Optical Frequency Domain Reflectometry System Research Articles

Optical frequency domain reflectometry-based high-performance distributed sensing empowered by a data and physics-driven neural network.

Optical frequency domain reflectometry (OFDR) based distributed strain sensors are the preferred choice for achieving accurate strain measurements over extensive sensing ranges while maintaining exceptional spatial resolution. However, the simultaneous realization of high spatial resolution, high strain resolution, large strain range, and an extended sensing range presents an exceedingly challenging endeavor. In this study, we introduce and experimentally demonstrate a data and physics-driven neural network-empowered OFDR system designed to attain high-performance distributed sensing. In our experiments, we successfully maintained an impressive sensing resolution of sub-microstrain (0.91 μ ε) alongside a sharp spatial resolution of sub-millimeter (0.857 mm) across a 140-m sensing range. To the best of our knowledge, this marks the inaugural experimental demonstration of OFDR-based distributed sensing, combining sub-millimeter spatial resolution and sub-μ ε strain resolution across a lengthy sensing range over a hundred meters. This pioneering work unveils new pathways for the development of ultra-high-performance optical fiber sensing systems, paving the way for the next generation of intelligent systems tailored for diverse smart industrial applications.

Long-range distributed vibration sensing based on internal-modulation OFDR

Presented here is long-range distributed vibration sensing based on internal-modulation optical frequency domain reflectometry (OFDR). In the proposed system with internal modulation, a silicon-based photonic-chip laser is used as the laser source, and by controlling the output voltage curve of an arbitrary waveform generator to induce temperature change in the external cavity of the laser, a 10-GHz optical frequency tuning range is achieved. The complexity of the proposed internal-modulation system is lower than that of the traditional external-modulation OFDR system that combines a narrow-linewidth laser with a single-sideband modulator to achieve wavelength tuning. Cross-correlation analysis is used as a sensing mechanism to evaluate the similarity between Rayleigh scatter signals and to achieve vibration event localization. Experimental comparison is made of the vibration sensing performance of the external- and internal-modulation systems, and for a vibration event generated at a distance of 100.95 km, they locate it with a sensing spatial resolution of 43.0 m and 16.8 m, respectively. The results indicates that the proposed distributed vibration sensing based on internal modulation has better sensing performance and lower complexity compared to the traditional external-modulation system. In addition, the proposed system is single-ended and involves no optical amplification, which makes it very suitable for ultra-long-range sensing.

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.

Optical frequency domain reflectometry with broadened frequency sweep range assisted by a dual electro-optic frequency comb.

We propose an optical frequency domain reflectometry (OFDR) with the assistance of a dual electro-optic frequency comb (EOFC), which is intended to improve the system spatial resolution. As the spatial resolution of an OFDR system is inversely proportional to the frequency sweep range, the EOFC acts as a multi-frequency light source for collecting Rayleigh backscattering signals, which are combined to extend the effective frequency sweep range. By utilizing this technique, we have successfully expanded the experimental frequency sweep range to hundreds of gigahertz, achieving a sub-millimeter spatial resolution.

Long-Distance OFDR With High Spatial Resolution Based on Time Division Multiplexing

In this study, a long-distance optical frequency domain reflectometry (OFDR) with high spatial resolution based on time division multiplexing is proposed and experimentally demonstrated. Distributed strain sensing with high spatial resolution over long sensing range can be realized by reconfiguring the system layout in a time-division-multiplexed manner by re-routing the measurement signals for segmented processing with extra circulators and couplers only, rather than any other mechanisms for performance improvement of laser source or algorithms for nonlinear errors compensation. Through time-division-multiplexed reconfiguration, the sensing distance is no longer restricted by the path difference of the auxiliary interferometer which is commonly employed in OFDR systems for compensating laser frequency tuning nonlinearity. Compared with the traditional system layout, the proposed system allows distributed measurement with flexible spatial resolutions in each sub-sensing unit without any crosstalk among them. In experiments, distributed strain sensing with spatial resolution up to 5mm is achieved over an active sensing distance of 225m by the proposed system. Furthermore, the spatial resolution flexibility of the proposed system is further verified by successfully obtaining the strain information with 2 mm spatial resolution at the end of a 235-m-long fiber.

Optical Frequency-Domain Reflectometry Based Distributed Temperature Sensing Using Rayleigh Backscattering Enhanced Fiber

An innovative optical frequency-domain reflectometry (OFDR)-based distributed temperature sensing method is proposed that utilizes a Rayleigh backscattering enhanced fiber (RBEF) as the sensing medium. The RBEF features randomly high backscattering points; the analysis of the fiber position shift of these points before and after the temperature change along the fiber is achieved using the sliding cross-correlation method. The fiber position and temperature variation can be accurately demodulated by calibrating the mathematical relationship between the high backscattering point position along the RBEF and the temperature variation. Experimental results reveal a linear relationship between temperature variation and the total position displacement of high backscattering points. The temperature sensing sensitivity coefficient is 7.814 μm/(m·°C), with an average relative error temperature measurement of −1.12% and positioning error as low as 0.02 m for the temperature-influenced fiber segment. In the proposed demodulation method, the spatial resolution of temperature sensing is determined by the distribution of high backscattering points. The temperature sensing resolution depends on the spatial resolution of the OFDR system and the length of the temperature-influenced fiber. With an OFDR system spatial resolution of 12.5 μm, the temperature sensing resolution reaches 0.418 °C per meter of RBEF under test.

Signal Processing in Optical Frequency Domain Reflectometry Systems Based on Self-Sweeping Fiber Laser with Continuous-Wave Intensity Dynamics

We report on the development of an optical frequency domain reflectometry (OFDR) system based on a continuous-wave Er-doped self-sweeping fiber laser. In this work, we investigate the influence of the input data processing procedure in an OFDR system on the resulting reflectograms and noise level. In particular, several types of signal averaging (in time and frequency domain) and Fourier analysis are applied. We demonstrate that the averaging in the frequency domain can be applied to evaluate absolute values of the local scattering amplitudes related to the Rayleigh light scattering (RLS), which is associated with the interference of scattering signals on microscopic inhomogeneities in optical fibers. We found that the RLS signal remains unchanged in the case of signal averaging in time domain, while the noise floor level decreases by 30 dB with an increasing number of points from 1 to ~450. At the same time, it becomes possible to detect the spectral composition of the scattering at each point of the fiber using windowed Fourier transform. As a result, the sensitivity of the developed system allows us to measure the RLS signal at a level of about 20 dB above the noise floor. The described analysis methods can be useful in the development of distributed sensors based on Rayleigh OFDR systems.

Optical Frequency Domain Reflectometry Based on Multilayer Perceptron

We proposed an optical frequency domain reflectometry based on a multilayer perceptron. A classification multilayer perceptron was applied to train and grasp the fingerprint features of Rayleigh scattering spectrum in the optical fiber. The training set was constructed by moving the reference spectrum and adding the supplementary spectrum. Strain measurement was employed to verify the feasibility of the method. Compared with the traditional cross-correlation algorithm, the multilayer perceptron achieves a larger measurement range, better measurement accuracy, and is less time-consuming. To our knowledge, this is the first time that machine learning has been introduced into an optical frequency domain reflectometry system. Such thoughts and results would bring new knowledge and optimization to the optical frequency domain reflectometer system.

Recent Advancements in Optical Frequency-Domain Reflectometry: A Review

Optical frequency-domain reflectometry (OFDR) based on Rayleigh scattering has been widely used for distributed sensing applications for either static or dynamic parameter measurements, such as temperature, strain, and vibration due to its unique capability. In this review article, we report the recent advancements in OFDR-based distributed sensing techniques. First, the sensing principles and signal processing methods of the OFDR system are introduced. Various nonlinear compensation methods are also reviewed and compared to show their advantages and disadvantages. Then, key technologies that are aimed to improve the sensing range, spatial resolution, strain measurement range, and frequency response range are surveyed in detail. Finally, practical applications of the OFDR-based distributed sensing system are introduced, including the measurement of humidity, refractive index, magnetic field, flow rate, gas sensing, radiation sensing, shape sensing, biomedical fields, and so on.

Real-time performance improvement approach based on FPGA in OFDR system

This paper presents a novel approach to accelerate the data processing rate in optical frequency-domain reflectometry (OFDR) system. In conventional solution, data is sampled by data acquisition card and then sent to personal computer (PC) for data processing through high-speed interface such as peripheral component interconnect express (PCIE) and so on. But frequently-used general laptop can’t provide this interface. What’s more, long time is necessary to complete the calculation in PC due to the limitations of serial processing in central processing unit (CPU). To improve the real-time performance of OFDR system, in our work, field-programmable gate array (FPGA) is used as hardware circuit to complete the calculation task and then send the results to PC only to display through universal asynchronous receiver/transmitter (UART) interface in the OFDR system simultaneously. In this way, the refresh rate of output can be accelerated about 50 times, which means the real-time performance of the system is improved a lot.

A Universal Phase Error Analysis for Optical Frequency Tuning Lasers Utilized in Fiber Sensing with OFDR

As optical fiber sensing has attracted increasing attention due to its advantages such as high accuracy, low costs, and stability, its optical source judgment has become an attractive issue by which to characterize its performance. Optical frequency-domain reflectometry (OFDR) has been demonstrated as a means of the fiber identification (ID) of optical fibers; however, the linearity of the optical frequency tuning rate determines both the spatial resolution and detection range. In this paper, the results from various simulations and experiments show that the phase error from the initial frequency and tuning rate can affect the performance of the OFDR system, which directs the future improvement direction of fiber sensing based on such technology.

Reconstruction error model of distributed shape sensing based on the reentered frame in OFDR.

At present, the reconstruction error of optical fiber shape sensing is commonly represented by Euclidean distance error. However, the Euclidian error of shape reconstruction will be dependent on the shape complexity, which depends on length, curvature and torsion. In this paper, we establish a reconstruction error model of distributed shape sensing in optical frequency domain reflectometry (OFDR) based on the Frenet-Serret frame and the error delivering theory, which illustrates the relationship between the reconstruction error and parameters such as curvature, torsion, fiber length and strain measurement error. We experimentally verify the feasibility and applicability of the proposed reconstruction error model by distributed optical fiber shape sensing system based on OFDR. The proposed reconstruction error model can provide a prediction of the maximal reconstruction error when the estimated range of curvature, torsion, fiber length of a shape needs to be reconstructed and strain measurement errors of OFDR system are known. It is very useful to judge whether the shape reconstruction error meets the requirement according to the shape to be reconstructed.

Elimination of Side Lobe Ghost Peak Using Wiener Deconvolution Filter in OFDR

Ghost peaks refer to these peaks on backscattering trace measured by a reflectometer similar to Fresnel reflection peaks appearing at the position of non-reflection events in optical fiber or optical waveguide networks testing, which will cause ambiguity in the interpretation of reflection events on backscattering trace. We present a simple and effective method to eliminate side lobe ghost peaks in optical frequency domain reflectometry (OFDR) using a Wiener deconvolution filter. An OFDR system usually contains a main interferometer that is used to measure backscattering trace of fiber umber test (FUT) and an auxiliary interferometer that is used to measure the phase of tunable laser source (TLS) to compensate tuning nonlinearity. Multi-peaks in the spectrum of TLS and reflections in the auxiliary interferometer will generate side lobes on point spread function (PSF) of OFDR system, which will cause ghost peaks on FUT trace. By performing a Wiener deconvolution filtering on the auxiliary interferometer, we can obtain the PSF of an auxiliary interferometer, which is used as a convolution kernel to perform a Wiener deconvolution filtering on FUT trace from the main interferometer. Comparing the main interferometer spatial signal before and after deconvolution filtering, the peak amplitude whose reduction higher than threshold is identified to a ghost peak and is eliminated. By this ghost peak elimination method, we can acquire a pure FUT trace by an OFDR using a low-cost distributed feedback (DFB) TLS with a 26 m test length and an mm level spatial resolution.

High-spatial-resolution OFDR with single interferometer using self-compensation method

A high-spatial-resolution optical frequency domain reflectometry (OFDR) with single interferometer was proposed by using self-compensation method (SCM). The auxiliary interferometer (AI) of conventional OFDR was replaced by a processed arc end of the fiber under test (FUT) to generate a reflection signal with appropriate intensity. The actual instantaneous optical frequency (IOF) was successfully extracted from the arc end of the FUT. Combining the arc end and SCM, the phase noise of the proposed OFDR was successfully eliminated to obtain the compensated signal with high signal-to-noise ratio (SNR). Moreover, the influence of delay fiber (DF) length on the compensation effect was theoretically and experimentally analyzed. The proposed OFDR achieved the distributed temperature sensing with a high-spatial-resolution of 3 mm without sacrificing measurement performance, where the measured distance of the FUT was ∼ 108 m. The high-spatial-resolution of 5 mm was still achieved when the measured distance of the FUT was increased to ∼170 m. Such an OFDR system with low cost and good performance has development potential in the field of distributed measurement.

Beyond a 107 range-resolution-1 product in an OFDR based on a periodic phase noise estimation method.

We present and demonstrate a method based on a periodic phase noise estimation in an optical frequency domain reflectometry (OFDR) capable of a beyond 107 range-resolution-1 product (RRP) for the first time, which corresponds to 2.5 × improvement compared with the state-of-the-art. The moving average filter is employed to suppress the amplification of noise in the derivation process. Further, with the help of a third-order Taylor expansion, this method provides a highly precise estimation of periodic phase noise, which is the main factor impacting the performance of OFDR systems with medium-to-long measurement range combined with a submillimeter spatial resolution. A spatial resolution within 535 μm over the measurement range of 8 km is obtained. The proposed method offers a promising technique for fiber network monitoring and sensing applications.

Wide Measurement Range Distributed Strain Sensing With Phase-Accumulation Optical Frequency Domain Reflectometry

In this paper, a wide measurement range strain sensing method based on phase-accumulation optical frequency domain reflectometry (OFDR) is proposed. Different from the traditional phase demodulation method that analyzes the relative phase between the measurement signal and the reference signal, we achieve large measurement range strain sensing by accumulating the relative phases of adjacent scanning cycles. The proposal can break through the limitation of phase unwrapping in the traditional phase method, and realize the low measurement error in the accumulation process. In the experiments, when the wavelength scanning range is only 0.138 nm, we achieve quasi-static strain measurement with a spatial resolution of 1.8 cm, a maximum strain value of about 14000 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">μϵ</i> , and the noise level of about 0.601 rad representing the strain resolution of 0.48 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">μϵ.</i> We also analyze the influence of random noise on the method, and verify the good robustness of the phase accumulation method. The research of this paper provides a useful reference for the development of low-cost OFDR system for distributed strain/temperature measurement.

Multi-parameter measurement of a multi-point high-frequency vibration signal in an OFDR system.

The phase cross-correlation function of an optical frequency domain reflectometry (OFDR) system is proposed to detect a multi-point vibration event, which is verified in detail by theoretical simulation and experiment. An OFDR system based on a non-tunable laser source with digital sweep frequency is developed. It is verified experimentally that the location and frequency resolution of multi-point high-frequency vibration can be detected by analyzing the phase cross-correlation function of the sensing signal. High-frequency signals of 50 kHz and 20 kHz are located and separated on the 8 km optical fiber. The frequency resolution is 1.26 kHz, and the minimum spatial error is 11.5 m.

Quasi-Distributed Magnetic Field Fiber Sensors Integrated with Magnetostrictive Rod in OFDR System

We have proposed and designed a fiber-optic magnetic field sensors based on magnetostriction, of which the magnetostrictive induced strain of magnetostrictive rod attached to an optical fiber can be measured by optical frequency-domain reflectometry (OFDR). By analyzing the stress transfer process at the interface between the magnetostrictive rod and the sensing optical fiber, we find that the sensor sensitivity is mainly related to the magnetostrictive material and bond width. The experimental results show the sensor performance under different magnetostrictive rods and radiuses. The sensitivity of the Fe-Ga-based sensor is up to 5.05 με/mT, while the sensitivity of the Tb-Dy-Fe-based sensor is up to 3.42 με/mT. The proposed sensor can easily construct a sensor network for quasi-distributed fiber-optic magnetic field sensing, which can be used to monitor magnetic fields at more than one point.

Internet of Things Infrastructure Based on Fast, High Spatial Resolution, and Wide Measurement Range Distributed Optic-Fiber Sensors

Information sensor can realize the ubiquitous connection between objects and humans by the distributed fiber-optic sensor array, i.e., emerging infrastructure for Internet of Things, in which optical frequency domain reflectometry (OFDR) with its high spatial resolution plays an important role in achieving intelligent perception and identification of the objects. However, given a certain fiber length, there are tradeoffs among the processing speed, the spatial resolution, and the strain measurement range. In this article, a time optimization interpolation method and a distance domain compensation method are reported and experimentally verified to break the aforementioned tradeoffs for the first time. First, a full theoretical analysis on how to reduce the interpolation number and the reason why it is difficult to achieve high spatial resolution and large strain measurement and how to resolve it is made. Second in the proof-of-concept experiment, measurements of large strains up to 10 000 <inline-formula> <tex-math notation="LaTeX">$\boldsymbol{\mu } \boldsymbol{\varepsilon }$ </tex-math></inline-formula> are realized along the sensing fiber with a spatial resolution of 2 mm using a conventional OFDR system. Also, the processing time is shortened by 76 times compared with the traditional processing method. This article makes a significant step toward high-performance OFDR system with fast processing, high spatial resolution, and wide strain measurement.

In-Fiber Auxiliary Interferometer to Compensate Laser Nonlinear Tuning in Simplified OFDR

The traditional optical frequency domain reflectometry usually requires a separate auxiliary interferometer to compensate for laser nonlinear tuning effect. In order to break through this limitation, this paper proposes a simplified optical frequency domain reflectometry system by integrating an in-fiber auxiliary interferometer before the sensing fiber. We experimentally characterized the laser nonlinear tuning effect, and then verified the feasibility of the proposed system in compensating the laser nonlinear tuning effect by both simulation and experiment. Finally, the capabilities of the distributed sensing of our proposed system was confirmed by the temperature and strain sensing experiments.