Fiber optic acousto-ultrasonic transducers offer numerous applications as embedded sensors for
impact and damage detection in industrial and aerospace applications as well as non-destructive
evaluation. Superficial contact transducers with a sheet of fiber optic Bragg gratings has been
demonstrated for guided wave ultrasound based measurements. It is reported here that this method
of measurement provides highly reproducible guided ultrasound data of the test composite
component, despite the optical fiber transducers not being permanently embedded in it.
The advantages of optical fiber sensing in harsh electromagnetic as well as physical stress environments make them
uniquely suited for structural health monitoring and non-destructive testing. In addition to aerospace applications they
are making a strong footprint in geophysical monitoring and exploration applications for higher temperature and pressure
environments, due to the high temperature resilience of fused silica glass sensors. Deeper oil searches and geothermal
exploration and harvesting are possible with these novel capabilities. Progress in components and technologies that are
enabling these systems to be fieldworthy are reviewed and emerging techniques summarized that could leapfrog the
system performance and reliability.
Fiber optic hydrophones are useful for a variety of underwater monitoring applications, as they offer high sensitivity,
signal-to-noise ratio and multiplexing ability for marine acoustics and offer longer term reliability than conventional
electronic hydrophones. The frequency response of packaged fibers depends strongly on the material and mechanical
parameters of the sensor design. Two fiber bragg-grating (FBG)-based hydrophones are described and their frequency
response is measured. One consists of a diaphragm-linked FBG, and another is a polymer coated FBG. While the
diaphragm-linked FBG has predictable resonances, resonance features on the simpler coated FBG hydrophone are
observed as well.
Guided wave ultrasound is well suited for inspection of laminate composite structures. Compared to nearly flat or gently
curved composites, performing accurate NDT on sharply curved structures is more complex with standard ultrasound
test methodologies, such as pulse-echo methods. Ultrasound propagation in curved composite structures is investigated
for sharply curved geometries. Responses are predicted based on dispersion models. Experimental results are presented
on 0.25 inch thick curved carbon fiber reinforced plastic (CFRP) composite geometry of an aircraft structural component
and compared with predicted values.
Passive acoustic monitoring hydrophones with low power consumption for autonomous underwater vehicles (AUVs) are
desirable for long term unmanned monitoring of ocean acoustics including marine mammal acoustics as well as those
due to human activity. Fiber-optic hydrophones offer wider bandwidth and high sensitivity alternatives to conventional
piezoelectric transducer (PZT) devices. Deployment on board AUVs requires operation under a wide range of
temperature and pressure conditions that change with depth and location, hence, maintaining the sensitivity and
reliability over the operating range is crucial.
A read-out mechanism for a resonant hydrophone using a fiber Bragg grating (FBG) transducer is described. The read-out
uses a temperature tuned DFB laser diode to compensate for FBG changes with temperature and depth, enabling
operation over a wide temperature range. Its compact footprint and battery-powered readout system operation enables
portability on AUVs.
Fiber optic sensors offer several advantages over their electrical counterparts, especially for applications in hostile,
spark-sensitive environments, because no electrical power is required at the sensors. In addition, the installation of fiber
sensors external to fluid carrying conduits facilitates access for troubleshooting and replacement, unlike in-line
diaphragm-based sensors. Furthermore, glass fiber pressure sensors have a much higher operating temperature range,
which makes them more practical for flammability-prone environments. Multiple fiber Bragg grating (FBG) sensors can
be multiplexed along a single fiber optic cable, as opposed to traditional resistive strain gauges, which require individual
shielded metal cabling. Applications for such fiber-optic pressure detection systems include the pressure monitoring of
flow in fuel lines and their pressure valves. This paper characterizes the application of FBG sensors, with remote access
capability, for the nonintrusive pressure monitoring of different types of metallic pipes. We show that pressure changes
smaller than one psi can be detected with a tunable diode laser-based detection system. Standard metal pipes of steel,
inconel, copper-nickel alloy and titanium are characterized, and the resilience of FBG sensors to an overpressure of up to
1500 psi is demonstrated.
Passive hydrophones with a minimal footprint are useful for a variety of underwater monitoring applications; a fiber
optic hydrophone is being investigated. Fiber optic hydrophones have been used in offshore moored applications, and
have the capability to cover large areas. A resonant hydrophone using a fiber Bragg grating (FBG) transducer for limited
bandwidth operation is described and compared with predicted diaphragm resonances. A battery-power readout system
using a laser diode source that can typically operate for a full day, and can be used in off-mooring applications such as in
autonomous underwater vehicles, is outlined.
Fiber optic sensors have matured to allow the detection of chemical and physical parameters at multiple points, or in a
distributed fashion, along the length of a single optical fiber strand. The electro-optic readout unit connected to the fiber
needs to be tailored to the specific sensor application to balance the requirements for spatial and frequency resolution,
interrogation intervals, readout time, noise suppression, system size and cost, and other parameters. This paper will
discuss different fiber optic sensor configurations and suitable readout strategies to meet the requirements of the sensing
Non-destructive testing of critical structural components is time consuming, while necessary for maintaining safe
operation. Large aerospace structures, such as the vertical stabilizers of aircraft undergo inspection at regular intervals
for damage diagnostics. However, conventional techniques for damage detection and identification before repair can be
scheduled are conducted off-line and therefore can take weeks. The use of guided ultrasound waves is being investigated
to expedite damage detection in composites. We measure the frequency dependent loss of ultrasonic guided waves for a
structure comprising a boron-nitride composite skin sandwiching an aluminum honeycomb. A wide range of ultrasound
frequencies propagate as measured using PZTs, with the lowest attenuation observed about 200-250 kHz. These
measurements are confirmed using optical fiber Bragg grating arrays used as ultrasound transducers.
Radiation doses used in radiation therapy are calculated during the course of treatment planning. Cross-validation of
calculated dose versus received dose is performed mostly in-vitro and may not represent actual therapy doses. In vivo
measurements are at best typically limited to a few surface points. Presently, dose is measured primarily with diodes,
thermoluminescent or MOSFET dosimeters. Their outer sizes are in the range of 3 mm, which are unpractical for in
vivo internal use, in particular for interstitial or intracavital brachytherapy. In addition, diode and MOSFET sensors
are individually tethered to cables and are therefore inconvenient for making multiple point measurements.
Feasibility of multiple point radiation dosimetry using luminescent optical fibers for in vivo dosimetry during
radiation therapy is described that overcomes these difficulties. The spectral response of a candidate rare-earth doped
optical fiber dosimetric probe is reported, having 0.5 rads/cm sensitivity. This sensor capability would enable
continuous radiation monitoring of dose and dose rate during therapy at multiple locations along the sensor fiber.
There is much interest in using optical fiber sensors for strain measurement because they are lightweight and are
insensitive to electromagnetic interference. Structural monitoring applications can employ fiber Bragg gratings (FBGs)
for point-by-point measurements, whereas continuous sensors, based on Brillouin scattering, can be used to measure
strain along entire lengths of un-modified optical fibers. These two strain measurement methodologies, and their
practical limitations, are compared. Accurate and reliable static strain measurement with FBGs, a turnkey
instrumentation with multichannel detection ability, and embedding techniques with repeatable calibration ability are
reported. Embedding techniques that survive high temperature cycling, and high humidity under salt water
environments, have been developed.
A system for interrogation of high-frequency events with an FBG array in the presence of large quasi-static fluctuations
has been developed at Intelligent Optical Systems (IOS). The system allows highly sensitive detection of periodic or
transient events up to the MHz range while automatically compensating for slow changes in the FBG center frequency
using a closed loop tracking system. Both the high frequency signal and the low frequency parts of the sensor spectrum
are available for further processing. The system components, setup, and applications are presented and discussed.
We report here on bacterial biofilm detection with an optical fiber probe and a compact detection system. This probe was tested on cells of the Pseudomonas aeruginosa and other species of bacteria in planktonic and sessile forms. Optical signal changes corresponding to the number density of the bacterial cells were measured.
We report initial experimental results on the fabrication of multicore near-field optical (NFO) fiber probes after investigating several multicore NFO probe designs. Fluorescence measurement data demonstrating optical distinction between individual adjacent channels of a three- core NFO is presented. Our results demonstrate that simultaneous multiparameter sensing in side single living cells shall soon be a reality.
An adaptive multilayer optical neural network with all-optical forward propagation including optical thresholding by a liquid crystal light valve (LCLV) is described. It has a large number of modifiable optical interconnections that are implemented by liquid crystal television screens, and it has a modular structure enabling the cascading of layers, each layer with its own light source. Sigmoid fits to response curves of four LCLVs are evaluated and their suitability as optical thresholding functions is examined on the basis of neural network simulations.
The paper describes a high-speed digital ellipsometer, called 'discrete retardation modulation ellipsometer', designed for investigating fiber-optic sensor systems. The new device uses an electrooptic modulator to extract all four Stokes parameters and, thus, does not require lock-in amplifiers. The ellipsometer was used to measure the response of an optical fiber current sensor to test current at 4 kHz. The result was found to be within 10 percent from the expected value.
Electro-optic modulators (EOM's) can be used for polarization modulation
of light at frequencies up to hundreds of megahertz and are therefore strong
candidates for high speed effipsometer systems.
The characteristic of this device is the half wave voltage, V ,which is a function
of the wavelength of light and the temperature. A description of the particular
ADP EOM we are using, and a discussion of the effect of laser linewidth
and temperature on the measurement of V,, is presented. For the laser diode
wavelength of )t = 830 nm, V, and the extinction ratio are measured at different
The efficiency of plasma assisted deposition processes such as reactive ion plating with regard to the densification of the deposited thin films depends strongly on the arrival rate ratio of ionized species (atoms molecules) to the total influx of vaporized coating material. While it is relatively easy to measure ion beam currents produced by an auxiliary ion beam source (for example a Kaufman gun) it is much more difficult to measure ion currents in plasma processes. Typical probe measurements change the plasma conditions almost instantaneously. An optical fiber Faraday ring ammeter initially developed for space plasma research provides for a non-invasive measurement of ion currents present in plasma assisted deposition processes. We will be discussing the principles and the potential use of such a device in a high vacuum box coater equipped for low voltage reactive ion plating deposition. 1.