The development of both “soft” and “hard” fabrication techniques for the patterning of nonlinear photonic devices in ionically self-assembled monolayer (ISAM) films is reported. A combination of electron beam lithography and reactive ion etching was used to pattern two-dimensional holes with a lattice of 710 nm and diameters ranging from 550 to 650 nm. A soft alternative to this fabrication was also demonstrated. Nanoimprint lithography was successfully employed to pattern similar photonic structures with average hole diameters of 490 nm and a lattice spacing of 750 nm, as well as Bragg gratings with a period of 620 nm. Potential impact of this fabrication process on the chemical composition and nonlinear properties of the ISAM films was assessed using Fourier transform infrared spectroscopy, x-ray photoelectron spectroscopy, and second harmonic generation. The spectroscopy techniques confirmed that the chemical composition and bonding of the ISAM films was not adversely affected by the thermal cycles required for nanoimprinting. Second harmonic generation analysis also confirmed that the nanoimprinting process did not affect the nonlinear properties of the material, PCBS/PAH ISAM films, further indicating the suitability of such materials for the nanoimprinting of nonlinear optical photonic structures.
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.
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.
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.
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.
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.
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.
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.
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