The Guangzhou New TV Tower (GNTVT), currently being constructed in Guangzhou, China, is a supertall structure with a height of 610 m. This tube-in-tube structure comprises a reinforced concrete inner tube and a steel outer tube adopting concrete-filled-tube columns. A sophisticated structural health monitoring (SHM) system consisting of over 600 sensors has been designed and is being implemented by The Hong Kong Polytechnic University to GNTVT for both in-construction and in-service real-time monitoring. This paper outlines the technology innovation in developing and implementing this SHM system, which includes (i) modular design of the SHM system, (ii) integration of the in-construction monitoring system and the in-service monitoring system, (iii) wireless-based data acquisition and Internet-based remote data transmission, (iv) design and implementation of a fiber Bragg grating sensing system,(v) structural health and condition assessment using static and dynamic monitoring data, (vi) verification of the effectiveness of vibration control devices by the SHM system, and (vii) development of an SHM benchmark problem by taking GNTVT as a test bed and using real-world measurement data. Preliminary monitoring data including those obtained during the Wenchuan earthquake and recent typhoons are also presented. Copyright © 2008 John Wiley & Sons, Ltd.

Technology innovation in developing the structural health monitoring system for Guangzhou New TV Tower

Optical fibers have been involved in the area of sensing applications for more than four decades. Moreover, interferometric optical fiber sensors have attracted broad interest for their prospective applications in sensing temperature, refractive index, strain measurement, pressure, acoustic wave, vibration, magnetic field, and voltage. During this time, numerous types of interferometers have been developed such as Fabry-Perot, Michelson, Mach-Zehnder, Sagnac Fiber, and Common-path interferometers. Fabry-Perot interferometer (FPI) fiber-optic sensors have been extensively investigated for their exceedingly effective, simple fabrication as well as low cost aspects. In this study, a wide variety of FPI sensors are reviewed in terms of fabrication methods, principle of operation and their sensing applications. The chronology of the development of FPI sensors and their implementation in various applications are discussed.

https://www.mdpi.com/1424-8220/14/4/7451/pdf-vor

Chronology of Fabry-Perot interferometer fiber-optic sensors and their applications: a review.

This paper presents a systematic review of long period fiber gratings (LPFGs) written by the CO2 laser irradiation technique. First, various fabrication techniques based on CO2 laser irradiations are demonstrated to write LPFGs in different types of optical fibers such as conventional glass fibers, solid-core photonic crystal fibers, and air-core photonic bandgap fibers. Second, possible mechanisms, e.g., residual stress relaxation, glass structure changes, and physical deformation, of refractive index modulations in the CO2-laser-induced LPFGs are analyzed. Third, asymmetrical mode coupling, resulting from single-side laser irradiation, is discussed to understand unique optical properties of the CO2-laser-induced LPFGs. Fourthly, several pretreament and post-treatment techniques are proposed to enhance the efficiency of grating fabrications. Fifthly, sensing applications of the CO2-laser-induced LPFGs are investigated to develop various LPFG-based temperature, strain, bend, torsion, pressure, and biochemical sensors. Finally, communication applications of the CO2-laser-induced LPFGs are investigated to develop various LPFG-based band-rejection filters, gain equalizers, polarizers, and couplers.

/pdf/review-of-long-period-fiber-gratings-written-by-co2-laser-19six9u5nt.pdf

Review of long period fiber gratings written by CO2 laser

We demonstrate a sub-micron silica diaphragm-based fiber-tip Fabry–Perot interferometer for pressure sensing applications. The thinnest silica diaphragm, with a thickness of ∼320  nm, has been achieved by use of an improved electrical arc discharge technique. Such a sub-micron silica diaphragm breaks the sensitivity limitation imposed by traditional all-silica Fabry–Perot interferometric pressure sensors and, as a result, a high pressure sensitivity of ∼1036  pm/MPa at 1550 nm and a low temperature cross-sensitivity of ∼960  Pa/°C are achieved when a silica diaphragm of ∼500  nm in thickness is used. Moreover, the all-silica spherical structure enhanced the mechanical strength of the micro-cavity sensor, making it suitable for high sensitivity pressure sensing in harsh environments.

Sub-micron silica diaphragm-based fiber-tip Fabry-Perot interferometer for pressure measurement.

In this study, the authors present an experimental and theoretical description of the use of first order Raman amplification to improve the performance of a Phase-sensitive optical time domain reflectometer (φOTDR) when used for vibration measurements over very long distances. A special emphasis is given to the noise which is carefully characterized and minimized along the setup. A semiconductor optical amplifier and an optical switch are used to greatly decrease the intra-band coherent noise of the setup and balanced detection is used to minimize the effects of RIN transferred from the Raman pumps. The sensor was able to detect vibrations of up to 250 Hz (close to the limits set by the time of flight of light pulses) with a resolution of 10 m in a range of 125 km. To achieve the above performance, no post-processing was required in the φOTDR signal. The evolution of the φOTDR signal along the fiber is also shown to have a good agreement with the theoretical model.

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Phase-sensitive Optical Time Domain Reflectometer Assisted by First-order Raman Amplification for Distributed Vibration Sensing Over >100 km

A novel fibre-optic sensing system used for temperature-strain discrimination is presented. This system consists of an extrinsic Fabry-Perot interferometric sensor (EFPI) and a chirped in-fibre Bragg grating (CFBG) in series. The EFPI and the CFBG are wavelength-division-multiplexed (WDM) to provide strain and temperature information, respectively. The wavelength-shift of the CFBG induced by temperature change in the 1.55 µm region is interrogated with an intensity-based scheme, allowing fast measurement of temperature. The cavity length change of the EFPI is measured in the 1.3 µm region, allowing strain to be measured without cross-talk from the temperature sensor, i.e. the CFBG. Experimental results show that the achieved accuracies for strain and temperature measurement are ±20×10-6 and ±2°C, respectively.

https://iopscience.iop.org/article/10.1088/0256-307X/18/5/307/pdf

Temperature-Strain Discrimination Sensor Using a WDM Chirped in-Fibre Bragg Grating and an Extrinsic Fabry-Pérot

A novel technique for simultaneous measurement of static strain, temperature and vibration in health monitoring of structures is demonstrated using an integrated in-fibre Bragg grating (FBG)/extrinsic Fabry-Perot interferometer (EFPI) sensor. The EFPI sensor provides static strain and vibration information simultaneously by using the channel-spectrum method and the low-coherence interferometric technique, respectively. The FBG sensor is used for temperature measurement. The experimental results show that a static-strain accuracy of ±20µe, a temperature accuracy of ±1°C and a vibration resolution of 1 nm have been achieved with a good repeatability.

https://iopscience.iop.org/article/10.1088/0256-307X/18/12/323/pdf

Simultaneous Static Strain, Temperature and Vibration Measurement Using an Integrated FBG/EFPI Sensor

A novel tunable fiber ring laser configuration with a combination of Raman amplification and erbium-doped fiber amplification (EDFA) is proposed for realizing high signal-to-noise ratio (SNR), ultra-long distance (300 km) fiber Bragg grating (FBG) sensor systems The hybrid Raman–EDF amplification configuration can enhance the optical SNR of FBG sensor signals significantly, so the optical power reflected by the fiber Bragg grating can be compensated A good optical SNR of 15 dB is achieved for a 275 km transmission distance, which is the best performance to our knowledge, and about 4 dB at 300 km In the static strain experiments with fiber lengths of 100,200,250,300 km, the linearity is greater than 0999 This system can be widely used in the ultra-long-distance sensing applications such as railways, oil(gas) pipelines and coastline

Ultra-Long Distance Fiber Bragg Grating Sensor System

An all-fibre dynamic gain equalizer based on a novel long-period fibre grating (LPFG) written on a photosensitive fibre by high-frequency CO2 laser pulses is demonstrated for the dynamic gain flattening of Er-doped fibre amplifiers. The resonant peak amplitude of such a dynamic gain equalizer can be adjusted linearly by bending the LPFG. The gain profile of the Er-doped fibre amplifier constructed has been flattened to within ±0.7 dB over a 32 nm bandwidth using such a device.

An All-Fibre Dynamic Gain Equalizer Based on a Novel Long-Period Fibre Grating Written by High-Frequency CO2 Laser Pulses

A novel theory, namely, Fourier mode coupling (FMC) theory for fiber Bragg gratings (FBGs) is proposed in this paper. During analyzing coupled modes of FBGs, the Fourier transform relations among the amplitude coefficients of coupled modes are found for the first time. The general expressions of reflective and transmissive spectra of FBGs are deduced from the combination of Fourier transform with the well-known coupled-mode theory. In the proposed FMC theory, the spectral characteristics of the FBG are achieved by the calculation of coupled modes in the spatial domain spectrum, which is the Fourier transform result of refractive index perturbation in the FBG. The FBG spectrum based on the FMC theory is simulated here, and compared with those obtained from the coupled mode theory and pure Fourier transform. The comparison shows that the FMC theory for and the derived spactra of FBGs are in accordance with the coupled mode theory and the practical spectra of the FBG respectively. The FMC theory has many features, these being simple, clear, direct, accurate and fast, which could be used as a universal tool for fast spectrum analysis of any FBG with an arbitrary distribution of refractive index perturbation along the fiber axis.

Rao Yun-Jiang

Papers

Temperature-Strain Discrimination Sensor Using a WDM Chirped in-Fibre Bragg Grating and an Extrinsic Fabry-Pérot

Simultaneous Static Strain, Temperature and Vibration Measurement Using an Integrated FBG/EFPI Sensor

Ultra-Long Distance Fiber Bragg Grating Sensor System

An All-Fibre Dynamic Gain Equalizer Based on a Novel Long-Period Fibre Grating Written by High-Frequency CO2 Laser Pulses

Theory of Fourier mode coupling for fiber Bragg gratings