Fiber optic sensors may soon play a crucial role in preventing the derailment of freight and passenger trains. The sensors monitor the weight of a passing train, defects in its wheels and stress resulting from a cracked or broken rail. Fiber optic sensors developed at the University of Illinois may prevent train derailments. The sensors monitor the weight of a passing train, defects in its wheels and stress resulting from a cracked or broken rail. Shun Lien Chuang of the University of Illinois and his research assistants Alan Hsu and Erik Young have developed a series of sensors based on fiber-coupled semiconductor lasers. Photodetectors record the transmission of the laser light through the fiber on a rail, and a computer identifies and records the location of changes or interruptions in optical transmission. One sensor uses optical time-domain reflectometry to measure the signal loss in the fiber as a function of distance, using optical pulses to pinpoint the site of a broken rail. Another takes advantage of a "microbending" effect to measure the weight and force associated with a train's presence. "The microbending effect is caused by the fact that a bent fiber will have part of the guided light leaking into the surrounding [area], therefore creating a decrease in optical transmission intensity," Chuang explained. Using a loading machine, the team calibrated the optical transmission coefficient as a function of the applied stress. By introducing microbending into the fiber, the researchers can measure additional pressures, such as that of passing rail cars. This sensor has also proved effective in locating flat spots on wheels, because it measures the impact force between the wheel and rail. Work on the sensors, which was sponsored by the Association of American Railroads and the National Academy of Sciences, was begun after the association expressed interest in alternatives to the electronic systems currently in place on most railways. The team selected optical fiber sensors for their insulating properties, light weight, immunity to electromagnetic interference, and sensitivity to strain and stress, but there are additional benefits. "A conventional track circuit depends on the electric current passing through the track," Chuang said. "In some cases, the rail track has only a crack instead of an open, broken gap, [and] the track circuit cannot pick up the broken rail signal since the conduction current path is not interrupted." Fiber sensors can pick up these smaller defects. Some railroad companies have expressed interest in field-testing the fibers, and the research group hopes to make the sensors commercially available in the near future.