Optical Frequency Domain Reflectometry Research Articles

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.

Distributed strain monitoring for different composites structures with high resolution based on optical fiber sensing

Distributed strain monitoring for different composites structures has been accomplished with high resolution based on optical fiber frequency domain reflectometry (OFDR) system. The dynamic measurement range and spatial resolution of the system can be adjusted according to the objects to be monitored. In order to verify the reliability of the system, theoretical and experimental studies have been carried out. The physical model structures have been established. The sensing fiber was pasted onto the physical structure. The finite element analysis software ANSYS has been used to build 3D models of the structures in experiments, including the aluminum alloy cantilever beam model and the carbon fiber reinforced composite plate model. The simulated strain curves have been acquired based on the composite material analysis and the static analysis of module simulation according to the actual load. The experimental strain curves have been obtained by the established OFDR system. The theoretical and experimental results show that the system can achieve the spatial resolution of 1.3 mm within the measurement range of 50 m. Under certain load conditions, the maximum error between the experimental measurement results and the theoretical simulation results of the strain distribution is 2.62% for the aluminum alloy cantilever beam, and that is 2.65% for the carbon fiber reinforced composite sheet. The results verified the reliability of the OFDR system for distributed monitoring of strain.

Improvement of Strain Measurement Range via Image Processing Methods in OFDR System

We have experimentally analyzed and compared the performance of optical frequency domain reflectometry (OFDR) assisted by image processing methods including wavelet transform and Gaussian filter arithmetic in terms of strain measurement range. The incomplete space similarity in the distributed strain measurement is used to recover the accurate strain gradient information by the image processing methods. The distributed spectrum shifts of the cross-correlation calculation along the sensing fiber between reference signal and measurement signal is regarded as a two-dimension (2D) image which is disposed by the proposed methods rather than extracting the spectrum shifts immediately. Compared with the traditional processing method, the proposed image processing method is able to remove and smoothen the fake peaks or outliers caused by the spatial mismatch of stretched fiber in OFDR system. In the experiments, strain gradient information up to 7000 μϵ can be identified by the proposed methods with a 4 mm spatial resolution. Furthermore, the wavelet transform is discussed in detail in order to determine the value of the suitable decomposition level in OFDR data processing scheme. The proposed methods provide a new idea to achieve the wide strain measurement range in OFDR system, which is a significant step toward a high-performance distributed sensing system, especially for occasions when a wide range of strain measurement is required.

Experimental Research on Strain Transfer Behavior of Fiber-Optic Cable Embedded in Soil Using Distributed Strain Sensing

The strain transfer between fiber-optic cable and soil plays a critical role in the deformation characteristics of a cable–soil interface. Existing findings cannot provide a clear understanding of the effects of key influencing factors including horizontal confining pressures, anchorage property of cables, and saturation of soils on the strain transfer and shear characteristics at the interface. A group of pullout tests of cables in soil were conducted to examine the strain transfer efficiency using the optical frequency domain reflectometry (OFDR) technique. Two kinds of cables were pulled out from sandy soil and sand–gravel–clay mixtures under the confining pressures of 0–1.2 MPa. Typical strain-hardening behavior was observed for cables with confining pressure and anchorage, and the interface shear strength could not be evaluated within the strain measurement range. To address this problem, the so-called interface shear coefficient was adopted, and the key influential factors were discussed quantitatively. The interface shear coefficient keeps a linear relationship with the confining pressure, and that of the anchored cable in saturated backfill mixtures is 2–3 times than that of the unanchored cable. These findings will guide the methods of gaining reliable data for revealing the failure mechanism of geostructures via distributed strain sensing.

Identification of external load information using distributed optical fiber sensors embedded in an existing road pavement

This paper describes field applications of distributed optical fiber sensors (DOFS) for monitoring the traffic load on an existing road. To investigate the optimal embedment depth for DOFS for pavement monitoring, a series of field tests was conducted by considering three embedment depths (10, 30, and 40 mm) and three moving loads (pedestrian, car and truck). The main objectives were to examine the performance of DOFS embedded in the pavement and investigate its suitability and sensitivity for obtaining information about moving loads at various depths from the road surface. Among the existing backscattering light technology, Rayleigh scattering-based Optical Frequency Domain Reflectometry (OFDR) was chosen in order to achieve a measurement of high-speed, high range, and good repeatability with a very high spatial resolution of 2.56 mm, all of which are necessary for monitoring dynamic responses due to moving loads. In addition, a data-processing method was developed based on a combination of a Hampel identifier and a low-pass filter, which proved to be effective in removing outliers and high-frequency noise from the raw measurement data, respectively. The results showed that it is not only possible to detect a moving load on a road, but that a wide range of information can also be obtained from the strains measured by DOFS, including the type of load, its speed, its weight, the number of axles, the axle spacings (in the case of a vehicle) and the traffic flow. With low maintenance costs, DOFS based on Rayleigh scattering OFDR could be used for collection of accurate and reliable traffic data when embedded in a pavement.

Detection, visualization, quantification, and warning of pipe corrosion using distributed fiber optic sensors

This paper presents a distributed monitoring approach for detection, visualization, quantification, and warning for pipe corrosion using a single-mode telecommunication-grade fiber optic cable as a distributed sensor. The distributed sensor can be deployed on the surface of a pipe to measure corrosion along the pipe based on optical frequency domain reflectometry. To validate this distributed pipe corrosion monitoring approach, steel pipes were immersed in sodium chloride solution (concentration: 3.5 wt%) for corrosion tests. Strain distributions were measured and used to detect and map corrosion along the pipe. A meso-scale analytical model was developed to link the measured strains to the mass loss of pipe under corrosion. The effects of cable thickness, spatial resolution, and deployment pattern on corrosion assessment were investigated systematically. A threshold-based warning method is proposed based on the unique sensor data.

The Influence of Germanium Concentration in the Fiber Core on Temperature Sensitivity in Rayleigh Scattering-Based OFDR

This paper presents a linear dependence of temperature sensitivity in Rayleigh scattering-based optical frequency domain reflectometry (OFDR) on the concentration of germanium in the fiber core. Eight different commercially available single-mode fibers have been investigated and compared with the standard SMF-28 Corning fiber. We have experimentally analyzed the influence of germanium concentration in the fiber core, the protective coating, the degree of reduction of the fiber cladding's diameter, and the amount of humidity on Rayleigh backscatter in response to temperatures ranging from 10-90°C. Maximum sensitivity was 1.343 GHz/°C for 20.78 mol % of the germanium-doped bare fiber and it does not depend on changes in humidity. The experimental results were compared and qualitatively agreed with the numerical simulations.

High Spatial Resolution Investigation of OFDR Based on Image Denoising Methods

Measurement accuracy of the optical frequency domain reflectometry (OFDR) will be deteriorated severely when the spatial resolution is improved due to the influence of the random noise, especially in long range distributed fiber optical sensing system. In order to eliminate the random noise, we have experimentally compared and analyzed the performance of OFDR system with and without the image processing methods in term of spatial resolution. The data arrays of the cross-correlation results are obtained after processing the collected distributed Rayleigh scattering signals along the sensing fiber, which can be used to construct a two-dimension (2D) image with the spectrum shift information induced by the strain. Thus, the image denoising methods can be applied to remove the random noise and improve the measurement accuracy. Based on this principle, two image denoising methods including the total variation method and the 2D Gaussian filter arithmetic are introduced to realize distributed sensing with high spatial resolution in long distance OFDR system. Experimental results show that the random noise can be suppressed and the outliers can be removed by the proposed methods. The strain gradients with high spatial resolution are identified effectively when the image denoising methods are used. Furthermore, spatial resolution with 1.3mm is achieved by the proposed methods with a wavelength sweeping rate of 20nm/s in 52m sensing fiber. The proposed methods provide a potential solution for OFDR system in long distance sensing with high spatial resolution, which is a significant step toward a high-performance OFDR system for practical applications.

Temperature-compensated distributed refractive index sensor based on an etched multi-core fiber in optical frequency domain reflectometry.

We proposed a temperature-compensated distributed refractive index (RI) sensor using an etched multi-core fiber (MCF) in optical frequency domain reflectometry. The MCF contains inner and outer cores and is etched until the outer core is exposed. Therefore, the outer core can be used for distributed RI sensing, and the inner core can be used for temperature compensation. The sensing length of 19 cm and the spatial resolution of 5.3 mm are achieved in the experiment. The RI sensing range is as wide as 1.33-1.44 refractive index units (RIU), and the maximum sensitivity of 47 nm/RIU is obtained around 1.44 RIU. Additionally, the temperature sensitivity is 9.8 pm/°C. Using this sensor, we successfully detected the glycerol diffusion process in water.

Compact multifunction digital OFDR system without using an auxiliary interferometer.

This paper describes an integrated, accurate, and inexpensive semiconductor laser -based optical frequency domain reflectometry (OFDR) system design. The system utilizes the fiber under test for both sensing and frequency sweep linearization functions, allowing the system to mitigate and compensate for phase errors without the need for an auxiliary interferometer, as is the case for traditional OFDR systems. Benefiting from the unique and embedded design, this system reaches the minimal OFDR system with only one optical interferometer and its corresponding optic-electric components without sacrificing accuracy. In addition, conventional design requires an external auxiliary interferometer, which may experience different noises from the main measuring interferometer, deteriorating the overall performance. Experimental results demonstrate the enhanced performance of the compact design as compared with the former methods, as well as the reduced complexity and improved cost-effectiveness.

Demonstration of Large Curvature Radius Shape Sensing Using Optical Frequency Domain Reflectometry in Multi-Core Fibers

In distributed shape sensing, the shape reconstruction error is more and more sensitive to the strain measuring error along with curvature radius of reconstructed shape increasing, which causes a notable challenge for large curvature radius shape reconstructing. In this paper, we demonstrate a large curvature radius shape sensing using optical frequency domain reflectometry (OFDR) in multi-core fibers. We construct a theoretical model of strain measuring error and curvature radius reconstructing errors under different curvature radius. In the experiment, by this reconstructing error model, we optimally select the measurable strain resolution and the sensing spatial resolution to realize the shape reconstruction with a large curvature radius and reconstruct two-dimensional (2D) circle shapes of curvature radii from 5 cm to 100 cm. To verify the accuracy of three-dimensional (3D) shape reconstruction, we present a 3D shape sensing validation method based on 3D printing technology. We fabricate a 3D phantom containing a groove with a variable curvature radius of 5 cm to 100 cm. The presented distributed shape sensing system realize to reconstruct this complicated 3D shape. The root-mean-square error of curvature radius between the reconstructed and designed 3D space curves is 7.2 mm and the mean Euclidean distance is 3.4 mm.

Dynamic Range Enhanced Optical Frequency Domain Reflectometry Using Dual-Loop Composite Optical Phase-Locking

We report on a dynamic range enhanced optical frequency domain reflectometry distributed backscattering interrogator based on dual-loop composite optical phase-locked loop (OPLL). Exploiting simultaneously an acousto-optic frequency shifter based an external modulation loop and a piezo based direct modulation loop, the proposed composite OPLL allows offering a larger loop bandwidth and gain, permitting a more efficient coherence enhancement as well as sweep linearization. A high fidelity frequency sweep of ~8.2 GHz at 164 GHz/s sweep rate is generated with a peak-to-peak frequency error as low as ~ 120 kHz. It leads to a dynamic range enhancement of more than 3 dB for the measured power loss compared to the case when only piezo loop is applied. This corresponds to ~15 km extension for the measurement range of Rayleigh backscattering without any spatial resolution penalties. Fourier transform-limited spatial resolution has been demonstrated at a range window more than about 28 times of the intrinsic coherence length of the adopted fiber laser. The proposed method provides a straightforward optimization of the real-time sweep control and is expected to be a useful tool in industrial and commercial applications.

Frequency-comb-calibrated swept-wavelength interferometry.

Lasers are often used to characterize samples in a non-destructive manner and retrieve sensing information transduced in changes in amplitude and phase. In swept wavelength interferometry, a wavelength-tunable laser is used to measure the complex response (i.e. in amplitude and phase) of an optical sample. This technique leverages continuous advances in rapidly tunable lasers and is widely used for sensing, bioimaging and testing of photonic integrated components. However, the tunable laser requires an additional calibration step because, in practice, it does not tune at a constant rate. In this work, we use a self-referenced frequency comb as an optical ruler to calibrate the laser used in swept-wavelength interferometry and optical frequency domain reflectometry. This allows for realizing high-resolution complex spectroscopy over a bandwidth exceeding 10 THz. We apply the technique to the characterization of low-loss integrated photonic devices and demonstrate that the phase information can disentangle intrinsic from coupling losses in the characterization of high-Q microresonators. We also demonstrate the technique in reflection mode, where it can resolve attenuation and dispersion characteristics in integrated long spiral waveguides.

Ultimate Spatial Resolution Realisation in Optical Frequency Domain Reflectometry with Equal Frequency Resampling.

A method based on equal frequency resampling is proposed to suppress laser nonlinear frequency sweeping for the ultimate spatial resolution in optical frequency domain reflectometry. Estimation inaccuracy of the sweeping frequency distribution caused by the finite sampling rate in the auxiliary interferometer can be efficiently compensated by the equal frequency resampling method. With the sweeping range of 130 nm, a 12.1 µm spatial resolution is experimentally obtained. In addition, the sampling limitation of the auxiliary interferometer-based correction is discussed. With a 200 m optical path delay in the auxiliary interferometer, a 21.3 µm spatial resolution is realised at the 191 m fibre end. By employing the proposed resampling and a drawing tower FBG array to enhance the Rayleigh backscattering, a distributed temperature sensing over a 105 m fibre with a sensing resolution of 1 cm is achieved. The measured temperature uncertainty is limited to ±0.15 °C.

Distributed refractive index sensing based on bending-induced multimodal interference and Rayleigh backscattering spectrum.

A distributed refractive index (RI) sensor based on high-performance optical frequency domain reflectometry was developed by bending a piece of standard single-mode fiber to excite sets of higher-order modes that penetrate the surrounding medium. External variations in RI modifies the profiles of the sets of excited higher-order modes, which are then partially coupled back into the fiber core and interfere with the fundamental mode. Accordingly, the fundamental mode carries the outer varied RI information, and RI sensing can be achieved by monitoring the wavelength shift of the local Rayleigh backscattered spectra. In the experiment, an RI sensitivity of 39.08 nm/RIU was achieved by bending a single-mode fiber to a radius of 4 mm. Additionally, the proposed sensor maintains its buffer coating intact, which boosts its practicability and application adaptability.

Performance Study of a Zirconia-Doped Fiber for Distributed Temperature Sensing by OFDR at 800 °C.

Optical Frequency Domain Reflectometry (OFDR) is used to make temperature distributed sensing measurements along a fiber by exploiting Rayleigh backscattering. This technique presents high spatial and high temperature resolutions on temperature ranges of several hundred of degrees Celsius. With standard telecommunications fibers, measurement errors coming from the correlation between a high temperature Rayleigh trace and the one taken as a reference at room temperature could be present at extremely high temperatures. These correlation errors, due to low backscattering signal amplitude and unstable backscattering signal, induce temperature measurement errors. Thus, for high temperature measurement ranges and at extremely high temperatures (e.g., at 800 °C), a known solution is to use fibers with femtosecond laser inscribed nanograting. These fs-laser-insolated fibers have a high amplitude and thermally stable scattering signal, and they exhibit lower correlation errors. In this article, temperature sensing at 800 °C is reported by using an annealed zirconia-doped optical fiber with an initial 40.5-dB enhanced scattering signal. The zirconia-doped fiber presents initially OFDR losses of 2.8 dB/m and low OFDR signal drift at 800 °C. The ZrO2-doped fiber is an alternative to nanograting-inscribed fiber to make OFDR distributed fiber sensing on several meters with gauge lengths of 1 cm at high temperatures.

Source Location and Suppression of Phase Induced Intensity Noise in Fiber-Based Continuous-Wave Coherent Doppler Lidar

The phase induced intensity noise (PIIN) may seriously reduce the signal-to-noise ratio (SNR) of continuous-wave coherent Doppler lidar (CW-CDL), thereby limiting its performance in practical applications. In this paper, we propose to use optical frequency domain reflectometry (OFDR) technique to locate the PIIN sources and evaluate their intensities. According to the OFDR information, we demonstrate that the PIIN in our CW-CDL system is mainly induced by the crosstalk of polarization beam splitter. Then, the PIIN is reduced by setting fiber delay line, polarization control, and AR-coating. Finally, a wind Doppler signal at a distance of 80 meters is obtained on a clear day, and a typical SNR of 19.4 dB is achieved.

Deformation measurement based on high resolution distributed optical fiber sensing and conjugated beam method

Structural deformation is in general an effective indicator of unforeseen damage accumulation. It is difficult to monitor deformation distribution by traditional single point measurement methods. Based on a high strain sensing resolution technique of optical frequency domain reflectometry (OFDR), this paper presents a hybrid method for structural deformation measurement. The main advantage is the explicit relationship between structural deformation and monitored strain data thus the calculation process can be conducted free from load identification and stiffness measurement. The accuracy of this method is investigated via both theoretical and experimental study. In lab experiments, simple support beam with fix supports and spring supports was designed. Compared with classical double integration method, the accuracy of the conjugated beam method (CBM) improved 5%. In concentrated and uniform distributed load conditions, the average relative error ranges from 4% to 7%. Furthermore, the average relative errors for several points are no more than 3% in complex load conditions.

Characterizing microring resonators using optical frequency domain reflectometry.

A novel, to the best of our knowledge, method to extract optical microring resonators' loss characteristics is proposed and demonstrated using optical frequency domain reflectometry (OFDR). Compared with the traditional optical transmission measurement method, the spatial-resolved backscattering optical signals obtained from the OFDR can clearly show the resonance mode's increased optical path length due to its circulation inside the resonator. By further processing the backscattered optical signals, loaded $Q$-factors of several resonators can be accurately determined. A calculation model is proposed to derive the resonance mode's intrinsic $Q$-factor from OFDR measurements of a series of loaded resonators.

Dynamic structural health monitoring of a model wind turbine tower using distributed acoustic sensing (DAS)

Maintenance of wind turbine towers is currently a manual process that requires visual inspection and bolt tightening yearly. This process is costly to energy companies and its necessity is not well-defined. In this study, two Rayleigh-based distributed fiber optic sensing technologies are evaluated and compared for their ability to monitor the dynamic structural behavior of a model wind turbine tower subject to free and forced vibration. They are further tested for their ability to detect structural phenomena associated with loose bolts and material damage within the tower. The two technologies examined are optical frequency domain reflectometry (OFDR) and phase-based optical time domain reflectometry (phi-OTDR), which is a technology used in distributed acoustic sensing (DAS). OFDR is a tested and proven strain measurement technology commonly used for structural health monitoring but can only make strain measurements over short distances (10 s of meters). OFDR was used to validate the measurements made with phi-OTDR which can measure over much longer distances (several kilometers). Due to its sensing distance capability, phi-OTDR is a promising technology for monitoring many wind turbines networked together with a single fiber optic cable. This study presents a first-of-its-kind use of phi-OTDR for structural health monitoring to demonstrate its capabilities.