Fiber optic sensors may be used to monitor strain and temperature in composite materials. These measurements can be useful in determining rate and degree of cure of composite. Multi-dimensional strain measurements enabled by fiber gratings written onto polarization maintaining optical fiber enable monitoring changes in transverse strain, transverse strain gradients, and shear strain internal to composites and adhesive joints. This paper provides a brief historical overview of the usage of fiber sensors to provide strain measurements in composite parts, leading eventually to multi-axis strain sensing.

This paper introduces the application of commercial FFPI (Fiber Fabry-Perot Interferometer) sensors to harsh environmental machinery monitoring (temperature up to 400°C). Examples of dynamic pressure, temperature, vibration, rotational speed and bearing condition in engine, pump and bearing application with the multi-sensor operation (same or combination of different kinds of parameters monitoring up to 24 channels) are demonstrated.

Fiber Bragg grating sensors have attracted considerable attention for measurement applications due to their greatly reduced size, low weight, and immunity to electromagnetic interference in comparison with traditional sensing methods. Dynamic measurement of industrial machine tools is useful for gauging surface accuracy, monitoring tool condition, and predicting process stability, but requires a robust sensing scheme. The small size and high natural frequencies of micro machining tools coupled with a harsh manufacturing environment can render traditional sensors ineffective. This work presents a new method for measuring tool motion with fiber Bragg grating strain sensors. The feasibility of the sensing scheme is first demonstrated with a simple bench-top cantilever beam experiment. Then, a method for potting the sensors in the through coolant holes of a 1/8” carbide end mill with a high-viscosity gap-filling cyanoacrylate is demonstrated. Comparative structural analysis tests demonstrate the effectiveness of the sensors. Measurements of tool motion during cutting are presented. Finally, methods of noise reduction and improving signal accuracy are discussed.

Optical dynamic pressure and velocity sensors are under development for oil exploration and reservoir monitoring. The abilities of these sensors to operate in high temperature and static pressure environments, without nearby electronics, offers the potential for enhanced reliability with significant reduction in life-cycle costs. To aid in the design of these sensors, the minimum sensitivity requirements are derived in terms of the minimum detectable signals by the signal conditioning equipment and background noise levels in their operating environments. These requirements are compared with the performance achieved with commercially available electronic sensors. The results indicate that conventional moving coil geophones may have serious limitations in performing wide frequency-bandwidth measurements of seismic wave particle velocity in typical borehole noise environments. Accelerometer response, on the other hand, increases linearly with frequency, and consequently the useful bandwidth of an accelerometer generally exceeds that of the geophone.

Two fiber optic DTS systems were deployed and evaluated in oil and gas wells. Extended testing at 5,000 and 3,000 foot depths was performed to characterize the stability of the sensors. Long-term stability of less than 0.4°C rms and short-term stability of 0.05°C rms were observed. Ease of fiber optic sensor array deployment was proven, as well as the feasibility of remote real time monitoring of downhole temperature distributions.

During the past four years, Optoplan/Weatherford have developed and qualified high responsivity high bandwidth fiber interferometric sensors, FBG based fiber networks and a high resolution electro-optic instrumentation system for high temperature and high pressure in-well seismic imaging and monitoring applications. During this period, several field trials have been conducted, and recently a six level, three component (3-C) accelerometer system was installed permanently in a well at a gas storage field for TotalFinaElf in southwestern France. Results obtained from qualification and field testing of the sensors, network and instrumentation system are presented.

Typical fiber Bragg sensing systems, in the 1500nm to 1600nm wavelength range, use gratings spaced approximately 5nm apart. This large spacing is preferred since optical power illuminating the gratings is limited and monitoring systems often have low performance and hence have difficulty in differentiating between closely spaced gratings. In this paper, we present a high power ASE source in combination with an extremely high performance Optical Spectrum Analyzer (OSA), that allows sensing gratings to be closely spaced so many sensing elements can be monitored while maintaining very good wavelength and power detection. The integrated unit can be provided on a PCI card, in a box or as an OEM module to enable a robust, handheld unit. The OSA is built around proprietary Compliant Micro Electro Mechanical (CMEMs) tunable filters, primarily developed for very densely populated Telecommunications applications. The OSA is wavelength calibrated and can be matched to the ASE source to compensate for any spectral power characteristic of the source. This integrated OSA and ASE source will allow the sensing industry to increase grating array sizes while maintaining low cost.

Polymer technology for use in Inertial Sensor systems can provide a method for a much higher level of integration as compared to systems using lithium niobate modulator technology. Performance approaches the same level as LiNbO₃ while additionally providing a much lower cost of manufacture. Integration of polymer structures for couplers and modulators are discussed with some concepts for complete monolithic integration of all components and prototype assemblies.

Continuously tunable diode laser sources coupled to single-mode fiber are useful sources for fiber Bragg grating (FBG) strain sensors. A low-cost 1300 nm laser diode is anti-reflection coated making it nearly an LED. By coupling this coated laser to a short length of single-mode fiber containing a low-reflection FBG, which serves as the new tunable laser output coupler, a tunable, fiber-coupled laser source is possible. The laser diode package is mounted in a temperature-controlled mount, and the FBG is mounted so it can be strained. By controlling both temperature and FBG strain, the laser can be continuously tuned across a 10 nm spectral range giving a narrow line laser source for use in strain sensors.

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.

Newly developed advanced aircraft structures are utilizing composite technology for improving stiffness, strength and weight properties. Such structures are commonly found in inaccessible regions where current NDE techniques are limited. The development of low profile, distributed, embeddable, real-time, optical fiber sensors capable of detecting the onset of composite failure in aircraft structures would eliminate a significant portion of related maintenance costs. Notable composite failures that are difficult to assess include delaminations and moisture ingression issues. Optical fiber-based sensors add the inherent advantages of being lightweight, low profile, immune to EMI, resistant to harsh environments, and highly sensitive to a variety of physical and chemical measurements. Optical fiber-based sensors can also be embedded directly into the composite part during manufacturing and co-cured. This creates a monitoring system that has little impact on the properties of the final part while providing significant benefits. Fiber optics embedded in composite honeycomb panels were fabricated and tested using ground - air - ground thermal cycles to determine moisture ingression monitoring capabilities of the sensors. Two different types of moisture sensing fiber optics were measured. One type of installed moisture sensor is based off of a Bragg grating system, while the other moisture sensor is based off of a long period grating system. Presented herein is a comparison of the two different types of fiber optic sensors that monitored the moisture ingression in honeycomb panels.

An optical fiber calcium ion sensor is developed through the exploitation of the natural selectivity of the Ca⁺² binding properties of the fluorescent probe Calcium Orange (Molecular Probes, Eugene, OR). A multi-mode optical fiber is used to detect calcium in solution. There is a two and a half fold increase observed between a 1 mM EGTA + buffer solution and a 1 mM Ca²⁺ solution. A variety of different methods of attaching the molecular probe to the end of the fiber are explored.

Over the past few years Blue Road Research and the University of California at San Diego have been collaborating to develop a bridge health monitoring system using long gage length fiber optic strain sensors and modal analysis. Two programs supporting this effort have been funded by the National Science Foundation and from this work several papers have been published showing its strong progress^1-5. In 2002, the Federal Highway Administration and Caltrans performed a full-scale test on some of the components that will be used for the planned I-5/Gilman Advanced technology Bridge in California, USA. As a part of this test Blue Road Research used its developmental system to validate the use of this damage detection technique and to compare the results with conventional modal analysis tools.

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.

The feasibility of using multi-axis fiber grating strain sensors to monitor transverse strain and transverse strain gradients to complex, woven composite structures has been evaluated. This paper overviews the multi-axis fiber optic grating strain sensors and how they can be applied to measuring multidimensional strain fields interior to composite parts with complex composite weave structures. Experimental results are given for the case of a bi-axially woven composite coupon as well as for an E-glass/epoxy composite sample.

Optical fiber sensors must compete in performance with traditional electronic sensors, such as quartz crystal pressure and temperature monitors. The precision of commercial electronic sensors can reach the parts-per-billion (ppb) level. To test the precision of a laser based spectrometer system, repeated measurements of an absorption line of a molecular gas cell were made. The Allan deviation is computed, and it is shown that the laser interrogation system, built completely out of commercially available components, can achieve precision at the 10-ppb level.