Roger G. Duncan

Swept-wavelength interferometric interrogation of fiber Rayleigh scatter for distributed sensing applications

We review recent advancements in making high resolution distributed strain and temperature measurements using swept-wavelength interferometry to observe the spectral characteristics of Rayleigh scatter in optical fibers. Current methods available for distributed strain or temperature sensing in optical fiber include techniques based on Raman, Brillouin, and Rayleigh scattering. These techniques typically employ optical time domain reflectometry and are thus limited in spatial resolution to 0.1 to 1 m. Fiber Bragg gratings can yield higher spatial resolution but are difficult to multiplex in large numbers for applications requiring wide scale coverage. Swept-wavelength interferometry allows the Rayleigh scatter amplitude and phase to be sampled with very high spatial resolution (10s of microns). The Rayleigh scatter complex amplitude can be Fourier Transformed to obtain the Rayleigh scatter optical spectrum and shifts in the spectral pattern can related to changes in strain or temperature. This technique results in distributed strain measurements with 1 με resolution or temperature measurements with 0.1 C resolution. These measurements can be made with sub-cm spatial resolution over a 100 m measurement range or with sub-10 cm resolution over a 1 Km range. A principle advantage of this technique is that it does not require specialty fiber. Thus, measurements can be made in pre-installed single mode or multimode fibers, including those used for telecommunication networks. Applications range from fault monitoring in short range communications networks, structural health monitoring, shape sensing, pipeline and electrical transmission line monitoring, to perimeter security. Several examples are discussed in detail.

High-accuracy fiber-optic shape sensing

We describe the results of a study of the performance characteristics of a monolithic fiber-optic shape sensor array. Distributed strain measurements in a multi-core optical fiber interrogated with the optical frequency domain reflectometry technique are used to deduce the shape of the optical fiber; referencing to a coordinate system yields position information. Two sensing techniques are discussed herein: the first employing fiber Bragg gratings and the second employing the intrinsic Rayleigh backscatter of the optical fiber. We have measured shape and position under a variety of circumstances and report the accuracy and precision of these measurements. A discussion of error sources is included.

High-resolution extended distance distributed fiber-optic sensing using rayleigh backscatter

We describe the use of swept-wavelength interferometry for distributed fiber-optic sensing in single- and multimode optical fiber using intrinsic Rayleigh backscatter. The interrogation technique is based on measuring the spectral shift of the intrinsic Rayleigh backscatter signal along an unaltered standard telecommunications grade optical fiber and converting the spectral shift to strain or temperature. This technique shows great utility as a method for highly distributed sensing over great distances with existing, pre-installed optical fiber. Results from sensing lengths greater than 1 km of optical fiber with spatial resolutions better than 10 cm are reported.

Fiber-Optic Shape Sensing and Distributed Strain Measurements on a Morphing Chevron

Boeing has recently flight tested a Variable Geometry Chevron (VGC) system which used shape memory alloy (NiTinol) actuators to drive changes in shape. At take off, the VGCs immersed into the fan stream to reduce jet noise; at cruise they were actively morphed to investigate shock cell noise and performance losses. A set of three strain gages mounted on each chevron provided estimates of its tip position (shape) and feedback to the on-board control system. During the development of the VGC flight system, Luna Innovations instrumented two VGC test articles with shape probes, a new technology in which multi-core fiber provides distributed and axially co-located differential strain measurements to generate complex shape data. This technology shows promise of providing a more direct correlation of NiTinol actuation to chevron shape and tip immersion. Additionally, Luna Innovations instrumented each VGC with high density distributed strain fiber to provide hundreds of discrete strain measurements over the surface of the chevron.

Characterization of a fiber-optic shape and position sensor

We report the results of a study of the performance characteristics of a distributed fiber-optic shape and position sensor. Strain measurements from distributed fiber Bragg gratings in a multi-core optical fiber multiplexed via the frequency domain reflectometry technique are used to deduce the shape of the optical fiber. We have measured a range of two- and three-dimensional shapes using a multi-core fiber with a sensor spacing of 1.0 cm and a gage length of 0.5 cm and have reported the accuracy and precision of these measurements. A discussion of error sources is also included.

Use of High Spatial Resolution Fiber‐Optic Shape Sensors to Monitor the Shape of Deployable Space Structures

We report the use of a fiber‐optic distributed sensing system to monitor the shape of light‐weight deployable space structures. This technique involves using optical frequency domain reflectometry to demodulate the reflected signal from multiplexed Bragg gratings that have been photoetched in the core of an optical fiber. In this work, high‐resolution optical shape sensors were applied to the surface of isogrid booms and used to monitor the shape of the structure subjected to various static and dynamic loading conditions. Data from the fiber‐optic sensors correlates strongly with expected results.

Distributed Bragg grating sensing technique for test article damage detection and monitoring

We report the use of a fiber-optic distributed sensing system to monitor structural fatigue on an aircraft undergoing a full scale fatigue test. This technique involves using optical frequency domain reflectometry to demodulate the reflected signals from multiplexed Bragg gratings that have been photoetched in the core of an optical fiber. The optical fibers, containing a high density of Bragg gratings, were applied along the surface of a Lockheed Martin P-3C Orion fatigue test article to assess the suitability of this technique for long-term structural damage detection and monitoring. Preliminary results indicate good agreement with quasi-collocated foil strain gauges and demonstrate great potential for supplementing or replacing conventional non-destructive evaluation techniques.

Structural integrity monitoring of aircraft panels using a distributed Bragg grating sensing technique

We report the use of a fiber-optic distributed sensing system to monitor crack growth on aircraft panels. The system utilizes optical frequency domain reflectometry to demodulate the reflected signals from up to thousands of weakly reflecting gratings photoetched along a single optical fiber. In our experiment, data from a regular array of sensors attached to an aircraft panel were recorded as the panel was subjected to increasing loads. Strain contour maps generated from these data enable clear visualization of the crack growth over time. A similar experiment was also performed using fiber-optic strain sensors embedded in aircraft composite repair patches. The results of these experiments demonstrate the viability of distributed fiber-optic sensing for crack growth monitoring.

High spatial-resolution temperature monitoring of an industrial motor using a distributed fiber-optic sensing technique

We report the use of a fiber-optic distributed sensing system to monitor temperature at a multitude of discrete points on an industrial motor undergoing qualification after a rewinding. This technique involves using optical frequency domain reflectometry to demodulate the reflected signal from multiplexed Bragg gratings that have been photoetched in the core of an optical fiber. In this work, high-resolution optical sensing fiber was applied along the stator windings and end-windings of the motor to assess their suitability for long-term temperature monitoring. Performance tests were conducted at different heat loads representing different electrical conditions. Results indicate excellent agreement with collocated Resistance Temperature Devices (RTDs) and demonstrate significant potential for mitigating costly motor failure due to insulation breakdown resulting from highly localized hotspots.

Fabrication of nonlinear optical devices in ionically self-assembled monolayers

Abstract. The development of both “soft” and “hard” fabrication tech-niques for the patterning of nonlinear photonic devices in ionically self-assembled monolayer ISAM films is reported.Acombination of electronbeam lithography and reactive ion etching was used to pattern two-dimensional holes with a lattice of 710 nm and diameters ranging from550 to 650 nm. A soft alternative to this fabrication was also demon-strated. Nanoimprint lithography was successfully employed to patternsimilar photonic structures with average hole diameters of 490 nm and alattice spacing of 750 nm, as well as Bragg gratings with a period of620 nm. Potential impact of this fabrication process on the chemicalcomposition and nonlinear properties of the ISAM films was assessedusing Fourier transform infrared spectroscopy, x-ray photoelectron spec-troscopy, and second harmonic generation. The spectroscopy tech-niques confirmed that the chemical composition and bonding of theISAM films was not adversely affected by the thermal cycles required fornanoimprinting. Second harmonic generation analysis also confirmedthat the nanoimprinting process did not affect the nonlinear properties ofthe material, PCBS/PAH ISAM films, further indicating the suitability ofsuch materials for the nanoimprinting of nonlinear optical photonicstructures.