A fiber-optic distributed acoustic sensing method based on phase-sensitive optical time domain reflectometry combined with dual-chirped pulse and micro-reflective fiber Bragg grating (FBG) is proposed. The sensing fiber consists of a micro-reflective FBG with uniform spatial interval. The micro-reflective FBG help to gain a high signalto-noise ratio light signal, comparing with the Rayleigh backscattering (RBS) light of optical fiber itself. The dualchirped pulses are generated by a time delay, whose corresponding spatial interval approximately equal to twice the spatial interval of adjacent micro-reflective FBG. A beating signal is generated due to the interference of the two identical chirped pulses reflected by the micro-reflective FBG array. Acoustic disturbance between the microreflective fiber gratings will change the phase of the beating signal and the interference waveform will shift. Quantitative measurement can be achieved by directly demodulating the beating signal through using a crosscorrelation algorithm. By using such a method to perform the sensing for the micro-reflective FBG array, distributed quantitative measurement can be realized with only direct detection scheme and simple demodulation algorithm. Experiment are carried out with 2km fiber and PZT vibration simulation and the results verified the effectiveness of our method.
A hybrid fiber grating for refractive index (RI) measurement with temperature compensation is reported and experimentally validated. The sensor is fabricated successively by inscribing long period grating (LPG) and tilted fiber Bragg grating (TFBG) in the same region of an optical fiber in sequence. The measured RI at different temperature agreed well with the standard value. The RI sensitivities is 579.36 nm/RIU.
Coherent Anti-Stokes Raman Scattering (CARS) microscopy has attracted lots of attention because of the advantages, such as noninvasive, label-free, chemical specificity, intrinsic three-dimension spatial resolution and so on. However, the temporal overlap of pump and Stokes has not been solved owing to the ultrafast optical pulse used in CARS microscopy. We combine interference spectrum of residual pump in Stokes path and nonlinear Schrodinger equation (NLSE) to realize the temporal overlap of pump pulse and Stokes pulse. At first, based on the interference spectrum of pump pulse and residual pump in Stokes path, the optical delay is defined when optical path difference between pump path and Stokes path is zero. Then the relative optical delay between Stokes pulse and residual pump in PCF can be calculated by NLSE. According to the spectrum interference and NLSE, temporal overlap of pump pulse and Stokes pulse will be realized easily and the imaging speed will be improved in CARS microscopy.
In this paper, a double-sideband heterogeneous with suppressed carrier (DSBH-SC) pulse modulation method for fiber-optic distributed acoustic sensing is proposed. An electro-optic in-phase/quadrature (I/Q) modulator is used to realize carrier-suppressed double-sideband heterogeneous pulse modulation in which the positive and the negative optical sidebands can carry independent modulation signals. Due to the modulation curve of the electro-optic I/Q modulator irregularly, the factors that influence the performance of the DSBH-SC are analyzed from modulation amplitude and frequency. The analysis shows that the constant frequency modulation on the lower optical sideband while a stable wide band linear frequency chirping on the upper optical sideband can be obtain in appropriate modulation conditions. It presents a method of digital subcarrier modulation for distributed optical sensing.
In digital coherent optical time domain reflectometer (coherent-OTDR) system, a dual-parallel Mach-Zehnder modulator (DP-MZM) is employed to modulate the signal light and to generate frequency-shifted pulse light. However, the environment temperature strongly influent the stability of the DP-MZM. To stabilize the quality of the frequency-shifted pulse light, we proposed a bias control method to keep the modulator at the optimum bias. This bias control method search for the optimum bias by changing three bias voltages at the same time based on chaotic particle swarm optimization algorithm(PSO). The experimental results show obvious effect on locating the optimum bias voltages for the DP-MZM.
Wavelength division multiplex technology can enhance the sensing capacity by detecting various samples simultaneously. Whisper-Gallery-Mode (WGM) can be selected simulated in the micro-bubble by a directional coupler made by Si3N4 grating. Some grating parameters, including period, width, and refractive index modulation are numerically simulated by FDTD solution software to find their impacts on the WGM selected process. Grating with a particular period can simulated a WGM in micro-resonator on purpose. The interference of different bubble resonators is also discussed in this paper.
Microbubble resonators combine the unique properties of whispering gallery mode resonators with the capability of integrated microfluidics. The microbubble resonator is fabricated by heating the tapered tip of a pressurized glass capillary with oxyhydrogen flame. Firstly, a microtube with a diameter of 250um is stretched under heating of oxyhydrogen flame, the heating zone length is set to be 20mm and the length of stretch is set to be 7000um.Then nitrogen will be pumped in to the tapered microtube with the pressure of 0.1Mpa, the tapered tip will be heated by the oxyhydrogen flame continuously until a microbubble forms. An optical fiber taper with a diameter of 2 um, fabricated by stretching a single-mode optical fiber under flame was brought in contact with the microbubble to couple the light from a 1550nm tunable diode laser into the whispering gallery mode. The microbubble resonator has a Q factors up to 1.5 × 10<sup>7</sup> around 1550nm. Different concentrations of ethanol solution (from 5% to 30%) are filled into it in order to test the refractive index sensing capabilities of such resonator, which shows a sensitivity of 82nm/RIU.
A demodulation algorithm for Low-coherence Interferometry is proposed. The demodulation includes a Lithium Niobate
wedge, which index is tunable via optics-electric property. This method is based on the theorem that second harmonic
term is orthogonal with the fundamental term in Fourier transform. An alternative external electric field is imposed on
the wedge and tune the position of Low-coherence Interferometry is proposed. For every point on the CCD camera, the
variation of the detected light power is Fourier transformed and obtains the ratio between second-harmonic term and
fundamental term. This paper demonstrate theoretically that proposed demodulation method can enhance the accuracy of
pressure sensing for more than 35 times.
Optical fiber sensor has great advantage for applications dealing with extreme environment. We developed a high
precision optical pressure sensor for aviation industry. The optical pressure sensor is based on two-beam interference of
microcavity and is fabricated with Micro-electromechanical systems (MEMS) and laser fusion technology. The cavity
length variation resulting from external pressure is demodulated with spatial polarization low coherence interference unit
and a high stable phase demodulation algorithm. The effect of light source output parameter is also investigated. We
carried out research on optical fiber strain, temperature and acoustic vibration sensor for aerospace application. The
optical fiber sensors for strain and temperature measurement are based on fiber Bragg grating(FBG).Both bare FBG and
packaged FBG performances under cryogenic temperature and high vacuum are investigated. An eight-channel parallel
FBG wavelength interrogator is developed. The optical fiber acoustic vibration sensor is based on two-beam interference
of microcavity and use intensity demodulation method for high speed response. The mutiple-parameter and multiplepoint
measurement instrument is successfully applied to status monitoring of water sublimator.