Optic Extrinsic Fabry-Perot Interferometric Sensors Research Articles

Fourier transform white-light interferometry based on nonlinear wavelength sampling

Fourier transform white-light interferometry possesses high resolution and wide dynamic range for the absolute measurement of fiber optic interferometric sensors. However, the white-light optical spectrum distributed along wavelength is a chirp signal because the phase of the optical spectrum has a nonlinear relationship with the scanning wavelength. The chirped spectrum is considered as a constant period signal when it is Fourier transformed. The chirp in the period would bring errors into the phase shift and reduce the measurement resolution. A nonlinear wavelength sampling algorithm is proposed in this paper. The chirp characteristics of the white-light optical spectrum are considered, and the nonlinear wavelength sampling intervals vary with the wavelength. By using the nonlinear wavelength sampling algorithm, the errors in the phase shift can be reduced effectively, whereas the chirp characteristics of the signals can be retained entirely for filtering and extracting the chirped optical spectrum signals from the composite signal. The experimental results show that the standard deviation decreases from 0.016 to 0.005 μm by using the nonlinear wavelength sampling, when a fiber optic extrinsic Fabry-Perot interferometric sensor with a cavity length of 1512.2 μm is interrogated.

Microwave assisted reconstruction of optical interferograms for distributed fiber optic sensing

This paper reports a distributed fiber optic sensing technique through microwave assisted separation and reconstruction of optical interferograms in spectrum domain. The approach involves sending a microwave-modulated optical signal through cascaded fiber optic interferometers. The microwave signal was used to resolve the position and reflectivity of each sensor along the optical fiber. By sweeping the optical wavelength and detecting the modulation signal, the optical spectrum of each sensor can be reconstructed. Three cascaded fiber optic extrinsic Fabry-Perot interferometric sensors were used to prove the concept. Their microwave-reconstructed interferogram matched well with those recorded individually using an optical spectrum analyzer. The application in distributed strain measurement has also been demonstrated.

High-resolution interrogation technique for fiber optic extrinsic Fabry-Perot interferometric sensors by the peak-to-peak method

An improved peak-to-peak method is developed for interrogating the absolute cavity length of fiber optic extrinsic Fabry-Perot interferometric (EFPI) sensors with high resolution. A fiber Fabry-Perot tunable filter (FFP-TF) is used to scan the optical spectrum of an EFPI, and the problems caused by the nonlinear performance and poor repeatability of the FFP-TF are removed by using a wavelength calibration technique. A linear fitting is used to calculate the wavelength spacing between two adjacent apexes in the optical spectrum, and the cavity length can be retrieved using this wavelength spacing. The experimental results show that the measuring resolution is improved from 25 to 1 microm, and a linear output is also obtained.

Extrinsic Fabry-Perot sensor for strain and crack opening displacement measurements from -200 to 900 degrees C

The authors present the design of a fiber optic extrinsic Fabry-Perot interferometric sensor element and its operation in several applications. The sensor has been demonstrated in a coal-fired combustor for the measurement of both thermally induced strains and crack opening displacements in ceramic materials. As a strain sensor, the device is demonstrated at temperatures ranging from -200 to 900 degrees C. It is shown that a Fabry-Perot strain gage with a one centimetre gage length can be operated in differential mode with 0.01 microstrain resolution in real-time, and in an absolute mode with 0.5 microstrain resolution with a three-second scan time.