Nils Nöther

Incoherent optical frequency domain reflectometry and distributed strain detection in polymer optical fibers

We present, to our knowledge for the first time, the possibility of measuring the backscatter signal of perfluorinated polymer optical fibers (POF) using an incoherent optical frequency domain reflectometry (OFDR) technique. The OFDR setup is described and it is shown that the dynamic range and measurement speed are superior to standard OTDR systems. It is shown for the first time that distributed detection of strain in POF is possible using the OFDR technique.

A distributed fiber optic sensor system for dike monitoring using Brillouin frequency domain analysis

We report on the development of a distributed sensor system for strain measurement using Brillouin optical frequency domain analysis (BOFDA) in single-mode silica optical fibers. Our research aims at the application of the sensor system in flood protection. The sensing fibers are embedded into the soil body of river dikes, where they perform early detection of critical soil displacement. We present a BOFDA setup that performs strain measurements with a spatial resolution better than 3 meters over a length of 2 km. Its accuracy is verified by measurements on a calibrated strain profile as well as several laboratory tests that emulate the stressing of the optical fibers by soil movement. It is shown that the BOFDA approach offers feasible solutions to known critical issues of Brillouin sensing such as spectral broadening at high spatial resolution, digital filtering for enhancement of the dynamic range, and fluctuations of the Brillouin gain due to birefringence.

Stimulated Brillouin scattering in graded index multimode optical fiber by excitation of the fundamental mode only

We investigate the suitability of silica graded index multimode fibers (MMF) for distributed Brillouin sensing in structural health monitoring, where the measurement range is limited by small bendings that appear during the integration process of the sensing fibers into the structures. For the investigation of stimulated Brillouin scattering (SBS) in MMF, we use an MMF connected on both ends to the SMF measurement setup by fusion splices to ensure that only the fundamental mode is transmitted. The SBS spectra in MMF are recorded using a 1319 nm single frequency (line width 5 kHz) laser. Results found for standard singlemode fibers and the fundamental mode in multimode silica optical fibers are compared. We present the gain spectra showing the dependence of frequency shift, attenuation and modal noise to both temperature and strain. The dependence of the attenuation due to bending is shown. Finally, the perspective of the excitation of SBS in polymer optical fibers is discussed against the background of our research on SBS in MMF.

A distributed fiber optic sensor system for dike monitoring using Brillouin optical frequency domain analysis

We report on the development of a complete system for spatially resolved detection of critical soil displacement in river embankments. The system uses Brillouin frequency domain analysis (BOFDA) for distributed measurement of strain in silica optical fibers. Our development consists of the measurement unit, an adequate coating for the optical fibers and a technique to integrate the coated optical fibers into geotextiles as they are commonly used in dike construction. We present several laboratory and field tests that prove the capability of the system to detect areas of soil displacement as small as 2 meters. These are the first tests of truly distributed strain measurements on optical fibers embedded into geosynthetics.

Structural Health Monitoring by Distributed Fiber Optic Sensors Embedded into Technical Textiles

Abstract Technical textiles with embedded distributed fiber optic sensors have been developed for the purposes of structural health monitoring in geotechnical and civil engineering. The distributed fiber optic sensors are based on Brillouin scattering in silica optical fibers and OTDR in polymer optical fibers. Such “smart” technical textiles are used for reinforcement of geotechnical and masonry structures. The embedded fiber optic sensors provide online information about the condition of the structure and about the occurrence and location of any damage or degradation. Zusammenfassung Technische Textilien mit integrierten faseroptischen Sensoren eröffnen neue Möglichkeiten der Zustandsüberwachung (structural health monitoring) in Geotechnik und Ingenieurbau. Die verteilt messenden Sensoren basieren auf der Brillouin-Streuung in Glasfasern und auf der OTDR in polymeroptischen Fasern. Derartige “intelligente” technische Textilien werden in erster Line zur Verstärkung von geotechnischen Bauwerken und von Gebäuden genutzt. Die integrierten Sensoren liefern eine zeitnahe Information über den bestimmungsgemäßen Zustand des Bauwerks sowie über die Entstehung und den Ort von lokalen Bauwerksschäden.

Performance of digital incoherent OFDR and prospects for optical fiber sensing applications

We propose a digital implementation of the incoherent optical frequency domain reflectometry (I-OFDR) technique for precise backscatter measurement and optical fiber sensing applications. Specific performance parameters of the I-OFDR are discussed and compared to an analog vector network analyzer-based I-OFDR system. Improved sensitivity, dynamic range and signal stability of the digital I-OFDR is presented and demonstrated by means of quasi-distributed length change measurement.

A novel signal restoration method in the frequency domain to enhance spatial resolution in distributed Brillouin sensing

This article demonstrates how spatial resolution of distributed Brillouin sensing systems can be significantly enhanced by measuring the spatially resolved Brillouin gain spectra of an optical fiber in the frequency domain. We employ a novel signal processing method to overcome the known problem of spectral broadening that occurs at centimeter-range spatial resolutions. The method is based on an analytical description of stimulated Brillouin scattering in optical fibers for the case of harmonically modulated optical signals. From this analysis, the artifacts that degrade the measurement resolution were modeled; for the first time to our knowledge, undegraded gain spectra could be restored by means of deconvolution techniques adopted from digital image processing.