Nian Fang

A distributed optical fiber sensing system based on a bidirectional chaotic fiber ring laser has been proposed. The output waveforms induced by an external disturbance in some period are different from those in other time. This period equals the time difference between two counter-propagating beams arriving at the semiconductor optical amplifier (SOA) from the disturbance point. It is utilized for the disturbance location. In this paper, the location mechanism is explored by investigating optical interaction in the SOA in two simulation systems with a continuous wave optical source. One system is for studying the interaction of a continuous wave beam and a phase modulation beam input into the SOA from the left and right sides respectively. The other system is for investigating the interaction of two phase modulation beams counter-arriving at the SOA with a certain time difference. Under a small SOA current, only the transformation of the phase modulation to intensity modulation occurs due to the interference caused by facets reflection of the SOA. With the increase of current, the cross-gain modulation effect of the SOA makes the interference signal in one beam copy to the other one reversely in the phase, which generates the time difference characteristic. For the chaotic sensing system the situation is similar to the large current case in the second simulation system, only the conversion of the modulation format is achieved by the sensitivity to initial values of chaotic systems. The cross-gain modulation effect in the SOA contributes to the time difference location method.

Welcome to the WIT Press eLibrary - the home of the Transactions of the Wessex Institute collection, providing on-line access to papers presented at the Institute's prestigious international conferences and from its State-of-the-Art in Science & Engineering publications.

A novel fiber-optic acoustic sensor using an erbium-doped fiber amplifier (EDFA)-based fiber ring laser and a balanced Sagnac interferometer for acoustic sensing of the partial discharge (PD) in power transformers is proposed and demonstrated. As a technical background, an experimental investigation on how the variations of the fiber birefringence affect the sensor performances was carried out, and the results are discussed. The operation principles are described, and the relevant formulas are derived. The analytical results show that an EDFA-based fiber ring laser operating in chaotic mode can provide a degree of polarization (DOP) tunable light beam for effectively suppressing polarization fading noises. The balanced Sagnac interferometer can eliminate command intensity noises and enhance the signal-to-noise ratio (SNR). Furthermore, it inherently operates at the quadrature point of the response curve without any active stabilizations. Several experiments are conducted for evaluating the performances of the sensor system, as well as for investigating the ability of the detection of high-frequency acoustic emission signals. The experimental results demonstrate that the DOP of the laser beam can be continuously tuned from 0.2% to 100%, and the power fluctuation in the whole DOP tuning range is less than 0.05 dBm. A high-frequency response up to 300 kHz is reached, and the high sensing sensitivity for detections of weak corona discharges, as well as partial discharges also is verified.

We improved the local projection denoising effect of chaotic time series with high noise level by adding a Kernel Principal component analysis (PCA) preprocessing to reduce the dependence of the denoising effect to the neighborhood radius. The case study results in the Lorenz system show that the proposed method is effective.

We present an optical fiber sensing system for discrete multi-target’s on-off states detection and location. The sensing system consists of multiple cascaded 1x2 fiber couplers (FCs). Each FC is connected to a reflective optical switch sensor (ROSS). A ROSS is corresponding to a target to be detected. The system can remotely detect the on-off states of multiple ROSSs, as well as accurately and rapidly locate the ROSSs in alarm states. By detecting the various intensities of light pulses reflected by each ROSS, their on-off states can be monitored. The ROSSs in alarm states can be located by directly measuring the reflected light pulse intensity at corresponding time axis or the declined peaks of short-time crosscorrelation of reflection pulse train during adjacent optical pulse period in noisy case. The simulation results with the same and different ROSS intervals show that the detection and location methods proposed are feasible and effective. The system can be used to detect and locate different kinds of physical and chemical targets discretely distributed at the same time, which can prompt state changes of the ROSSs. Although the proposed system is similar to the optical time domain reflectometry (OTDR) in structure, it has better real time performance and lower cost than OTDR.

A recognition method based on the gait characteristic for walking intrusion signal of a fiber fence system is presented, which can reduce the rate of false alarm. The gait characteristic of human is mainly embodied in an intermittent and quasi‐periodic contact with the floor when he walks. It can be expressed in terms of step period and foot‐strike time. They are, on average, about 0.63 s and 0.2 s for a normal walker in the nature state, respectively. According this intermittent and quasi‐periodic feature and local characteristic gait times imbedded in a fiber fence system output waveform, a disturbance source can be adjudged as a human intrusion, not as an animal or other random one. Two types of experimental results show that the proposed method is effective and universal for different systems and races. © 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52:2381–2384, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.25437

A novel fiber-optic distribution monitoring system using birefringent optical circuit synthesis (BOCS) method to monitor the external influences on the fiber is demonstrated. The detection mechanism is based on the synthesis of the transfer function of sensing fiber by using the measurement values of Stokes parameters of transmission light with a optical frequency scanning. The sensing fiber is modeled as one with the birefringence distribution characteristic and is divided into a series of birefringent lattice segments. The mode coupling angle at each birefringent segment is taken as the sensing parameter and its variation distribution as the perturbation distribution along the fiber length can be derived from two profiles of mode coupling angle obtained from the BOCS. The operation principles of the proposed system is described and several experimental results are demonstrated. At last, the discussions on the experimental results and the limitations of the proposed detection method are presented.

A novel laser with tunable and stable degree of polarization (DOP) is introduced. It consists of a polarization beam splitter (PBS) and a bidirectional semiconductor fiber ring, which is composed of a semiconductor optical amplifier (SOA), a polarization controller (PC) and a 2 × 2 wideband fiber coupler. By adjusting the PC or the injected current of the SOA, the time averaged DOP of the laser output varies optionally from 0.16 to 99%. Employing the bidirectional ring structure and PBS polarization combination method, DOP of the laser can be kept stable at any level in that range. © 2009 Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 1669–1671, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24403