Four-Wavelength Passive Demodulation Technique for the Measurement of Static/Dynamic Composite Signals

A four-wavelength passive demodulation technique for the measurement of static/dynamic composite signals is proposed and experimentally demonstrated. Those previously reported passive demodulation methods for extrinsic Fabry–Perot interferometrics (EFPIs) will collapse if the cavity length changes significantly during signal demodulation. However, the proposed technique can demodulate the measurand even if the cavity length changes significantly. The phase difference between different interferometric signals is first calculated by using four interferometric signals. The phase compensation of these interferometric signals is then implemented to eliminate the influence of cavity length variations on the demodulation. The change in the cavity length can then be recovered. Dynamic signals, static signals, and static/dynamic composite signals were interrogated by the demodulation technique with relative errors of $\sim $ 0.1%. An EFPI with a cavity length ranging from 32.53 to $1219.29~\mu \text{m}$ is detected by the same demodulator. The fast measurement of a cavity length change with an amplitude of $\sim $ 80 $\mu \text{m}$ is achieved. The technique is applicable to different types of signals applied on EFPIs with an unsteady cavity length.

I. Introduction

Fiber optical sensors, especially extrinsic Fabry–Perot interferometric (EFPI) sensors, have been extensively applied in various industrial and scientific research areas because they are immune to electromagnetic interference, compact in size, extremely sensitive, and operate well in harsh environments [1], [2], [3], [4], [5], [6]. Therefore, demodulation techniques for EFPI sensors have also attracted abundant interest [6], [7], [8], [9], [10], [11], [12], [13]. However, the demodulation system exhibited a significant difference between the absolute measurement used for static signals [10], [11], [12] and the relative measurement used for dynamic signals [6], [7], [8], [9]. As a result, these measurement techniques can only be used to detect dynamic signals [6] or static signals [14], [15] separately and cannot measure static/dynamic composite signals. Some of the multiparameter measurement techniques use different demodulators to measure static and dynamic signals through time-division multiplexed techniques [16], rather than one demodulator for static/dynamic composite signal measurements.