Coherent Optical Frequency Domain Reflectometry Research Articles

Rayleigh-based crack monitoring with distributed fiber optic sensors: experimental study on the interaction of spatial resolution and sensor type

AbstractCracks can negatively affect the durability of concrete structures, making effective crack monitoring crucial for maintenance. Utilizing coherent optical frequency domain reflectometry, it is possible to accurately localize cracks and measure their widths. However, long distributed fiber optic sensors (DFOS) lengths, required for structural health monitoring applications, are accompanied by a reduced spatial resolution due to technical limitations of the interrogator. This study investigates the impact of spatial resolution on signal quality in areas with large strain gradients and demonstrates its implications for crack monitoring. First, tensile tests were conducted on a calibration rig to determine the measurable strain gradient for different gage pitches (gps) and further influencing parameters. Subsequently, four types of DFOS – categorized as robust and filigree – were installed on a 4 m long reinforced concrete beam subjected to a 4-point bending test. Crack widths up to 0.5 mm were measured with varying gps to assess how spatial resolution affects crack width calculations. The results indicate that a reduced spatial resolution impairs the ability to accurately record regions with large strain gradients, leading to diminished signal quality and reduced measurable crack widths. While strain gradients of approximately $$1500\,\upmu \varepsilon$$ 1500 μ ε /mm could be measured with the finest gp of 0.65 mm, this threshold dropped to about $$60\,\upmu \varepsilon$$ 60 μ ε /mm for a gp of 5.20 mm. For the DFOS/adhesive combinations tested, and at the highest spatial resolution available for sensor lengths greater than 50 m (gp = 2.60 mm), reliable crack monitoring of crack widths larger than 0.2 mm was not possible. This limitation poses challenges for practical applications. A method for determining the required gp based on the expected crack width and the DFOS/adhesive combination used is proposed to ensure reliable crack monitoring.

Shape Sensing of Resin Pipe with a Helical-winding Optical Fiber in Bending Test

In this study, we developed a shape-sensing technique for a pipe. An optical fiber was helically bonded on the outer surface of a resin pipe whose length, outer diameter and inner diameter are 3200 mm, 34 mm, and 26.6 mm, respectively. Firstly, four grooves were made on the surface along the pipe in a helical pattern with the same pitch of 360 mm. At the one pipe end, adjacent grooves were offset by 90 degrees and the offset was maintained along the pipe. The single-mode fiber was put into a groove and fixed by epoxy and then it was embedded into the adjacent groove. Consequently, the optical fiber was going back and forth twice along the pipe. At the pipe ends, there were stress-free fibers between the grooves. Shape sensing for the pipe is conducted as follows. 1. Strain distributions along the grooves are measured by a distributed fiber-optic strain sensing system. 2. The bending strain is extracted from the measured strain to calculate the curvature and bending angle. 3. The shape of the pipe is reconstructed based on the curvature and bending angle using the Frenet-Serret formulas. Three-point bending tests were conducted. In the tests, the pipe ends were clamped to be the fixed ends and a point load was applied to the pipe center. Strain distributions were measured by tunable wavelength coherent optical time domain reflectometry (TW-COTDR) or optical frequency domain reflectometry (OFDR). Then the pipe shape was reconstructed as described above. We compared the reconstructed shape with the deformation calculated by finite element analysis and the agreement was excellent. If we insert the developed pipe into a bedrock and reconstruct the pipe shape continuously, it is expected that we can detect cracks or damage in the bedrock. This approach is expected to make rock excavation constructions safer.

Birefringence characterization in a dual-hole microstructured optical fiber using an OFDR method.

The birefringence in a dual-hole microstructured optical fiber is numerically calculated and characterized with an optical frequency domain reflectometry (OFDR) method. Due to the asymmetric dual air holes in the cross-section, the polarized L P01x and L P01y modes propagate with different group velocities and time delays. Through a polarized coherent OFDR system in experiment, the Fresnel reflection peaks for each mode are separated in the frequency domain with their corresponding beat frequency. Thus, the group birefringence -9.68×10-4 is calculated with a beat frequency difference of 50.03Hz between the L P01x and L P01y modes at a 6.2m fiber end, which is in good agreement with that of -9.54×10-4 from the theoretical simulation. Our demonstration provides an accurate and flexible method for group birefringence characterization in microstructured optical fibers with complex cross-sectional structures.

Laser vibrometer-rangefinder based on self-sweeping fiber laser.

A vibrometer-rangefinder based on principles of coherent optical frequency-domain reflectometry (C-OFDR) is experimentally demonstrated. A self-sweeping ytterbium-doped fiber laser, which does not require any spectrally selective elements and drivers for wavelength tuning, with a sweeping range of 1056-1074 nm is used as a tunable source of probe radiation for the C-OFDR measurements. We demonstrate the possibility of measuring target vibrations in the frequency range from 2 Hz to 5 kHz with an amplitude of down to ∼5 nm at a distance of up to ∼13 m. The maximum measurable vibration frequency is limited by the instability of the self-sweeping laser parameters in the time domain and is estimated as ∼7.5 kHz.

Optical Fiber-Based Continuous Liquid Level Sensor Based on Rayleigh Backscattering.

This work reports an optical fiber–based continuous liquid level sensor for cryogenic propellant mass gauging, which has significant advantages over the existing liquid level sensors in terms of accuracy, simplicity, and reliability. Based on Rayleigh backscattering coherent optical frequency domain reflectometry, every point of the sensing fiber is a liquid sensor which is able to distinguish liquid and vapor. We obtained a measurement accuracy of 1 mm for the optical fiber sensor by measuring both liquid nitrogen and water levels. For the first time, for practical applications, we experimentally studied the influence of ambient temperature and strain changes on the sensing performance as well as the repeatability of the optical fiber–based liquid level sensor’s measurements.

Fundamental Studies on the Use of Distributed Fibre Optical Sensing on Concrete and Reinforcing Bars.

Distributed fibre optical sensing (DFOS) allows for quasi-continuous strain measurement in a broad range of gauge lengths and measurement frequencies. In particular, Rayleigh backscatter-based coherent optical frequency domain reflectometry has recently registered a significant application increase in structural concrete research and monitoring thanks to its numerous merits, such as high resolution and low invasiveness. However, it is not a plug-and-play technique. The quality of the acquired data depends highly on the choice of the fibre optical sensor and the methods of instrumentation and post-processing. Furthermore, its unprecedented resolution and sensitivity allow capturing local effects not well documented so far. This paper analyses the suitability of DFOS based on Rayleigh backscatter for reliably measuring strains and discusses the origin and structural relevance of local variations in the results. A series of experimental investigations are presented, comprising tensile tests on bare reinforcing bars and concrete compression tests. A critical analysis of the results leads to a best practice for applying DFOS to reinforcing bars and concrete, which establishes a basis for reliable, accurate measurements in structural concrete applications with bonded reinforcement.

Fiber Cavities Design for a Multipoint Fiber Refractometer

A methodology to design a multipoint fiber refractometer based on the Coherent Optical Frequency Domain Reflectometry (C-OFDR) technique is presented; in this refractometer, the nonlinearity effect generated by the scanning in optical frequency is reduced by a proper design of each sensing head; eliminating the need for the auxiliary interferometer commonly used for reducing this effect. Experimental demonstration on the nonlinearity effect in the sensing signal is presented. This effect is reduced by cutting the length of the processed signal in a cycle of scanning; to demonstrate this behavior, 100%, 50%, and 25% of the sensing signal from a scanning cycle are processed. However, it is not only about reducing the length of the sensing signal, but proper sensing head design also allows the reduced signal to be form by an integer number of cycles from each interference signal generated at the sensing heads, which in turns permit concatenation of the reduced signal without losing information. These three considerations, reduction of the signal in a scanning cycle, an integer number of cycles, and concatenation, generate stable and strong sensing signals. In this sense, the design of the sensing heads allows reducing the nonlinearity effect and possibility of a multipoint refractometer as well. Theoretical description and experimental results to demonstrate the effectiveness of the proposed method are presented. Design, construction, characterization and simultaneous implementation of three sensing heads are demonstrated.

Doppler optical frequency domain reflectometry for remote fiber sensing.

Coherent optical frequency domain reflectometry has been widely used to locate static reflectors with high spatial resolution. Here, we present a new type of Doppler optical frequency domain reflectometry that offers simultaneous measurement of the position and speed of moving objects. The system is exploited to track optically levitated "flying" particles inside a hollow-core photonic crystal fiber. As an example, we demonstrate distributed temperature sensing with sub-mm-scale spatial resolution and a standard deviation of ∼10°C up to 200°C.

Temperature monitoring of tumor hyperthermal treatments with optical fibers: comparison of distributed and quasi-distributed techniques

Abstract Optical fiber based sensors capable of measuring temperature distribution over a given length are particularly attractive in the biomedical field, especially in the case of real-time monitoring of minimally invasive hyperthermal treatments of tumors, such as laser ablation, given their intrinsic multiplexing along a single fiber span and the unique properties of optical fibers in terms of flexibility, size and electromagnetic compatibility. The paper compares two sensing approaches: one is based on an array of Bragg gratings inscribed in the core of a single-mode fiber, the other detects Rayleigh backscattering from an unmodified single-mode fiber using coherent optical frequency domain reflectometry. Following an introduction, which describes the most distinctive features of the two approaches, the paper presents three comparative experiments, each devised to highlight a peculiar aspect of the sensors. In the first experiment the sensors are exposed to various constant temperature distributions to evaluate the uniformity of the readings along the fiber length. In the second experiment, a known temperature profile is generated through an ad hoc setup to evaluate the actual capability of the two approaches to reconstruct the temperature distribution. Finally, in the third experiment the two sensors are employed in a simulated tumor laser ablation procedure to measure the temperature distribution in the irradiated area. Both sensors provide results whose error, mainly due to cross sensitivity between temperature and strain, is acceptable for the considered biomedical application.

Fiber Bragg gratings fabricated by femtosecond lasers for narrow-linewidth fiber laser

A narrow-linewidth erbium-doped fiber laser based on the fiber Bragg gratings (FBGs) fabricated was proposed and demonstrated experimentally. A pair of FBGs with reflectivity of 92 % and 10 %, respectively, at the center wavelength of 1580 nm were inscribed in standard SMF-28 fibers without hydrogen loading by femtosecond laser point-by-point directly writing method, which was used as mirrors for wavelength selection of fiber laser, gratings full width at half maximum (FWHM) was less than 0.15 nm. A simple linear-cavity fiber laser was realized based on gain medium, pump source and FBGs. When gain medium of Er-doped fiber is selected 8 m, a stable single wavelength of 1579 nm laser with a 3 dB linewidth of 0.06 nm was obtained. Its output power is 24.1 mW when the maximum pump power is 600 mA, and its single wavelength of 1579 nm is consistent with the central wavelength of the fabricated FBGs. This type of laser source shows real promise for applications in coherent optical frequency domain reflectometry and high resolution fiber sensor systems.

Application of Distributed Fiber Optic Sensing Technique to Monitor Stability of a Geogrid-Reinforced Model Slope

The development of the smart geosynthetics in recent years has shown tremendous potential with regard to its capability to strengthen geotechnical structures and, meanwhile, evaluate the local strains/stresses. This paper introduced the application of a distributed monitoring system to monitor a laboratory model slope reinforced with smart geogrids, in which the coherent optical frequency domain reflectometry technology (C-OFDR) was used to continuously monitor the geogrid deformations under different surcharge loadings. It showed that the measured results using C-OFDR technology were generally consistent to those from the fiber Bragg grating (FBG) sensors; however, C-OFDR has other significant advantages over the FBG technology. Empirical relationships between the geogrid characteristic strain measured by the C-OFDR sensors and the factor of safety calculated by the conventional limit equilibrium method were established, making it possible to use the geogrid characteristic strain to monitor the slope stability. This study proved the effectiveness of the distributed C-OFDR sensing technology in monitoring the geogrid-reinforced slope stability in the laboratory scale, a critical stepping stone to extend this technology to the field.

Resolution Improvement in a Multi-Point Fiber Refractometer Based on Coherent-OFDR

In this letter, we propose a signal processing method for improving the resolution of a multi-point fiber refractometer from 10 -4 to 10 -6 . The fiber refractometer uses the Fresnel reflection at the fiber tip to measure the refractive index (RI), based on the coherent optical frequency domain reflectometry (C-OFDR) technique; using a standard four-pin coaxial pigtailed DFB diode laser as the optical source. The proposed method is divided into two steps; first the raw signal is processed to reduce the nonlinearity generated by the optical frequency sweeping laser, and then a pattern that repeats itself in the sensing signal is identified, extracted and replicated several times to improve the sensing signal used to calculate the RI. In the experimental results, using 30 ms of effective acquisition time, and the proposed method, an improvement of two orders of magnitude in the resolution of the system was obtained.

Short-Range Non-Bending Fully Distributed Water/Humidity Sensors

Existing sensing technologies lack the ability to spatially resolve multiple sources of water or humidity without relying on the deployment of numerous inline sensors. A fully distributed approach has the potential to unlock a diverse range of applications, such as humidity mapping and liquid-depth measurements. We have explored a new direction toward what is, to the best of our knowledge, the first non-bending fully distributed water/humidity sensors. This new class of sensors was made possible from the first combination of small-core exposed-core fiber, a hydrophilic polyelectrolyte multilayer coating, and coherent optical frequency-domain reflectometry. Their non-bending nature enables deployment in a wider range of environments compared to the bending type based on water-induced fiber bending. The sensing mechanism involves monitoring back-reflected optical signals created by changes in the local reflectivity due to water-induced reduction in the local refractive-index of the coating. The demonstrated average sensitivity of the sensing fiber with 10.0 bilayer polyelectrolyte multilayer coating to relative humidity varies from 0.060 to 0.001/%RH (0–38 cm distance) within a dynamic range of 26–95%RH. The distance-dependent detection limit varies between 0.3–10.0%RH (0–38 cm distance), and the spatial resolution of 4.6 mm is the smallest demonstrated for exposed-core fibers and can be vastly improved by simply broadening the swept range. The response time is 4–6 s, and the recovery time is 3–5 s. The sensing range (i.e., distance) is −0.5 m, which is more suitable for water-depth monitoring.

Oscillations and hysteresis during hydrocarbon oxidation on a diesel oxidation catalyst: Predictive model

Dodecane oxidation on zeolite beta (BEA) was studied on a diesel oxidation catalyst (DOC) comprising Pt-Pd/Al2O3. Oscillatory CO2 formation under steady-state and transient conditions was studied by spatially-resolved mass spectrometry (SpaciMS) and coherent optical frequency domain reflectometry (c-OFDR) (Peng et al., 2018). The CO2 oscillations were accompanied by local temperature fluctuations during the steady state and temperature-programmed oxidation. The coupling between hydrocarbon sorption and oxidation, the proposed underlying mechanism for the oscillatory behavior, is investigated using a monolith reactor model. Using independently measured dodecane oxidation and sorption kinetics, the model predicts most of the experimental features, including the existence and amplitude of the oscillations. The model also correctly predicts the existence of an observed hysteresis during temperature ramp-up/ramp-down experiments. A non-dimensional map provides a predictive guide for the existence of oscillations. In addition to providing understanding and prediction of the spatiotemporal phenomena, the model analysis suggests a way to decrease the hydrocarbon slip (emissions) during temperature ramp-down.

Measurement Range Enhancement of Rayleigh-Based Optical Frequency Domain Reflectometry With Bidirectional Determination

We report a noble coherent optical frequency domain reflectometry (OFDR) system that simply enables measurement range enhancement up to full coherence length of a laser source. The proposed technique is based on bidirectional determination of Rayleigh backscattered signal along a fiber. To do this, complex OFDR signals are acquired with the help of $\pi / 2$ phase-shifting interferometry in a detection part and an additional delay fiber in a reference arm. Through a bidirectional determination process using the complex OFDR signals, mirrored signals appearing at both sides of the spatial domain were completely discriminated, so that ambiguity arising due to a positional origin can be removed. Bidirectional distributed strain and temperature sensing is successfully performed for the first time. Spectral shifts by applying strain to and heating of the test fiber were found to be independent on both sides of the reference position, indicating that twofold enhancement of the measurement range can be obtained compared to previous systems.

Extended-bandwidth frequency sweeps of a distributed feedback laser using combined injection current and temperature modulation.

This article details the generation of an extended-bandwidth frequency sweep using a single, communication grade distributed feedback (DFB) laser. The frequency sweep is generated using a two-step technique. In the first step, injection current modulation is employed as a means of varying the output frequency of a DFB laser over a bandwidth of 99.26 GHz. A digital optical phase lock loop is used to lock the frequency sweep speed during current modulation, resulting in a linear frequency chirp. In the second step, the temperature of the DFB laser is modulated, resulting in a shifted starting laser output frequency. A laser frequency chirp is again generated beginning at this shifted starting frequency, resulting in a frequency-shifted spectrum relative to the first recorded data. This process is then repeated across a range of starting temperatures, resulting in a series of partially overlapping, frequency-shifted spectra. These spectra are then aligned using cross-correlation and combined using averaging to form a single, broadband spectrum with a total bandwidth of 510.9 GHz. In order to investigate the utility of this technique, experimental testing was performed in which the approach was used as the swept-frequency source of a coherent optical frequency domain reflectometry system. This system was used to interrogate an optical fiber containing a 20 point, 1-mm pitch length fiber Bragg grating, corresponding to a period of 100 GHz. Using this technique, both the periodicity of the grating in the frequency domain and the individual reflector elements of the structure in the time domain were resolved, demonstrating the technique's potential as a method of extending the sweeping bandwidth of semiconductor lasers for frequency-based sensing applications.

Signal conditioning for compensating nonlinearity and nonrepeatability of an optical frequency scanning laser implemented in a C-OFDR system.

Reported systems using coherent optical frequency-domain reflectometry, C-OFDR, rely on the linearity of the scanning rate of the tunable laser and sometimes on its repeatability. In this work, we present the implementation of signal-conditioning algorithms for a fiber temperature sensor system based on coherent optical frequency-domain reflectometry. Postacquisition signal conditioning removes any nonlinearity and nonrepeatability effects and allows synchronization of the whole system. A low reflectivity, 0.1%, fiber Bragg grating, placed in a reference interferometer, is implemented for removing the nonrepeatability of the optical source. The spatial resolution achieved by the temperature fiber sensor is 0.36 mm, with repeatability of 0.032°C, while using telecommunication-grade components.

Assessing intrusion by the capillary during spatially resolved mass spectrometry measurement

The impact of the capillary probe of a spatially-resolved mass spectrometer system (SpaciMS) on the measured reactant conversion is reported using propylene oxidation over Pt/Al2O3 washcoated monoliths. The findings suggest that the invasive nature of SpaciMS depends on its configuration and application. Using monoliths with a range of cell densities (100–600 cells per square inch, CPSI), the concentration profiles of propylene sampled with probes of two different outer diameters (170 and 363μm) are compared with the temperature measured using coherent optical frequency domain reflectometry (c-OFDR). The comparison indicates that flow blockage has a negligible effect if the limiting propylene is depleted in the downstream reactor section. Suction by the probe compensates for the blockage for certain combinations of the channel diameter and probe size. In such cases the profile measured by a probe with the larger outer diameter (363μm) is similar to that measured by a smaller capillary (170μm). The experiments reveal that the axial position of the probe does not influence the flow profile in a 100 CPSI monolith channel, nor does it affect the amount of flow deflected to surrounding channels for a 600 CPSI monolith. Under some conditions the results are impacted by transverse concentration gradients. Their existence complicates the interpretation of the SpaciMS data. The difference between the location of propylene depletion and temperature maximum provides a useful metric for capturing the collective impacts of flow deflection and transverse gradients. The complexity of the flow, transport and reaction suggests that at least three dimensionless groups are needed to bound the operating regions in which the presence of the probe has minimal impact on the measured concentration.

Long-range measurement of Rayleigh scatter signature beyond laser coherence length based on coherent optical frequency domain reflectometry

Long-range C-OFDR measurement of fiber Rayleigh scatter signature is described. The Rayleigh scatter signature, which is an interference pattern of backscatters from the random refractive indices in fibers, is known to be applicable to fiber identification and temperature or strain sensing by measuring its repeatability and its spectral shift. However, these applications have not been realized at ranges beyond the laser coherence length since laser phase noise degrades its repeatability. This paper proposes and demonstrates a method for analyzing the optical power spectrum of local Rayleigh backscatter to overcome the limitation imposed by laser phase noise. The measurable range and spatial performance are also investigated experimentally with respect to the remaining phase noise and noise reduction by signal averaging with the proposed method. The feasibility of Rayleigh scatter signature measurement for long-range applications is confirmed.

Spatiotemporal behavior of Pt/Rh/CeO2/BaO catalyst during lean–rich cycling

The experimental system that utilized an optical fiber and a spatially resolved capillary inlet mass spectrometer to study the dynamic oxygen storage capacity of ceria containing LNT catalyst during periodic lean (oxygen)–rich (propylene) operation. • Combined measurement of intrachannel temperature and concentration. • Detailed insight about the dynamics of a transient exothermic catalytic reaction. • Oxidation and reduction chemistry with ceria plays important role during cycling. • Ceria enables a more efficient oxidation to total oxidation products. • Cycling leads to much higher production of partial oxidation products H 2 and CO. Experiments were conducted to determine the impact of key operating variables (ceria loading, space velocity, cycle time, and rich pulse intensity) on the oxygen storage and release process of a Pt/Rh/CeO 2 /BaO monolithic catalyst during periodic lean (oxygen)–rich (propylene) operation. The concentrations were measured by spatially-resolved capillary inlet mass spectrometry (SpaciMS), and the temperature profile was measured by coherent optical frequency domain reflectometry (c-OFDR), providing detailed insight into the spatio-temporal features of the reaction system. The experiments revealed that the addition of ceria increased the breakthrough time of the propylene during a lean-to-rich transition due to an increased oxidation rate. Hydrogen was formed in the upstream and was consumed in the downstream section of the catalyst by reaction with ceria. Increasing the space velocity increased the upstream hydrogen formation rate and decreased its downstream oxidation rate. Increasing the lean and rich durations (while keeping their duration ratio constant) increased the oxygen uptake but resulted in propylene breakthrough and decreased the solid temperature. Increasing the rich period (at a fixed rich feed and total cycle time) allowed the catalyst to release more oxygen. A comparison of the ceria-containing catalyst to one without ceria revealed a more efficient complete oxidation for the former, while a comparison of the periodic to stationary operation revealed that less than half the propylene was converted to CO 2 by the former.