In-service structural health monitoring (SHM) of engineering structures has assumed a significant role in assessing their safety and integrity. Fibre Bragg grating (FBG) sensors have emerged as a reliable, in situ, non-destructive tool for monitoring, diagnostics and control in civil structures. The versatility of FBG sensors represents a key advantage over other technologies in the structural sensing field. In this article, the recent research and development activities in structural health monitoring using FBG sensors have been critically reviewed, highlighting the areas where further work is needed. A few packaging schemes for FBG strain sensors are also discussed. Finally a few limitations and market barriers associated with the use of these sensors have been addressed.

Fibre Bragg gratings in structural health monitoring—Present status and applications

We demonstrate a novel sensor which uses the difference in strain and temperature response of fiber Bragg gratings and a long period fiber grating to discriminate between strain and temperature induced wavelength shifts. Sensor interrogation is performed entirely on the fiber Bragg grating reflection signals. Strain and temperature were simultaneously measured to /spl plusmn/9 /spl mu/strain and /spl plusmn/1.5/spl deg/C over a wide range of conditions.

Hybrid fiber Bragg grating/long period fiber grating sensor for strain/temperature discrimination

At least one parameter of at least one fluid in a pipe (12) is measured using a spatial array of acoustic pressure sensors (14, 16, 18) placed at predetermined axial locations x1,x2,x3 along the pipe (12). The pressure sensors (14, 16, 18) provide acoustic pressure signals P1(t), P2(t), P3(t) on lines (20, 22, 24) which are provided to signal processing logic (60) which determines the speed of sound amix of the fluid (or mixture) in the pipe (12) using acoustic spatial array signal processing techniques with the direction of propagation of the acoustic signals along the longitudinal axis of the pipe (12). Numerous spatial array processing techniques may be employed to determined the speed of sound amix. The speed of sound amix is provided to logic (48) which calculates the percent composition of the mixture, e.g., water fraction, or any other parameter of the mixture or fluid which is related to the sound speed amix. The logic (60) may also determine the mach number (MX) of the fluid. The acoustic pressure signals P1(t), P2(t), P3(t) measured are lower frequency (and longer wavelength) signals than those used for ultrasonic flow meters, and thus is more tolerant to inhomogeneities in the flow. No external source is required and thus may operate using passive listening. The invention will work with arbitrary sensor spacing and with as few as two sensors if certain information is known about the acoustic properties of the system.

Fluid parameter measurement in pipes using acoustic pressures

A pressure sensor capable of measuring pressure in a harsh environment includes at least one intrinsic fiber optic sensor element formed within a core of an optical fiber. A length of the optical fiber containing the sensor element is attached to a pressure sensitive structure. A dimension of the pressure sensitive structure changes in response to changes in the pressure of a pressure field of the environment. The sensor element is responsive to an input signal and to a strain caused by changes in the dimension of the pressure sensitive structure for providing a pressure signal indicative of the pressure. A temperature compensation sensor is also formed in the fiber near the location of the pressure sensor. The temperature sensor is isolated from strain associated with the pressure for providing temperature compensation of the pressure sensor. Alternatively, temperature compensation is provided by an intrinsic fiber optic sensor element mounted to experience an equal but opposite strain associated with changes in the dimension of the pressure sensitive structure.

High sensitivity fiber optic pressure sensor for use in harsh environments

The present invention provides a wavelength division multiplexed optical communication system with dynamically stabilized wavelength selectors. To accurately correlate the wavelength of a wavelength selector to the wavelength emitted by an optical transmission source, the present invention uses an optical detector and feedback loop to optimize the wavelength of an optical selection element in accordance with a wavelength of an incident optical channel. In one embodiment, the optical selection element is a Bragg grating associated with a grating wavelength controller, such as a temperature regulator or strain tuning system, to adjust the wavelength band of maximum reflectivity of the grating. The feedback loop communicates with the optical detector and the wavelength controller to modify the grating's reflection wavelength band in accordance with the wavelength of an incident optical channel.

WDM optical communication systems with wavelength stabilized optical selectors

An embedded optical sensor has a plurality of layers (10-20) and an optical fiber (21) with a fiber grating (28), disposed between the layers (14, 16). The layers (10-20) comprise filaments (22) and resin (24) which have different thermal expansion coefficients and the filaments (22) are oriented so as to create unequal transverse residual stresses that act through the geometry of a resin-rich region that surrounds on the grating (28) in the fiber (21). The unequal transverse residual stresses cause birefringence in the grating (28), thereby causing the grating (28) to reflect light (32) having two wavelengths with a predetermined separation, each along a different polarization axis. The wavelength separation and average wavelength between such separation have different sensitivities to temperature and strain, thereby allowing independent temperature and strain measurements using only a single grating. The birefringence is maximized when the filaments (22) of the adjacent layers (10, 12) are oriented at 90 degrees with respect to the longitudinal (Z-axis) of the fiber (21).

Embedded optical sensor capable of strain and temperature measurement using a single diffraction grating

A system based on integrated optical technologies for the measurement and diagnostics of physical parameters on whatever structure, by the use of optical sensors, made by the fiber embedded Bragg grating method and by the use of a planar integrated optics device for the analysis of the optical signal. The sensors may be embedded or bonded to the structure, allowing the measurement of parameters like strain and temperature, in either a static or dynamic regime. The system pertains to the technical field of the diagnostics and measurements of mechanical or thermal parameters and to the application field of ground, water and aerospace transportation and also to the application field of construction.

Integrated optical instrumentation for the diagnostics of parts by embedded or surface attached optical sensors

On the holographic detection of core-to-skin disbonds in composite sandwich structures

Eingeschlossener optischer sensor zur messung von spannung und temperatur mit einem einzigen diffraktionsgitter

Strumentazione basata sulla tecnologia dell'ottica integrata per la misura e la diagnostica di parametri fisici su qualsiasi struttura, con l'impiego di sensori ottici a reticolo di Bragg realizzati nel nucleo della fibra ottica e di un dispositivo planare ad ottica integrata per l'analisi del segnale ottico I sensori possono essere fissati alla superficie di una struttura, o inclusi in essa, per consentire la misura di grandezze, quali ad esempio deformazione e temperatura, sia in regime statico sia in regime dinamico L'invenzione puo essere classificata nel campo tecnico della diagnostica e misura di grandezze meccaniche e termiche o nel campo applicativo delle opere civili, nautiche, dei trasporti in generale e delle costruzioni aerospaziali

Claudio Voto

Papers

Embedded optical sensor capable of strain and temperature measurement using a single diffraction grating

Integrated optical instrumentation for the diagnostics of parts by embedded or surface attached optical sensors

On the holographic detection of core-to-skin disbonds in composite sandwich structures

Eingeschlossener optischer sensor zur messung von spannung und temperatur mit einem einzigen diffraktionsgitter

Strumentazione utilizzante componenti in ottica integrata per la diagnostica di parti con sensori a fibra ottica inclusi o fissati sulla superficie.