This paper presents the full-spectral measurement of fiber Bragg grating sensor responses during impact testing of
composite laminates. The sensors are embedded in carbon fiber/epoxy laminates which are subjected to multiple low
velocity impacts until perforation of the laminate occurs. Applying a recently developed high-speed interrogator, the
Bragg grating sensor interrogation is demonstrated at 534 Hz over a 14.9 nm bandwidth. The measurement of the
transient response of the grating sensors during impact reveals unique spectral signatures that could not be detected
through peak-wavelength monitoring or post-impact full-spectral scanning of the sensors, including local relaxation of
This paper presents a new means for collecting fiber Bragg grating (FBG) data
during drop tower measurements used to assess damage to composite structures. The
high repetition-rate collection process reveals transient features that cannot be
resolved in quasi-static measurements. The experiments made at a repetition rate of
about 500 Hz show that the detected FBG spectrum broadens for a short period of
time and relaxes quickly to a narrower static state. Furthermore, this relaxation time
increases dramatically as the strike count increases. The information gained by such
measurements will enhance the ability to characterize and distinguish failure modes
and predict remaining lifetime in composite laminate structures.
Proc. SPIE. 6933, Smart Sensor Phenomena, Technology, Networks, and Systems 2008
KEYWORDS: Optical fibers, Optical filters, Fiber Bragg gratings, Sensors, Digital filtering, Field programmable gate arrays, Fiber lasers, Analog electronics, Data communications, Temperature metrology
A fiber Brag grating sensor interrogator has been developed which is capable of gathering vectors of information from
individual fiber Bragg gratings by capturing the full optical spectrum 3 kHz. Using a field programmable gate array
with high speed digital-to-analog converters and analog-to-digital components, plus a kilohertz rate MEMS optical filter,
the optical spectrum can be scanned at rates in excess of 10 million nanometers per second, allowing sensor sampling
rates of many kilohertz while maintaining the necessary resolution to understand sensor changes. The autonomous
system design performs all necessary detection and processing of multiple sensors and allows spectral measurements to
be exported as fast as Ethernet, USB, or RS232 devices can receive it through a memory mapped interface. The high
speed - full spectrum - fiber Bragg grating sensor interrogator enables advanced interrogation of dynamic strain and
temperature gradients along the length of a sensor, as well as the use of each sensor for multiple stimuli, such as in
temperature compensation. Two examples are described, showing interrogation of rapid laser heating in an optical fiber,
as well as complex strain effects in a beam that had an engineered defect.
Advancements in portability and performance are described for a fiber optic sensor readout system capable of
monitoring wavelength-multiplexed sensors. The handheld sensor interrogator was designed to readily interface with
conglomerate sensor systems as a smart sensor node and process all spectral data within its own system in real time at 20
Hz for +/- 13 picometers resolution mode. Portability was demonstrated by flying the system on a miniature aerial
vehicle (MAV) which collected strain and temperature flight data for broadcast to a ground station. Additional
improvements upgraded the sensor measurement speed by two orders of magnitude.
In a quest for fiber optic sensors that could monitor soil moisture, Blue Road Research implemented fiber Bragg grating sensors in such a way that they could detect humidity, soil moisture evaporation rates, and pressure changes from soil weight. These were then used to monitor soil in controlled flood tests to determine the moisture levels in a soil test bed. Fiber optic sensors seem well suited for humidity and soil moisture monitoring since they can easily be multiplexed with many sensors on one fiber line, and they have distinct longevity advantages that enable their use in applications involving wet environments, remote locations or long distances, electromagnetic interference, flammability, or other harsh environmental conditions that may degrade ordinary electronic sensors or their measurements. This paper describes the workings of a highly accurate optical humidity sensor that can be multiplexed on a fiber optic strand to monitor humidity that may lead to corrosion, soil moisture levels and changes, weather conditions, etc. as well as means to record such data.
Polyimide coated fiber Bragg gratings have a linear response to changes in relative humidity and temperature. Blue Road Research is using this technology to monitor relative humidity and using acrylate-coated gratings to monitor temperature. This paper describes some of the sensors and readout systems for simple and multiplexed relative
humidity and temperature sensors. Additionally, an indirect method for monitoring soil moisture is described.
Blue Road Research has demonstrated the use of fiber optic Bragg grating sensors in roads and highways to develop traffic sensors that could count and classify traffic usage on roadways, providing statistical information for maintenance, safety, and growth. This paper reviews the progress by Blue Road Research that led to installation of traffic sensors on the I-84 freeway and outlines the benefits of developing a fiber optic weigh-in-motion sensor.
Previous studies have shown the capability of fiber Bragg gratings (FBGs) to monitor components of strain on bridges and structures. In past months, Blue Road Research and the Oregon Department of Transportation embedded long-gage FBG sensors into the Interstate 84 freeway east of Portland, Oregon to determine the feasibility of retro-install and for use of these sensors in monitoring freeway traffic speeds under conditions similar to loop inductors, piezo-ceramic weigh-in-motion (WIM) systems, and other vehicle monitoring devices. The objective of the study was to develop a working traffic sensor system with the potential to be more durable, reliable, informative, and cost-effective than currently available traffic sensors. A primary purpose of the freeway installation was to test the sensors for vehicle classifier and counter applications. In addition to discussion of the advantages of using FBGs for traffic classifiers and systems over conventional sensing methods, this paper overviews the installation and summarizes the use of FBG traffic sensors for vehicle counting and classification.