Alex K. Sang

Distributed strain and temperature sensing in plastic optical fiber using Rayleigh scatter

In recent years we have demonstrated the ability to analyze Rayleigh scatter in single- and multi-mode fused silica fibers to deduce strain and temperature shifts, yielding sensitivity and resolution similar to that obtained using Fiber Bragg Gratings. This technique employs scanning laser interferometry to obtain high spatial resolution Rayleigh scatter spectral information. One of the promising aspects of using Rayleigh scatter for distributed sensing is that the technique should work for any fiber that exhibits discernable Rayleigh scatter. We now demonstrate that distributed sensing with mm-range spatial resolution in off-the-shelf plastic multi-mode optical fiber is feasible. We report temperature and strain sensitivity, and comment on measurement range and hysteresis level. Distributed Rayleigh scatter sensing in plastic optical fiber may offer a valuable alternative to sensing in fused silica fibers because of plastics low cost and differing mechanical and chemical properties.

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.

One Centimeter Spatial Resolution Temperature Measurements in a Nuclear Reactor Using Rayleigh Scatter in Optical Fiber

We present the use of swept wavelength interferometry for distributed fiber-optic temperature measurements in a nuclear reactor. The sensors consisted of 2-m segments of commercially available, single mode optical fibers. The interrogation technique is based on measuring the spectral shift of the intrinsic Rayleigh backscatter signal along the optical fiber and converting the spectral shift to temperature.

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.

Multiple Fiber Loop Strain Rosettes in a Single Fiber Using High Resolution Distributed Sensing

Simple, circular loops in a single optical fiber bonded to a metal test sample are used to form multiple strain gauge rosettes. Strain measurements are made using an optical backscatter reflectometer to detect changes in the phase of the Rayleigh Scatter of the fiber with 160-μm spatial resolution along the length of the fiber. Applied strain levels, as well as the axis along which the loads are applied, are measured. Thermal gradients are also detected. The high spatial resolution and strain sensitivity of this technique enable highly functional fiber rosettes formed of small diameter loops of unaltered low-bend-loss optical fiber. Using this technique, multiple rosettes can be formed in a single fiber to created distributed measurements of principal strains.

High resolution, high sensitivity, dynamic distributed structural monitoring using optical frequency domain reflectometry

Fiber-optic ultrasonic transducers are an important component of an active ultrasonic testing system for structural health monitoring. Fiber-optic transducers have several advantages such as small size, light weight, and immunity to electromagnetic interference that make them much more attractive than the current available piezoelectric transducers, especially as embedded and permanent transducers in active ultrasonic testing for structural health monitoring. In this paper, a distributed fiber-optic laser-ultrasound generation based on the ghost-mode of tilted fiber Bragg gratings is studied. The influences of the laser power and laser pulse duration on the laser-ultrasound generation are investigated. The results of this paper are helpful to understand the working principle of this laser-ultrasound method and improve the ultrasonic generation efficiency.

Strain measurements of a fiber loop rosette using high spatial resolution Rayleigh scatter distributed sensing

Strain is measured with high spatial resolution on fiber loops bonded to a metal test sample to form a fiber rosette. Strain measurements are made using an Optical Backscatter Reflectometer to detect changes in the phase of the Rayleigh Scatter of the fiber with 160 μm spatial resolution along the length of the fiber. Using this experimental set-up, applied strain levels as well as the axis along which the loads are applied are measured. Thermal gradients are also detected. The high spatial resolution and strain sensitivity of this technique enable highly functional fiber rosettes formed of small diameter loops of standard low-bend-loss optical fiber.

Distributed Fiber Optic Strain Measurement Using Rayleigh Scatter in Composite Structures

This paper presents the use of distributed fiber optic sensing to achieve centimeter level resolution strain data along the entire length of a large composite beam. A 6.5 meter long composite beam, designed for use in a corrosive flue gas desulfurization (FGD) unit, was instrumented. A section of optical fiber was embedded into a fiberglass rope, which in turn was embedded into the composite beam during the manufacturing process. The beam was experimentally tested in four-point bending at the North Carolina State University Constructed Facilities Laboratory, and the strain profile along the entire length was measured using the embedded optical fiber. Strains of up to 6500 microstrain were measured at over 300 unique positions along the span by monitoring changes in the spectral shift of the Rayleigh scatter in the optical fiber using optical frequency domain reflectometry (OFDR). The fiber used in this test was optically equivalent to standard telecommunication fiber, allowing for low-cost, high-density strain measurements on large structures. The experiment confirms the potential of embedded fiber optic distributed sensing to be used for real-time health monitoring, or as a process feedback in an instrumented structural system. Benefits of employing distributed fiber optic sensing in structures such as the composite FGD unit include the ability to monitor and detect deterioration and damage, minimize the chance of unplanned downtime or failure, and limit exposure to consequences such as environmental contamination.

High-resolution distributed fiber-optic sensing for dynamic structural monitoring

First-generation fiber-optic sensing instruments had scan and data transmission times of several seconds.1–3 By eliminating data processing bottlenecks we have dramatically decreased the scan time and latency while maintaining high sensitivity and spatial resolution. Decreased scan time helps us accurately track fast-moving mechanical responses to an abrupt stimulus such as an impact. When the sensor output is used as feedback to control a structure (for example, when using fiber-optic shape sensor data to remotely guide a catheter to a desired location in the body), low latency allows the control system to respond more quickly and precisely to abrupt environmental stimuli or user inputs. Taken in combination with optical fiber’s light weight, small size, easy installation onto (or within) complex parts, and high-resolution continuous sensing capability, these higher acquisition rates open up a wide range of dynamic sensing opportunities. Potential applications include monitoring the mechanical response to changing loading forces as well as detecting and tracking structural defects in wind turbine blades, aircraft wings and fuselages, and composite pressure vessels. Luna also uses this technology to monitor strain in multi-core optical fibers in order to compute shape and track position in minimally invasive surgical devices. The scanning-laser interferometric technique measures the sensor’s backscattering amplitude and phase versus distance down the fiber with micron-level spatial resolution. The reflected pattern from Rayleigh backscattering or Fiber Bragg Gratings (FBGs) can be analyzed to determine local changes in optical phase induced by a strain or temperature change along the fiber. Rayleigh backscattering in optical fiber is caused by Figure 1. Strain spatial distribution on club A with the sensing fiber mounted in a straight line along the side of the golf club shaft. Strain is read out from Rayleigh scattering at 250Hz; trace is taken at the bottom of the front swing.