One of the most common fiber optic sensor (FOS) types used are fiber Bragg gratings (FBG), and the most frequently measured parameter is strain. Hence, FBG strain sensors are one of the most prevalent FOS devices in use today in structural sensing and monitoring in civil engineering, aerospace, marine, oil and gas, composites and smart structure applications. However, since FBGs are simultaneously sensitive to both temperature and strain, it becomes essential to utilize sensors that are either fully temperature insensitive or, alternatively, properly temperature compensated to avoid erroneous measurements. In this paper, we introduce the concept of measured “total strain”, which is inherent and unique to optical strain sensors. We review and analyze the temperature and strain sensitivities of FBG strain sensors and decompose the total measured strain into thermal and non-thermal components. We explore the differences between substrate CTE and System Thermal Response Coefficients, which govern the type and quality of thermal strain decomposition analysis. Finally, we present specific guidelines to achieve proper temperature-insensitive strain measurements by combining adequate installation, sensor packaging and data correction techniques.
We report in this paper on the design and development of a novel
on-line structural health monitoring and fire detection
system based on an array of optical fiber Bragg grating (FBG) sensors and interrogation system installed on a new, precommercial
compact aircraft. A combined total of 17 FBG sensors - strain, temperature and high-temperature - were
installed at critical locations in an around the wings, fuselage and engine compartment of a prototype, Comp Air CA 12
all-composite, ten-passenger personal airplane powered by a 1,650 hp turbine engine. The sensors are interrogated online
and in real time by a swept laser FBG interrogator (Micron Optics sm125-700) mounted on board the plane. Sensors
readings are then combined with the plane's avionics system and displayed on the pilot's aviation control panel. This
system represents the first of its kind in commercial, small frame, airplanes and a first for optical fiber sensors.
The design and development of a novel opto-mechanical strain sensor-called FlexPatch-based on the use of an
optical fiber Bragg grating (FBG) mounted into a custom-made miniature metallic flexure is reported. The FBG
sensing element is attached to a carrier flexure using proprietary bonding process which ensures a linear, drift-free
and repeatable strain response even under severe moisture and temperature conditions. The sensor is uncompensated
for temperature effects, but has undergone extensive mechanical and environmental testing and is qualified for use in
a strain range of +/- 2,500&mgr;&egr; with a gage factor of 1.2pm/&mgr;&egr; over a temperature range from -40° to 120°C, and a
fatigue life >100x106 cycles. The FlexPatch is intended for use in diverse sensing and monitoring applications and
can be installed onto surfaces by epoxy bonding or spot welding.
Fiber optic-based chemical sensors are created by coating fiber Bragg gratings (FBG) with the glassy polymer cellulose
acetate (CA). CA is a polymeric matrix capable of localizing or concentrating chemical constituents within its structure.
Some typical properties of CA include good rigidity (high modulus) and high transparency. With CA acting as a sensor
element, immersion of the gratings in various chemical solutions causes the polymer to expand and mechanically strain
the glass fiber. This elongation of the fiber sections containing the grating causes a corresponding change in the
periodicity of the grating that subsequently results in a change in the Bragg-reflected wavelengths. A high-resolution
tunable fiber ring laser interrogator is used to obtain room-temperature reflectance spectrograms from two fiber gratings
at two different wavelengths - 1540nm and 1550nm. The graphical representation from this device enables the display
of spectral shape, and not merely shifts in FBG central wavelength, thereby allowing for more comprehensive analysis of
how different physical conditions cause the reflectance profile to move and alter overall form. Wavelength shifts on the
order of 1 to 80 pm in the FBG transition edges and changes in spectral shape are observed in both sensors upon
immersion in a diverse selection of chemical analytes.
In this paper, we report the development of a new bonding agent and method for the surface mounting of optical fiber
Bragg grating (FBG) strain and temperature sensors for use in high temperature environments - where there is a
presence of water, moisture, dust, susceptibility to corrosion and/or elevated temperatures up to 800°C. To ensure a
stable reflectivity response of FBGs and their survival at elevated temperatures, we are using surface relief fiber Bragg
gratings (SR-FBG). These gratings, instead of being written in the core of a photosensitive or hydrogen-loaded fiber,
are formed by introducing a periodic surface relief - through photolithographic and etching processes - in the cladding
above the core. Samples of SR-FBGs were successfully encapsulated and mounted onto metal shims. The packaged
sensors displayed a linear response with temperature and a sensitivity factor of 11pm/°C.
In this paper, we report the development of a new bonding agent and method for the surface mounting of optical fiber Bragg grating (FBG) strain and temperature sensors for use in high temperature environments--where there is a presence of water, moisture, dust, susceptibility to corrosion and/or elevated temperatures up to 800°C. To ensure a stable reflectivity response of FBGs and their survival at elevated temperatures, we are using chemical composition gratings (CCGs). The refractive index modulation in these gratings is caused by a chemical change, which results in a higher activation energy and stable behavior up to 1000°C. Samples of CCGs were successfully encapsulated and mounted onto metal shims. The packaged sensors were tested for strain (+/- 1000με) and temperature (to +400 °C) response. The encapsulated sensors display a linear response with an increase in the temperature sensitivity of the FBG, with a factor of ~ 28.34pm/°C, and a strain gauge factor of 1.7pm/με.
Long-gauge SOFO sensors have been in use for the last 10 years for the monitoring of civil, geotechnical, oil & Fiber optic sensing systems are increasingly recognized as a very attractive choice for structural health monitoring. Moving form demonstration project to industrial applications requires an integrated approach where the most appropriate technologies are combined to meet the user's requirements. In this context it is often necessary and desirable to combine different sensing technologies in the same project. A bridge-monitoring project might for example require long-gauge interferometric sensors to monitor the concrete deck, interferometric inclinometers for the piles and fiber Bragg grating sensors for the monitoring of the strains in the steel beams and for measuring temperatures. Although fiber optic sensors relying on different technologies can easily be combined at the packaging and cable levels, they often require dedicated instruments to be demodulated. A unified demodulation system would therefore be very attractive.
This paper describes a technique relying on the analysis of reflected spectra and allowing the demodulation of interferometric (Michelson or Faby-Perot) sensors and fiber Bragg grating sensors with a single measurement system. It also compares the obtained performance in terms of resolution and dynamic range with the available dedicated systems.
In this paper, we report the development of a new bonding agent and method for the surface mounting of optical fiber Bragg grating strain and temperature sensors for use in harsh environments. The compound is based on a combination of ceramic fillers with an epoxy binder that is applied with a brush technique. Samples of optical fiber Bragg gratings were successfully encapsulated and mounted on metal shims. The packaged sensors were tested for strain (+/- 1000µε) and temperature (-20 to +120 °C) response. The encapsulated sensors display a linear response with an increase in the temperature sensitivity of the FBG, with a factor of 24.37pm/°C, and a strain gauge factor of 1.25pm/με.
Owners must manage and ensure the safety of their civil structures even as use of many structures extends well beyond their design lifetime. Traditionally, most structures rely on strict maintenance procedures, visual inspections, and very few sensors. But maintenance is very expensive, visual inspections can miss critical problems, and conventional sensors can fail in harsh environments. Can fiber-optic sensing (FOS) address these issues? This is not a new question, but there are some new answers. This paper highlights several structures where FOS is used, and describes the associated successes and challenges for each application. Many successes are coupled to improved FOS tools: better sensor packages, simpler and less expensive instrumentation, improved installation techniques, and more efficient data analysis tools. Examples of each are provided. Particular attention is given to the economics of instrumenting civil structures - when and how it pays. Conclusions include recommendations for future developments that will further accelerate FOS acceptance and use.