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Flexible wire device, fastened along outside of fiber bundle from viewing portion to tip, positions calibrated adjustable gage in field of view. Scale factor is determined from known magnification characteristics of fiber optics system or from graduations on gage tip.
A novel fiber optic pressure sensor based on the photoelastic effects has been developed for extremely high temperature gases. At temperatures varying from 25 to 650 C, the sensor experiences no change in the peak pressure of the transfer function and only a 10 percent drop in dynamic range. Refinement of the sensor has resulted in an optoelectronic interface and processor software which can calculate pressure values within 1 percent of full scale at any temperature within the full calibrated temperature range.
A fiber-optic Raman diagnostic system for H2/O2 rocket flows is currently under development. This system is designed for measurements of temperature and major species concentration in the combustion chamber and part of the nozzle of a 100 Newton thrust rocket currently undergoing tests. This paper describes a measurement system based on the spontaneous Raman scattering phenomenon. An analysis of the principles behind the technique is given. Software is developed to measure temperature and major species concentration by comparing theoretical Raman scattering spectra with experimentally obtained spectra. Equipment selection and experimental approach are summarized. This experimental effort is part of a program, which is in progress, to evaluate Navier-Stokes based analyses for this class of rockets.
Report discusses electromagnetic propagation characteristics of weakly guiding optical-fiber structures having complicated shapes with cross-sectional dimensions of order of wavelength. Coupling, power-dividing, and transition dielectric-waveguide structures analyzed. Basic data computed by scalar-wave, fast-Fourier-transform (SW-FFT) technique, based on numerical solution of scalar version of wave equation by forward-marching fast-Fourier-transform method.
A complete working prototype of the fiber optic blade tip sensor was first tested in the laboratory, followed by a thorough evaluation at NASA W8 Single Compressor Stage Facility in Lewis Research Center. Subsequently, a complete system with three parallel channels was fabricated and delivered to Dr. Kurkov. The final system was tested in the Subsonic Wind Tunnel Facility, in parallel with The General Electric Company's light probe system. The results at all operating speeds were comparable. This report provides a brief description of the system and presents a summary of the experimental results.
In an optical fiber having NA greater than 1, light may be internally reflected when it strikes the fiber end at a fiber-air interface. This problem may be overcome by modification of the fiber by reverse tapering the core. Light is redirected by the taper to strike the interface at an angle closer to normal. This allows light to exit the fiber end that would by internally reflected in an untapered fiber of NA greater than 1. The novelty of the present invention lies in the tapering of the fiber core for increased through transmission of light. Prior art devices have made use of fiber tapers to achieve mode control or fiber coupling. The problem of internal reflection has not been addressed as it is one that is not as important in fibers having NA less than 1, which are more common. In chemical sensing it is advantageous to make use of fibers having higher NA due to an increased sensitivity. However the advantages in sensitivity are diminished due to the loss of signal at the fiber-air interface. The present invention overcomes the problem of loss at the interface, thus facilitating the use of high NA fibers for chemical sensing.
This paper addresses two fiber optic sensor development programs in the Photonics Laboratory, NASA Ames Research Center, one in progress and the other being initiated. The ongoing program involves development of advanced acoustic sensors for wind tunnel applications. The new undertaking involves development of a novel sensor technique for studies of aerodynamic transition from laminar to turbulent flow.
A three-component fiber-optic based Doppler Global Velocimeter was constructed, evaluated and used to measure shock structures about a low-sonic boom model in a Mach 2 flow. The system was designed to have maximum flexibility in its ability to measure flows with restricted optical access and in various facilities. System layout is described along with techniques developed for production supersonic testing. System evaluation in the Unitary Plan Wind Tunnel showed a common acceptance angle of f4 among the three views with velocity measurement resolutions comparable with free-space systems. Flow field measurements of shock structures above a flat plate with an attached ellipsoid-cylinder store and a low-sonic boom model are presented to demonstrate the capabilities of the system during production testing.
The Photonics Group at NASA Goddard Space Flight Center in the Electrical Engineering Division of the Advanced Engineering and Technologies Directorate has been involved in the design, development, characterization, qualification, manufacturing, integration and anomaly analysis of optical fiber subsystems for over a decade. The group supports a variety of instrumentation across NASA and outside entities that build flight systems. Among the projects currently supported are: The Lunar Reconnaissance Orbiter, the Mars Science Laboratory, the James Webb Space Telescope, the Express Logistics Carrier for the International Space Station and the NASA Electronic Parts. and Packaging Program. A collection of the most pertinent information gathered during project support over the past year in regards to space flight performance of optical fiber components is presented here. The objective is to provide guidance for future space flight designs of instrumentation and communication systems.
The X-56A aircraft is a remotely-piloted aircraft with flutter modes intentionally designed into the flight envelope. The X-56A program must demonstrate flight control while suppressing all unstable modes. A previous X-56A model study demonstrated a distributed-sensing-based active shape and active flutter suppression controller. The controller relies on an estimator which is sensitive to bias. This estimator is improved herein, and a real-time robust estimator is derived and demonstrated on 1530 fiber optic sensors. It is shown in simulation that the estimator can simultaneously reject 230 worst-case fiber optic sensor failures automatically. These sensor failures include locations with high leverage (or importance). To reduce the impact of leverage outliers, concentration based on a Mahalanobis trim criterion is introduced. A redescending M-estimator with Tukey bisquare weights is used to improve location and dispersion estimates within each concentration step in the presence of asymmetry (or leverage). A dynamic simulation is used to compare the concentrated robust estimator to a state-of-the-art real-time robust multivariate estimator. The estimators support a previously-derived mu-optimal shape controller. It is found that during the failure scenario, the concentrated modal estimator keeps the system stable.
The X-56A aircraft is a remotely-piloted aircraft with flutter modes intentionally designed into the flight envelope. The X-56A program must demonstrate flight control while suppressing all unstable modes. A previous X-56A model study demonstrated a distributed-sensing-based active shape and active flutter suppression controller. The controller relies on an estimator which is sensitive to bias. This estimator is improved herein, and a real-time robust estimator is derived and demonstrated on 1530 fiber optic sensors. It is shown in simulation that the estimator can simultaneously reject 230 worst-case fiber optic sensor failures automatically. These sensor failures include locations with high leverage (or importance). To reduce the impact of leverage outliers, concentration based on a Mahalanobis trim criterion is introduced. A redescending M-estimator with Tukey bisquare weights is used to improve location and dispersion estimates within each concentration step in the presence of asymmetry (or leverage). A dynamic simulation is used to compare the concentrated robust estimator to a state-of-the-art real-time robust multivariate estimator. The estimators support a previously-derived mu-optimal shape controller. It is found that during the failure scenario, the concentrated modal estimator keeps the system stable.
A wavelength division multiplexing (WDM) method for fiber optic sensors is proposed which uses a broadband light source and narrow bandpass thin film optical filter coatings on cylindrical graded index lenses. In the WDM system described here, all bits are multiplexed onto a single signal return fiber by assigning each bit a unique wavelength. A multielement photodetector array is used as the encoded position information is in parallel. Preliminary prototype test results are presented.
The NASA Glenn Research Center is interested in determining structural damage in engine components during flight to evaluate the health of aerospace propulsion systems. On the ground, we can use holography to detect structural damage by examining the characteristic mode shapes and frequencies of vibrating objects. We are studying the feasibility of using embedded fiber Bragg gratings (FBGs) to accomplish this goal in a flight-worthy system, by using the minimal intrusion and high sensitivity afforded by fiber optics. We have recently compared holographically imaged modes of vibrating plates with the corresponding dynamic strains detected by embedded FBGs. We constructed an experimental setup for studying the responses of FBGs to dynamic excitations. One of the plates was made of a polymer matrix composite (PMC) with an FBG embedded in it, and the other one was made of copper with surface-mounted FBGs. The instrumented plates were mounted and vibrated, and time-averaged holography was used to measure their surface displacements. Simultaneously, the signals from the FBGs were detected and sent via fiber-optic cable to a quiet location about 20 m away for interrogation. The the test configuration used for the PMC plate is shown. Experimental results are also shown. The FBG was embedded in the middle of the PMC plates, roughly within the center circular fringe in each of the interferograms shown. Two resonant excitation frequencies were used: 706 and 3062 Hz. The plot in this paper shows a larger FBG signal at the higher frequency; this is because the plate bends more at higher order resonant modes, causing higher strain. This contrasts to the smaller displacements characteristic of higher frequencies, which are measured by holographic techniques.
The status of on-going NASA tasks involving fiber optic data transmission, and related topics is given. Ground based applications, including a multiplexed wideband 2 km prototype link and a building-to-building video link, are described. In connection with the use of fibers in space, the effects to be expected from the space environment are touched on, particularly radiation darkening of fibers and temperature effects. Laboratory results on performance of fibers at cryogenic temperatures are also presented. Finally, some thoughts on future applications are given.
Recently, scientists and engineers have investigated the advantages of smart materials and structures by including actuators in material systems for controlling and altering the response of structural environments. Applications of these materials systems include vibration suppression/isolation, precision positioning, damage detection and tunable devices. Some of the embedded materials being investigated for accomplishing these tasks include piezoelectric ceramics, shape memory alloys, and fiber optics. These materials have some benefits and some shortcomings; each is being studied for use in active material design in the SPICES (Synthesis and Processing of Intelligent Cost Effective Structures) Consortium. The focus of this paper concerns the manufacturing aspects of smart structures by incorporating piezoelectric ceramics, shape memory alloys and fiber optics in a reinforced thermoset matrix via resin transfer molding (RTM).
Link phase noise and Allan deviation do not degrade the hydrogen maser performance using 800 meters of LTC fiber optic cable.
State-of-the-art instrumentation techniques have provided an opportunity to obtain greater insight into structural wing loads during flight. An investigation was undertaken to research new wing instrumentation and load sensing techniques for measuring accurate in-flight spanwise load distributions on wing structures. A straight tapered wing was instrumented with both conventional foil strain gages at five spanwise wing stations and fiber optic strain sensors at every half inch along the entire wing span. Thirty-nine unique load cases were applied to the wing lower surface using hydraulic actuators to obtain various shear, bending moment, and torque load distributions on the wing. This paper will highlight three load calibration approaches. Conventional linear regression calibration methods were applied to foil strain gages providing a single wing station vertical shear, bending moment, and torque load. Linear regression methods were applied to a fiber optic sensing system to provide bending moment and torque spanwise load distributions. A load sensing scheme using strain derived wing shape information derived from a single load case provided vertical shear and bending moment spanwise load distribution information. Aspects of the three different approaches will be compared and contrasted to inform the reader of the benefits or disadvantages of each. Instrument installation, sensor characteristics, test execution aspects, and recommended calibration techniques will be discussed.
Local Area Networks are in common use for data communications and have enjoyed great success. Recently, there is a growing interest in using a single network to support many applications in addition to traditional data traffic. These additional applications introduce new requirements in terms of volume of traffic and real-time delivery of data which are not met by existing networks. To satisfy these requirements, a high-bandwidth tranmission medium, such as fiber optics, and a distributed channel access scheme for the efficient sharing of the bandwidth among the various applications are needed. As far as the throughput-delay requirements of the various application are concerned, a network structure along with a distributed channel access are proposed which incorporate appropriate scheduling policies for the transmission of outstanding messages on the network. A dynamic scheduling policy was devised which outperforms all existing policies in terms of minimizing the expected cost per message. A broadcast mechanism was devised for the efficient dissemination of all relevant information. Fiber optic technology is considered for the high-bandwidth transmisison medium.