Locating Excess Mode-Dependent Loss in Coupled Multi-Core Fibers based on Coherent OTDR

In order to meet the length calibration requirements of coherent optical time domain reflectometer(COTDR) in state monitoring of long-distance communication, a new coherent optical time domain reflectometer length calibration method was proposed. The method utilized the principle that light circulated continuously in the fiber ring. And based on the principle, the COTDR length calibration device was developed. The device used the pulse frequency and pulse width emitted by the function generator to control the breaking time of the optical switch. Thereby it could realize the control of the propagation time of the light in the fiber loop. If a fiber amplifier was added to the fiber loop, simulation of fiber links of different lengths could be realized. Finally, the length calibration of the coherent optical time domain reflectometer was achieved. The experimental results showed that the length of the fiber used in the device is 150km, and the optical pulse could be rotated up to 7 times in the fiber loop. Therefore, the maximum range of the coherent optical time domain reflectometer length calibration was 1050km. Its extended uncertainty was 7.0m, and the containing factor was 2. This method could not only promote the development of coherent optical time domain reflectometry technology, but also provided a good idea for COTDR length calibration.

The nonlinear effects that limit the performance of the multi-frequency probe (MFP) based coherent optical time domain reflectometry (C-OTDR) are investigated. Based on theoretical analysis and experimental results, compared with conventional C-OTDR, when the probe pulse has power gradient within the pulse width, self-phase modulation (SPM) and cross-phase modulation (XPM) are strengthened in the new C-OTDR scheme. The generation of four-wave mixing (FWM) is dependent on SPM and XPM, and with modulation frequency of phase modulator higher than 40 MHz, the stimulated Brillouin scattering (SBS) threshold can be enhanced by more than 5 dB, which benefits the maximum dynamic range of the MFP C-OTDR.

Signal fading severely affects the performance of optical fiber distributed acoustic sensing (DAS) systems based on coherent optical time domain reflectometry (COTDR), especially in terms of differentiated responses between channels. To eliminate fading, a polarization diversity COTDR system is established, which generates two pulse feedbacks with different fading components for P and S polarization. The rayleigh backscattered (RBS) signals from the two separate probing pulses exhibit different fading positions under the influence of external interference but have the same phase change rate. Through an improved rotational vector summation (IRVS) algorithm, the complex RBS signals of the two polarization states are effectively synthesized, simultaneously suppressing fading phenomena caused by orthogonal polarization. Experimental results demonstrate that the demodulated signal-to-noise ratio (SNR) can reach 65 dB@200 Hz. Moreover, by using 20% of the mean amplitude as a threshold, the probability of fade events decreases from 4.9% to 0.12%. The proposed fade mitigation distributed sensing system exhibits excellent performance and plays a crucial role in practical DAS systems.

The inter-channel crosstalk (XT) in frequency division multiplexing coherent optical time domain reflectometry (FDM-OTDR) is described theoretically. The inter-channel XT degrades the spatial resolution of the OTDR trace at the position, where the reflectivity changes precipitously, and is dependent on the frequency channel spacing, pulse waveform, and weighting function of the signal processing. We show that the deterioration of the OTDR trace can be fully simulated by using impulse response analysis, and that apodization of the probe pulse shape and weighting function effectively suppress the inter-channel XT. Moreover, we propose a novel nonlinear self-phase modulation (SPM) suppression scheme to adapt apodized probe pulse in our experiment. The experimental results for the impulse response, OTDR trace, and SPM suppression are discussed.

Fiber optic distributed acoustic sensing (DAS) is a potential method to achieve partial discharge (PD) detection and fast localization in gas insulated metal enclosed transmission lines (GIL). In order to detect and locate the weak partial discharge signals in the GIL as early as possible, this paper proposes an integrated distributed fiber optic sensor installation method for the GIL, as well as a distributed partial discharge acoustic detection and localization system. Based on the fully insulating nature of the optical fiber, the optical fiber is installed inside the GIL to improve the sensitivity to partial discharge acoustic signals, and the length of the sensor unit is increased by repeated winding to further improve the detection performance of the acoustic signals. A true-to-life GIL partial discharge test platform was built to investigate the multi-point distributed sensing performance of the proposed coherent optical time domain reflectometer system for metal tip discharge. The test results show that the sensing distance of the sensing system can reach 120m, which can achieve the multipoint detection and localization of the partial discharge acoustic signal of 29.7pC. The proposed sensing method and system provide a new solution for GIL weak partial discharge optical fiber distributed acoustic signal detection.

This paper describes a coherent optical time domain reflectometer (OTDR) with a high dynamic range for locating faults in optical cable systems incorporating repeaters based on erbium-doped fiber amplifiers (EDFA). We also present experimental results achieved with transmission lines containing several fiber spans and EDFAs. >

In this work, we propose the use of Coherent Optical Time-Domain Reflectometry (C-OTDR) in order to discriminate the medium surrounding one or more Co2+-doped fiber sections. The proposed approach can be used to perform vibration measurements along the whole fiber by means of C-OTDR measurements at 850 nm, as well as liquid detection in selected positions by active thermometry at 1550 nm. By pumping the fiber with a modulated optical pump, the induced fiber temperature modulation is monitored through C-OTDR, as the latter is sensitive to optical phase changes occurring along the sensing fiber. As the induced phase modulation (i.e. the C-OTDR signal) is affected by the thermal conductivity of the surrounding medium, the latter can be retrieved from the phase signal. In our tests, a 5 Hz modulation frequency was chosen to discriminate between air, water, and acetone.

Optical fiber distributed acoustic sensing (DAS) is a potential approach to realize on-line monitoring and rapid localization of power cable partial discharges (PDs). Distributed detection of discharges requires a DAS system with a wide frequency band and low noise level. For this purpose, an improved coherent optical time-domain reflectometry (COTDR) method based on dual-frequency probe pulses was developed. The method doubled the length-bandwidth product (LBP) of traditional DAS systems without significantly increasing the complexity of signal demodulation. A discharge signal extraction algorithm was also proposed based on the detection results of the dual-frequency pulses to exclude the effects of fading noises and vibrations. The improved COTDR system demonstrated the frequency band extension by detecting and recovering the 50 kHz sinusoidal vibration. The sensing distance was 1.1 km and the spatial resolution was 7.15 m. Afterward, the multi-point weak spark discharge acoustic signals were correctly measured and localized, and the frequency components up to 71.36 kHz were detected. The proposed COTDR method provides a prospective solution for distributed PD detection for power cables.

In this paper, we analyze the performance of a distributed acoustic sensor at two different interrogation wavelengths. We show theoretically that, in a coherent optical time-domain reflectometry (OTDR) operating at 850 nm, the dynamic signal-to-noise ratio (SNR) is enhanced, compared to an identical configuration operating at 1550 nm. Such enhancement is maximum at the interrogating pulse input section, while decreasing along the fiber in virtue of the higher loss. Experimental tests, carried out using two heterodyne C-OTDR detection schemes operating at the analyzed wavelengths, confirm the SNR improvement.

We designed and fabricated all-fiber fan-in/fan-out (FI/FO) devices using a fused taper technique for square lattice structure coupled-core multicore fiber (CC-MCF). A vanishing core bridge fiber with a diameter of 80 μm was tailor-designed to minimize both the mode field mismatch and the required tapering ratio at the same time. In experiment, we succeeded in fabricating FI/FO devices with a core pitch as small as 17 μm and achieving practically low insertion losses of 1.43 dB and 2.28 dB for fan-in and fan-out devices, respectively. The fabricated FI/FO devices can well multiplex and demultiplex 1550 nm light into and out from the CC-MCF without obvious total power fluctuation even when an external perturbation is intentionally imposed on the CC-MCF. The proposed FI/FO device is very useful for both the study and application of CC-MCFs which usually have ultra-small core pitches.

Distributed acoustic sensors (DAS) based in coherent optical time-domain reflectometry (C-OTDR) provide a cost-effective solution for intrusion monitoring of large civil infrastructures like pipelines, railways or motorways. Although detection of events is well demonstrated, an estimation of the amplitude of these events is difficult to achieve. We propose a new method to recover the amplitude of the vibration from the conventional C-OTDR backscattered power traces. It is based on the FFT analysis of the DAS signals. Using a discrete accelerometer as a reference, we have calibrated the response of an optical fiber DAS using known stimuli. A correlation between the amplitude of the vibration and the ratios of the amplitudes of harmonics to the fundamental of the DAS signals is demonstrated. This analysis overcomes the main issues of the amplitude measurement, related to the interaction between the stimulus and the interference pattern.

In this paper we study a multi-function and high precision submarine optical fiber monitorsystem. This design is based on the core technology of coherent optical time-domain reflectometry (COTDR) which is used for submarine optical fiber monitor. The design provides multi-functionwhich including: undersea amplifier gain monitor, output power monitor; undersea fiber events monitor (such as fiber cut, attenuation as well as reflection) and also the fiber fault location which may identify the accurate location of the failurepoint (distance from failure point to its upstream amplifier). With these functions, the designcan facilitate the staff to inquiry and debug, and it also improves the maintainability and operability of the undersea equipment.

Nonlinear interference (NLI) is a performance limiting factor for long-haul transmission in coupled-core multi-core fiber ( CC-MCF). We p ropose t o u se subcarrier multiplexing (SCM) technique to reduce the amount of generated NLI increasing the maximum reachable distance.

Distributed fiber-optic vibration sensing based on phase extraction from optical reflectometry is discussed in this paper. The principles on the phase extraction from coherent phase-sensitive optical time-domain reflectometry and time-gated digital optical frequency-domain reflectometry are introduced, and the experimental results of distributed vibration measurements based on the two techniques are demonstrated. To have a better understanding on the performance of the techniques, the phase extraction noise in the phase extraction process is analyzed. To solve the problem of existing measurement dead zones, the influence of Rayleigh fading phenomenon is investigated, and a solution of using statistical analysis to overcome this drawback is introduced and experimentally verified. With these improvements, long-range nondead-zone distributed vibration sensing techniques with linear response to external vibration signals based on phase extraction from optical reflectometry are realized.

We summarize important linear and nonlinear transmission properties of uncoupled-core multicore, multimode and coupled-core multicore fibers. The role of linear coupling between spatial modes on nonlinear transmission is discussed.