Johannes Roths

Gauge factors of fibre Bragg grating strain sensors in different types of optical fibres

Gauge factors of fibre Bragg grating (FBG)-based strain sensors that had been inscribed into three different types of optical fibres, which differ in core diameters and doping concentrations, were determined at room temperature with high accuracy. Repeated measurements were carried out with several samples of each type of fibre to allow statistical evaluations. For each type, the gauge factors were measured in two configurations: when the bare fibres were glued on a specimen at the location of the FBG and when they were vertically suspended and not bonded to any structure at the location of the FBG. By combining the results of both configurations, the strain transfer ratio of the gluing process and the strain-optic coefficient, peff, of the different types of fibres were determined. The strain-optic coefficient was found to vary up to 1.5% for the different types of optical fibres. The strain transfer ratio was obtained to be close to unity (>99%), showing the high quality of the gluing technique employed. The investigations demonstrate that highly accurate strain sensing is possible with fibre-optic strain sensors. The results are important for the development of accurate and reliable attaching techniques for coated sensor fibres and fibre-optic sensor patches.

Strain calibration of optical FBG-based strain sensors

A facility for strain sensitivity calibration of optical FBG-based strain sensors according to the German VDI/VDE 2660 guideline was established and characterized. Statistical analysis of several calibration measurement series performed with one single type of FBG strain sensor and application technique showed a reproducibility of 0.15%. Strain sensitivities for FBGs inscribed in two different types of optical fibres (GF1B and PR2008) showed significantly different strain sensitivities of k = 0.7885±0.0026 and k = 0.7758±0.0024, respectively.

Polarization dependence of the strain sensitivity of fiber Bragggratings inscribed in highly birefringent optical fibers

It can be expected that the range of applications for FBG-based strain and temperature sensors would expand if the accuracy of this sensor technique was improved. In this study, polarization effects of FBG sensors, which contribute significantly to the measurement uncertainty of this technique, were investigated. Therefore, FBGs were inscribed into highly birefringent optical fibers. These sensor elements were attached to a specimen with defined orientations of the fibers slow and fast axes with regard the specimens surface. We observed a change of the fibers birefringence in the order of 5 10-5 as a consequence of the gluing process, that was employed to attach the fiber onto the specimen. The strain sensitivities were determined for each polarization mode and for different fiber orientations using a highly accurate strain calibration facility. It was found that in all experiments the strain sensitivity for the slow axis was significantly higher (about 0.8%) than for the fast axis. The strain sensitivity also depends on the orientation of the fibers birefringent axes with regard to the surface of the specimen. Although the investigations were performed with FBGs inscribed into birefringent fibers, the findings are still of importance for understanding the polarization-dependant accuracy limits of FBGs in standard single-mode fibers.

High-precision thermal strain measurements using surface-mounted fiber Bragg grating sensors

Thermal strain measurements by fiber Bragg grating (FBG) sensors mounted onto different host materials are demonstrated for low coefficients of thermal expansion (CTE). Such low CTEs are typically found in carbon fiber reinforced plastics (CFRP). This work has application potential for FBG sensor networks in the highprecision control of thermal deformations in structures or in curing monitoring. For this purpose, a thermal error model of the FBG sensor, which accounts for the thermo-optic coefficient and the thermal expansion of the FBG, was characterized experimentally. The error-model characterization method is based on reference measurements of FBGs bonded to ZERODUR ceramics. Using this error model, thermal strain can be measured by surface-mounted FBGs on any given host structure using an external temperature reference and the FBGs wavelength shift. This method is demonstrated successfully for unidirectional layers of CFRP with a CTE of -0.4 · 10-6 1/K in fiber direction and for steel (316 Ti), which is commonly used in cryogenic applications. Measurements are performed for temperatures from 100K to 320K and the results are verified by high-precision dilatometer measurements. Accuracy limits of the FBG-based thermal strain measurements are discussed, as well as the minimization of errors induced by the FBGs structural interface. Further, the reduction of errors in the adhesive bonding is discussed. This work expands the understanding of the separation of thermal and mechanical effects in the signals obtained by FBGs.

Comparison of transverse load sensitivities of fibre Bragg gratings in different types of optical fibres

The response of optical fibre Bragg gratings (FBG) to transverse load was found to be significantly different for FBGs inscribed in two different types of single mode optical fibres. The transverse load sensitivity, defined as the relative wavelength separation of the birefringence-induced FBG double-peaks per transverse line-force, was found to be Cq = (45.1 ± 1.5)10-9 1/N/m for a moderately GeO2 doped (GF1B) and Cq = (48.5 ± 1.0)10-9 1/N/m for a highly GeO2 doped (PR2008) optical fibre. These data are important for a complete characterization of the opto-mechanical behavior of FBG sensor elements inscribed in different types of single mode optical fibres.

Determination of strain sensitivity of free fiber Bragg gratings

Strain sensitivities of free, uncoated fiber Bragg gratings at λB ~1535 nm in a SMF28 standard telecommunication fiber and in a highly GeO2 doped photosensitive fiber (F86) were determined at room temperature. Both fibers showed similar strain sensitivities of kSMF28 = (0.7951 ± 0.0041) for the SMF28 and kF86 = (0.7912 ± 0.0023) for the F86 fiber. The stress sensitivities of both fibers were found to be slightly different with the values of (Δλ/F)SMF28 = (1.347 ± 0.006) nm/N for the SMF 28 fiber and (Δλ/F)F86 = (1.309 ± 0.001) nm/N for the F86 fiber.

Temperature Dependence of Glue-Induced Birefringence in Surface-Attached FBG Strain Sensors

The accuracy of surface attached fiber Bragg gratings (FBG) as strain sensors is affected by glue-induced birefringence, especially for applications in a large temperature range. In this study, we investigated the effects of the gluing technique with anew approach using FBGs in PM fibers that were azimuthally aligned relatively to the surface they were attached to. With this methodology, the glue-induced birefringence induced during the thermal curing process and its temperature dependence was investigated in the range of -30 to 170 °C. The used adhesive showed viscoelastic behavior up to 150 °C. The glue-induced birefringence increased with decreasing temperature reaching a value in the order of B = 6 · 10-5 at -30 °C. Measurable reversible stress relaxation processes took place, even at low temperatures below the glues glass transition temperature, which made the glue-induced birefringence dependent on time, temperature, and its prehistory. The same amount of glue-induced birefringence must be expected when FBGs in single mode fibers with the same gluing technique are used, and have to be taken into account in highly accurate measurements.

Regenerated Bragg Gratings in Panda Fibers for Simultaneous Temperature and Force Measurements at High Temperatures

Simultaneous temperature and force measurements in an extended temperature range up to 500 °C were shown with a regenerated fiber Bragg grating in a polarisation maintaining fiber of type Panda. Type I gratings in Panda fibers were regenerated under application of a high temperature annealing process. During the first temperature cycle, a distinct hysteresis of the fibers birefringence with temperature was observed, but reproducible dependencies of the birefringence on temperature were shown for subsequent temperature cycles. The birefringence of the fiber at room temperature was nearly doubled compared to the pristine fiber, and showed linear temperature dependencies below and above 500 °C, but with different sensitivities in both temperature ranges. A change in the temperature dependence of the birefringence can be explained by the crossing of the transition temperature of the stress applying parts (SAP) at around 500 °C. Below this temperature, measurements of temperature and applied axial forces showed reproducible and approximately linear responses of the sensor element, allowing a 2 × 2 matrix approach for simultaneous temperature and longitudinal load measurements to be applied in this temperature range. Above 500 °C, in contrast, the SAPs become viscous, resulting in a viscoelastic behaviour of the fiber.

Herstellung von Faser-Bragg-Gitter in hoch-doppelbrechenden Glasfasern

Zusammenfassung Die Verbreitung von Faser-Bragg-Gittern (FBG) als Sensoren zur Messung von Dehnung und Temperatur führt auch zu einem wachsenden Bedarf an hochgenauen Sensorelementen. Die hier präsentierte Studie zeigt die Grenzen der Standard-FBG-Sensoren (FBGS) auf und stellt ein Konzept für hoch genaue FBGS vor. FBG in hochdoppelbrechenden Glasfasern (HiBi) haben vielversprechende Eigenschaften, die eine höhere Genauigkeit als Standard-FBG ermöglichen. Im Rahmen dieser Studie wurden FBG in unterschiedlichen HiBi-Fasern eingeschrieben und die effektiven Brechungsindizes sowie die Doppelbrechung bestimmt. Diese Daten sind von großer Bedeutung für die reproduzierbare Herstellung von HiBi-FBG als hochgenaue Sensoren. Abstract With spreading of the fibre-Bragg-grating (FBG) based sensor technology the demand for highly accurate sensors is growing. This study shows the limitation of the accuracy of standard FBG-sensors (FBGS) and an approach is presented to the development of highly accurate FBGS. FBGs inscribed into highly birefringent (HiBi) optical fibres show promising properties to reach an accuracy higher than Standard-FBG. In this study FBGs were inscribed into several HiBi-fibres and the effective refractive indices as well as the birefringence were determined. These investigations are important to establish a reproducible fabrication process for HiBi-FBGs that can be used for highly accurate fibre based measurements.

Nonlinear temperature dependence of glue-induced birefringence in polarization maintaining FBG sensors

Glue-induced stresses decrease the accuracy of surface-mounted fiber Bragg gratings (FBG). Significant temperature dependent glue-induced birefringence was verified when a thermally cured epoxy-based bonding technique had been used. Determining the peak separation of two azimuthally aligned FBGs in PM fibers combined with a polarization resolved measurement set-up in a temperature range between -30°C and 150°C revealed high glue-induced stresses at low temperatures. Peak separations of about 60 pm and a nonlinear temperature dependence of the glue-induced birefringence due to stress relaxation processes and a visco-elastic behavior of the used adhesive have been shown.