Suppression Of Fading Research Articles

A Space-Division Multiplexing Method for Fading Noise Suppression in the Φ-OTDR System.

Phase-sensitive time-domain reflectometry (Φ-OTDR) can be used for fully distributed long-distance vibration monitoring. There is a fading phenolmenon in the Φ-OTDR, which will cause the signal intensity somewhere to be too low to extract the phase of the signal without distortion. In this paper, the Φ-OTDR based on space-division multiplexing (SDM) is proposed to suppress fading and we used multi-core optical fiber (MCF) to realize SDM. While inheriting the previous optimization strategy, we proposed a strategy based on frequency spectral similarity to process multiple independent signals obtained by SDM. And we compared the two methods. Through the experiments, the distortion rate can be reduced from an average level of 9.34% to less than 2% under continuous running of 270 s, which proves that SDM is a reliable technical route to achieve fading suppression. This method can effectively improve the fading suppression capability of the existed commercial systems.

Recent Progress in Distributed Fiber Acoustic Sensing with Φ-OTDR.

Distributed fiber acoustic sensing (DAS) technology can continuously spatially detect disturbances along the sensing fiber over long distance in real time. It has many unique advantages, including, large coverage, high time-and-space resolution, convenient implementation, strong environment adaptability, etc. Nowadays, DAS becomes a versatile technology in many fields, such as, intrusion detection, railway transportation, seismology, structure health monitoring, etc. In this paper, the sensing principle and some common performance indexes are introduced, and a brief overview of recent DAS researches in Shanghai Institute of Optics and Fine Mechanics (SIOM) is presented. Some representative research advances are explained, including, quantitative demodulation, interference fading suppression, frequency response boost, high spatial resolution, and distributed multi-dimension localization. The engineering applications of DAS, carried out by SIOM and other groups, are summarized and reviewed. Finally, possible future directions are discussed and concluded. It is believed that, DAS has great development potential and application prospect.

Performance enhancement method for phase-sensitive optical time-domain reflectometer system based on suppression of fading induced false alarms

The phase-sensitive optical time-domain reflectometry (Φ-OTDR) has been developed rapidly as a fully fiber-optic distributed vibration sensing technology. However, the demodulation technique based on the phase term would induce a serious false alarm problem due to the signal fading effect. An effective method to suppress fading-induced false alarms in the Φ-OTDR system is proposed, which is based on the suppression mask and numerical relationship between phase and amplitudes of Rayleigh backscattering. The performance of the proposed method has been experimentally demonstrated in both laboratory environment and in-field situation test. Without any hardware addition in a traditional Φ-OTDR equipment, false alarms rate can be reduced from 4.81% to 0.15%, whereas low missing alarms rate can be achieved at the same time. In-field results show that this work provides a low-cost solution to enhance the performance for real-life engineering application of the phase-discrimination Φ-OTDR system.

Continuous Fading Suppression Method for Φ-OTDR Systems Using Optimum Tracking Over Multiple Probe Frequencies

The demand for distributed acoustic sensors, which are capable of reconstructing the amplitude, frequency, and phase of an acoustic field, is increasing. These sensors are typically realized by phase-sensitive optical time-domain reflectometry (Φ-OTDR). However, common Φ-OTDR systems suffer from a fading phenomenon, which causes amplitude fluctuation on Rayleigh backscattering traces, and the scattered light may not have enough intensity in some regions. These areas have low amplitude backscattering that may be even lower than the system noise floor during some periods of time. Therefore, we cannot reconstruct the phase signal properly in such low intensity areas. Systems with multiple frequencies have been proposed to achieve fading suppression. However, online data processing of such schemes remains a challenge, especially for continuous in-field applications. Prediction of fading must be performed in real time. Any delay caused by massive calculations is not acceptable. In this paper, a continuous fading suppression method based on a Φ-OTDR system with three different probe frequencies is presented as well as a tracking algorithm for selecting the optimum probe signal for any time, which predicts the occurrence of fading before it actually occurs. The performance of the proposed method has been experimentally evaluated and statistically analyzed. The distortion induced by the fading effect could be suppressed to 1.15% under continuous real-time running. The method is highly effective and repeatable, which makes it suitable for practical online purposes.

Rayleigh Fading Suppression in One-Dimensional Optical Scatters

Highly coherent wave is favorable for applications in which phase retrieval is necessary, yet a high coherent wave is prone to encounter Rayleigh fading phenomenon as it passes through a medium of random scatters. As an exemplary case, phase-sensitive optical time-domain reflectometry (\Phi-OTDR) utilizes coherent interference of backscattering light along a fiber to achieve ultra-sensitive acoustic sensing, but sensing locations with fading won't be functional. Apart from the sensing domain, fading is also ubiquitous in optical imaging and wireless telecommunication, therefore it is of great interest. In this paper, we theoretically describe and experimentally verify how the fading phenomena in one-dimension optical scatters will be suppressed with arbitrary number of independent probing channels. We initially theoretically explained why fading would cause severe noise in the demodulated phase of \Phi-OTDR; then M-degree summation of incoherent scattered light-waves is studied for the purpose of eliminating fading. Finally, the gain of the retrieved phase signal-to-noise-ratio and its fluctuations were analytically derived and experimentally verified. This work provides a guideline for fading elimination in one-dimension optical scatters, and it also provides insight for optical imaging and wireless telecommunication.

Real-Time Polarimetric Optical High-Voltage Sensor Using Phase-Controlled Demodulation

Modern smart grid designs demand reliable sensors throughout the power delivery systems, not only to monitor the quality of the electrical energy generated by different technologies (wind, solar, hydroelectric, thermoelectric, nuclear), but also as an integral part of the generation, transmission, and distribution networks control systems. Laser polarimetric interferometry is a very suitable technique for the implementation of high sensitivity measurement instruments for various physical quantities, in particular, for high-voltage sensing. However, because the polarimetric technique is extremely sensitive to small refractive indexes variations induced in electro-optic materials by the electric field or voltage to be measured, it is also sensitive to spurious environmental quantities such as temperature variations. In this work, a novel optical voltage sensor (OVS) in conjunction with an efficient real-time method for optical phase demodulation and a PI feedback control method for signal fading suppression is demonstrated. Linearity for sinusoidal voltages having amplitudes up to 8.2 kV was evaluated, with an accuracy of 0.2% in the OVS nominal operating range. The OVS bandwidth of 5 kHz is sufficient for most electrical power quality sensor applications. Nonsinusoidal signals with high harmonic content were tested resulting in a total harmonic distortion that differed by only 0.12% from that measured by an HV reference probe. Responses to transient disturbances were also determined, showing the capability of the system to detect spikes.

The Fading Characteristics of the Top-Loaded WCAU Antenna

The electrical characteristics of the original guyed cantilever antenna in use at WCAU are reviewed. Fading characteristics taken at several points in the service area are shown and analyzed. Probability considerations are used to place the fading phenomena on a quantitative basis. Model tests are described which led to the use of a capacitive area at the top of the tower. This capacitance consists of a set of outriggers insulated from the tower. The outriggers are then connected to the tower through an inductance. The first hat so constructed was found to be too large for good suppression of fading. The hat was reduced in size until a desirable condition was reached. Analysis of fading records, as well as listening tests, shows that the primary night service area is more than doubled and is now practically the same as the day service area. This has been accomplished with no increase in power and only a slight increase in ground signal.

A Critical Study of the Characteristics of Broadcast Antennas as Affected by Antenna Current Distribution

The action of broadcast antennas with various current distributions is examined in an endeavor to determine the combinations which are most likely to be useful. In each case, the vertical radiation characteristics, the radiation resistance, and the electric field intensity at the surface of the earth one mile from the antenna are determined. The relative merits with respect to fading suppression are also considered. Of the several arrangements examined, some have been very disappointing. In no case where the total antenna height is at all reasonable for broadcast use does the field strength at one mile go to exceptionally high values. For heights of the order of a half wavelength, it is hard to find anything better than a straight vertical wire. Some of the arrangements save a small amount in height, but in no other respect do they improve on the straight wire. The analysis of the antenna with decreased velocity and antennas spread in the horizontal plane shows that both are definitely ruled out of the picture. The results show that the Franklin antenna would be very useful if it were not for the great heights required.