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  • 1
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    THUNDER Piezoelectric Actuators as a Method of Stretch-Tuning an Optical Fiber Grating (2000)
    In:  CASI
    Details
    Publication Date: 2018-06-05
    Description: A method of stretching optical fiber holds interest for measuring strain in smart structures where the physical displacement may be used to tune optical fiber lasers. A small, light weight, low power tunable fiber laser is ideal for demodulating strain in optical fiber Bragg gratings attached to smart structures such as the re-usable launch vehicle that is being developed by NASA. A method is presented for stretching optical fibers using the THUNDER piezoelectric actuators invented at NASA Langley Research Center. THUNDER actuators use a piezoelectric layer bonded to a metal backing to enable the actuators to produce displacements larger than the unbonded piezoelectric material. The shift in reflected optical wavelength resulting from stretching the fiber Bragg grating is presented. Means of adapting THUNDER actuators for stretching optical fibers is discussed, including ferrules, ferrule clamp blocks, and plastic hinges made with stereo lithography.
    Keywords: Optics
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    NASA TECHNICAL REPORTS
  • 2
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    Use of 3000 Bragg Grating Strain Sensors Distributed on Four Eight-meter Optical Fibers During Static Load Tests of a Composite Structure (2001)
    In:  CASI
    Details
    Publication Date: 2018-06-05
    Description: This paper describes the use of a fiber optic system to measure strain at thousands of locations along optical fibers where weakly reflecting Bragg gratings have been photoetched. The optical fibers were applied to an advanced composite transport wing along with conventional foil strain gages. A comparison of the fiber optic and foil gage systems used for this test will be presented including: a brief description of both strain data systems; a discussion of the process used for installation of the optical fiber; comparative data from the composite wing test; the processes used for the location and display of the high density fiber optic data. Calibration data demonstrating the potential accuracy of the fiber optic system will also be presented. The opportunities for industrial and commercial applications will be discussed. The fiber optic technique is shown to be a valuable augmentation to foil strain gages providing insight to structural behavior previously requiring reliance on modeling.
    Keywords: Aircraft Design, Testing and Performance
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    NASA TECHNICAL REPORTS
  • 3
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    Improvements to pulsed phase locked loop strain monitor (1993)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: A pulse phase locked loop system according to the present invention is described. A frequency generator such as a voltage controlled oscillator (VCO) generates an output signal and a reference signal having a frequency equal to that of the output signal. A transmitting gate gates the output frequency signal and this gated signal drives a transmitting transducer which transmits an acoustic wave through a material. A sample/hold samples a signal indicative of the transmitted wave which is received by a receiving transducer. Divide-by-n counters control these gating and sampling functions in response to the reference signal of the frequency generator. Specifically, the output signal is gated at a rate of F/h, wherein F is the frequency of the output signal and h is an integer; and the received signal is sampled at a delay of F/n wherein n is an integer.
    Keywords: INSTRUMENTATION AND PHOTOGRAPHY
    Type: NAS 1.71:LAR-14887-1
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    NASA TECHNICAL REPORTS
  • 4
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    Three-Axis Distributed Fiber Optic Strain Measurement in 3D Woven Composite Structures (2013)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Recent advancements in composite materials technologies have broken further from traditional designs and require advanced instrumentation and analysis capabilities. Success or failure is highly dependent on design analysis and manufacturing processes. By monitoring smart structures throughout manufacturing and service life, residual and operational stresses can be assessed and structural integrity maintained. Composite smart structures can be manufactured by integrating fiber optic sensors into existing composite materials processes such as ply layup, filament winding and three-dimensional weaving. In this work optical fiber was integrated into 3D woven composite parts at a commercial woven products manufacturing facility. The fiber was then used to monitor the structures during a VARTM manufacturing process, and subsequent static and dynamic testing. Low cost telecommunications-grade optical fiber acts as the sensor using a high resolution commercial Optical Frequency Domain Reflectometer (OFDR) system providing distributed strain measurement at spatial resolutions as low as 2mm. Strain measurements using the optical fiber sensors are correlated to resistive strain gage measurements during static structural loading. Keywords: fiber optic, distributed strain sensing, Rayleigh scatter, optical frequency domain reflectometry
    Keywords: Composite Materials
    Type: JSC-CN-28273 , 2013 Smart Structures/NDE; Mar 10, 2013 - Mar 14, 2013; San Diego, CA; United States
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  • 5
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    System and Method for Measuring the Transfer Function of a Guided Wave Device (2002)
    In:  CASI
    Details
    Publication Date: 2019-07-10
    Description: A method/system are provided for measuring the NxN scalar transfer function elements for an N-port guided wave device. Optical energy of a selected wavelength is generated at a source and directed along N reference optical paths having N reference path lengths. Each reference optical path terminates in one of N detectors such that N reference signals are produced at the N detectors. The reference signals are indicative of amplitude, phase and frequency of the optical energy carried along the N reference optical paths. The optical energy from the source is also directed to the N-ports of the guided wave device and then on to each of the N detectors such that N measurement optical paths are defined between the source and each of the N detectors. A portion of the optical energy is modified in terms of at least one of the amplitude and phase to produce N modified signals at each of the N detectors. At each of the N detectors, each of the N modified signals is combined with a corresponding one of the N reference signals to produce corresponding N combined signals at each of the N detectors. A total of N(sup 2) measurement signals are generated by the N detectors. Each of the N(sup 2) measurement signals is sampled at a wave number increment (Delta)k so that N(sup 2) sampled signals are produced. The NxN transfer function elements are generated using the N(sup 2) sampled signals. Reference and measurement path length constraints are defined such that the N combined signals at each of the N detectors are spatially separated from one another in the time domain.
    Keywords: Optics
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  • 6
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    Apparatus and Method for Measuring Strain in Optical Fibers using Rayleigh Scatter (2003)
    In:  CASI
    Details
    Publication Date: 2019-08-13
    Description: An apparatus and method for measuring strain in an optical fiber using the spectral shift of Rayleigh scattered light. The interference pattern produced by an air gap reflector and backscattered radiation is measured. Using Fourier Transforms, the spectrum of any section of fiber can be extracted. Cross correlation with an unstrained measurement produces a correlation peak. The location of the correlation peak indicates the strain level in the selected portion of optical fiber.
    Keywords: Optics
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  • 7
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    Edge Triggered Apparatus and Method for Measuring Strain in Bragg Gratings (2003)
    In:  CASI
    Details
    Publication Date: 2019-08-13
    Description: An apparatus and method for measuring strain of gratings written into an optical fiber. Optical radiation is transmitted over one or more contiguous predetermined wavelength ranges into a reference optical fiber network and an optical fiber network under test to produce a plurality of reference interference fringes and measurement interference fringes, respectively. The reference and measurement fringes are detected, and the reference fringes trigger the sampling of the measurement fringes. This results in the measurement fringes being sampled at 2(pi) increments of the reference fringes. Each sampled measurement fringe of each wavelength sweep is transformed into a spatial domain waveform. The spatial domain waveforms are summed to form a summation spatial domain waveform that is used to determine location of each grating with respect to a reference reflector. A portion of each spatial domain waveform that corresponds to a particular grating is determined and transformed into a corresponding frequency spectrum representation. The strain on the grating at each wavelength of optical radiation is determined by determining the difference between the current wavelength and an earlier, zero-strain wavelength measurement.
    Keywords: Instrumentation and Photography
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  • 8
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    Apparatus and method for measuring relative phase of signals in a multiple-echo system (1998)
    In:  CASI
    Details
    Publication Date: 2019-08-27
    Description: An apparatus and method for measuring the relative phase of echo signals in a multiple-echo system. A signal generator generates an exciting tone burst and subsequent tone bursts delayed in phase from the exciting tone burst. The exciting tone burst is gated into a transducer coupled to the multiple-echo system. Each subsequent tone burst is converted into a series of pulses. Each pulse triggers a sample/hold circuit to sample each echo signal. The samples of the echo signal are averaged and then compared to a reference value. The signal generator is controlled to vary the subsequent tone burst phase delay to determine for each echo signal a subsequent tone burst phase delay that causes the average of the samples of the echo signal to be substantially equal to the reference value wherein the determined subsequent tone burst phase delay is the phase delay of the echo signal with respect to the exciting tone burst. The control circuit then determines the difference in phase delay between echo signals to determine the relative phase delay of the echo signals.
    Keywords: Communications and Radar
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  • 9
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    Pulsed phase locked loop strain monitor (1995)
    In:  CASI
    Details
    Publication Date: 2019-08-28
    Description: A pulse phase locked loop system according to the present invention is described. A frequency generator such as a voltage controlled oscillator (VCO) generates an output signal and a reference signal having a frequency equal to that of the output signal. A transmitting gate gates the output frequency signal and this gated signal drives a transmitting transducer which transmits an acoustic wave through a material. A sample/hold samples a signal indicative of the transmitted wave which is received by a receiving transducer. Divide-by-n counters control these gating and sampling functions in response to the reference signal of the frequency generator. Specifically, the output signal is gated at a rate of F/h, wherein F is the frequency of the output signal and h is an integer; and the received signal is sampled at a delay of F/n wherein n is an integer.
    Keywords: Electronics and Electrical Engineering
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  • 10
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    High Precision Wavelength Monitor for Tunable Laser Systems (2002)
    In:  CASI
    Details
    Publication Date: 2019-07-10
    Description: A solid-state apparatus for tracking the wavelength of a laser emission has a power splitter that divides the laser emission into at least three equal components. Differing phase shifts are detected and processed to track variations of the laser emission.
    Keywords: Lasers and Masers
    Format: text
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