Rayleigh Backscattering Light Research Articles

Fading suppression in the Ф-OTDR system based on a phase-modulated optical frequency comb.

In this paper, what we believe to be a novel method is proposed to suppress the fading effect of the phase-sensitive optical time domain reflectometer (Ф-OTDR) by using a phase-modulated optical frequency comb. In the Ф-OTDR system, intensity distributions of Rayleigh backscattering (RBS) light are different for pulsed probe lights with different central frequencies, therefore the locations of the fading points corresponding to signals of different frequencies are differently distributed, allowing the use of frequency division multiplexing to suppress the fading effects. In the experimental system of this paper, a continuous light in the form of a frequency comb is firstly generated through phase modulation. It is then modulated into a pulsed probe light and injected into the sensing fiber to produce different RBS intensity distributions. Finally, the extracted phase is processed by using the amplitude evaluation method, so that the distorted phase can be eliminated. Fading suppression is achieved using our system, and the effect of suppression is evaluated. By using an equal-amplitude optical frequency comb containing seven frequency components, the fading probability density of the system is dramatically reduced from the range of 5.49%-9.83% to 0.08%. Compared with the conventional system using a single acoustic-optic modulator to generate the frequency shift, the method proposed in this paper features a larger modulation bandwidth and more flexible frequency combination scheme to better suppress the fading effect. This method does not sacrifice the response bandwidth of the system, and the phase delay can be precisely controlled, which helps to fully suppress the fading effect.

Polarization Properties of Coherently Superposed Rayleigh Backscattered Light in Single-Mode Fibers.

The properties of the state of polarization (SOP) and the degree of polarization (DOP) of Rayleigh backscattered light (RBL) in single-mode fibers (SMF) are investigated theoretically and experimentally when the incident probe is a perfectly coherent continuous-wave (CW) light. It is concluded that the instantaneous DOP of the coherently superposed RBL is always 100%, and the instantaneous SOP is determined by the distributions of the birefringence and the optical phase along the SMF. Therefore, the instantaneous SOP of the coherently superposed RBL does not have a constant relationship with the SOP of the incident CW probe. Furthermore, the instantaneous SOP varies randomly with time because the optical phase is very sensitive to ambient temperature and vibration even in the lab environment. Further theoretical derivation and experimental verification demonstrate, for the first time, that the temporally averaged SOP of the coherently superposed RBL has a simple constant relationship with the SOP of the incident CW probe, and the temporally averaged DOP is 1/3 in an SMF with low and randomly distributed birefringence. The derived formulas and obtained findings can be used to enhance the modelling and improve the performances of phase-sensitive optical time-domain reflectometry and other Rayleigh backscattering based fiber-optic sensors.

Scanning-free hybrid Rayleigh-Brillouin distributed fiber-optic sensing system.

Hybrid systems based on Brillouin optical time domain analysis (BOTDA) utilizing Rayleigh backscattering light wave as a probe have enabled single-end and long-range distributed sensing for multiple parameters. However, the spatial resolution for dynamic parameter measurement is limited, and the frequency scanning process of BOTDA is time-consuming. To address these challenges, we propose a hybrid system that combines BOTDA and time-gated digital optical frequency domain reflectometry (TGD-OFDR), aiming to enhance the spatial resolution of dynamic measurements without compromising the system's signal-to-noise ratio and eliminate the frequency scanning process of BOTDA. In the experimental setup, we conducted measurements on a 9.52 km single-mode fiber. A sinusoidal vibration with a frequency of 3 kHz was measured with a spatial resolution of 3 m, achieving a noise floor of 0.05 nε/√Hz. Furthermore, temperature measurements with a spatial resolution of 10 m and a Brillouin frequency shift (BFS) measurement accuracy of 0.74 MHz were successfully obtained using the scanning-free single-end BOTDA technique. This hybrid system shows promising potential for various applications in distributed fiber-optic sensing.

Noise analysis in direct detection and coherent detection phase-sensitive optical time-domain reflectometry systems.

This study compares noise and signal-to-noise ratio (SNR) in direct detection and coherent detection fiber-based distributed acoustic sensing (DAS) systems. Both detection schemes employ the dynamic analysis of Rayleigh-backscattered light in phase-sensitive optical time-domain reflectometry (ΦOTDR) systems. Through theoretical and experimental analysis, it is determined that for photodetection filters with a sufficiently narrow bandwidth, the SNR performance of both detection schemes is comparable. However, for filters with poor selectivity, coherent detection was found to exhibit superior performance. These findings provide crucial guidelines for the design of high-performance time-domain DAS systems.

Digital phase shift based simulated coherence phase demodulation technology for [formula omitted]-OTDR

In this paper, we propose a digital phase demodulation scheme for phase-sensitive optical time-domain reflectometer (Φ-OTDR) systems based on simulated coherence for digital phase shift, in which three-path signals with a phase interval of 120° can be obtained by employing digital simulated orthogonal coherence for phase shift instead of using a hardware 3×3 coupler and three detectors, while the high-frequency component can be eliminated through cross differential between simulated coherence signals. On this basis, the phase variation of Rayleigh backscattering (RBS) light can be analyzed by differential cross multiplication (DCM). The performance of the proposed scheme is evaluated through experiments by detecting a series of vibration signals with various sensing distances, vibration frequencies, and waveforms. The experimental results demonstrate that the phase variation caused by external vibration can be effectively reconstructed and a strain resolution of 0.5 nɛ and the SNR of 51.03 dB can be achieved through the proposed demodulation method, which significantly simplifies the system and effectively suppresses the deviation of phase interval and noise caused by hardware demodulation.

Continuous Time-Domain Measurement Method of Rayleigh Backscattered Light With Coherent Detection

This paper shows the results of continuous time-domain measurement of Rayleigh backscattered light. Experimental results show that we can measure the variation of the phase of the backscattered light from a specific point of the fibers where we applied sinusoidally varying stress at a frequency of 140 kHz. We expect that the method will find application to continuously measure the rapid variation of external perturbation in the time domain at a specific point in the fibers.

Distributed Partial Discharge Locating and Detecting Scheme Based on Optical Fiber Rayleigh Backscattering Light Interference.

Optical fiber sensors are used for partial discharge detection in many applications due their advantage of strong anti-electromagnetic interference capability. Multi-point distributed partial discharge detection and location are important for electrical equipment. In this paper, a distributed partial discharge location and detection scheme based on optical fiber Rayleigh backscattering light interference is experimentally demonstrated. At the same time, the location and extraction algorithm is used to demodulate the partial discharge signal; furthermore, the high-pass filter is used to reduce the system low-frequency noise and environment noise. It is clear that the proposed system can detect a partial discharge signal generated by metal needle sensitivity, and the detectable frequency range is 0-2.5 kHz. We carried out 10 locating tests for two sensing units, the experimental results show that the maximum location error is 1.0 m, and the maximum standard deviation is 0.3795. At same time, the signal-to-noise ratio (SNR) of sensing unit 1 and sensing unit 2 are greatly improved after demodulation, which are 39.7 and 38.8, respectively. This provides a new method for a multipoint-distributed optical fiber sensor used for detecting and locating a long-distance electrical equipment partial discharge signal.

Interference fading suppression in φ-OTDR using space-division multiplexed probes

We propose and experimentally demonstrate a novel interference fading suppression method for phase-sensitive optical time domain reflectometry (φ-OTDR) using space-division multiplexed (SDM) pulse probes in a few-mode fiber. The SDM probes consist of multiple different modes, and three spatial modes (LP01, LP11a, and LP11b) are used in this work for the proof of concept. Firstly, the Rayleigh backscattering light of different modes is experimentally characterized, and it turns out that the waveforms of the φ-OTDR traces for distinct modes are all different and independent. Thanks to the spatial difference of the fading positions for distinct modes, multiple probes from spatially multiplexed modes can be used to suppress the interference fading in φ-OTDR. Then, the performances of the φ-OTDR systems using a single probe and multiple probes are evaluated and compared. Specifically, the statistical analysis shows that the fading probabilities over both the fiber length and the time scale are reduced significantly by using multiple SDM probes, which verifies the significant performance improvement on fading suppression. By introducing the concept of SDM to φ-OTDR, the proposed novel interference fading suppression method avoids the complicated frequency or phase modulation, which has the advantages of simplicity, good effectiveness and high reliability.

Distributed vibration and temperature simultaneous sensing using one optical fiber

Optical fiber distributed vibration and temperature sensor are very important for many applications. It is challenging to design a hybrid vibration and temperature sensing system because multi-parameter simultaneous sensing is the common requirement in most applications. In this paper, we demonstrated a novel distributed vibration and temperature simultaneous sensing system using standard optical fiber, which based on Rayleigh and Raman backscattering light. We have performed several temperature and vibration tests under different circumstances. The maximum temperature measurement deviation is 0.6 °C and maximum consistency deviation of three sensing units is 0.9 °C. The continuous vibration signal has been detected by using standard audio signal as the vibration source and the detectable frequency range is 0 Hz–5 kHz. In particular, we have carried simultaneous sensing test, which the vibration and temperature signals are detected successfully by one sensing unit at the same time. This new optical fiber distributed multi-parameter simultaneous sensing system is potentially used in pipeline and power cable for temperature and vibration monitoring.

Investigation of a tunable and switchable multi-wavelength bidirectional Brillouin-erbium fiber ring laser

A multi-wavelength bidirectional Brillouin-erbium fiber ring laser with switchable Brillouin frequency spacing (BFS) is proposed and experimentally demonstrated. In the presented Brillouin-erbium ring laser, including an optical amplifier and a highly nonlinear fiber, and without any optical isolator, due to Rayleigh scattering, stimulated Brillouin scattering, and cascaded four-wave mixing initiated successively by the Brillouin pump (BP) light, the odd- and even-order Stokes lines are generated and circulate in the opposite direction in the ring cavity. The BP light and Stokes-induced Rayleigh backscattering light also simultaneously circulate in the ring cavity. Only by adjusting BP power, the gain competition between Brillouin based Stokes and cavity modes’ oscillation can be controlled, the laser output can be conveniently switched between single BFS and odd- or even-order double BFS. In addition, under the certain BP power conditions, the proposed multi-wavelength Brillouin-erbium fiber laser also can realize switchable odd- or even-order Stokes generation and Stokes generation with single BFS, with an increasing wavelength number in turn, only by simply adjusting pump power of the erbium-doped fiber amplifier. Stability and wavelength tunability of the proposed multi-wavelength bidirectional Brillouin-erbium fiber ring laser are also investigated, respectively.

Hybrid electronically addressable random fiber laser.

We report here a novel architecture for a random fiber laser exploiting the combination of a semiconductor optical amplifier (SOA) and an erbium doped fiber (EDF). The EDF was optically biased by a continuous wave pump laser, whereas the SOA was arranged in a fiber loop-mirror and driven by nanosecond duration current pulses. Laser pulses were obtained by synchronizing the SOA driver to the returning amplified Rayleigh back-scattered light from a selected short section of the EDF. By tuning the SOA pulse rate, random lasing was achieved by addressing selected meter-long sections of the 81-m long EDF, which was open-ended. Laser oscillation can be potentially obtained with SOA modulation frequencies from several kHz to the MHz regime. We discuss the mechanism leading to the hybrid random laser emission, connecting with phase sensitive optical time domain reflectometry and envision potential applications of this electronically addressable random laser.

Phase demodulation method based on a dual-identical-chirped-pulse and weak fiber Bragg gratings for quasi-distributed acoustic sensing

A phase demodulation method for quasi-distributed acoustic sensing (DAS) systems based on a dual-identical-chirped-pulse and weak fiber Bragg gratings (WFBGs) is proposed. Compared to the use of Rayleigh backscattering light in optical fibers, the implementation of WFBGs can contribute to obtaining an optical signal with a higher signal-to-noise ratio (SNR). The dual-identical-chirped-pulse is generated by a time-delay fiber, and the sinusoidal carrier is generated by the interference between the two chirped pulses reflected by adjacent WFBGs. The phase of the sinusoidal carrier represents the dynamic strain change posed on the sensing fiber. Discrete Fourier transform is used to directly retrieve the phase information. The performance of the phase demodulation from interference signals under different sinusoidal carrier frequencies and SNRs is numerically investigated. The piezoelectric transducer is employed to emulate the sound in the experiment to verify the effectiveness of our method. It is shown that the dynamic strain can be well reconstructed at the end of a 101.64 km fiber when the signal SNR is down to 3.234 dB. Our proposed method enables the application of the long-distance sensing in quasi-DAS systems.

Hybrid demodulation method for distributed acoustic sensing based on coherent detection and pulse pair

A hybrid demodulation method based on coherent detection and pulse pair for distributed acoustic sensing is proposed. The Rayleigh backscattering light of pulse pair is coherent amplified by local reference light. A 3 × 3 fiber coupler is utilized to produce stable 2π/3 phase shift for phase extraction. The digital mixing of the beat signals of two pulses suppress the noise caused by phase drift and vibration on the local reference fiber. Several simulated acoustic waveforms are detected and retrieved. Experimental results show that the system has good linear response. And the equivalent strain of noise is as low as 0.4nε.

A Novel Weak-Scattering Michelson Interferometer Based on PBS for Long-Distance Disturbance Localization

Optical fiber interferometer could achieve various dynamic measurements by transforming phase change into intensity change. In this paper, a novel Michelson interferometer (MI) based on two polarization beam splitters (PBSs) is proposed. Two thirds of Rayleigh backscattering (RB) is separated from the signal light by using PBS and Faraday rotating mirror (FRM) due to the difference polarization properties between RB and signal light in optical fibers. And the signal-to-noise ratio (SNR) of system has been significantly improved. Within the detection range of 50 km, the localization experiments on this new interferometer show that the SNR is increased by 6.69 dB compared with traditional MI on the condition of 500 kHz sampling rate. And the positioning accuracy can achieve ±100 m in the first-order null-frequency. The sensing distance also increases from 36.17 km to 49.42 km with a SNR of 15 dB. This proposed sensing system exhibits high sensitivity in the low frequency region, which is of great importance for long-distance buried optical cable detection and location.

Fault detection of few-mode fiber based on high-order mode with high fault detection sensitivity.

We report on the fusion splice loss characteristics of each mode in few-mode fiber (FMF). The fusion fault loss characteristics of all modes of the six-mode step-index and graded-index fibers are measured, respectively. The high-order modes with high fault detection sensitivity are presented. The fault events with different loss amplitudes can be accurately characterized by high-order modes. On this basis, a fault detection and location method for FMF based on backscattered light of high-order modes is proposed and experimentally demonstrated. Rayleigh backscattering light of high-order modes is utilized to detect the faults of 7.2km six-mode step-index fiber with three fusion splice points with different fusion quality, the detection results of each mode are compared. The high-order modes LP21a, LP21b, and LP02 with high fault detection sensitivity are shown, three fault points located around 1.03, 3.1, and 3.2km of the FMF are successfully detected. While the LP01, LP11a, and LP11b modes can detect only one fault point, which is located around 3.19km of the FMF.

Characterization of vibration relative amplitude in direct detection Φ-OTDR using variable gain method

Vibration relative amplitude characterization has always been a tough problem in direct detection phase-sensitive optical time-domain reflectometry (Φ-OTDR) system due to the nonlinear relationship between the optical intensity and vibration amplitude. In this paper, a novel variable gain method is proposed to characterize the relative amplitude of external vibration. The mathematical model of Φ-OTDR is established, and the influence of external vibration on detected coherent Rayleigh back-scattering light in a single-mode fiber is simulated. Then, the vibration signals of different amplitudes imposed on the sensing fiber are further investigated, and the relationship between the interference amplitude peak power and vibration amplitude is derived. On the basis of the theoretical analysis, the variable gain method is carried out and implemented in Φ-OTDR system. The experimental results demonstrate that the calibrated interference peak power increases linearly with the applied vibration amplitude, and a high correlation coefficient (R2 = 0.93071) for linear fitting is obtained.

Coherent Kramers-Kronig Receiver for $\boldsymbol \Phi$-OTDR

The phase-sensitive optical time-domain reflectometry ( $\Phi$ -OTDR), which could locate and demodulate perturbation along tens of kilometers sensing fiber with high spatial resolution and high sensitivity, has been applied in many fields, such as seismic wave detection, structure health monitoring, and so on. The phase retrieval of the Rayleigh backscattering light is the most critical process of $\Phi$ -OTDR; therefore, the research on the phase retrieval of $\Phi$ -OTDR has attracted lots of attentions. It should be noted that in fiber-optic communications, the phase retrieval is also a crucial process for phase-encoded schemes. Particularly, as an effective phase retrieval scheme, the Kramers-Kronig (KK) receiver proposed recently has become a hot topic in fiber-optic communications for its high spectral efficiency and low requirement of continuous-wave-to-signal power ratio. Therefore, this paper discusses the feasibility of applying the KK receiver into $\Phi$ -OTDR and analyzes the signal retrieval error with KK relation in $\Phi$ -OTDR; then, its performance in phase retrieval is verified through numerical simulations and experiments. All the results show that generally the KK receiver is a good phase retrieval scheme for heterodyne $\Phi$ -OTDR.

Event Discrimination Using Phase Correlation in Φ-OTDR System Based on Coherent Detection

We report a novel method of event discrimination using phase correlation in Φ-OTDR system based on coherent detection. The phase change demodulated from the Rayleigh backscattering light is the result of the event acting on the optical fiber. Then, the correlation coefficient between the phase change of every point along the fiber and that of the fiber end is obtained. A vibration event acting on the fiber induces the change of the correlation coefficient. So, the number of vibration events is determined by the number of changes of the correlation coefficient. In addition, it proves that our proposed method is able to discriminate two close events even when they are in the range of half-pulsewidth.

Distributed optical fiber vibration sensing using phase-generated carrier demodulation algorithm

A novel optical fiber-distributed vibration-sensing system is proposed, which is based on self-interference of Rayleigh backscattering with phase-generated carrier (PGC) demodulation algorithm. Pulsed lights are sent into the sensing fiber and the Rayleigh backscattering light from a certain position along the sensing fiber would interfere through an unbalanced Michelson interferometry to generate the interference light. An improved PGC demodulation algorithm is carried out to recover the phase information of the interference signal, which carries the sensing information. Three vibration events were applied simultaneously to different positions over 2000 m sensing fiber and demodulated correctly. The spatial resolution is 10 m, and the noise level of the Φ-OTDR system we proposed is about 10−3 rad/ $$\surd {\text{Hz}}$$ , and the signal-to-noise ratio is about 30.34 dB.

A high performance distributed optical fiber sensor based on Φ-OTDR for dynamic strain measurement

A high-performance distributed optical fiber sensor based on phase-sensitive optical time-domain reflectometry (Φ-OTDR) for dynamic strain measurement has been proposed. In this newly designed system, ultraweak fiber Bragg gratings (UWFBGs) are sparsely embedded in the sensing fiber. Through an unbalanced 3 × 3 coupler structure, the reflected lights from two adjacent UWFBGs would be mixed with each other and then split into three parts for phase demodulation. Then, a novelty table-look-up scheme is introduced to precisely demodulate the frequency, amplitude, and phase of dynamic strain. Rayleigh back-scattering light is utilized to identify and locate the accurate position of vibration event between two adjacent UWFBGs. The experimental results show that the proposed system is capable of reconstructing the vibration with a linear intensity response of R 2 = 0.9986 and a fiber length variation sensitivity of 117 pm/(Hz)1/2. Meanwhile, a wide frequency response band from 50 to 2075 Hz is achieved with high signal-to-noise ratio above 56 dB.