The standard extrinsic Fabry-Perot interferometric optical fiber sensor (EFPI OFS) is excellent in sensitivity and resolution, but it has disadvantage to be susceptible to directional ambiguity because of using the only fringe count. This research suggests the EFPI OFS with the structure of the transmission-type, which is different from the ordinary one with boundary faces of a single-mode fiber/air and a multimode fiber/air as reflectors. The transmission-type (T-type) EFPI OFS uses not only the interferometric fringe count on the cavity length change but also the level change of the output signal for the measurement. This makes the simple and effective distinction of the direction possible.
The output signal of this sensor was analyzed by using (1) the uniform plane wave-based model, (2) the SMF power distribution-based model and (3) the splice loss-based model and verified by the experiment. Based on the result of the experiment, the type of optical fibers and the initial length and the range of the gap length were determined for the optimum measurement. The strain measurement with the fabricated T-type EFPI OFS's was performed, and its comparison with the resistance-based strain gauge showed good agreement.
For the temperature measurement and investigation of the sensor characteristic on the temperature, the verified T-type EFPI OFS was attached to the cantilevered specimen of aluminum alloy and heated. The comparison of the result of temperature measurement between OFS and thermocouple shows good agreement. However T-type EFPI OFS shows measuring limit of temperature due to the glass transition temperature of epoxy used as adhesive.
