Sidney G. Allison

Novel piezoelectric actuators for tuning an optical fiber Bragg grating

A method of stretching optical fiber holds interest for measur- ing strain in smart structures where the physical displacement may be used to tune optical fiber lasers. A small, lightweight, tunable fiber laser is ideal for demodulating strain in optical fiber Bragg gratings (FBGs) at- tached to smart structures such as the reusable launch vehicle that is being developed by NASA. A method is presented for stretching optical fibers using the thin-layer composite unimorph ferroelectric driver and sensor (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 FBG is presented. Means of adapting THUNDER actuators for stretching optical fibers are discussed, including ferrules, ferrule clamp blocks, and plastic hinges made with stereo lithography.

Pulsed phase-locked loop technology improvements for greater accuracy and expanded capabilities

This paper presents the design for a new pulsed phase-locked loop (PPLL) instrument. Earlier phase detection methods are analyzed for potential error sources, and an alternative phase detection method with fewer sources of error is described. This alternative method of phase detection is incorporated into a fully automated PPLL instrument. Data taken with the new system is compared to data taken with an earlier PPLL instrument. This comparison shows that the alternative phase detection does reduce error. The control loop is analyzed for stability and optimization, and a method of automating this optimization is presented. A method of wave-form acquisition using the automated PPLL that permits automatic location of the received echo is also described.

Interrupted ultrasonic bolt load measurements using the pulsed phase-locked loop system

A method of reacquiring a previous phase lock-point using the pulsed phase-locked loop (PPLL) ultrasonic system in situations where the measurement is interrupted such as by removing and recoupling the transducer is developed. Operation of the PPLL is mathematically analyzed in the time domain rather than in the conventional frequency domain, providing a method of characterizing multiple lock-points. The general lock-point reacquisition method that follows from the model relies on measurement over a frequency range that spans several lock-points and is demonstrated to work well for interrupted measurements on threaded fasteners. The method is analyzed for sensitivity to signal noise, and an equation is derived, expressing the number of lock-points over which to measure for successful lock-point reacquisition as a function of signal noise level. Experimental verification on a glass block shows that theoretical values obtained with this model are in good agreement with measurements. Automation of the method presented has been demonstrated, showing that it offers a practical, objective approach to making interrupted load measurements, thereby greatly widening the range of applications of the PPLL.

Optical fiber distributed sensing structural health monitoring (SHM) strain measurements taken during cryotank Y-joint test article load cycling at liquid helium temperatures

This paper outlines cryogenic Y-joint testing at Langley Research Center (LaRC) to validate the performance of optical fiber Bragg grating strain sensors for measuring strain at liquid helium temperature (-240°C). This testing also verified survivability of fiber sensors after experiencing 10 thermal cool-down, warm-up cycles and 400 limit load cycles. Graphite composite skins bonded to a honeycomb substrate in a sandwich configuration comprised the Y-joint specimens. To enable SHM of composite cryotanks for consideration to future spacecraft, a light-weight, durable monitoring technology is needed. The fiber optic distributed Bragg grating strain sensing system developed at LaRC is a viable substitute for conventional strain gauges which are not practical for SHM. This distributed sensing technology uses an Optical Frequency Domain Reflectometer (OFDR). This measurement approach has the advantage that it can measure hundreds of Bragg grating sensors per fiber and the sensors are all written at one frequency, greatly simplifying fiber manufacturing. Fiber optic strain measurements compared well to conventional strain gauge measurements obtained during these tests. These results demonstrated a high potential for a successful implementation of a SHM system incorporating LaRCs fiber optic sensing system on the composite cryotank and other future cryogenic applications.

Pulsed phase-locked loop calibration over frequency

This paper presents a method of correcting for unwanted phase shifts introduced by interchanging ultrasonic transducers and other measurement system components when using the pulsed phase-locked loop (PPLL) ultrasonic system. Theory is derived mathematically separating phase errors into their constituents. The relationship to a previously derived method of determining the number of periods into the tone-burst, at which the system samples, is established, and a method of adjusting this number to be an integer is presented. Implementation of the technology on a computer is demonstrated through experimental results on transducers of differing frequencies and diameters. Sensitivity to path-length is tested. This work has resulted in a straightforward means of calibrating transducers in conjunction with PPLL measurements using a single, fixed reference block.

Tunable Fiber Bragg Grating Ring Lasers using Macro Fiber Composite Actuators

The research reported herein includes the fabrication of a tunable optical fiber Bragg grating (FBG) fiber ring laser (FRL)1 from commercially available components as a high-speed alternative tunable laser source for NASA Langleys optical frequency domain reflectometer (OFDR) interrogator, which reads low reflectivity FBG sensors. A Macro-Fiber Composite (MFC) actuator invented at NASA Langley Research Center (LaRC) was selected to tune the laser. MFC actuators use a piezoelectric sheet cut into uniaxially aligned rectangular piezo-fibers surrounded by a polymer matrix and incorporate interdigitated electrodes to deliver electric fields along the length of the piezo-fibers. This configuration enables MFC actuators to produce displacements larger than the original uncut piezoelectric sheet. The FBG filter was sandwiched between two MFC actuators, and when strained, produced approximately 3.62 nm of wavelength shift in the FRL when biasing the MFC actuators from -500 V to 2000 V. This tunability range is comparable to that of other tunable lasers and is adequate for interrogating FBG sensors using OFDR technology. Three different FRL configurations were studied. Configuration A examined the importance of erbium-doped fiber length and output coupling. Configuration B demonstrated the importance of the FBG filter. Configuration C added an output coupler to increase the output power and to isolate the filter. Only configuration C was tuned because it offered the best optical power output of the three configurations. Use of Plastic Optical Fiber (POF) FBGs holds promise for enhanced tunability in future research.

Stretch-tuning optical fiber Bragg gratings using macro-fiber composite (MFC) piezoelectric actuators

The demand for high safety and reliability standards for aerospace vehicles has resulted in time-consuming periodic on-ground inspections. These inspections usually call for the disassembling and reassembling of the vehicle, which can lead to damage or degradation of structures or auxiliary systems. In order to increase aerospace vehicle safety and reliability while reducing the cost of inspection, an on-board real-time structural health monitoring sensing system is required. There are a number of systems that can be used to monitor the structures of aerospace vehicles. Fiber optic sensors have been at the forefront of the health monitoring sensing system research. Most of the research has been focused on the development of Bragg grating-based fiber optic sensors. Along with the development of fiber Bragg grating sensors has been the development of a grating measurement technique based on the principle of optical frequency domain reflectometry (OFDR), which enables the interrogation of hundreds of low reflectivity Bragg gratings. One drawback of these measurement systems is the 1 - 3 Hz measurement speed, which is limited by commercially available tunable lasers. The development of high-speed fiber stretching mechanisms to provide high rate tunable Erbium-doped optical fiber lasers can alleviate this drawback. One successful approach used a thin-layer composite unimorph ferroelectric driver and sensor (THUNDER) piezoelectric actuator, and obtained 5.3-nm wavelength shift. To eliminate the mechanical complexity of the THUNDER actuator, the research reported herein uses the NASA Langley Research Center (LaRC) Macro-Fiber Composite (MFC) actuator to tune Bragg grating based optical fibers.

Ultrasonic Measurement of Aircraft Strut Hydraulic Fluid Level

An ultrasonic method is presented for non-intrusively measuring hydraulic fluid level in aircraft struts in the field quickly and easily without modifying the strut or aircraft. The technique interrogates the strut with ultrasonic waves generated and received by a removable ultrasonic transducer hand-held on the outside of the strut in a fashion that is in the presence or absence of hydraulic fluid inside the strut. This technique was successfully demonstrated on an A-6 aircraft strut on the carriage at the Aircraft Landing Dynamics Research Facility at NASA Langley Research Center. Conventional practice upon detection of strut problem symptoms is to remove aircraft from service for extensive maintenance to determine fluid level. No practical technique like the method presented herein for locating strut hydraulic fluid level is currently known to be used.

Integrated regenerative multicellular optical fiber grating control system

A new technique is presented for active distributed fiber sensing for interrogating structural integrity and environmental monitoring using an innovation in low power integrated compact tunable tiber optic laser capability.