Hao Liao

Fiber-Optic Michelson Interferometric Acoustic Sensor Based on a PP/PET Diaphragm

A novel acoustic sensor based on a polypropylene/poly (ethylene terephthalate) (PP/PET) diaphragm is demonstrated. The Michelson interferometer is formed by two beams of light that are reflected into optical fiber collimators by both sides of the PP/PET film. The deformation of diaphragm caused by acoustic signal will be magnified twice in the optical path of proposed sensor. The sensitivity of our proposed sensor is more than -128 dB re 1 rad/μ Pa in the frequency range of 90-4000 Hz, and a signal-to-noise ratio of ~ 42dB is achieved at 600 Hz. Due to its superiorities of low cost, small size, high sensitivity, and easy fabrication, the proposed sensor exhibits a potential for low-frequency acoustic sensing and healthy monitoring.

An Infrasound Sensor Based on Extrinsic Fiber-Optic Fabry–Perot Interferometer Structure

An infrasound sensor based on an extrinsic fiber-optic Fabry-Pérot interferometer structure is reported and demonstrated. The transducer part of this sensor is a composite film which is composed of a 3-μm-thick small round aluminum foil and a ~50-μm-thick polymer polyethylene terephthalate film. The infrasound interference will introduce the vibration of the film and result in the changes of the FP cavity length synchronously, as well as the interference spectral shift. As a consequence, we will obtain the synchronous changes of reflected power by inputting a single wavelength laser. Theoretical simulation and experimental validation are both carried out to test the performance of our fiber infrasound sensor. Results show that low-frequency infrasound signal from 1 to 20 Hz can be detected with low distortion. The proposed sensor has its superiorities and can be useful for low-frequency infrasound sensing.

Bending Direction Detective Fiber Sensor for Dual-Parameter Sensing Based on an Asymmetrical Thin-Core Long-Period Fiber Grating

An asymmetrical thin-core long-period fiber grating (ATC-LPFG) cascaded with an inline Mach-Zehnder interferometer (MZI) for dual-parameter sensing has been demonstrated. In addition, bending direction is also determined at the same time. A section of thin-core fiber (TCF) is fusion spliced between two single-mode fibers (SMFs). ATC-LPFG is 4 cm away from the first splicing point to form an inline MZI. The transmission spectrum consists of four dominant resonant wavelengths generated by multimode interference and loss peaks of ATC-LPFG. Two resonant wavelengths are the main loss peaks of ATC-LPFG, and additional two resonant wavelengths are caused by multimode interference. Bending direction is determined by former two resonant wavelengths; curvature and temperature sensitivities are measured by the resonant wavelength of multimode interference and ATC-LPFG. Cross sensitivity can be overcome because the resonant wavelength is generated by different mechanisms. The experimental results indicate that the sensitivities of curvature and temperature are 28.82 dB/m-1 and 67.3 pm/°C, respectively.

Passively mode-locked fiber laser sensor for acoustic pressure sensing

The development is reported of a multi-longitudinal mode fiber laser sensor based on passive mode locking employing carbon nanotubes in the laser cavity. A polymer membrane is employed beneath the pre-strained erbium-doped fiber (EDF) to convert the sound pressure disturbance into axial strain, alter the cavity length, and induce a shift of the longitudinal modes beat. Hence, acoustic pressure measurement can be carried out by detecting the shift of the beat frequency. Experimental results show comparable strain and sound pressure sensitivity of ~0.5 kHz/με and 147.2 Hz/Pa, respectively. The proposed sensor is an alternative for the measurement of acoustic pressure and possesses the advantages of good stability and ease of interrogation.

Sensitivity amplification of fiber-optic in-line Mach–Zehnder Interferometer sensors with modified Vernier-effect

In this paper, a novel sensitivity amplification method for fiber-optic in-line Mach-Zehnder interferometer (MZI) sensors has been proposed and demonstrated. The sensitivity magnification is achieved through a modified Vernier-effect. Two cascaded in-line MZIs based on offset splicing of single mode fiber (SMF) have been used to verify the effect of sensitivity amplification. Vernier-effect is generated due to the small free spectral range (FSR) difference between the cascaded in-line MZIs. Frequency component corresponding to the envelope of the superimposed spectrum is extracted to take Inverse Fast Fourier Transform (IFFT). Thus we can obtain the envelope precisely from the messy superimposed spectrum. Experimental results show that a maximum sensitivity amplification factor of nearly 9 is realized. The proposed sensitivity amplification method is universal for the vast majority of in-line MZIs.

Single hole twin eccentric core fiber sensor based on anti-resonant effect combined with inline Mach-Zehnder interferometer

A novel fiber curvature sensor without temperature cross interference based on a single hole twin eccentric core fiber has been proposed. Anti-resonant mechanism combined with inline Mach-Zehnder interference (MZI) structure are applied to the measurands detection. The spectrum is composed of a comb spectrum caused by the inline MZI and several dominant resonant wavelengths induced by anti-resonant effect. The curvature sensitivity of -1.54dB/m-1 can be achieved by intensity demodulation of the selected dip of Gaussian fitting. Similarly, the temperature sensitivity of 70.71pm/°C and 34.17pm/°C are respectively achieved by tracking coherent decrease point obtained by the FFT band pass filter method and Gaussian fit dip. Consequently, a relatively higher resolution of temperature measurement can be realized by the two methods mentioned above. The proposed sensor has a great potential for structural health monitoring, such as buildings, towers, bridges, and many other infrastructures due to its compact structure, easy fabrication and without cross impacts.

Fiber Acoustic Sensor Based on Polarization-Maintaining Photonic Crystal Fiber Cascaded with a Long Period Grating in a Sagnac Loop

A fiber acoustic sensor was proposed. A long period grating is the sensing element with a plastic membrane as a transducer. In experiment acoustic signals of 100-4000Hz were measured and the sensitivity is about 3mV/Pa.

Demodulation of diaphragm based fiber-optic acoustic sensor using with symmetric 3×3 coupler

A diaphragm based Michelson interferometric fiber-optic acoustic sensor based on symmetric 3×3 coupler is demonstrated in this paper. An ellipse fitting differential cross multiplication algorithm is proposed for signal interrogating. Experimental results show that a minimum detectable pressure (MDP) level of ∼500 μPa/Hz1/2 is obtained.

Phase demodulation of Fabry-Perot interferometer-based acoustic sensor utilizing tunable filter with two quadrature wavelengths

A phase demodulation method for short-cavity extrinsic Fabry-Perot interferometer (EFPI) based on two orthogonal wavelengths via a tunable optical filter is proposed in this paper. A broadband light is launched into the EFPI sensor and two monochromatic beams with 3dB bandwidth of 0.2nm are selected out from the reflected light of the EFPI sensor. A phase bias is induced between the two interferential signals due to the wavelength difference of the two beams. The wavelength difference will have an affect on the sensitivity of demodulated signal, which has been theoretically and experimentally demonstrated. The maximum sensitivity can be obtained when the phase bias is 0.5π corresponding to the wavelength difference of 1/4 FSR of the EFPI spectrum. The acoustic wave induced phase variation can be interrogated through an optimized differential cross multiplication (DCM) method. A normalization process is induced into the traditional DCM method to eliminate the influence of ambient temperature and pressure fluctuation induced spectrum shift on output signal. This means that, once the wavelength difference is fixed, the wavelength variation of each individual beam will have little influence on the amplitude of demodulated signal. The EFPI sensing head is formed by a 3μm-thick aluminum diaphragm, which has a SNR of more than 53dB. Through the proposed demodulation scheme, a large dynamic range and good linearity is acquired and Q-point drift problem of traditional EFPI sensor can be solved. The demodulation scheme can be applied to other kinds of short-cavity EFPI based acoustic sensors.

Spectrum interrogation of fiber acoustic sensor based on self-fitting and differential method

In this article, we propose an interrogation method of fiber acoustic sensor to recover the time-domain signal from the sensor spectrum. The optical spectrum of the sensor will show a ripple waveform when responding to acoustic signal due to the scanning process in a certain wavelength range. The reason behind this phenomenon is the dynamic variation of the sensor spectrum while the intensity of different wavelength is acquired at different time in a scanning period. The frequency components can be extracted from the ripple spectrum assisted by the wavelength scanning speed. The signal is able to be recovered by differential between the ripple spectrum and its self-fitted curve. The differential process can eliminate the interference caused by environmental perturbations such as temperature or refractive index (RI), etc. The proposed method is appropriate for fiber acoustic sensors based on gratings or interferometers. A long period grating (LPG) is adopted as an acoustic sensor head to prove the feasibility of the interrogation method in experiment. The ability to compensate the environmental fluctuations is also demonstrated.