Toyota CRDL Demonstrates Inexpensive Bidirectional Fiber Communications

Brech Hitz

Plastic optical fibers are beginning to replace electrical wiring harnesses in many automotive applications because of their low cost, light weight and immunity to electromagnetic interference. Recently, engineers at Toyota Central R&D Laboratories (CRDL) Inc. in Nagakute and at Toyoda Gosei Ltd. in Inazawa, both in Japan, demonstrated an inexpensive, bidirectional system capable of transmitting 250 Mb/s over 20 m of plastic fiber.

Figure 1. The small plastic multiplexing modules are connected with up to 20 m of plastic optical fiber. Images ©OSA.

The system comprises two small (8 × 9 × 7 mm) plastic multiplexing modules and the plastic fiber to connect them (Figure 1). Each module has an LED (one module’s is red, and the other’s is green), a photodetector and a filter to separate the two wavelengths. The filter transmits about 85 percent of the light from the green LED and reflects about 96 percent of the light from the red one.

Figure 2. Fabricating the multiplexing modules was straightforward in thelaboratory (see text for details), and presumably could be fast and inexpensive in an industrial environment.

Fabrication of the multiplexing modules was straightforward and inexpensive. First, the engineers inserted one end of a plastic optical fiber and the wavelength filter into precut slots in a small plastic enclosure (Figure 2a). They filled the enclosure with a photopolymerizing resin (Figure 2b), which they hardened to form a waveguide by exposing it through the plastic fiber to 5 mW of laser light at 457 nm (Figure 2c). The filter reflected about half the incident 457-nm light, so two branches of the waveguide were formed. The waveguide has a constant diameter because the laser light was self-trapped by total internal reflection as it formed the waveguide. An exposure time of approximately 30 seconds was sufficient to form the waveguide.

Finally, the engineers removed the unhardened resin with a solvent (Figure 2d), refilled the enclosure with a low-index ultraviolet resin and hardened it with ultraviolet light (Figure 2e). The result was a compact, rugged, transparent module, which they subsequently subjected to bit-error-rate testing (Figure 3).

Figure 3. The small plastic multiplexing modules are compact, rugged and transparent.

They measured the insertion loss of the modules to be 2.1 dB for the light emitted by the green LED (λ = 495 nm) and 2.2 dB for the red (λ = 650 nm). But because crosstalk from the local LED in each module was too great, they had to add a filter in front of the photodetectors before measuring the error rate. They expect that design improvements will remove the need for these filters.

At 250 Mb/s and with the extra filters in place, they measured at a photodetector power of −20.6 dBm for a bit error rate of 10⁻¹² for the red LED, and a similar photodetector power of −17.4 dBm for the green LED.