To meet the high-precision positioning requirements of dam health monitoring,this paper designs a high spatial resolution distributed Brillouin dam health monitoring scheme based on DPP-BOTDA (Differential Pulse Pair Brillouin Optical Time Domain Analysis) technology.Due to the large pulse width and strong pulse energy of the two differential pulses used in the Brillouin system, the high-precision measurement of Brillouin frequency shift can be achieved; Moreover, the difference in pulse width between the differential pulse pair is small, which can achieve high spatial resolution and meet the high-precision positioning and measurement requirements for dam health monitoring. The rise and fall time of the pulse signals is a key factor affecting the spatial resolution of DPP-BOTDA systems. To achieve centimeter level spatial resolution, this paper designs a SOA (semiconductor optical amplifiers) narrow pulse driving circuit that can generate pulse signals with rise and fall times less than 1 ns (ps level pulse edge). An experimental setup for the DPP-BOTDA system is built. The SOA narrow pulse driving circuit mentioned above is used to generate pulse signals with pulse widths of 48ns and 50ns, respectively, for differential Brillouin tests. By analyzing the spatial resolution test curve of the system with the differential pulse signals demodulation, it can be concluded that the system can achieve a spatial resolution of 0.2m. It meets the high spatial resolution requirements for dam health monitoring and is of great significance in the field of distributed Brillouin fiber optic sensing applications.
KEYWORDS: Demodulation, Temperature metrology, Spatial resolution, Pulse signals, Frequency response, Data processing, Time-frequency analysis, Signal detection
Traditional Brillouin optical time domain reflectometer (BOTDR) generally uses frequency scanning to obtain the Brillouin gain spectrum. And the measurement speed of the sweeping frequency (SF) method is slow, usually on the order of minutes. In this paper, the mechanism of rapid BOTDR measurement based on sloped-assisted (SA) technology is analyzed, the measurement frequency of SA-BOTDR is theoretically calculated, and the system scheme is proposed and designed. Through experimental research, the temperature measurement range of the system is 25°C~70°C at the end of the 203m sensing fiber. The temperature measurement accuracy is 1.38°C, the spatial resolution is 1.21 m, and the measurement frequency is 11.49 Hz.
In this paper, we introduce our recent progress in resolution improvement of distributed temperature sensors by employing narrow pulse laser and wideband avalanche photoelectric detector (APD). The narrow pulse laser generates the light pulse with 4 ns with at its half maximum, which represents 40 cm spatial resolution. the wideband APD is employed to acquire the backscattering light at Raman wavelength and meet the need of spatial resolution of the narrow pulse laser. High spatial resolution is attributive to narrow pulse width, but also leads to the deterioration of the signal to noise ratio of demodulated temperature curve. Thus the APD with both high sensitivity and wideband width is employed to achieve high SNR and high spatial resolution. Temperature tests were taken out to evaluate the performance of proposed sensor. Results showed that the performance of our proposed spatial resolution enhanced distributed temperature sensor could achieve the temperature error of 1.2°C, temperature resolution of 0.49°C, sensing length of 1 km and spatial resolution of 0.5 m.
In this paper, we introduce an optimal peak search strategy of Brillouin gain spectrum (BGS) and denoising method of central wavelength of BGS based on Brillouin optical time domain analysis (BOTDA) system. The system utilizes both up shifted and down shifted continuous-wave light to generate Brillouin scattering light, while using the amplified pulsed light to boost the signal by stimulated Brillouin scatting effect. In order to obtain an accurate strain change of fiber under test (FUT), the peak of each Brillouin scattering spectrum must be accurate and precise. In addition, the intensity of shifted light should be stable as well. Therefore, we propose a novel peak search algorithm of BGS of a BOTDA system, which keeps the right central wavelength even if the scan step of the frequency is in large condition. Besides that, we also proposed a time domain denoising method to enhance the signal to noise ratio. We also implement static experiments to verify our proposed scheme. Strain/temperature tests were taken out evaluate the performance of the BOTDA. Results showed that the resolution of BOTDA system could reach 13με/0.65°C at 1 km fiber cable.
The application of distributed optical fiber sensing technology in nuclear island safety monitoring is mainly studied in this paper. The anti-radiation ability of the system is an important index. The distributed optical fiber system in this paper is designed with a special anti-radiation optical fiber. The temperature of power supply cables and other facilities in the nuclear island containment can be distributed monitored in full period in real time. Radiation tests are carried out to validate the anti-radiation performance of the system. 60Co is used as a gamma ray radiation source to generate continuous pulses with an average energy of 1.25 MeV. The rate of radiation dose is 1800 Gy/h and the total radiation dose is 1950 Gy. The anti-radiation ability of anti-radiation optical fiber and common optical fiber is compared. It is proved by tests that common optical fiber sensor has great fiber loss in radiation environment, about 0.5dBm, and the loss is decreasing continuously. Anti-radiation optical fiber has little fiber loss in radiation environment, about 0.12 dBm, and the loss tends to be saturated. During the radiation test, the temperature measurement performance of the distributed optical fiber sensing system using anti-radiation optical fiber and the one using common optical fiber is tested in real time. The results show that the anti-radiation distributed optical fiber sensing system performs well in the whole process, which meets the requirements of temperature monitoring for 1339.2 Gy total radiation dose in the refueling cycle of nuclear island.
Considering the shortcomings of the traditional security technology system, such as poor stability, limited installation environment, high false alarm and missing report rate, this paper proposes a hanging fiber detector and video monitor system for perimeter security based on Michelson Interference Technology, and carries out functional test in the oil terminal of Petrochina Company Limited. In this system, an ordinary single-mode communication optical cable is arranged along the top of the fence in wavy shape. When the intruder climbs or turns over the fence, the system triggers the alarm event and emits an alarm. Then the high-speed dome camera will be controlled to turn and shoot the location of the invasion through the network video recorder and network switch. In the functional test, the system has realized climbing alarm, knocking the wall alarm, multi-point simultaneous intrusion alarm, false alarm learning, broken fiber alarm and other functions, which has better sensitivity, lower false alarm rate and better learning function compared with the products based on Sagnac principle and double MZ principle.
In this paper, we introduce an optimal bias voltage searching strategy and maintaining method in BOTDA system based on dual sideband modulation. The system utilizes both up shifted and down shifted continuous wave light to generate Brillouin scattering light, while using the amplified pulsed light to boost the signal by stimulated Brillouin scatting effect. In order to obtain a clean Brillouin shifted curve along the fiber under test, the probe light must be in good signal to noise ratio. In addition, the intensity of shifted light should be stable as well. Therefore, we propose a novel control method of frequency shift in sideband modulation of a BOTDA system, which keeps the first order frequency-shifted light at its maximum signal to noise ratio and minimum light intensity fluctuation. We also implement static experiments to verify our proposed scheme. Strain/temperature tests were taken out to evaluate the performance of the BOTDA. Results showed that the resolution of BOTDA system could reach 13με/0.65°C at 50 km fiber cable with spatial resolution of 0.5m.
An algorithm of Rayleigh noise compensation in dual-end configured distributed temperature sensor (DE-DTS) is proposed in this paper. A 2 km long multi-mode fiber is used to calculate the attenuation of the light within Anti-Stokes and Stokes bandwidth, and figure out the isolation of the wavelength division multiplex through both Anti-Stokes channel and Stokes channel. Experiments are taken out to validate the proposed Rayleigh compensation algorithm. As a result, the Rayleigh noise in both Anti-Stokes component and Stokes component is compensated, and the temperature error of the dual-end configured distributed temperature sensor is revised. With the help of the proposed algorithm, a dual-end DTS can reach to the absolute accuracy of 1.09°C (RMSE) between 180°C to 300°C, which significantly monishes the temperature error compared to the sensor without Rayleigh noise compensation.
KEYWORDS: Raman spectroscopy, Temperature metrology, Signal attenuation, Backscatter, Temperature sensors, Data acquisition, Cesium, Sensing systems, Pulsed laser operation
Numerical assessment of temperature uncertainty of Raman-based distributed temperature sensor is taken out in this paper. The sensing system utilized a modified loop-configuration to avoid stimulated Raman scattering and wavelength dependent loss. The experimental results show that the temperature accuracy reaches 0.74°C (RMSE) between -65°C to 300°C with 2 km fiber under test.
Distributed optical fiber temperature sensing system (DTS) is a sensing technology for real-time sensor of spatial temperature field distribution. The technology is based on Raman scattering and optical time domain reflection (OTDR),, and is composed of a demodulation host and a temperature sensing cable. The system obtains the ground temperature field change through the temperature sensing cable, calculates the geothermal energy replenishment amount and the recharge rate, and determines the recovery of the geothermal field after heating. It provides according to setting the allowable amount of geothermal energy to be mined and ensuring the long-term sustainable operation of the geothermal heating system.
We proposed a gain compensation method to overcome the amplitude fading induced by the gain-bandwidth product (GBP) of the detector, which will seriously deteriorate the positioning accuracy of the distributed disturbance sensor at a long sensing range. To guarantee the performance of this method, we used the time-frequency distribution of the interference signal to distinguish the normal signal and the one need to compensate. A positioning measurement experiment using an asymmetric dual Mach-Zehnder interferometer (ADMZI) was carried out to verify the effectiveness of the proposed method. The experiment result showed that the sensing range can reach 121km, which was improved by over 40% compared to the traditional positioning method without gain compensation.
A novel method to enhance the positioning resolution of distributed disturbance fiber sensor is proposed in this paper. The proposed scheme combines a high speed data acquisition system and a modified time delay estimate algorithm. The sensor performance is significantly improved by eliminating the impact of fluctuation of the interference signal generated by the environment disturbance. Theoretical analysis shows that with the proposed spatial resolution enhanced method, the disturbance sensing system is more suitable for various environments and provides low uncertainty in long term operation with meter-scale spatial resolution. Compared with the traditional time delay estimate method in distributed disturbance sensing system based on the criterion of spatial resolution, the positioning error of the sensor using our proposed method has been reduced at least an order of magnitude.
KEYWORDS: Sensing systems, Error analysis, Signal detection, Signal to noise ratio, Backscatter, Data acquisition, Interferometry, Interference (communication), Interferometers, Polarization
A positioning algorithm for two-wavelength dual Mach-Zehnder interferometry (TDMZI) vibration sensing system is proposed. We employ the reciprocal of the interval between neighboring zero-crossing (NZC) points to represent the frequency distribution of the interference signal in time domain. Meanwhile, these reciprocal points are used to fit a curve and we use cross correlation to estimate the time delay of the two fitted curves. Finally we analysis the positioning error caused by the proposed positioning algorithm and experimentally demonstrate that the algorithm can be used to locating with positioning error of ±50m. This algorithm has a promising potential in long distance two-wavelength vibration sensing system.
A novel method to auto-correct the fluctuation of calibration in Raman distributed temperature sensor is proposed in this paper. The proposed scheme combines a fiber coil configuration with a Pt-resistance in the front section to cancel out the impact of fluctuation generated by perturbations of the laser and APD, instability of power supply and environment temperature changes. The sensor performance is significantly improved by exploiting the fiber coil and Pt-resistance to correct the temperature calibration. Our theoretical analysis shows that with the calibration of temperature autocorrection method the sensing system is more suitable for various environments and provides low uncertainty in long term operation, and it has the potential to accurate temperature calibration with simple equipment and to reduce costs of the system.
KEYWORDS: Correlation function, Super resolution, Sensors, Interferometry, Signal to noise ratio, Fourier transforms, Data acquisition, Zoom lenses, Signal detection, Optoelectronics
A novel positioning algorithm based on super-resolution time delay estimation in dual Mach-Zehnder interferometry disturbance sensor is employed. We first compute the twice correlation function of the two output signal of DMZI by using modified chirp z-transform. Then fine interpolation of correlation peak is adopted to compute waveform of the main correlation peak only using the main segment of the cross power spectrum to improve the resolution of the twice correlation function. At last, in order to enhance the capacity of peak detection, we calculate the difference between the correlation function and its Hilbert transform to sharpen the peak of the correlation function. We have experimentally demonstrated that the proposed positioning algorithm can improve the positioning resolution and accuracy, and it has the potential to accurate positioning in low sampling rate and reduce costs of the system.
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