Non-contacting interferometric fiber optic sensors offer a minimally invasive, high-accuracy means of measuring a
structure's kinematic response to loading. The performance of interferometric sensors is often dictated by the technique
employed for demodulating the kinematic measurand of interest from phase in the observed optical signal. In this paper a
white-light extrinsic Fabry-Perot interferometer is implemented, offering robust displacement sensing performance.
Displacement data is extracted from an estimate of the power spectral density, calculated from the interferometer's
received optical power measured as a function of optical transmission frequency, and the sensor's performance is
dictated by the details surrounding the implementation of this power spectral density estimation.
One advantage of this particular type of interferometric sensor is that many of its control parameters (e.g., frequency
range, frequency sampling density, sampling rate, etc.) may be chosen to so that the sensor satisfies application-specific
performance needs in metrics such as bandwidth, axial displacement range, displacement resolution, and accuracy. A
suite of user-controlled input values is investigated for estimating the spectrum of power versus wavelength data, and the
relationships between performance metrics and input parameters are described in an effort to characterize the sensor's
operational performance limitations. This work has been approved by Los Alamos National Laboratory for unlimited
public release (LA-UR 12-01512).
Bundled intensity-modulated fiber optic displacement sensors offer high-speed (kHz-MHz) performance with
micrometer-level accuracy over a broad range of axial displacements, and they are particularly well-suited for
applications where minimally invasive, non-contacting sensing is desired. Furthermore, differential versions of these
sensors have the potential to contribute robustness to fluctuating environmental conditions. The performance limitations
of these sensors are governed by the relationship between axial displacement and measured power at the locations of
receiving fibers within a bundled probe. Since the propagating transmission's power level is spatially non-uniform, the
relative locations of receiving fibers within a bundled probe are related to the sensor's output, and in this way fiber
location is related to sensor performance.
In this paper, measured power levels are simulated using a validated optical transmission model, and a genetic algorithm
is employed for searching the intensity-modulated bundled displacement sensor's design space for bundle configurations
that offer high-overall combinations of desired performance metrics (e.g., linearity, sensitivity, accuracy, axial
displacement range, etc...). The genetic algorithm determines arrangements of fibers within the bundled probe that
optimize a performance-based cost function and have the potential to offer high-performance operation. Multiple
converged results of the genetic algorithm generated using different cost function structures are compared. Two
optimized configurations are prototyped, and experimental sensor performance is related to simulated performance
levels. The prototypes' linearity, sensitivity, accuracy, axial displacement range, and sensor robustness are described, and
sensor bandwidth limitations are discussed. This paper has been approved by Los Alamos National Laboratory for
unlimited public distribution (LA-UR 12-00642).
A testbed simulating an intensity-modulated fiber optic displacement sensor is experimentally characterized, and the
implications regarding sensor design are discussed. Of interest are the intensity distribution of the transmitted optical
signal and the relationships between sensor architecture and performance. Particularly, an intensity-modulated sensor's
sensitivity, linearity, displacement range, and resolution are functions of the relative positioning of its transmitting and
receiving fibers. In this paper, sensor architectures with various combinations of these performance metrics are
discussed. A sensor capable of micrometer resolution is reported, and it is concluded that this work could lead to an
improved methodology for sensor design. This paper has been approved by Los Alamos National Laboratory for
unlimited public distribution (LA-UR 10-06637).
The intensity distribution of a transmission from a single mode optical fiber is often approximated using a Gaussian-shaped
curve. While this approximation is useful for some applications such as fiber alignment, it does not accurately
describe transmission behavior off the axis of propagation. In this paper, another model is presented, which describes the
intensity distribution of the transmission from a single mode optical fiber. A simple experimental setup is used to verify
the model's accuracy, and agreement between model and experiment is established both on and off the axis of
propagation. Displacement sensor designs based on the extrinsic optical lever architecture are presented. The behavior of
the transmission off the axis of propagation dictates the performance of sensor architectures where large lateral offsets
(25-1500 μm) exist between transmitting and receiving fibers. The practical implications of modeling accuracy over this
lateral offset region are discussed as they relate to the development of high-performance intensity modulated optical
displacement sensors. In particular, the sensitivity, linearity, resolution, and displacement range of a sensor are functions
of the relative positioning of the sensor's transmitting and receiving fibers. Sensor architectures with high combinations
of sensitivity and displacement range are discussed. It is concluded that the utility of the accurate model is in its
predicative capability and that this research could lead to an improved methodology for high-performance sensor design.
Interferometric fiber optic accelerometers constitute a high-responsivity, high-resolution sensing architecture, with
achievable sensitivities of several rad/g and resolutions in the micro-g range, depending on the specific configuration.
Fiber Bragg grating (FBG) optical accelerometers offer ease of multiplexing but are inherently less sensitive than their
interferometric counterparts. Fiber-based accelerometers have the usual optical advantages of being lightweight,
electromagnetically immune, and non-spark emitting over traditional (piezo-electric) accelerometer architectures.
Among fiber optic sensing methodologies, both interferometric and FBG accelerometers can be interrogated using
phase-based demodulation, which offers advantages over intensity-based sensing schemes such as increased linearity,
repeatability, and insensitivity to extraneous measurands.
The performance of an accelerometer is often characterized in terms of its bandwidth, sensitivity, and resolution, all of
which depend on the specific transducer design (the mechanical architecture) as well as the optical interrogation
architecture. For a given optical interrogation architecture, a fundamental tradeoff exists in accelerometer transducer
design between bandwidth and sensitivity; attempts to increase bandwidth will generally result in a decrease in
sensitivity. This paper investigates the frequency and displacement characteristics that govern this tradeoff for several
transducer configurations, in order to determine a pair of configurations that offer the greatest sensitivity for a given
optical interrogation methodology (interferometric or FBG), at a prescribed bandwidth. The feasibility of several
mechanical architectures is assessed based on the physical dimensions required for a given configuration to achieve a
primary resonance of at least 15 kHz. The deflection of those configurations under their own self-weight is then
considered a measure of accelerometer sensitivity in the measurement band below primary resonance. This paper has
been reviewed by Los Alamos National Laboratory and received the following release number: LA-UR 10-00671.
In this paper, we present experimental investigations using energy harvesting and wireless energy transmission to
operate embedded structural health monitoring sensor nodes. The goal of this study is to develop sensing systems
that can be permanently embedded within a host structure without the need for an on-board power source. With this
approach the required energy will be harvested from the ambient environment, or periodically delivered by a RF
energy source to supplement conventional harvesting approaches. This approach combines several transducer types
to harvest energy from multiple sources, providing a more robust solution that does not rely on a single energy
source. Both piezoelectric and thermoelectric transducers are considered as energy harvesters to extract the ambient
energy commonly available on civil structures such as bridges. Methods of increasing the efficiency, energy storage
medium, target applications and the integrated use of energy harvesting sources with wireless energy transmission
will be discussed.