Pulse Delivered by Hollow Optical Fiber Creates Spark
Research shows that significant improvements in combustion efficiency can result when the igniting spark in an internal combustion engine is in the center of the cylinder rather than along the walls or near the electrodes, where the ignition kernel can be thermally quenched. One way to get the spark away from the cylinder walls is to use a laser-created spark. However, to be practical, open-path beam delivery should be avoided.
A potential solution is to use an optical fiber to pipe in the high-intensity laser pulse. Although this technique currently is impractical economically for automobiles and other small engines, it does hold promise for large, natural-gas engines such as those used for power generation and natural gas compression.
A coalition of researchers from Colorado State University in Fort Collins, and from Tohoku University in Sendai, Japan, recently demonstrated, for what it believes is the first time, the creation of an optical spark in a gas (atmospheric-pressure air) using nanosecond laser pulses delivered with a flexible fiber. It is more difficult to create a spark in gas than in a liquid or solid, because the optical intensity required is greater.
Figure 1. The 1-m-long hollow fiber transmitted the pulses from a Q-switched Nd:YAG laser. Illustrations ©OSA.
Specifically, power densities on the order of 100 to 1000 GW/cm2 are required, according to previous research. Besides engine ignition, the fiber optic spark delivery may be useful in laser-induced breakdown spectroscopy and in medical applications.
The researchers used a hollow fiber whose inner surface was coated with a reflective silver layer and a layer of cyclic olefin polymer (Figure 1). The fiber, which had an inner diameter of 700 µm, could withstand temperatures of up to ~500 K, making it suitable for use in engine environments. The optical source was a Q-switched, 1.064-µm Nd:YAG laser from Continuum Inc. of Santa Clara, Calif., that produced 8-ns pulses at a 5-Hz repetition rate.
Figure 2. A lens focused the laser light into the hollow fiber, and a pair of lenses focused the light emerging from the far end to a small spot where the spark was formed.
A lens at the input end of the fiber focused the laser pulse into the fiber (Figure 2). A pair of lenses at the fiber output focused the pulse exiting from the fiber to a spot where the spark was generated. At high pulse energies, the researchers observed gas breakdown at the fiber input, an obstacle that they believe could be avoided by operating in a vacuum.
Figure 3. A photograph captures the spark delivered through the hollow-core optical fiber.
They experimented with different lenses at the input end of the fiber, all of which were sufficient to focus the incoming pulse to less than 700 µm, the inner diameter of the hollow fiber. They found that the lens that produced the most-collimated beam entering the fiber -- that is, the weakest lens that got all the light into the fiber -- produced the most reliable sparking at the other end. That was because the most-collimated beam that entered the fiber produced the most-collimated beam exiting the other end, and the most-collimated beam exiting the fiber was focused to the smallest spot by the two focusing lenses.
In this case, the intensity of the focused spot was approximately 300 GW/cm2, and 97 percent of the pulses created a spark. The transmission through the fiber decreased from ~80 percent for low-energy pulses to ~70 percent for the highest-energy pulses containing 47 mJ, corresponding to an intensity of 1.8 GW/cm2 at the fiber exit.
Although these results were obtained with a straight fiber, the researchers also experimented with bent fibers. They found that a 1.4-m bend of radius decreased the intensity at the focused spot by about 36 percent and decreased the likelihood of a spark from 97 percent to 85 percent. A sharper bend of about 0.8 m reduced the intensity to 120 GW/cm2 and produced almost no sparking.
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