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.
A novel, multiplexed optical fiber differential-pressure transducer is described for the real-time pressure measurement of airflow in applications involving actuator- and SMA-controlled airfoils and multi-parameter skin friction measurements. The design of the pressure transducer is based upon extrinsic Fabry-Perot interferometry (EFPI) and uses a micromachined silicon diaphragm to modulate the sensing cavity. The pressure transducer was designed to operate from minus 10 to 10 psig and have a resolution of greater than 0.01 psi. Ten pressure transducers were spatially multiplexed and tested for smart wing applications. Results are also reported for an integrated skin friction balance/optical fiber pressure transducer tested in Virginia Tech's Supersonic Tunnel (VTSST).
A fiber optic optical time domain reflectometer-based (OTDR) sensing system is described for detecting the extent and/or location of damage incurred by a composite material following an impact event. Two multi-mode fibers are embedded within a graphite composite to form a 'checkerboard' fiber mesh with 0.5 inch spacing. The measurement system consists of a PC- based virtual instrument, a high resolution optical time domain reflectometer, and two fiber optic sensors. Using the OTDR and processing algorithms, the location of impact can be determined within the composite panel by scanning the length of each fiber from both ends. The OTDR scanning process yields four reference points which can be used to determine the location of the impact referenced to a coordinate axes designated within the composite panel.
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.
Extrinsic Fabry-Perot interferometric (EFPI) sensors have previously been demonstrated for relative strain and temperature measurements for smart structure applications. Inherent difficulties in the signal processing of these devices has created the need for absolute measurement capabilities. In this paper, we present an absolute measurement technique based upon white-light interferometric path matching. The system matches a reference gap to the sensing gap of an EFPI. When the difference of these two lengths is within the coherence length of the source, an intensity envelope is created in the system output. Determination of the corresponding path mismatch indicates the size of the sensor gap and hence strain can be determined. This measurement technique is capable of multiplexing an array of EFPI sensors and data will be presented demonstrating four multiplexed devices. Theoretical considerations for system optimization are also presented. As the only fiber-optic sensors subcontractor to Northrop Corporation on the Navy/Air Force-sponsored Smart Metallic Structures (SMS) program, Fiber & Sensor Technologies (F&S) is developing the optical fiber fatigue gage instrumentation for a multiplexed, in situ structural health monitoring system for aging aircraft. In March, 1995, F&S successfully demonstrated the system on a full-size F/A-18 wing-box spar fully instrumented with 12 of F&S' patented EFPI optical fiber strain gages. F&S is now in process of up-scaling the signal processing system in addition to the optics and intends to demonstrate a second generation multipoint sensor system capable of simultaneously monitoring strains at up to 60 different sites throughout the aircraft later in 1995 or early 1996.
Extrinsic Fabry-Perot interferometric (EFPI) sensors have previously been demonstrated for relative strain and temperature measurements for smart structure applications. Inherent difficulties in the signal processing of these devices has created the need for absolute measurement capabilities. In this paper, we present an absolute measurement technique based upon white-light interferometric path matching. The system matches a reference gap to the sensing gap of an EFPI. When the difference of these two lengths is within the coherence length of the source, an intensity envelope is created in the system output. Determination of the corresponding path mismatch indicates the size of the sensor gap and hence strain can be determined. This measurement technique is capable of multiplexing an array of EFPI sensors and data will be presented demonstrating four multiplexed devices. Theoretical considerations for system optimization are also presented.
A modified design of the extrinsic Fabry-Perot interferometric (EFPI) optical fiber sensor, for complete characterization of multi-component strain fields, is proposed. The novel EFPI includes a sensor head with two input fibers such that the respective reflections from the output multimode fibers end face are in quadrature. Any strain field possessing a component along the line joining the axes of the two fibers causes the initial phase difference to be modulated. The measured changes in phase difference is employed to determine this transverse component of the strain field. Absolute measurements are possible using the AEFPI sensing system. Applications of the modified sensor-head to smart materials and structures are discussed.
The ability of future materials to autonomously sense and respond to environmental stimuli has been proposed for several years [1, 2, 3]. Some investigators envision Hie large-scale, "smart” integrated function structures of Fifty years from now gradually evolving from the discretely instrumented and actuated structures of today and the near future for on-line, nondestructive evaluation.
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