KEYWORDS: Scattering, Single mode fibers, Modulation, Fiber optics sensors, Optical amplifiers, Light scattering, Temperature metrology, Signal attenuation, Data acquisition, Sensing systems
This paper presents a kind of highly nonlinear fiber (HNLF) sensor for distributed monitoring. In order to realize the high performance of the stimulated Brillouin scattering (SBS) in HNLF, the experiments diagram based on the topological structure of Brillouin optical time domain analysis (BOTDA) is optimized through two-stage modulation by two electro optical modulators (EOMs) to realize high extinction ratio. The SBS characteristics of the HNLF have been studied, including the Brillouin gain bandwidth, Brillouin gain center frequency and SBS threshold. It is found that the HNLF has a wider Brillouin gain bandwidth. The SBS threshold of HNLF is larger than that of single mode fiber, which are 78 mW and 7.9 mW respectively. The larger SBS threshold is useful to avoid SBS in some situations that Spontaneous Brillouin Scattering is necessary. Based on the Brillouin gain center frequency, the Brillouin frequency shift coefficients of strain and temperature in HNLF have been studied, which are 0.0308 MHz/με and 0.413 MHz/ °C respectively.
There are many similarities between Brillouin optical time-domain analysis and phase-sensitive optical time-domain reflectometer in system topological structure. A multifunctional distributed optical fiber sensing system based on their similarities and the combination of their topological structure is presented. The system can monitor strain and temperature, as well as detect and locate the intrusions or disturbances along the sensing fiber. Suppose that X is the width of the optical pulse incident into the sensing fiber with the unit of nanosecond and Y is the minimum detectable distance with the unit of meter between two intrusions that are being detected simultaneously. The experiment results show that y changes linearly with x according to the equation: y=0.103x−0.452. By optimizing device parameters of the system, strain measurement accuracy of 3.17 μϵ and the temperature measurement accuracy of 0.45°C have been realized when the optical pulse width is 30 ns. On the other hand, two intrusions have been detected and located simultaneously with this multifunctional system. The results show that the pulse width and the minimum distance between two intrusions (Lmin) have a linear relationship. The theoretical analyses and the experiment results show it is applicable to use this multifunctional system for strain and temperature monitoring, and intrusion detecting.
Phase-sensitive optical time domain reflectometer (Φ-OTDR) has been a research hotspot in recent years. For Φ-OTDR, the Rayleigh backscattering light is monitored with a photodetector, and the plotted trace is a curve of the returned optical power versus time. The time is proportional to the distance from the location along the sensing fiber to the optical pulses’ incident end. The trace presents the form of speckle-like profile owing to the coherent addition of the amplitudes of the light backscattered from different locations of the fiber. Based on this, the influences of the intrusion signal’s characteristics (occurrence location and phase variation) on the detection performances of the distributed sensor based on Φ-OTDR are investigated theoretically and demonstrated experimentally, especially on signal-to-noise ratio (SNR), missing report, and location accuracy. Results show that if intrusions occur at locations corresponding to the curve’s peak or subpeak and when its phase variation is (2k+1)π(k=1,2,3⋯), SNR will be the maximum. If intrusions occur at locations corresponding to the curve’s troughs, a missing report will occur no matter what value is taken by the phase variation of the intrusion signal. On the other hand, experimental results show most missing reports can be uncovered by using a frequency tunable laser and/or optical pulses with an alternative repetition frequency.
A novel fiber Bragg grating (FBG) sensor for the measurement of high temperature is studied. The sensing head is comprised of one FBG and two metal strips with different lengths and different coefficients of thermal-expansion (CTE). The temperature to be measured can be converted into the strain of the FBG. One can know the temperature according to the peak wavelength shift of the FBG induced by the strain. A dynamic range of 800°C and a resolution of 1°C can be obtained by this sensor system. The experiment results well agree with the theoretical analysis.
A novel fiber Bragg grating temperature sensor is proposed and experimentally demonstrated with a long-period grating as a linear response edge filter to convert wavelength into intensity-encoded information for interrogation. The sensor is embedded into an aluminum substrate with a larger coefficient of thermal expansion to enhance its temperature sensitivity. A large dynamic range of 110 °C and a high resolution of 0.02 °C are obtained in the experiments. The technique can be used for multiplexed measurements with one broadband source and one long-period grating, and therefore is low cost.
In this paper, a novel fiber Bragg grating (FBG) temperature sensor system with low cost demodulation mechanism was proposed. The broadband light source was demodulated by a long period fiber grating (LPG) with an appropriate attenuation band. The sensing FBG was encapsulated with aluminum groove to make the FBG have a higher thermal expansion coefficient. The sensor with enhanced sensitivity 0.02°C was realized.
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