<title>Transmission/reflection-type hybrid extrinsic Fabry-Perot interferometric optical fiber sensors</title>

Abstract

Most interferometric optical fiber sensors, including the extrinsic Fabry-Perot interferometric (EFPI) sensor, measure strain or temperature with only interferometric fringes. Thus the measurement direction such as tension/compression of strain or increase/decrease of temperature is difficult to be distinguished. The EFPI optical fiber sensor has been constructed with the transmission-type structure to compensate this disadvantage. In the transmission-type EFPI (TEFPI) optical fiber sensor, the additional signal level variation in the fringe signal presents the measurement direction information. However, the TEFPI optical fiber sensor has low visibility of the interferometric fringes, and requires somewhat sophisticated signal processing to distinguish the measurement direction changes. In this study, the hybrid extrinsic Fabry-Perot interferometric optical fiber sensor is presented, from which the transmission-type and reflection-type sensor signals can be simultaneously acquired. To detect peaks and valleys in the interferometric fringes, the reflection-type signal with relatively higher visibility is processed. The measurement direction is determined by the linear combination of the transmission-type and reflection-type signals, which presents the signal level variation with minimum fringe amplitude. Because the hybrid sensor uses the separated signals containing measurement quantity and direction information, its signal processing is more robust than one of the TEFPI sensor.