Distributed acoustic sensing technology has unique advantage in diverse applications, such as seismology, mineral exploration, and so on. We explored the methods to increase measurement range, distance and speed, which are the most important requirement for field deployment. Heterogeneous sideband linear frequency modulated optical pulse with different frequency modulated bandwidth was used to realize simultaneous measurement of acoustic events in different dynamic ranges. Weak fiber Bragg gratings and phase of constructed single frequency were proposed for long distance sensing. Fast processing algorithm based on a graphics processing unit for a linear frequency modulation pulse demodulation was realized. The experiment results verified our methods well.
An optical fiber refractive index (RI) sensor based on open microcavity Mach-Zehnder interferometer (OMZI) and fiber ring laser (FRL) is proposed and demonstrated experimentally. The OMZI is manufactured by splicing a tiny single mode fiber (SMF) segment with multi-mode fiber (MMF) joints laterally. The large offset structure forms an open microcavity which can be filled with the liquid under test. Through inserting the OMZI into an erbium-doped FRL, the RI measurement can be achieved by discriminating the lasing wavelength, and the detection limit (DL) can be effectively improved owing to the laser sensing spectrum with narrower 3-dB bandwidth and higher optical signal-to-noise ratio (OSNR). Experimental results show that the output laser wavelength has a linear response to the RI change with a sensitivity of −2947.818 nm/RIU during the range of 1.33302~1.33402, and the DL is as low as 5.89×10−6 RIU. Compared with other optical fiber RI sensors, the proposed fiber laser RI sensor with an open microcavity has the advantages of small size, high sensitivity and low DL, making itself a competitive candidate for the microfluidic RI measurement in biochemistry.
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.
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.
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