Zhenyang Ding

Long-range vibration sensor based on correlation analysis of optical frequency-domain reflectometry signals

We present a novel method to achieve a space-resolved long- range vibration detection system based on the correlation analysis of the optical frequency-domain reflectometry (OFDR) signals. By performing two separate measurements of the vibrated and non-vibrated states on a test fiber, the vibration frequency and position of a vibration event can be obtained by analyzing the cross-correlation between beat signals of the vibrated and non-vibrated states in a spatial domain, where the beat signals are generated from interferences between local Rayleigh backscattering signals of the test fiber and local light oscillator. Using the proposed technique, we constructed a standard single-mode fiber based vibration sensor that can have a dynamic range of 12 km and a measurable vibration frequency up to 2 kHz with a spatial resolution of 5 m. Moreover, preliminarily investigation results of two vibration events located at different positions along the test fiber are also reported.

Compensation of laser frequency tuning nonlinearity of a long range OFDR using deskew filter

We present a simple and effective method to compensate the optical frequency tuning nonlinearity of a tunable laser source (TLS) in a long range optical frequency-domain reflectometry (OFDR) by using the deskew filter, where a frequency tuning nonlinear phase obtained from an auxiliary interferometer is used to compensate the nonlinearity effect on the beating signals generated from a main OFDR interferometer. The method can be applied to the entire spatial domain of the OFDR signals at once with a high computational efficiency. With our proposed method we experimentally demonstrated a factor of 93 times improvement in spatial resolution by comparing the results of an OFDR system with and without nonlinearity compensation. In particular we achieved a measurement range of 80 km and a spatial resolution of 20 cm and 1.6 m at distances of 10 km and 80 km, respectively with a short signal processing time of less than 1 s for 5 × 10(6) data points. The improved performance of the OFDR with a high spatial resolution, a long measurement range and a short process time will lead to practical applications in the real-time monitoring, test and measurement of fiber optical communication networks and sensing systems.

Long Measurement Range OFDR Beyond Laser Coherence Length

We present a novel method to significantly extend the measurement range of an optical frequency domain reflectometry (OFDR) beyond the laser coherence length for both discrete reflection (such as Fresnel reflection) and Rayleigh backscattering measurements. A key to the method is to increase the laser frequency tuning speed while measuring the phase noise of the reflected optical signal that carries the location and reflectivity information. Using this technique in a conventional OFDR with a noise floor of -120 dB and a dynamic range of 53 dB, we achieved a measurable distance of 170 km for Fresnel reflection and 120 km for Rayleigh backscattering using a laser with a coherence length of only 13.6 km.

An Improved Positioning Algorithm With High Precision for Dual Mach–Zehnder Interferometry Disturbance Sensing System

An improved positioning algorithm for dual Mach-Zehnder interferometry (DMZI) disturbance sensing system is proposed. We employ zero-crossing method, which can be computed easily to extract the disturbance signal segment with maximum zero-crossing rate. Meanwhile, we use general cross correlation based on Wiener filtering and Gnn subtraction weighting function (WG-GCC) to estimate the time delay of the extracted signal, which is robust to the correlated noise. Finally, we experimentally demonstrate that the proposed positioning algorithm can greatly improve the positioning accuracy with positioning error of ±20 m. Compared with the traditional positioning algorithm, the positioning error has been reduced by an order of magnitude. This algorithm has a promising potential in real-time fence perimeter applications.

An Elimination Method of Polarization-Induced Phase Shift and Fading in Dual Mach–Zehnder Interferometry Disturbance Sensing System

We proposed an improved polarization control method to reduce polarization-induced phase shift (PIPS) and polarization-induced fading (PIF) in a dual Mach-Zehnder interferometry (DMZI) disturbance sensing system. PIPS and PIF will seriously affect the positioning accuracy of the DMZI sensor. This polarization control method uses two PCs to control the state of polarization at the inputs of clockwise Mach-Zehnder interferometer and counter-clockwise Mach-Zehnder interferometer, respectively, based on chaotic particle swarm optimization algorithm, which has advantages of high speed and easy implementation. We experimentally demonstrated that the polarization method can overcome the effect of PIPS and PIF, improve the positioning accuracy of the DMZI sensor efficiently with positioning error of ±20 m after polarization control. Compared with the traditional polarization control method in DMZI based on the criterion of visibility, the positioning error of the sensor using our proposed method has been reduced at least an order of magnitude.

Cryogenic Temperature Measurement Using Rayleigh Backscattering Spectra Shift by OFDR

We present a method to realize temperature variation measurement in the cryogenic environment (e.g., at 76 K) using Rayleigh backscattering spectra (RBS) shift in standard single mode optical fiber by optical frequency-domain reflectometry. By analyzing the relationship of effective sensing segment size of fiber (or sensing spatial resolution), minimal measurable temperature variation, and temperature response of RBS shift, we found minimal measurable temperature variation in the cryogenic environment can be improved by increasing effective sensing segment size. Our experiments show that at a relatively high temperature (e.g., above 195 K), minimal measurable temperature variation is 0.21 K with an effective sensing segment size of 8 cm. When the temperature is very low (e.g., at 76 K), minimal measurable temperature variation can still maintain 0.34 K by simply increasing the effective sensing segment size of fiber to 48 cm.

Long-Range Distributed Fiber Vibration Sensor Using an Asymmetric Dual Mach–Zehnder Interferometers

An asymmetric dual Mach-Zehnder interferometry (ADMZI) design is proposed to achieve distributed fiber vibration sensing at long range. In this structure, we utilized two distributed feedback laser beams and dense wavelength-division multiplexing to significantly reduce the influence of Rayleigh backscattering noise, which will seriously deteriorate the signal noise ratio (SNR) in traditional dual Mach=Zehnder interferometer (DMZI) sensor. At the sensing length of 61 km, the SNR achieved using the ADMZI design is 20 dB higher than that using the DMZI design. Using a positioning algorithm based on the time=frequency analysis, the positioning mean square error can reach 52.5 m at this distance.

A High-Efficiency Multiple Events Discrimination Method in Optical Fiber Perimeter Security System

This paper proposes an integrated scheme to distinguish invasive events in optical fiber dual Mach-Zehnder Interferometry based perimeter security system. This algorithm combined empirical mode decomposition, kurtosis characteristics with radial basis function neural network, which can improve the recognition rate of event discrimination and increase the variety of intrusion events. Several experiments demonstrate that the proposed scheme can discriminate four common invasive events (climbing the fence, knocking the cable, cutting the fence, and waggling the fence) with an average recognition rate above 85.75%, which can satisfy actual application requirements.

Wavelength-division-multiplexing method of polarized low-coherence interferometry for fiber Fabry–Perot interferometric sensors

We propose a new wavelength-division-multiplexing method for extrinsic fiber Fabry-Perot interferometric (EFPI) sensing in a polarized low-coherence interferometer configuration. In the proposed method, multiple LED sources are used with different center wavelengths, and each LED is used by a specific sensing channel, and therefore the spatial frequency of the low-coherence interferogram of each channel can be separated. A bandpass filter is used to extract the low-coherence interferogram of each EFPI channel, and thus the cavity length of each EFPI channel can be identified through demultiplexing. We successfully demonstrate the simultaneous demodulation of EFPI sensors with same nominal cavity length while maintaining high measurement precision.

Note: Improving spatial resolution of optical frequency-domain reflectometry against frequency tuning nonlinearity using non-uniform fast Fourier transform

We propose using non-uniform FFT to minimize the degrading effect of frequency tuning nonlinearity of a tunable laser source (TLS) in an optical frequency-domain reflectometry (OFDR) system. We use an auxiliary interferometer to obtain the required instantaneous optical frequency of the TLS and successfully demonstrate 100 times enhancement in spatial resolution of OFDR with only a 20% increase in computation time. The corresponding measurement reflectivity sensitivity is better than -80 dB, sufficient to detect bending induced index changes in an optical fiber.