We present a new computational algorithm of phase-shifting interferometry that can effectively eliminate the uncertainty errors of reference phases encountered when we obtain multiple interferograms. The algorithm treats the reference phases as additional unknowns and we determine their exact values by analyzing interferograms using the numerical least-squares technique. A series of simulations prove that this algorithm can improve measuring accuracy because it is unaffected by the nonlinear and random errors of phase shifters.

광위상간섭법에서 기준위상의 수치해석 보정 = Numerical correction of reference phases in phase-shifting interferometry

A common-path dual-wavelength phase demodulation technique for extrinsic Fabry–Perot interferometric (EFPI) sensors is proposed on the basis of a broadly tunable modulated grating Y-branch (MG-Y) laser. It can address the three main concerns of existing dual-wavelength phase interrogation methods: the imbalances and disturbances caused by two optical paths utilizing two lasers or two photodetectors, the restrictions between two operating wavelengths and the cavity length of EFPI, and the difficulty in eliminating the direct current (DC) component of the interferometric fringe. Dual-wavelength phase interrogation is achieved in a common optical path through high-speed wavelength switching. Taking advantage of the MG-Y laser’s full spectrum scanning ability (1527 ∼ 1567 nm), initial cavity length and DC component can be directly measured by white light interferometry. Two quadrature wavelengths are then selected to perform high speed phase demodulation scheme. Three polyethylene terephthalate (PET) diaphragm based EFPI acoustic sensors with cavity lengths of 127.954 µm, 148.366 µm and 497.300 µm, are used to demonstrate the effectiveness.

Common-path dual-wavelength quadrature phase demodulation of EFPI sensors using a broadly tunable MG-Y laser.

Extrinsic Fabry–Perot (FP) interferometric sensors are being intensively applied for partial discharge (PD) detection and localization. Previous research work has mainly focused on novel structures and materials to improve the sensitivity and linear response of these sensors. However, the directional response behavior of an FP ultrasonic sensor is also of particular importance in localizing the PD source, which is rarely considered. Here, the directional sensitivity of a microelectromechanical system (MEMS)-based FP ultrasonic sensor with a 5-μm-thick micromechanical vibrating diaphragm is experimentally investigated. Ultrasonic signals from a discharge source with varying incident angles and linear distances are measured and analyzed. The results show that the sensor has a 5.90 dB amplitude fluctuation over a ±60° incident range and an exciting capability to detect weak PD signals from 3 m away due to its high signal–noise ratio. The findings are expected to optimize the configuration of a sensor array and accurately localize the PD source.

https://www.mdpi.com/1424-8220/18/6/1975/pdf

Directional Sensitivity of a MEMS-Based Fiber-Optic Extrinsic Fabry–Perot Ultrasonic Sensor for Partial Discharge Detection

Flexible and stable demodulation techniques of large-scale fiber-optic Fabry–Perot (FP) acoustic sensors are highly desirable for accelerating their industrial applications. In this paper, we report a novel self-calibrating wavelength shifting interferometry (WSI) technique that enables simultaneous multi-point acoustic detection using diaphragm based fiber-optic FP acoustic sensors. A widely tunable modulated grating Y-branch (MG-Y) laser (1527∼1567 nm) performs high-speed wavelength switching, introducing phase-shifts in the wavelength domain for real-time phase retrieval. The proposed self-calibrating WSI is easily extended for multiplexing FP acoustic sensors by calibrating the corresponding phase-shift step of each sensor probe. Based on a modified Hariharan 5-step phase shifting algorithm, the phase-shift step for each channel can be calibrated in real-time, making the system robust in applications involving large environmental perturbations. An all-optical multi-point acoustic detection system based on WSI is proposed and experimentally demonstrated for the first time. Sound source localization experiments show that the multi-point acoustic detection system works stably and the positioning accuracy is about 2.42 cm.

Multiplexing fiber-optic Fabry-Perot acoustic sensors using self-calibrating wavelength shifting interferometry

A novel design method for fiber-optic Fabry-Perot (FP) acoustic sensor based on flywheel-like diaphragm has been proposed and experimentally demonstrated. Through theory analysis and simulation, we have established a sensing model based on the flywheel-like diaphragm and illustrated the acoustic sensor's resonant frequency regulation method. A flywheel-like diaphragm-based acoustic sensor has been fabricated and a dual-wavelength system is used to demodulate the phase fluctuation caused by acoustic wave. Experimental results show that the proposed acoustic sensor has an acoustic pressure sensitivity of 1.3967rad/Pa and noise-limited minimum detectable pressure (MDP) of ~16.43μPa/√Hz. The signal to noise ratio (SNR ) of 1kHz, 9kHz, and 20kHz are achieved to be 10.39dB, 60.51dB, and 37.14dB, respectively. Over a fairly broad response range (1 kHz to 20 kHz), the average SNR is up to 45.57dB. Moreover, our acoustic sensor's directivity map approximates to a heart shape. Its excellent frequency response characteristics, sensitivity, and directivity pattern is believed to have a bright application prospect.

https://iopscience.iop.org/article/10.1088/1361-6463/ab95bc/pdf

Flywheel-like diaphragm-based fiber-optic Fabry-Perot frequency tailored acoustic sensor

For the application of ultrasonic detection in low-pressure carbon dioxide medium, we propose a fiber-optic Fabry-Perot (F-P) ultrasonic sensor system based on an edge-stretched circular polymer diaphragm. The diaphragm is fabricated with pre-tension of 200 N/m. The demonstrated demodulation system is based on quadrature phase-shifted demodulation method. By setting the F-P cavity length and the wavelengths of distribute feedback (DFB) semiconductor lasers, two quadrature-phase interference signals can be obtained. Experimental results showed that the proposed sensor system can have a sensitivity of 10.4 nm/kPa and a signal-to-noise ratio (SNR) of 14.45 dB under gas pressure of 270 Pa in low-pressure carbon dioxide. The unique advantages of the proposed sensor system make it attractive for non-contact ultrasonic detection in low-pressure gas medium.

Noncontact Ultrasonic Detection in Low-Pressure Carbon Dioxide Medium Using High Sensitivity Fiber-Optic Fabry–Perot Sensor System

In this paper, the optical fiber Fabry-Perot (F-P) interferometer based on phase-shifting technique and birefringence crystals is proposed and demonstrated. We use the characteristics of birefringence and four birefringence crystals with different thicknesses to obtain the quadrature phase-shifted signals, which are demodulated by phase-shifting technique. Two types of sensing interferometers are used in the experiment. One is the optical fiber F-P sensor and the other is composed of the fiber end face and the glass surface fixed on the nanopositioning stage. The experimental results show that the normalized standard deviation (SD) of the calibration microphone centerline is 1.97 and 2.63 times larger than the optical fiber F-P interferometer under the sinusoidal sonic signals of 21 kHz and 40 kHz, and the interferometer is effective in avoiding phase ambiguity. The proposed interferometer has high stability and can adapt to a larger measurement range.

Optical fiber Fabry-Perot interferometer based on phase-shifting technique and birefringence crystals.

The invention discloses a birefringent crystal-based optical fiber Fabry-Perot acoustic vibration sensing device and a demodulation method. Optical signals emitted by an SLD broadband light source aretransmitted to an optical fiber Fabry-Perot acoustic vibration sensor and are modulated under the effect of external acoustic vibration signals; the modulated optical signals are reflected back to a1*2 coupler and are split into four paths of signals by a 1*4 coupler; the four paths of signals pass through an optical fiber collimator array used for emitting, and are modulated by a polarizer array, a birefringent crystal array and an analyzer array; the modulated signals enter an optical fiber collimator array for receiving; the four paths of the optical signals are converted into electricalsignals through a photodetector array; four birefringent crystals in the birefringent crystal array have different thicknesses, so that a quadrature phase difference exists between each paths of signals, so that four paths of interference signals are acquired; and phase information is obtained on the basis of four-quadrant arc tangent, so that the demodulation of the acoustic vibration signals canbe realized. Compared with the prior art, the birefringent crystal-based optical fiber Fabry-Perot acoustic vibration sensing device and the demodulation method of the present invention can ensure measurement accuracy.

Birefringent crystal-based optical fiber Fabry-Perot acoustic vibration sensing device and demodulation method

The invention discloses a polarization-interference-based optical path difference measuring device and method of a Mach-Zehnder interferometer. The measuring device is composed of a light source (1),a polarizer (2), first to sixth polarization-maintaining fibers (3) (5) (6) (7) (8) and (10), first and second polarization-maintaining couplers (4) and (9), a Wollaston prism (11), first and second photodetectors (12) and (13), a digital acquisition card (14) and a processing system (15). Light outputted by the light source (1) is polarized by the fiber polarizer (2); the light is split into twobeams of polarized light by the polarization-maintaining coupler (4); the two beams of polarized light enters the two arts, that is, the second and third polarization-maintaining fibers (5) and (6), of the Mach-Zehnder interferometer respectively, wherein the output polarization directions correspond to two groups of polarized light in quick axis and slow axis directions of the fourth and fifth polarization-maintaining fibers (7) and (8); the phase difference of the two groups of polarized light is pai/2 by adjusting the lengths of the fourth and fifth polarization-maintaining fibers (7); anda phase is calculated by using a quadrature phase demodulation method.

Zhao Zixu

Papers

Noncontact Ultrasonic Detection in Low-Pressure Carbon Dioxide Medium Using High Sensitivity Fiber-Optic Fabry–Perot Sensor System

Optical fiber Fabry-Perot interferometer based on phase-shifting technique and birefringence crystals.

Birefringent crystal-based optical fiber Fabry-Perot acoustic vibration sensing device and demodulation method

Polarization-interference-based optical path difference measuring device and method of Mach-Zehnder interferometer