Distributed fiber sensing (DFS) is a powerful tool for structural health monitoring (SHM), allowing continuous and seamless measurements of temperature and strain along the fiber. The spatial accuracy of a DFS interrogator, as a key parameter of the system, is vital for precisely locating structural perturbations or defects. Its evaluation and calibration methods however attract little attention. A fiber optic artefact based on a fiber loop is developed to evaluate distance accuracy and signal quality for both self-developed and commercial sensing systems based on Rayleigh, Raman, and Brillouin scattering effects, respectively. The measured distance is corrected to remove the influence of the pulse width. Additionally, the obtained SNRs are compared for different loop trips and pulse widths, assisting to assess signal quality for SHM applications.

The measurement and control of temperature plays a key role in achieving the European Green Deal targets for a low carbon energy system. Fibre-optic thermometry is an emerging technology that can improve temperature measurement in extreme environments for energy providers and industry due to its distributed sensing and immunity to electromagnetic fields. Various applications for optimisation and monitoring in the energy sector are described, covering the whole range from energy generation to transmission and consumption. However, fibre-optic thermometers have cross sensitivities to other quantities (e.g., strain and humidity) and ageing effects that need to be investigated, quantified and minimised to obtain traceable and reliable measurements. This is particularly important so that applications in critical infrastructure can benefit from future measurements that are not possible with conventional sensors. The European INFOTherm project aims to overcome the limitations that currently prevent the widespread use of fibre-optic thermometry by creating a dedicated European metrology infrastructure for research, development and calibration. First results on measurement uncertainty, improvement of measurement techniques and practical field tests are presented. Bei der Erreichung der Ziele des europ & auml;ischen Green Deal f & uuml;r ein kohlenstoffarmes Energiesystem spielen Temperaturmessung und -& uuml;berwachung eine Schl & uuml;sselrolle. Die faseroptische Thermometrie ist eine aufstrebende Technologie, die aufgrund ihrer verteilten Messung und ihrer Unempfindlichkeit gegen & uuml;ber elektromagnetischen Feldern die Temperaturmessung in extremen Umgebungen f & uuml;r Energieversorger und die Industrie verbessern kann. Es werden verschiedene Anwendungen zur Optimierung und & Uuml;berwachung im Energiesektor beschrieben, die den gesamten Bereich von der Energieerzeugung & uuml;ber die & Uuml;bertragung bis hin zum Verbrauch abdecken. Faseroptische Thermometer weisen jedoch Querempfindlichkeiten gegen & uuml;ber anderen Gr & ouml;ss en (z. B. Dehnung und Feuchtigkeit) und Alterungseffekte auf, die untersucht, quantifiziert und minimiert werden m & uuml;ssen, um r & uuml;ckf & uuml;hrbare und zuverl & auml;ssige Messungen zu erhalten. Dies ist besonders wichtig, damit Anwendungen in der kritischen Infrastruktur zuk & uuml;nftig von Messungen profitieren k & ouml;nnen, die mit konventionellen Sensoren nicht m & ouml;glich sind. Das europ & auml;ische Projekt INFOTherm zielt darauf ab, die Hemmnisse zu & uuml;berwinden, die einen breiten Einsatz der faseroptischen Thermometrie derzeit verhindern, indem eine dedizierte europ & auml;ische metrologische Infrastruktur f & uuml;r Forschung, Enwicklung und Kalibrierung geschaffen wird. Erste Ergebnisse zur Messunsicherheit, zur Verbesserung der Messverfahren und zu praktischen Feldtests werden vorgestellt.

Current temperature sensors require regular recalibration to maintain reliable temperature measurement. Photonic/quantum-based approaches have the potential to radically change the practice of thermometry through provision of in situ traceability, potentially through practical primary thermometry, without the need for sensor recalibration. This article gives an overview of the European Partnership in Metrology (EPM) project: Photonic and quantum sensors for practical integrated primary thermometry (PhoQuS-T), which aims to develop sensors based on photonic ring resonators and optomechanical resonators for robust, small-scale, integrated, and wide-range temperature measurement. The different phases of the project will be presented. The development of the integrated optical practical primary thermometer operating from 4 K to 500 K will be reached by a combination of different sensing techniques: with the optomechanical sensor, quantum thermometry below 10 K will provide a quantum reference for the optical noise thermometry (operating in the range 4 K to 300 K), whilst using the high-resolution photonic (ring resonator) sensor the temperature range to be extended from 80 K to 500 K. The important issues of robust fibre-to-chip coupling will be addressed, and application case studies of the developed sensors in ion-trap monitoring and quantum-based pressure standards will be discussed.

We investigate integrated silicon ring resonators with regard to the influence of design parameters and intra-wafer variations. First, we show the effect of different ring radii and gaps between ring and bus waveguide on optical properties (peak width, finesse, Q factor, and extinction ratio), from which we calculate the resonators’ coupling and loss coefficients. The dependence on the gap of these properties is discussed at the wafer scale. Second, by incorporating the spectra of 2242 resonators from 59 nominally identical dies on a 200 mm wafer, we show how these properties depend on the resonators’ position on the wafer. Third, we demonstrate how curve fitting of loss and coupling coefficients as a function of the gaps can be used to estimate the optimal gap that realizes critical coupling with a significantly reduced number of manufactured test structures needed to find optimal design parameters.

A novel method for transformer vibration monitoring using intensity-based distributed acoustic/vibration sensing (DAS/DVS) is presented. Fibre optic sensors were fully integrated into an oil-filled medium voltage distribution transformer. Open-circuit in-situ measurements were carried out using a single and three-phase supply with an electrical power of up to 30 kVA. Spectral analysis of the mechanical vibrations were performed by measuring the frequency response of the transformer using plain fibre-and transducer-based DVS approaches. The results were compared with the spectra of a microphone mounted on the outside of the transformer housing and the electrical measurements on the high voltage side. Using the fibre optic DVS based monitoring we were able to detect changes in operating power and frequency. Further it could be shown, that it is possible to identify anomalous behaviour such as electrical/mechanical noise sources using the determined vibration spectra.

We propose, and demonstrate, the use of a fiber-optical measurement artefact as a metrological tool for traceable distance calibration of distributed optical fiber sensors. The constructed fiber artefact consists of a lead-in fiber coupled to a fiber loop using a 3-dB coupler and is used to calibrate both a home-build Brillouin-OTDR setup and a custom version of a commercial Brillouin-OTDR interrogator build for distributed temperature sensing. For both interrogators, we demonstrate distance calibrations with 1-meter uncertainty (k=1) in the offset length and 0.1 % (1 m/km) uncertainty (k = 1) in the distance scale factor. In addition, it is shown that the fiber artefact can be used to assess undesired distance-dependent measurement biases. (c) 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

The paper reports the inscription of fiber Bragg gratings by 400nm femtosecond laser pulses using an interferometer setup. High temperatures probes were assembled, calibrated using temperature fixed-points and applied in industrial processes at T>1400°C.

A novel fibre-optic transducer is presented that is capable to convert a vibration signal into a spatially changing strain profile. Static loading experiments were performed using swept-wavelength interferometry (SWI) to characterise the distributed strain profile of the sensor. Based on this, the strain response of each fibre section was calculated for coherent optical time domain reflectometry (C-OTDR) as a function of pulse width and spatial sampling interval. The resulting strain response of the sensor spans over two orders of magnitude. With the help of this transducer and using only the intensity information of the C-OTDR, we were able to determine vibration spectra without additional harmonic frequencies for accelerations between 0.01 ms -2 and 0.2ms -2 .

Abstract Photonic sensors offer the possibility of purely optical measurement in contact thermometry. In this work, silicon-based ring resonators were used for this purpose. These can be manufactured with a high degree of reproducibility and uniformity due to the established semiconductor manufacturing process. For the precise characterisation of these photonic sensors, a measurement setup was developed which allows laser-based spectroscopy around 1550 nm and stable temperature control from 5 °C to 95 °C. This was characterised in detail and the resulting uncertainty influences of both the measuring set-up and the data processing were quantified. The determined temperature stability at 20 °C is better than 0.51 mK for the typical acquisition time of 10 s for a 100 nm spectrum. For a measurement of >24 h at 30 °C a standard deviation of 2.6 mK could be achieved. A hydrogen cyanide reference gas cell was used for traceable in-situ correction of the wavelength. The determined correction function has a typical uncertainty of 0.6 pm. The resonance peaks of the ring resonators showed a high optical quality of 157 000 in the average with a filter depth of up to 20 dB in the wavelength range from 1525 nm to 1565 nm. When comparing different methods for the determination of the central wavelength of the resonance peaks, an uncertainty of 0.3 pm could be identified. A temperature-dependent shift of the resonance peaks of approx. 72 pm/K was determined. This temperature sensitivity leads together with the analysed uncertainty contributions to a repeatability of better than 10 mK in the analysed temperature range from 10 °C to 90 °C.

data of Figure 3: Q-factor (a) and extinction ratio (b) of the three ring resonators (R1, R2, R3) between 1530 nm to 1570 nm for different temperatures (20°C, 50°C, 80°C)

Temperature is one of the most relevant physical quantities that affects almost all processes in nature. However, the realization of accurate temperature standards using current temperature references, like the triple point of water, is difficult due to the requirements on material purity and stability of the environment. In addition, in harsh environments, current temperature sensors with electrical readout, like platinum resistors, are difficult to implement, urging the development of optical temperature sensors. In 2018, the European consortium Photoquant, consisting of metrological institutes and academic partners, started investigating new temperature standards for self-calibrated, embedded optomechanical sensor applications, as well as optimised high resolution and high reliability photonic sensors, to measure temperature at the nano and meso-scales and as a possible replacement for the standard platinum resistant thermometers. This article presents an overview of the results obtained with sensor prototypes that exploit photonic and optomechanical techniques for sensing temperatures over a large temperature range (5 K to 300 K). Different concepts are demonstrated, including ring resonators, ladder-like resonators and suspended membrane optomechanical thermometers, highlighting initial performance and challenges, like self-heating that need to be overcome to realize photonic and optomechanical thermometry applications.

Fiber Bragg gratings inscribed in single crystalline multimode sapphire fibers (S-FBG) are suitable for monitoring applications in harsh environments up to 1900 °C. Despite many approaches to optimize the S-FBG sensor, a metrological investigation of the achievable temperature uncertainties is still missing. In this paper, we developed a hybrid optical temperature sensor using S-FBG and thermal radiation signals. In addition, the sensor also includes a thermocouple for reference and process control during a field test. We analyzed the influence of the thermal gradient and hotspot position along the sensor for all three detection methods using an industrial draw tower and fixed point cells. Moreover, the signal processing of the reflected S-FBG spectrum was investigated and enhanced to determine the reachable measurement repeatability and uncertainty. For that purpose, we developed an analytical expression for the long-wavelength edge of the peak. Our findings show a higher stability against mechanical-caused mode variations for this method to measure the wavelength shift compared to established methods. Additionally, our approach offers a high robustness against aging effects caused by high-temperature processes (above 1700 °C) or harsh environments. Using temperature-fixed points, directly traceable to the International System of Units, we calibrated the S-FBG and thermocouple of the hybrid sensor, including the corresponding uncertainty budgets. Within the scope of an over 3-weeks-long field trial, 25 production cycles of an industrial silicon manufacturing process with temperatures up to 1600 °C were monitored with over 100,000 single measurements. The absolute calibrated thermocouple (Uk=2≈1K…4K) and S-FBG (Uk=2≈10K…14K) measurements agreed within their combined uncertainty. We also discuss possible strategies to significantly reduce the uncertainty of the S-FBG calibration. A follow-up measurement of the sensor after the long-term operation at high temperatures and the transport of the measuring system together with the sensor resulted in a change of less than 0.5 K. Thus, both the presented hybrid sensor and the measuring principle are very robust for applications in harsh environments.

We present an analytical model for the dynamical self-heating effect in air-cladded optical microring resonators (ORRs). The spatially and time resolved temperature field is calculated by integrating the corresponding boundary value problem of the heat equation. It turns out that the self-heating amplitude is approximately proportional to the total absorbed power and anti-proportional to the thermal conductivity of the cladding material. Further, two-photon absorption plays a major role in the heating process, even for moderate input powers, due to the strong light confinement. Heating times are determined to be in the microsecond range and may limit the response time of ORR devices. The explicit formulas for the temperature fields allow a much faster determination of heating properties compared to elaborate finite element simulations. Thus, our model is predestinated for scanning large parameter spaces. We present such an analytical model for the self-heating effect in ORRs. For this purpose, we solve the heat equation on the ORRs geometrical domain. The heat source is caused by two effects, linear absorption from defect states and quadratic two-photon absorption (TPA). Due to the strong light confinement on resonance, very high light intensities are reachable in the resonator ring and the TPA might become a dominant heat source even for low excitation powers. We utilize insulating Neumann boundary conditions to calculate the temperature increase in the substrate region as a convolution between heat source and the corresponding Greens function. The temperature field in the ring structure is calculated by solving the corresponding eigenvalue problem that arises from a separation ansatz. The result is discussed in terms of maximum self-heating, response time and power dependence for ORRs with very high Q-factors of over 100 000. Finally, we compare the analytical calculations of the self-heating effect with finite element computations.

Summary: Photonic sensors offer new possibilities for the metrological temperature determination in specific applications including high electric fields or harsh environments. Within two EU projects the PTB develop and validate different photonic thermometers from 0 °C to over 1500 °C. The aim of this work is to develop and validate novel accurate photonic thermometers with uncertainties in the mK range and sensors for application within harsh environments at high temperatures. We show first results using silicon ring resonators which achieve high quality factors ( Q ≈ 160 000) and over 15 dB filter contrast.

We present the results of distributed fiber optic strain sensing for condition monitoring of a hybrid type IV composite fully wrapped pressure vessel using multilayer integrated optical fibers. Distributed strain sensing was performed for a total number of 252,000 load cycles until burst of the vessel. During this ageing test material fatigue could be monitored and spatially localized. Critical material changes were detected 17,000 cycles before material failure. Results have been validated by acoustic emission analysis.

We present results demonstrating several beneficial effects on distributed fiber optic vibration sensing (DVS) functionality and performance resulting from utilizing standard single mode optical fiber (SMF) with femtosecond laser-inscribed equally-spaced simple scattering dots. This modification is particularly useful when using traditional single-wavelength amplitude-based coherent optical time domain reflectometry (C-OTDR) as sensing method. Local sensitivity is increased in quasi-distributed interferometric sensing zones which are formed by the fiber segments between subsequent pairs of the scattering dots. The otherwise nonlinear transfer function is overwritten with that of an ordinary two-beam interferometer. This linearizes the phase response to monotonous temperature variations. Furthermore, sensitivity fading is mitigated and the demodulation of low-frequency signals is enabled. The modification also allows for the quantitative determination of local temperature gradients directly from the C-OTDR intensity traces. The dots' reflectivities and thus the induced attenuation can be tuned via the inscription process parameters. Our approach is a simple, robust and cost-effective way to gain these sensing improvements without the need for more sophisticated interrogator technology or more complex fiber structuring, e.g., based on ultra-weak FBG arrays. Our claims are substantiated by experimental evidence.

Due to their high strength-to-weight ratio and excellent fatigue resistance, glass fiber reinforced polymers (GFRP) are used as a construction material in a variety of applications including composite high-pressure gas storage vessels. Thus, an early damage detection of the composite material is of great importance. Material degradation can be determined via measuring the distributed strain profile of the GFRP structures. In this article, swept wavelength interferometry based distributed strain sensing (DSS) was applied for structural health monitoring of internal pressure loaded GFRP tube specimens. Measured strain profiles were compared to theoretical calculation considering Classical Lamination Theory. Reliable strain measurements with millimeter resolution were executed even at elongations of up to 3% in the radial direction caused by high internal pressure load. Material fatigue was localized by damaged-induced strain changes during operation, and detected already at 40% of burst pressure.

Due to high specific stiffness, fibre reinforced plastics are the dominant material group for the design of mobile pressure vessels. At the Federal Institute for Materials Research and Testing (BAM) aging process of composite pressure vessels is studied to be able to give more accurate lifetime predictions in future. Investigations are based on type III breathing air cylinders consisting of an aluminium tank which is fully wrapped with carbon fibre reinforced plastics. The goal is to detect changes of residual stresses over life time which directly affect fatigue strength. Within this paper an approach is presented to monitor residual stresses via an experimental modal analysis (EMA). First, the influence of changed stress conditions on modal parameters is analysed via a numerical study. Secondly, a test bench for an EMA is set up. To be able to analyse cylinders of different prestress condition, several specimens are prestress modified via high-temperature and high-pressure treatment. During the modification processes, specimens are monitored via optical fibres to control prestress modifications. Through experimental measurements of the modified specimens via EMA changes in prestressing can be detected. Finally, the validity and accuracy of the EMA is evaluated critically by comparing all numerically and experimentally obtained data.

Glass-fibre-reinforced plastics (GFRP) are commonly used for the construction of pressure vessels and tanks for example in automotive and aerospace industries. Especially for pressure vessels used for the storage of natural gas or hydrogen with operation pressures up to 700 bar, an early material fatigue detection is of great interest. Measuring the distributed strain profile of GFRP structures helps to understand and detect material fatigue. In this article, we demonstrate the great potential of swept wavelength interferometry (SWI) based distributed strain sensing for the monitoring of pressure vessels made from GFRP. A resin transfer molding (RTM) process was used to manufacture GFRP-tube specimens. Commercial optical fibres with polyimid coating were glued to the surface externally in circumferential and axial direction. A cyclic load of up to 150 bar was applied to the samples using a servo-hydraulic test bench. Comparing the loaded and unloaded test conditions, we determined up to 2 % elongation in circumferential direction. We demonstrate reliable distributed strain measurements with sub-centimetre spatial resolution. By monitoring these high-resolution strain profiles, we were able to detect local material degradation that manifested itself as localized strain changes. Crucially, the material degradation could be detected already after 75 % of the fatigue life before a crack appeared that led to leakage.