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Description:
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A new fiber optic sensing technology for measuring in -cylinder pressure in
automotive engines was investigated . The optic sensing element consists of two
mirrors in an in -line single mode fiber that are separated by some distance . To
withstand the harsh conditions inside an engine , the Fiber Fabry -Perot Interferometer
(FFPI ) element was coated with gold and copper . The metal -protected fiber sensor
was embedded into a small cut in the metal casing of the spark plug . At first , the
sensing element was dipped in liquid gold and cured . Then the gold -coated fiber
sensor was electroplated with copper . Finally , the metal -coated fiber sensor was
embedded in the spark plug .
The spark -plug -embedded FFPI sensor was monitored using a signal
conditioning unit . Field tests were carried out in a 3 -cylinder automotive engine with
a piezoelectric pressure sensor as a reference transducer up to about 3500 rpm . The
fiber optic sensor data generally matched those measured by the piezoelectric
reference sensor . The use of a Vertical Cavity Surface Emitting Laser (VCSEL ) diode as a light
source in an FFPI optic sensor system was investigated . Reflected light from the FFPI
sensing element was used to measure the optical path difference .
With a 1550nm VCSEL as the light source in a 12mm cavity length Fiber
Fabry -Perot Interferometer , spectral characteristics were examined to determine the
proper combination of dc bias current , modulation current amplitude and modulation
frequency . Single VCSEL operation and regular fringe patterns were achieved .
The laser tuning was -41 .2 GHz /mA and was determined from measurements
of the shift in the spectral peak of the VCSEL diode output as a function of dc bias
current . By testing the fringe movement as the FFPI sensor was heated , the
temperature tuning coefficient for the optical length was determined to be 11 x 10 -6 à ºC .
The results of these experiments indicate that the use of VCSEL diode as a
light source for the FFPI sensor offers a viable alternative to the use of Distributed
Feedback (DFB ) laser diodes for monitoring at a lower bias current and modulating
current amplitude . |