Nonlinear phase compensated OFDR based on Match Fourier Transform algorithm

In this paper, an algorithm based on homomorphic deconvolution is proposed to give an accurate estimation of nonlinear phase in the beat signal in optical frequency domain reflectometry (OFDR). Nonlinearities in the beat signal are obtained by using an auxiliary interferometer. After converting to cepstrum domain and filtering, the nonlinearity can be separated from the beat signal. Then, the deskew filter is used to eliminate the nonlinearity. In the proposed algorithm, no approximations are used, so the estimation is theoretically unbiased. Certain simulations are performed to verify the versatility and effectiveness of the proposed algorithm. The nonlinearities are accurately estimated and eliminated by the method, which improves the spatial resolution of the OFDR system.

A location deviation compensation algorithm based on maximum cross correlation is proposed for optical frequency domain reflectometry. The characteristic change of fiber under test and the auxiliary interferometer induced by strain/temperature causes location deviation in the optical frequency domain reflectometry system, which in turn leads to demodulation errors. Compared to current available compensated methods, this algorithm simultaneously compensates the location deviation caused by the fiber under test and the auxiliary interferometer. The temperature variation experiments of the fiber under test and the auxiliary interferometer were carried out, respectively, and results show that the algorithm minimized the demodulation errors introduced by the location deviation.

A new hardware configuration to correct nonlinear tuning affects in optical frequency domain reflectometry (OFDR) is proposed. Unlike conventional correction methods, the new configuration does not require a separate auxiliary interferometer detection, which may eliminate the need for an extra detection device and an acquisition channel. In the proposed method, an intentional beat signal is introduced at the beginning of the OFDR spectrum, which is treated as an auxiliary interferometer to acquire tunable laser phase information for the nonlinear tuning correction. The proposed method may reduce the overall system cost, reduce the data acquisition time and computational load, and make system configuration simpler by eliminating the need for extra components. In this article, the feasibility of the proposed configuration is demonstrated by correcting nonlinear tuning affects using simulations and experiments in a ∼50 m long optical fiber. Distributed strain and temperature sensing are also performed in a 50 m optical fiber with the proposed new configuration.

In modern society, the amount of data being transferred is growing at an exponential rate, about 2.5 quintillion bytes data created per day. Such data can be realized in the form of include the audio, video, or text. Furthermore, due to the many advantages of the optical fiber, including the low-cost, immunity to electromagnetic interface, light weight, and high carrier frequency, it is taking over the industry. The question then arises as how to diagnose the inner fault and maintain the integrity becomes the crux for the telecommunication system based on optical fiber. There are two main high-precision measurement methodologies to complete the task mentioned above: one is the optical time-domain reflectometry (OTDR), which has been widely applied in technologies such as light detection and ranging (LiDAR) in faraway situations. The other one is optical frequency-domain reflectometry (OFDR), which is also referred to as optical frequency modulated continuous wave (OFMCW) or chirped frequency measurement. Compared with the OTDR, the OFDR measures distance by comparing the phase change of the chirped incident optical signal and reflected signals to capture the distance information. Even though the OFDR has been proven to have many advantages over OTDR in terms of the high spatial resolution at μm level, low power consumption, and eye-friendliness, both the high complexity and cost significantly limit its applications. To overcome the above shortages in the OFDR system and expand this technology in the various applications, the following research was conducted. Firstly, since the optical tuning source utilized in OFDR is too expensive and complex to be implemented in-situ measurements, there are some researches completed in the tuning laser source control methods proposed to reduce the cost and complexity of this technology and fit the various applications. Additionally, to enhance the accuracy of the OFDR measuring, some tuning rate error compensating algorithms were introduced. Benefiting from the improved spatial resolution, the physical unclonable function (PUF) in the optical fiber can be achieved as its unique identification to sustain the data transferring security. Furthermore, even though the initial optical frequency (IOF) in OFDR has been disregarded in the distance measuring, it does influence the PUF reader precision. To fully analyze such effect, some research and IOF mitigating methods were proposed, in which a fast processing method for the PUF reader has been introduced by decreasing the calculating complexity. From the above research, a low-cost, accurate, and effective OFDR system has been

Optical frequency domain reflectometry (OFDR) measurements are performed by recording the interference pattern of light backscattered by density fluctuations or Bragg gratings along the length of an optical fiber. Changes in local temperature or strain in the fiber cause shifts in the backscattered light spectrum, which can be calibrated to the applied temperature or strain. Comparing the backscattered optical spectra from each reflection site with a reference (unstrained) spectrum allows for quantification of local spectral shifts. While mature OFDR-based technologies can provide high-precision spatially distributed measurements over kilometer lengths, the post-processing approaches used to recover spectral shifts from raw optical intensity data can limit the use of OFDR in harsh environments. This paper presents a novel approach to post-processing OFDR data which extends the usability of optical fibers exposed to harsh environments. This approach is the first to use the spectral shift quality, calculated for two OFDR measurements, to identify an appropriate reference scan from which the spectral shift can be determined. Three experimental data sets are used to test the algorithm: (1) an optical fiber heated to >950°C, (2) an aluminum-embedded optical fiber under strain from differential thermal expansion, and (3) an optical fiber exposed to a total neutron flux of $2\times 10^{13}$ n/cm2/s over 40 hours in a nuclear test reactor. Results show that using a metric of quality to select an appropriate reference measurement extends the functional range of OFDR strain and temperature sensors. Furthermore, this algorithm can be applied to existing OFDR data.

We present a distributed high sensitivity static strain sensing method based on all grating optical fiber in optical frequency domain reflectometry (OFDR).Through the use of all grating optical fiber, the backscattered light can be greatly improved, which increases the signal-to-noise ratio. A higher signal-to-noise ratio can help to obtain more accurate distance domain information, thereby obtaining better segmented optical frequency domain information for cross-correlation. Therefore, in the same cross-correlation method, our system obtains even larger range of FUT information due to the signal with higher signal-to-noise ratio. Using this system, we measured the change of strain at the end of a total 200 m FUT. The minimum measurable strain variation is 5 με and the spatial resolution of the system is 10cm. It should be noted that we use interpolation zero padding method on the basis of cross-correlation to obtain higher strain accuracy. In addition, we verified the stability of the system by fitting the results of different strains. The linearity of the fitting results is extremely high, and the R-square can reach more than 0.9. Compared with the OFDR system using the single mode fiber, the OFDR system using all grating optical fiber can provide a better sensing performance of measuring a minor strain variation under the same sensing spatial resolution. This is of great significance for the improvement of OFDR measurement applications.

We experimentally analyzed the internal reflection and loss of each component in a Si photonics frequency-modulated continuous-wave light detection and ranging (FMCW LiDAR) device using optical frequency domain reflectometry (OFDR) with a spatial resolution of better than 2.5 µm. Sweeping the incident laser wavelength by 120 nm, the reflections and losses of wire waveguides, widened waveguides, and optical switches on the chip were individually revealed. The slow-light grating (SLG) beam scanner, which has a limited working wavelength range, was evaluated with a spatial resolution of >10 µm by narrowing the wavelength sweep range. Consequently, a strong reflection was observed at the transition between the wire waveguide and the SLG, which can be a noise source in the FMCW LiDAR. Additionally, this study showed that the OFDR can be an important analysis tool for Si photonics integrated circuits. To our knowledge, this is the first demonstration, showing that the OFDR can be an important analysis tool for Si photonic integrated circuits.

A distributed optical fiber grating sensing system with large capacity and high spatial resolution is presented. Since highdensity identical weak grating array was utilized as sensing fiber, the multiplexing number was greatly increased, meanwhile, optical frequency domain reflectometry (OFDR) technology was used to implement high resolution distributed sensing system. In order to eliminate the nonlinear effect of tunable light source, a windowed FFT algorithm based on cubic spline interpolation was applied. The feasibility of the algorithm was experimentally testified, ultimately, the spatial resolution of system can reach mm-level. The influence of the crosstalk signal in the grating array on the OFDR system was analyzed. A method that a long enough delay fiber was added before the first FBG to remove crosstalk signal was proposed. The experiment was verified using an optical fiber with 113 uniform Bragg gratings at an interval of 10cm whose reflectivity are less than 1%. It demonstrates that crosstalk signal and measurement signal can be completely separated in the distance domain after adding a long enough delay fiber. Finally, the temperature experiment of distributed grating sensing system was carried out. The results display that each raster's center wavelength in the fiber link is independent of each other and the center wavelength drift has a good linear relationship with the temperature. The sensitivity of linear fitting is equal to 11.1pm/°C.

We proposed and experimentally demonstrated a high-spatial-resolution distributed acoustic sensor based on time-frequency-multiplexing (TFM) optical frequency domain reflectometry (OFDR). The TFM technique enhances the frequency response of OFDR by multiplexing the time-frequency channels and suppresses the crosstalk in the meantime. Phase demodulation is employed to achieve high sensitivity, and the impact of end effect in OFDR is studied and suppressed by a dedicated linear interpolation. In the results, a 10.5 kHz vibration is measured with 22 cm spatial resolution and 20 dB signal-to-noise ratio on a 1 km fiber. By adjusting the parameters, the system also shows a good DAS performance on a 33 kHz vibration with up to 200 kHz sampling rate.

Full-length strain and damage monitoring for carbon fiber reinforced polymer cable based on optical frequency domain reflectometry

We introduce a new optical frequency domain reflectometry (OFDR) system and processing method that utilize negative beat frequencies for the first time. The new approach enables efficient use of the available system bandwidth and facilitates distributed sensing with the maximum allowable update rate for a given fiber length. This is achieved by using a coherent optical-communications-type receiver that detects both the in-phase (I) and quadrature (Q) components of the backscatter field. The I and Q components are digitally combined to produce a complex backscatter signal whose Fourier transform is not necessarily symmetric. Judicious processing of the complex backscatter signal maps the reflection profile of one half of the sensing fiber to positive beat-frequencies and the profile of the other half to negative beat-frequencies. The new approach was tested via comprehensive computer simulations and experiment.

Measurement of radiation induced attenuation in irradiated active fibers using optical frequency domain reflectometry

In distributed microwave photonic (MWP) radar systems, the detection performance is highly sensitive to the quality of optical signals transmitted over the long fibers connecting the indoor and outdoor units. In this Letter, we propose an MWP radar system that, in addition to its standard range measurement capability, integrates optical frequency domain reflectometry (OFDR) for distributed vibration sensing along the optical fibers. A carrier-suppressed double-sideband (CS-DSB) optical signal is generated in the indoor unit, which serves as a light source and the local reference for the OFDR, as well as a transmitted signal for the radar transmitter. For radar detection, an echo received from an antenna is sent back to the indoor unit, where it is mixed with the local reference to perform de-chirping. For OFDR, a beat signal is produced by the coherent interference of the backscattered Rayleigh signal with the local reference, which realizes distributed optical fiber sensing. The dual functions of the system are evaluated by an experiment. For range measurement, a resolution of 4.8 cm at a transmitted bandwidth of 4 GHz is obtained. For distributed sensing, a reflection point detection with a resolution of 6.7 cm and a vibration measurement with a spatial resolution of 6 m at a vibration frequency of 4 kHz are demonstrated. The experiment also verified that utilizing OFDR to determine the vibration frequency, its impact on radar detection performance can be mitigated through digital signal processing.

Optical frequency domain reflectometry (OFDR) is an interferometric method which allows to determine the spatial distribution of the gain along the active fiber in a fiber amplifier. As a non-destructive tool it is ideal to find the optimum fiber length especially of co-directional pumped fiber amplifiers. In this paper we explain the OFDR method and discuss measurement results from different S-band TDFA setups.

In recent years, Optical Frequency Domain Reflectometry (OFDR) has been able to realize strain measurement with high sensitivity and high spatial resolution (SR) along the sensing fiber. Compared with the conventional cross-correlation method, OFDR based on phase-measuring method has the potential to obtain strain information with better SR. However, the accuracy of this method is severely affected by coherent fading. In this paper, a method combining multi-frequency detection and nearest neighbor analysis is proposed to solve this problem. By comparing the sub-phase curves in different wavelength regions, we successfully eliminate the interference of fading noise and obtain the undistorted phase signal. Finally, in the case of an effective wavelength scanning range of 3.6 nm (corresponding to the ranging SR of 0.22 mm), we realized the deformation measurement with standard deviation of 0.015 μm and SR of 1.1 mm, as well as the strain measurement with standard deviation of 0.55 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">μϵ</i> and SR of 5.6 cm. The experimental results also show that compared with the conventional multi-frequency average method, the proposed scheme can achieve similar phase measurement accuracy under less frequency division, so as to obtain better SR in the same scanning range.