Abstract
A passive homodyne phase demodulation technique based on a linear-fitting trigonometric-identity-transformation differential cross-multiplication (LF-TIT-DCM) algorithm is proposed. This technique relies on two interferometric signals whose interferometric phase difference is odd times of π. It is able to demodulate phase signals with a large dynamic range and wide frequency band. An anti-phase dual wavelength demodulation system is built to prove the LF-TIT-DCM algorithm. Comparing the traditional quadrature dual wavelength demodulation system with an ellipse fitting DCM (EF-DCM) algorithm, the phase difference of two interferometric signals of the anti-phase dual wavelength demodulation system is set to be π instead of π/2. This technique overcomes the drawback of EF-DCM—that it is not able to demodulate small signals since the ellipse degenerates into a straight line and the ellipse fitting algorithm is invalidated. Experimental results show that the dynamic range of the proposed anti-phase dual wavelength demodulation system is much larger than that of the traditional quadrature dual wavelength demodulation system. Moreover, the proposed anti-phase dual wavelength demodulation system is hardly influenced by optical power, and the laser wavelength should be strictly limited to lower the reference error.
Highlights
Fiber optic sensors have been widely researched because of their advantages such as high sensitivity, light weight, electromagnetic immunity, and multiplexing capabilities [1,2,3,4,5].Among fiber optic sensors, interferometric sensors play an important role including four types such as Michelson interferometric (MI) sensors [6], Mach-Zehnder interferometric (MZI) sensors [7], Sagnac interferometric (SI) sensors [8] and Fabry–Perot interferometric (FPI) sensors [9]
The extrinsic Fabry-Perot interferometric (EFPI) acoustic sensor is comprised of a vibrating diaphragm with a fixed boundary and an optical fiber which is perpendicular to the diaphragm to form an extrinsic Fabry–Perot interferometer
The combined light enters into the EFPI acoustic sensor through an optical circulator
Summary
Fiber optic sensors have been widely researched because of their advantages such as high sensitivity, light weight, electromagnetic immunity, and multiplexing capabilities [1,2,3,4,5].Among fiber optic sensors, interferometric sensors play an important role including four types such as Michelson interferometric (MI) sensors [6], Mach-Zehnder interferometric (MZI) sensors [7], Sagnac interferometric (SI) sensors [8] and Fabry–Perot interferometric (FPI) sensors [9]. Fiber optic sensors have been widely researched because of their advantages such as high sensitivity, light weight, electromagnetic immunity, and multiplexing capabilities [1,2,3,4,5]. The signal to be measured is demodulated in a phase of an interferometer, where a specific demodulation method is applied to the interrogation phase signal introduced by the signal to be measured. Homodyne phase demodulation has attracted much attention due to its large dynamic range, wide frequency band and immunity to phase noise from the laser source. It can be divided into two categories, which are active and passive homodyne phase demodulation. For passive homodyne phase demodulation, there are several different types, such as 3 × 3 coupler demodulation [13,14], phase generated carrier (PGC) demodulation [15,16,17,18] and quadrature dual wavelength demodulation [19,20,21,22]
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