Dawn K. Gifford

High resolution optical frequency domain reflectometry for characterization of components and assemblies

We describe a technique for polarization sensitive optical frequency domain reflectometry (OFDR) that achieves 22 micrometer two-point spatial resolution over 35 meters of optical length with -97 dB sensitivity in a single measurement taking only seconds. We demonstrate OFDRs versatility in both time- and frequency-domain metrology by analyzing a fiber Bragg grating (FBG) in both the spectral and impulse response domains. We also demonstrate how a polarization diversity receiver can be used in an OFDR system to track changes in the polarization state of light propagating through a birefringent component.

Optical vector network analyzer for single-scan measurements of loss, group delay, and polarization mode dispersion

We present a method for measuring the complete linear response, including amplitude, phase, and polarization, of a fiber-optic component or assembly that requires only a single scan of a tunable laser source. The method employs polarization-diverse swept-wavelength interferometry to measure the matrix transfer function of a device under test. We outline the theory of operation to establish how the transfer function is obtained. We demonstrate the enhanced accuracy, precision, and dynamic range of the technique through measurements of several components.

Characterization of Polarization-Maintaining Fiber Using High-Sensitivity Optical-Frequency-Domain Reflectometry

Optical-frequency-domain reflectometry is used to measure the group-index difference and the refractive-index difference (i.e., beat length) between the fast and slow modes in polarization-maintaining optical fiber. The Rayleigh scatter normally present in the fiber is measured in reflection. This measurement, in turn, enables a distributed measurement of the fibers birefringence that is rapid and completely nondestructive

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-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.

Waveguide dispersion effects in silicon-on-insulator coupled-resonator optical waveguides

The dispersion of the waveguides that constitute microring resonators can considerably affect the dispersion characteristics of coupled-resonator optical waveguides (CROWs). We derive expressions for CROW dispersion and group delay for silicon-on-insulator microring CROWs, showing both theoretically and experimentally the band-to-band dependence of the bandwidth and group delay on the dispersion properties of the constituent single-mode silicon waveguide.

Optical Frequency Domain Reflectometry for Single- and Multi-Mode Avionics Fiber-Optics Applications

In this talk, we will introduce a commercially available optical frequency domain reflectometry (OFDR) technique and its applications in avionics fiber-optics. We will provide multiple measurement examples within the avionics field not currently supported by conventional test tools or methods including high-resolution fault detection and distributed fiber sensing