Fiber Bragg grating (FBG) sensors are restricted to relatively slow speed applications due to current wavelength demodulation techniques. In this paper, we propose and model a demodulation technique using long period fiber grating (LPG) filters to address this problem.

Optical fiber sensors have numerous advantages over conventional sensing technologies. One such advantage is that optical fiber sensors can operate in high temperature environments. While most conventional electrical-based sensors do not operate reliably over 300 degrees C, fused silica based optical fiber sensors can survive up to 900 degrees C, and sapphire based optical fiber sensors can survive up to 2000 degrees C. Using both fused silica and sapphire technologies, we present result for high temperature strain, pressure, and temperature sensors using Extrinsic Fabry-Perot INterferometric-based and Bragg grating sensors. High temperature strain and temperature sensors were used to conduct fatigue testing of composite coupons at 600 degrees C. The results from these specific high temperature applications are presented along with future applications and directions for these sensors.

Optical fiber sensors are a novel and ideal approach for making chemical and physical measurements in a variety of harsh environments. They do not corrode, are resistant to most chemicals, immune to electromagnetic interference, light weight, inherently small and have a flexible geometry. This paper presents recent test results using optical fiber long-period grating (LPG) sensors to monitor corrosion precursors and by-products. With the appropriate coating, the LPG sensor can be designed to identify a variety of environmental target molecules, such as moisture, pH, sulfates, chlorates, nitrates and metal-ions in otherwise inaccessible regions of metallic structures. Detection of these chemicals can be used to determine if the environment within a particular area of an airplane or infrastructure is becoming conducive to corrosion or whether the corrosion process is active. The LPG sensors offer a clear advantage over similar electrochemical sensors since they can be rendered immune to temperature cross-sensitivity, multiplexed along a single fiber, and can be demodulated using a simple, low-cost spectrum analyzer. By coating the LPG sensor with specially designed affinity coatings that selectively absorb target molecules, selective, real-time monitoring of environmental conditions is possible. This sensing platform shows great promise for corrosion byproduct detection in pipe networks, civil infrastructure, process control, and petroleum production operations and can be applied as biological sensors for in-vitro detection of pathogens, and chemical sensors for environmental and industrial process monitoring.

Optical fiber sensing techniques are ideal for applications where high-temperature, electromagnetic interference, or vibration cause traditional electrical sensors to become unreliable. Gold-coated, silica-based optical fibers can withstand temperatures up to 900 degrees C and sapphire fibers can be employed for temperatures as high as 2000 degrees C. We present dynamic strain and temperature measurements of ceramic matrix composite specimens using extrinsic Fabry-Perot interferometric (EFPI) fiber optic strain and temperature sensors. The extremely low-mass and rugged construction of the sensors will allow them to survive high-cycle, high-temperature fatigue testing.

We report test results using optical fiber sensors to measure dynamic strain and temperature on-ceramic-matrix composite (CMC)specimens at temperatures up to 600 degrees C. For strain sensing we are employing extrinsic Fabry-Perot interferometric (EFPI) strain gages fabricated with gold-coated optical fibers and attached to the CMC specimens using high-temperature ceramic adhesive. For temperature measurements, specially fabricated Bragg and long-period grating (LPG) sensors are being employed.

Optical fiber sensors, because of their small size, low weight, extremely high information carrying capability, immunity to electromagnetic interference, and large operational temperature range, provide numerous advantages over conventional electrically based sensors, Fiber-based sensors have found numerous applications in industry for process control, and more recently for monitoring the health of advanced civil structures. This paper presents preliminary results from optical fiber sensor designs for monitoring acceleration and magnetic field.

Optical fiber corrosion sensors are being developed to address the high service costs associated with current structural maintenance procedures for civilian and military assets. A distributed optical fiber sensor system will help reduce the costs associated with corrosion damage and extend the lifetime of existing assets. Annual national losses in time, labor, materials and systems has been estimated in the billions of dollars. Additional costs arise from system downtime that results from disassembly procedures necessary to locate corrosion damage in remote locations. Furthermore, the potential to damage other system parts during maintenance is increased when disassembly and reassembly occurs. The development of on-line optical fiber sensors capable of detecting corrosion would eliminate a significant portion of the maintenance costs. We present recent test results using optical fiber long-period grating (LPG) corrosion sensors. With the appropriate coating, the sensors can be designed to detect water or metal ions in otherwise inaccessible regions of the aircraft. The LPG sensors can be designed with low temperature cross-sensitivity, multiplexed along a single fiber, and can be demodulated using a simple, low-cost spectrum analyzer.

Microelectromechanical systems or MEMS are miniature devices that have several advantages over conventional sensing and actuating technology. MEMS devices benefit form well developed integrated circuit production methods which ensure high volume, high yield processes that create low-cost sensors and actuators. OPtical fiber interconnected MEMS will provide new functionality in MEMS devices such as multiplexed operation for distributed sensing applications. This paper presents approaches in optical fiber to MEMS interfacing and some preliminary results.

In the past, optical fiber switches have typically been constructed from plastics or ceramics. However, the inability of these materials to operate effectively at high temperatures has greatly restricted the utilization of these devices. Recently, fiber optic switches have been manufactured from two thermally stable materials: carbon- carbon and BS50, a high temperature ceramic. The integration of these dimensionally stable materials into the fabrication of the optical switch will allow the switch to be utilized in an increased number of applications including optics, aerospace, mechanical, medical, and electronics. Preliminary testing included examining these new optical switches for structural damage due to the manufacturing process and testing the switches to demonstrate that the fibers could be realigned after processing. The tests concluded that no structural damage was induced, and the critical fiber realignment was achieved.

We present recent progress in the development of optical fiber sensors for early detection of corrosion on aging metallic aircraft. Optical fiber sensing techniques being investigated include fiber optic Bragg grating strain sensors to monitor the mass reduction of metal ''witness'' capillary tubes and extrinsic Fabry-Perot interferometric (EFPI) strain gages to monitor pillowing in lap joints.

Significant weight and space savings have been realized in avionic equipment and structures by using composite materials. Optical fiber provides improved communication between equipment and components on board the aircraft. The marriage of these two technologies by embedding optical fiber in the composites achieves improved signal transmission and reduced weight. The goal of this work is to provide reliable, low-profile, optical interconnects with composite-embedded optical fiber for communication with opto-electronic circuit cards and modules. The design and results of a prototype embedded multimode optical beamsplitter utilizing in-fiber 45 degree(s) dielectric coatings are presented.

Significant weight and space savings have been realized in avionic equipment and structures by using composite materials. Optical fiber provides improved communication between equipment and components on board the aircraft. The marriage of these two technologies by embedding optical fiber in the composites achieves improved signal transmission and reduced weight. The goal of this work is to provide reliable, low-profile, optical interconnects with composite-embedded optical fiber for communication with opto-electronic circuit cards and modules. The design and results of a prototype embedded multimode optical beamsplitter utilizing in-fiber 45 degrees dielectric coatings are presented.

A novel scheme for uncompensated measurements of static and dynamic strain in high temperature environments is proposed and implemented. This technique uses the high sensitivity of the extrinsic Fabry-Perot interferometric (EFPI) fiber-optic sensor to attain a resolution of 1 mu epsilon On an axially strained silicon carbide rod. The sensor head is fabricated from sapphire fibers and tubes, and hence, water cooling is not required even under high heat flux conditions produced by temperatures in excess of 1000 degrees C. Improvements in system design are suggested by employing the ''absolute'' EFPI implementation of the scheme. Future work in this area and applications to industrial materials and structures are discussed.

We propose the use of modal interferometers to detect changes in the transmitted signal in high-finesse extrinsic Fabry-Perot interferometric cavities for real-time, absolute strain and temperature measurements. The short length of the cavity ensures a large free spectral range of the resulting output Airy pattern and by tracking the wavelength shift of one peak, the applied perturbation may be completely characterized. The same principle is also proposed to detect the signal reflected from fiber Bragg gratings for strain and temperature sensing. The relative merits and demerits of this demodulation scheme are discussed and preliminary experimental results are presented.

We present modifications in the design and operation of the conventional optical fiber extrinsic Fabry-Perot interferometric (EFPI) sensor to obtain real-time, self-calibrated, on-line and absolute strain measurements. The absolute EFPI (AEFPI) system utilizes the concept of white light interferometry to interrogate a fiber Fabry-Perot cavity and demodulation of the output signal may be carried out using a number of simple techniques like path matching or optical spectrum analyzer detection. The limitations of the conventional EFPI strain sensors are listed and it is shown that the modified AEFPI system overcomes most of these drawbacks by virtue of the fact that the information is wavelength-encoded. The AEFPI system is used to determine the strain during loading tendons that are commonly utilized in pre-stressed concrete. Other major applications include strain measurements in high-performance aerospace materials and structures under extreme mechanical vibrations and temperature variations. Preliminary experimental results are presented and applications to smart structures are proposed.

Optical fiber gratings have recently emerged as attractive sensors for non-destructive evaluation of materials and structures. We present photoinduced Bragg and long-period gratings (LPGs) for monitoring corrosion in aging infrastructure. These two sensors are described based upon fabrication techniques, sensing mechanisms, sensitivities, and cross sensitivities. It will be demonstrated that while Bragg gratings need to be prestrained to detect corrosion of metals, the modulation of the evanescent field of the cladding modes in-long-period grating can be employed for corrosion monitoring. It will also be shown that sensitivities of LPG-based corrosion sensors to ambient temperature fluctuations can be reduced significantly.

This poster will highlight on-going research at the Virginia Tech Fiber & Electro-Optics Research Center (FEORC) in the area of thin films on optical fibers. Topics will include the sputter deposition of metals and metal; alloys onto optical fiber and fiber optic sensors for innovative applications. Specific information will be available on thin film fiber optic hydrogen sensors, corrosion sensors, and metal-coated optical fiber for high temperature aerospace applications.