those of plastic fibers, but it is the temperature performance that marks the most important difference between the two fiber types. Plastic fibers used for data links are formulated from polymethyl methacrylate, which keeps the upper limit of the fiber’s operating temperature to 85 °C, essentially confining its use to applications within the passenger area of the automobile. Plastic fiber also has a high attenuation level (~150 dB/km) that restricts the fiber’s length and its minimum bending radius (typically >25 mm).

Hard-clad silica fibers also have excellent resistance to heat, moisture and fatigue, which means that they help provide the durability and long-term reliability essential for automakers seeking to control warranty-related costs.

As a class of optical fiber available from several manufacturers, hard-clad silica continues to provide the necessary bandwidth and physical properties for many applications. The MOST protocol is just the latest example of how this type of fiber continues to be the choice for applications that require a balance of performance characteristics and the use of low-cost connectors and optical transceivers.

Meet the author

Robert Dauphinais is market manager for industrial products for OFS Specialty Photonics Div. in Avon, Conn.; e-mail: bdauphin@ofsoptics.com.