Pulse Waveform Manipulation in FDM-OTDR for Suppressing Inter-Channel Crosstalk

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.

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.

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.

We propose and experimentally demonstrate a novel method based on coherent optical time domain reflectometry for locating excess mode-dependent loss in coupled multi-core fibers for the first time.

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. >

The dependence of thickness and concentration product (TCP) of single-wall carbon nanotubes saturable absorber (SWCNTs SA) on stabilizing and shortening pulsewidth in mode-locked fiber lasers (MLFLs) was investigated. We found that an optimized TCP for pulse energy and nonlinear self-phase modulation (SPM) enabled to determine the shorter pulsewidth and broader 3-dB spectral linewidth of the MLFLs. The shortest MLFL pulsewidth of 418 fs and broad spectral linewidth of 6 nm were obtained as the optimized TCP was 70.93 (μm•wt%), which was in good agreement with the area theorem prediction. This significant effect of TCP on pulse energy, SPM, pulsewidth, and spectral linewidth of MLFLs suggests that the TCP represents the total amount of SWCNTs in SA, which can be used as one of important and key parameters for characterizing the passive MLFL pulsewidth.

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.

The most used computer codes for simulating the sound propagation in closed spaces are based on geometrical acoustics. Such an approach is known to be suitable when sound waves interference does not play a major role. For estimating a cross-over frequency above which this condition is met, acousticians use a simple formula for the so called “Schroeder frequency,” which in turn is related to modal overlap. In this work, two methods for observing experimentally the cross-over frequency above which wave interference may be disconsidered are presented. One of them is related to the statistical independence of room impulse responses (in space), while the other one is based on the analysis of impulse responses' phase information within a frequency band. Preliminary results for a shoebox room, based on analytically generated impulse responses, and measurements performed in an auditorium are shown, and indicate similar frequency values, also comparable to the estimated Schroeder frequencies.

A compact device with a two-level transfer function (TF) implemented with two semiconductor optical amplifier (SOA)-based stages is proposed and characterized. Each stage exploits nonlinear polarization rotation and self-phase modulation. The obtained improved TF with very flat top and bottom levels makes the scheme suitable for working as a reshaper in all-optical regeneration. The effectiveness of the device is verified in regenerating both nonreturn-to-zero (NRZ) and return-to-zero (RZ) data signals up to 40 Gb/s. Bit error rate measurements demonstrate increased threshold margin and extinction ratio improvement.

Polarization-multiplexed quaternary phase-shift keying (PM-QPSK) modulation associated with coherent detection and digital signal processing are the underlying technologies for 100-GE optical channels. Linear impairments such as chromatic dispersion (CD) and polarization mode dispersion (PMD) can now be easily overcome by adaptive digital filtering. Intra-channel nonlinear distortions such as the nonlinear phase noise (NLPN) and self-phase modulation (SPM) are the next limiting factors. Two of the highest performance intra-channel nonlinear equalizers proposed so far are the backward propagation split-step Fourier (BP-SSF) method and the frequency-domain Volterra series equalizer (FD-VSNE). In this paper we make a brief overview of digital signal processing for 100-GE optical carriers, and we test the performance limits of intra-channel nonlinear equalization algorithms, comparing the BP-SSF and FD-VSNE approaches in polarization-multiplexed transmission.

By using OPFs to transmit data in the form of light, fiber-optic communication has completely changed the way in-formation is transmitted. This technology's remarkable benefits, such as much lower attenuation, increased band-width capacity, and decreased crosstalk over extended distances, make it superior to conventional electrical trans-mission techniques. The deployment of Wavelength Division Multiplexing (WDM), which maximizes bandwidth usage by enabling several data channels to be carried simultaneously at various wavelengths, is one of the rapid breakthroughs in optical fiber (OPF) technology that are examined in this research. However, nonlinear event in-volved Self-Phase Modulation (S.P.M.), Four-Wave Mixing (FWM), and Stimulated Raman Scattering (S.R.S.) that are inherent in fiber optics frequently impair the performance of WDM systems. Signal distortion and energy loss may result from these causes. The study examines many digital modulation methods, such as Differential Phase Shift Keying (DPSK), Return-to-Zero (RZ), and Non-Return-to-Zero (NRZ), and evaluates how they affect system performance and efficiency at high bit-rates. The results point to a future path for optical communication system research and development, showing that DPSK provides improved sensitivity and lower bit error rates, while RZ modulation exhibits higher resilience to nonlinear effects.

Impulse response analysis in conditional quantile models with an application to monetary policy

Orcutt’s hypothesis in international economics implies that trade flows respond to exchange rate changes faster than to changes in relative prices. Most previous studies used import and export demand models and tested the hypothesis by imposing and comparing lag lengths on the exchange rate and relative prices. One recent study, however, employed impulse response of trade flows to one SD shock to the nominal exchange rate and one SD shock to relative prices and tested the Orcutt’s hypothesis for several industrial countries. In this article we follow this study and test the hypothesis for six developing countries using impulse response analysis. Like the other study for industrial countries, we do not find much support for the hypothesis.

Although one lung ventilation (OLV) is commonly used, little is known about the modulation of the autonomic nervous system with OLV while under general anesthesia. As the frequency domain and time domain analyses are powerful analytic tools, we investigated their modulation during OLV. Patients undergoing thoracic surgery were classified into two groups: those who did (group A, n=8) and those who did not (group N, n=8) receive atropine. After a double lumen tube was placed endotracheally, mechanical ventilation of both lungs (BLV) was established at 18 min(-1) while under isoflurane anesthesia. Electrocardiogram, systolic arterial pressure (SAP), and inspiratory flow (Finsp) were digitally recorded as follows: awake before anesthesia; BLV after anesthesia; BLV after intravenous 10 microg kg(-1) of atropine (group A) or not (group N); left OLV; and right OLV. Power spectral analyses of heart rate (HR) and SAP were computed by determining low-(LF, 0.04-0.15 Hz) and high-frequency (HF, 0.15-0.40 Hz) components, and impulse response analysis was executed among HR, SAP, and Finsp. Impulse responses were assessed by the maximum values in the time domain. In frequency domain analysis, atropine depressed LF and LF/HF but not HF in HR variability, while no difference was observed between right OLV and left OLV. The heart rate to SAP impulse response was maintained at a significantly higher level in group A than in group N (905+/-360 vs. 425+/-375 mmHg beats(-1)min(-1)) at right OLV. A significant difference was also observed between left and right OLV within group N. Impulse response analysis demonstrated that there is a greater effect on autonomic nervous system modulation during right OLV than in left OLV, which mainly results from a parasympathetic neural linkage origin.