Wenjun Ni

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.

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.

Ultrathin graphene diaphragm-based extrinsic Fabry-Perot interferometer for ultra-wideband fiber optic acoustic sensing

An ultra-wideband fiber optic acoustic sensor based on graphene diaphragm with a thickness of 10nm has been proposed and experimentally demonstrated. The two reflectors of the extrinsic Fabry-Perot interferometer is consist of fiber endface and graphene diaphragm, and the cavity is like a horn-shape. The radius of the effective area of the ultrathin graphene diaphragm is 1mm. Attributed to the strong van der Waals force between the diaphragm and the ceramic ferrule, the sensor head can be applied not only in the air but also underwater. Experimental results illustrate that ultra-wideband frequency response is from 5Hz to 0.8MHz, covering the range from infrasound to ultrasound. The noise-limited minimum detectable pressure level of 0.77Pa/Hz1/2@5Hz and 33.97μPa/Hz1/2@10kHz can be achieved, and the applied sound pressure is 114dB and 65.8dB, respectively. The fiber optic acoustic sensor may have a great potential in seismic wave monitoring, photoacoustic spectroscopy and photoacoustic imaging application due to its compact structure, simple manufacturing, and low cost.

A highly sensitive twist sensor without temperature cross sensitivity based on tapered single-thin-single fiber offset structure

A highly sensitive twist sensor without temperature cross sensitivity based on tapered single mode-thin core-single mode fiber offset structure is proposed and experimentally demonstrated. The two parameters mentioned above can be measured simultaneously without cross sensitivity. The twist sensitivity of 0.12dB/° is achieved by tracking power variation of the resonant wavelength, and the wavelength shift of the spectrum is ±0.01nm. The temperature sensitivity of 0.12nm/°C can be achieved by wavelength demodulation, and the power fluctuation of the spectrum is ±0.015dB. Therefore, the twist and temperature can be detected by the power and wavelength demodulation method, respectively.

Graphene diaphragm-based extrinsic Fabry-Perot interferometer for low frequency acoustic sensing

A low frequency fiber acoustic sensor based on the transduction of 10nm thickness graphene diaphragm has been experimentally demonstrated. The ultra thin graphene diaphragm is used as a reflecting surface to form an extrinsic Fabry-Perot interferometer. The diameter of the graphene diaphragm is selected as 125 pm, thus the resonant frequency of 25958Hz is obtained. The sensitivity of the acoustic pressure can be as high as 43.5dB re 1 V/Pa@60Hz. Moreover, the minimum detectable acoustic pressure of 171.6pPa/Hz1/2@60Hz is achieved. The compact structure, simple manufacture and low cost fiber acoustic sensor can be realized.

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.

Phase demodulation of interferometric fiber sensor based on fast Fourier analysis

A demodulation method for interferometric fiber sensors (IFSs) is proposed in this article. The phase variation induced by the measurands can be estimated by calculating the Fourier phase at the intrinsic spatial frequencies of the fiber sensor. Theoretical analysis of the demodulation method is discussed in detail. Numerical simulations are put forward to demonstrate the consistency of the demodulation results under different wavelength sampling interval and noise level, showing a better stability compared with the conventional peak wavelength tracking technique. The proposed method is also experimentally demonstrated by an inline multimode interferometer based on a single-mode fiber (SMF) offset-splicing structure. Experimental results indicate that the phase response of different cladding modes can be analyzed simultaneously. Simultaneous measurement of strain and temperature is realized in our confirmatory experiment by analyzing the phase sensitivities of two selected cladding modes.