We experimentally subject a fiber Bragg grating to an unknown, variable temperature gradient. We use the full-spectral response of the grating to determine the magnitude of the gradient over the length of the grating via the full width at quarter maximum bandwidth. The experimental bandwidth and spectrum deformation were compared with a numerical model consisting of an analytical heat transfer model, a finite element analysis model, and the transfer matrix (T-matrix) method. The numerical model showed excellent agreement with the experimental results when the T-matrix method was modified to include the slope of the gradient in addition to the magnitude of the gradient.
Future flight vehicles may comprise complex flight surfaces requiring coordinated in-situ sensing and actuation.
Inspired by the complexity of the flight surfaces on the wings and tail of a bird, it is argued that increasing the number of
interdependent flight surfaces from just a few, as is normal in an airplane, to many, as in the feathers of a bird, can
significantly enlarge the flight envelope. To enable elements of an eco-inspired Dynamic Servo-Elastic (DSE) flight
control system, IFOS is developing a multiple functionality-sensing element analogous to a feather, consisting of a very
thin tube with optical fiber based strain sensors and algorithms for deducing the shape of the “feather” by measuring
strain at multiple points. It is envisaged that the “feather” will act as a unit of sensing and/or actuation for establishing
shape, position, static and dynamic loads on flight surfaces and in critical parts. Advanced sensing hardware and
software control algorithms will enable the proposed DSE flight control concept. The hardware development involves
an array of optical fiber based sensorized needle tubes for attachment to key parts for dynamic flight surface
measurement. Once installed the optical fiber sensors, which can be interrogated over a wide frequency range, also allow
damage detection and structural health monitoring.
Fiber optic sensor systems can alleviate certain challenges faced by electronics sensors faced when monitoring structures
subject to marine and other harsh environments. Challenges in implementation of such systems include scalability,
interconnection and cabling. We describe a fiber Bragg grating (FBG) sensor system architecture based that is scalable
to support over 1000 electromagnetic interference immune sensors at high sampling rates for harsh environment
applications. A key enabler is a high performance FBG interrogator supporting subsection sampling rates ranging from
kHz to MHz. Results are presented for fast dynamic switching between multiple structural sections and the use of this
sensing system for dynamic load monitoring as well as the potential for acoustic emission and ultrasonic monitoring on
materials ranging from aluminum and composites to concrete subject to severe environments.
A Lamb wave-based damage identification method called damage imaging method for composite shells is presented. A
damage index (DI) is generated from the delay matrix of the Lamb wave response signals, and it is used to indicate the
location and approximate area of the damage. A piezoelectric actuator is employed to generate the Lamb waves that are
subsequently captured by a fiber Bragg grating (FBG) sensor element array multiplexed in a single fiber connected to a
high-speed fiber-optic sensor system. The high-speed sensing is enabled by an innovative parallel-architecture optical
interrogation system. The viability of this method is demonstrated by analyzing the numerical and experimental Lamb
wave response signals from laminated composite shells. The technique only requires the response signals from the plate
after damage, and it is capable of performing near real-time damage identification. This study sheds some light on the
application of a Lamb wave-based damage detection algorithm for curved plate/shell-type structures by using the
relatively low frequency (around 100 kHz) Lamb wave response and the high-speed FBG sensor system.
The development of a Magnetic Resonance Imaging (MRI) compatible optically-actuated active needle for guided percutaneous surgery and biopsy procedures is described. Electrically passive MRI-compatible actuation in the small diameter needle is provided by non-magnetic materials including a shape memory alloy (SMA) subject to precise fiber laser operation that can be from a remote (e.g., MRI control room) location. Characterization and optimization of the needle is facilitated using optical fiber Bragg grating (FBG) temperature sensors arrays. Active bending of the needle during insertion allows the needle to be accurately guided to even relatively small targets in an organ while avoiding obstacles and overcoming undesirable deviations away from the planned path due to unforeseen or unknowable tissue interactions. This feature makes the needle especially suitable for use in image-guided surgical procedures (ranging from MRI to CT and ultrasound) when accurate targeting is imperative for good treatment outcomes. Such interventions include reaching small tumors in biopsies, delineating freezing areas in, for example, cryosurgery and improving the accuracy of seed placement in brachytherapy. Particularly relevant are prostate procedures, which may be subject to pubic arch interference. Combining diagnostic imaging and actuation assisted biopsy into one treatment can obviate the need for a second exam for guided biopsy, shorten overall procedure times (thus increasing operating room efficiencies), address healthcare reimbursement constraints and, most importantly, improve patient comfort and clinical outcomes.
Electromagnetic interference (EMI) immune and light-weight, fiber-optic sensor based Structural Health Monitoring
(SHM) will find increasing application in aerospace structures ranging from aircraft wings to jet engine vanes. Intelligent
Fiber Optic Systems Corporation (IFOS) has been developing multi-functional fiber Bragg grating (FBG) sensor systems
including parallel processing FBG interrogators combined with advanced signal processing for SHM, structural state
sensing and load monitoring applications. This paper reports work with Auburn University on embedding and testing
FBG sensor arrays in a quarter scale model of a T38 composite wing. The wing was designed and manufactured using
fabric reinforced polymer matrix composites. FBG sensors were embedded under the top layer of the composite. Their
positions were chosen based on strain maps determined by finite element analysis. Static and dynamic testing confirmed
expected response from the FBGs. The demonstrated technology has the potential to be further developed into an
autonomous onboard system to perform load monitoring, SHM and Non-Destructive Evaluation (NDE) of composite
aerospace structures (wings and rotorcraft blades). This platform technology could also be applied to flight testing of
morphing and aero-elastic control surfaces.
Reliable Thermal Protection System (TPS) sensors are needed to achieve better designs for spacecraft (probe) heatshields
for missions requiring atmospheric aero-capture or entry/reentry. In particular, they will allow both reduced risk
and heat-shield mass minimization, which will facilitate more missions and allow increased payloads and returns. For
thermal measurements, Intelligent Fiber Optic Systems Corporation (IFOS) is providing a temperature monitoring
system involving innovative lightweight, EMI-immune, high-temperature resistant Fiber Bragg Grating (FBG) sensors
with a thermal mass near that of TPS materials together with fast FBG sensor interrogation. The IFOS fiber optic sensing
technology is highly sensitive and accurate. It is also low-cost and lends itself to high-volume production. Multiple
sensing FBGs can be fabricated as arrays on a single fiber for simplified design and reduced cost. In this paper, we
provide experimental results to demonstrate the temperature monitoring system using multi-sensor FBG arrays
embedded in small-size Super-Light Ablator (SLA) coupon, which was thermally loaded to temperatures in the vicinity
of the SLA charring temperature. In addition, a high temperature FBG array was fabricated and tested for 1000°C
Electromechanical impedance is a popular diagnostic method for assessing structural conditions at high frequencies. It has been utilized, and shown utility, in aeronautic, space, naval, civil, mechanical, and other types of structures. By contrast, fiber optic sensing initially found its niche in static strain measurement and low frequency structural dynamic testing. Any low frequency limitations of the fiber optic sensing, however, are mainly governed by its hardware elements. As hardware improves, so does the bandwidth (frequency range * number of sensors) provided by the appropriate enabling fiber optic sensor interrogation system. In this contribution we demonstrate simultaneous high frequency measurements using fiber optic and electromechanical impedance structural health monitoring technologies. A laboratory specimen imitating an aircraft wing structure, incorporating surfaces with adjustable boundary conditions, was instrumented with piezoelectric and fiber optic sensors. Experiments were conducted at different structural boundary conditions associated with deterioration of structural health. High frequency dynamic responses were collected at multiple locations on a laboratory wing specimen and conclusions were drawn about correspondence between structural damage and dynamic signatures as well as correlation between electromechanical impedance and fiber optic sensors spectra. Theoretical investigation of the effect of boundary conditions on electromechanical impedance spectra is presented and connection to low frequency structural dynamics is suggested. It is envisioned that acquisition of high frequency structural dynamic responses with multiple fiber optic sensors may open new diagnostic capabilities for fiber optic sensing technologies.
Structural dynamic characterization is important for ensuring reliability and operability of spacecraft payloads in harsh
environments. During the launch, a structure experiences dynamic loads, including acoustic excitation. Conventional
sensors are used to infer structural dynamic characteristics. Limitations of conventional strain sensors include low
frequency band, susceptibility to electro-magnetic interference, and use of multiple wires. To mitigate these deficiencies,
an innovative fiber optic strain measurement system is considered to obtain strain distribution at specific locations on a
payload. Theoretical models are suggested and compared with results of experimental testing. Limitations of analytical
models are discussed and comparisons with numerical models are presented. The research addresses the usability of
presented models in determining the dynamic response of a payload and variation due to distribution of components. It is
proposed that discussed experimental and theoretical procedures can be used in determining structural performance for a
variety of missions.
Fiber Bragg Grating-based (FBG) strain sensors have been widely used in engineering applications requiring small size,
light weight, amenability to multiplexing, and very fast response times. State-of-the-art FBG interrogators are capable of measuring as low as sub micro strains and as high as 1% fiber strain in tension and higher still under compression. In this paper, we will discuss the development of an FBG based real-time instrumentation system to conduct highly
dynamic strain measurements during an impact. A high-speed FBG interrogation system was used along with an FBG
sensor data analysis software for efficient post-processing. In order to capture high strain data during an impact event, one needs to conduct measurements at very fast speeds and simultaneously to maintain FBG sensor survivability. A high strain FBG fixture was designed accordingly. Such high strain fixture allows the FBG strain sensor to measure the actual field strain with a reduction factor K in order to expand the strain measurement range. Numerical simulation results using finite element analysis (FEA) were used to validate the high strain fixture design analysis. Finally, a proof-ofconcept FBG-based high strain measurement system has been demonstrated to measure dynamic strain data under impact tests. Experimental strain reduction factors were determined from the strain data and correlated well with FEA predicted values.
Optical fibers are small-in-diameter, light-in-weight, electromagnetic-interference immune, electrically passive,
chemically inert, flexible, embeddable into different materials, and distributed-sensing enabling, and can be temperature
and radiation tolerant. With appropriate processing and/or packaging, they can be very robust and well suited to
demanding environments. In this paper, we review a range of complete end-to-end fiber optic sensor systems that IFOS
has developed comprising not only (1) packaged sensors and mechanisms for integration with demanding environments,
but (2) ruggedized sensor interrogators, and (3) intelligent decision aid algorithms software systems. We examine the
• Fiber Bragg Grating (FBG) optical sensors systems supporting arrays of environmentally conditioned multiplexed
FBG point sensors on single or multiple optical fibers: In conjunction with advanced signal processing, decision aid
algorithms and reasoners, FBG sensor based structural health monitoring (SHM) systems are expected to play an
increasing role in extending the life and reducing costs of new generations of aerospace systems. Further, FBG
based structural state sensing systems have the potential to considerably enhance the performance of dynamic
structures interacting with their environment (including jet aircraft, unmanned aerial vehicles (UAVs), and medical
or extravehicular space robots).
• Raman based distributed temperature sensing systems: The complete length of optical fiber acts as a very long
distributed sensor which may be placed down an oil well or wrapped around a cryogenic tank.
Next generation navigation systems demand performance enhancements to support new applications with longer range
capabilities, provide robust operation in severe thermal and vibration environments while simultaneously reducing
weight, size and power dissipation. Compact, inexpensive, advanced guidance components are essential for such
applications. In particular, Inertial Reference Units (IRUs) that can provide high-resolution stabilization and accurate
inertial pointing knowledge are needed. For space applications, an added requirement is radiation hardening up to 300
krad over 5 to 15 years. Manufacturing specifications for the radiation-induced losses are not readily available and
empirical test data is required for all components in order to optimize the system performance.
Interferometric Fiber-Optic Gyroscopes (IFOGs) have proven to be a leading technology for tactical and navigational
systems. The sensors have no moving parts. This ensures high reliability and a long life compared to the mechanical
gyroscopes and dithered ring laser gyroscopes. However, the available architectures limit the potential size and cost of
The work reported here describes an innovative approach for the design, fabrication, and testing of the IFOG and enables
the production of a small, robust and low cost gyro with excellent noise and bandwidth characteristics with high
radiation tolerance. The development is aimed at achieving a sensor volume < 5 cubic inches. The new IFOS gyro uses
an open loop configuration, utilizes extremely small diameter radiation-hard fiber with customized all-digital signal
processing. The optics is packaged using a combination of highly-integrated optical component assemblies with an allfiber
approach that leads to a more flexible yet lower cost optical design.
The IFOS gyro prototypes are implemented using a distributed architecture, where the light source, electronics and
receiver are integrated in an external package, while the sensor head is integrated in a robust and environmentally rigid
package. The sensor package design is compatible with the most severe environmental requirements foreseen for the
This paper presents the current state-of-the-art performance of the prototype gyros and the potential for further reduction
of size with improved performance. The gyro sample and data rates are extremely high and can be close to the
modulation frequency (up to 80 kHz). IFOS has shown that the noise at high frequencies is not flattening out and
extremely high bandwidth operation is possible without any degradation of the operational stability.
IFOS has also demonstrated the potential for a future, smaller and extremely robust IFOG. The next phase design will
include highly radiation-resistant integrated, compact optical circuits based on InP technology that includes the light
source, splitter and receiver in one package, a gyro coil that utilizes small diameter, radiation-hard fiber and a small fiber
phase modulator with > 300 krad radiation tolerance. This gyro offers the low noise, low drift, low vibration sensitivity,
high accuracy, high bandwidth and high radiation tolerance solution required for next generation systems. We will
present both theoretical modeling and experimental results obtained to date
Immune to electromagnetic interference, Fiber Bragg Grating (FBG) optical sensors are multiplexable, highly sensitive
to minute strains, and can facilitate maximum smart structure functionality, with minimum weight and size. In
conjunction with advanced damage characterization algorithms, FBG sensor systems are expected to play an increasing
role in extending the life and reducing costs of new generations of civil, mechanical and aerospace systems. In this
paper, we discuss the development of fast parallel processing FBG interrogation systems. Comparison with other
structural health monitoring systems demonstrates better signal-to-noise, high-speed multi-sensor support and damage
Light-weight and multiplexable, Fiber Bragg Grating (FBG) optical sensors allow the integration of many sensors along
a single electrically passive and electromagnetic interference immune optical fiber. In this paper, we describe the use of
multiple FBGs for monitoring and early stage detection of fatigue crack growth. We provide experimental results over a
range of temperatures for fatigue crack monitoring in titanium alloy specimens subjected to periodic loading for tens of
thousands of cycles and demonstrate detection of early-stage fatigue crack extension.
We demonstrate for an unpolarized Fiber Optic Gyroscope (FOG) with open-loop electronics, that, by applying more
source power and conserving optical power in the optical path, we can achieve improved Angle Random Walk (ARW)
performance without enlarging loop or put in multiple turns of fiber. The predicted trends are demonstrated by the
experiment in terms of bandwidth. Power-law dependency is shown within the accuracy of the instrumentation.
An effort to develop a miniaturized multichannel optical fiber Bragg grating sensor interrogator was initiated in 2006 under the Small Business Innovative Research (SBIR) program. The goal was to develop an interrogator that would be sufficiently small and light to be incorporated into a health monitoring system for use on tactical missiles. Two companies, Intelligent Fiber Optic Systems Corporation (IFOS) and Redondo Optics, were funded in Phase I, and this paper describes the prototype interrogators that were developed. The two companies took very different approaches: IFOS focused on developing a unit that would have a high channel count and high resolution, using off-the-shelf components, while Redondo Optics chose to develop a unit that would be very small and lightweight, using custom designed integrated optical chips. It is believed that both approaches will result in interrogators that will be significantly small, lighter, and possibly even more precise than what is currently commercially available. This paper will also briefly describe some of the sensing concepts that may be used to interrogate the health of the solid rocket motors used in many missile systems. The sponsor of this program was NAVAIR PMA 280.
Fiber gratings are proving to provide versatile discrete sensor elements for structural health monitoring systems. For example, they outperform traditional resistive foil strain gages in terms of temperature resistance as well as multiplexing capability, relative ease of installation, electromagnetic interference immunity and electrical passivity. However, the fabrication method and post-fabrication processing influences both performance and survivability in extreme temperature environments. In this paper, we compare the performance and survivability when making strain measurements at elevated temperatures for a range of fabrication and processing conditions such as UV-laser and electric-arc writing and post-fabrication annealing. The optimum method or process will depend on the application temperatures (e.g., up to 300°C, 600°C or 1000°C), and times at these temperatures. As well, other sensing requirements, including the number of sensors, measurand and sensitivity may influence the grating choice (short or long period).
Structural Health Monitoring (SHM) is becoming an increasingly important tool for the maintenance, safety and integrity of aerospace structural systems. Immune to electromagnetic interference, Fiber Bragg Grating (FBG) optical sensor matrices are light-weight and multiplexable, allowing many sensors on a single fiber to be integrated into smart structures. Highly sensitive to minute strains, they can facilitate maximum SHM functionality, with minimum weight and size. Consequently, these optical systems, in conjunction with advanced damage characterization algorithms, are expected to play an increasing role in extending the life and reducing costs of new generations of structures and airframes. In this paper, we discuss the development of both hardware and algorithms to detect, locate and quantify delamination in composite laminated beam structures. We present an integrated SHM system including (a) the capability of interrogating over 50 FBG sensors simultaneously with sub-picometer resolution at over 50 kHz, (b) an FBG-sensor/piezo-actuator matrix smart skin design and methodology, and (c) damage detection location and quantification algorithms based on mode shape or other relevant advanced algorithmic-based damage diagnosis and prognosis techniques. Comparison with other SHM systems (e.g., based on piezo-electric (PVDF) and Scanning Laser Vibrometer sensors) demonstrates better signal-to-noise and damage detection for our FBG system.