Improvement of Performance for Raman Assisted BOTDR by Analyzing Brillouin Gain Spectrum

We propose a simplified partitioned Brillouin gain spectrum (BGS) analysis method to enhance the spatial resolution and measurement accuracy of a Brillouin optical time-domain reflectometer (BOTDR) assisted by a first-order Raman pump. We theoretically derive the mathematical model of the partitioned BGS and analyze the superposition process of sub-Brillouin signals within a theoretical spatial resolution range. We unified all the unknown constant parameters of the calculation process to simplify the partitioned BGS analysis method and the value of the uniform parameter is attained through the system test data and numerical analysis. Moreover, to automate data processing, the starting point of the temperature/strain change is determined by the first occurrence of the maximum Brillouin frequency shift (BFS), then the position where the partitioned BGS analysis method calculation begins is obtained. Using a 100 ns probe pulse and partitioned BGS analysis method, we obtain a spatial resolution of 0.4 m in the 78.45-km-long Raman-assisted BOTDR system, and the measurement accuracy is significantly improved. In addition, we achieve a strain accuracy of 5.6 με and a spatial resolution of 0.4 m in the 28.5-km-long BOTDR without Raman amplification.

High SNR Φ-OTDR with Multi-Transverse Modes Heterodyne Matched-Filtering Technology

Phase-sensitive optical time domain reflectometer (Φ-OTDR) has attracted attention in scientific research and industry because of its distributed dynamic linear response to external disturbances. However, the signal-to-noise ratio (SNR) of Φ-OTDR is still a limited factor by the weak Rayleigh Backscattering coefficient. Here, the multi-transverse modes heterodyne matched-filtering technology is proposed to improve the system SNR. The capture efficiency and nonlinear threshold are increased with multiple transverse modes in few-mode fibers; the incident light energy is permitted to be enlarged by a wider probe pulse by using heterodyne matched-filtering without spatial resolution being deteriorated. As far as we know, this is the first time that both multi-transverse modes integration method and digital heterodyne matched filtering method have been used to improve the SNR of Φ-OTDR simultaneously. Experimental results show that the noise floor is reduced by 11.4 dB, while the target signal is kept. We believe that this proposed method will help DAS find important applications in marine acoustic detection and seismic detection.

Optical OFDM Error Floor Estimation by Means of OTDR Enhanced by Front-End Optical Preamplifier

Optical time-domain reflectometer (OTDR) enables simple identification and localization of a plethora of refractive and reflective events on a fiber link, including splices, connectors and breaks, and measuring insertion/return loss. Specifically, large enough OTDR dynamic range (DR) and thus high signal-to-noise-ratio (SNR) enable clear far-end visibility of longer fibers. We point out here that, under such conditions, the optical bit-error-rate (BER) floor is dominantly determined by reflective events that introduce significant return loss. This complements the OTDR legacy tests by appropriate optical BER floor estimation in the field. As high SNR implies inter-symbol interference as dominating error generating mechanism, we could apply the classical time-dispersion channel model for the optical BER floor determined by the root-mean-square (rms) delay spread of the actual fiber channel power-delay profile. However, as the high-SNR condition is not always fulfilled mostly due to insufficient DR, we propose here inserting a low-noise optical preamplifier as the OTDR front-end to reduce noise floor and amplify the backscattered signal. In order to verify the model for the exemplar test situation, we measured BER on the same fiber link to find very good matching between the measured BER floor values and the ones predicted from the OTDR trace.

Research on Optical Fiber Sensor Signal Processing Technology Based on Variational Mode Decomposition and Singular Spectrum Analysis

To better apply the phase-sensitive optical time-domain reflectometer (Φ-OTDR) in projects with complex environments, given the problem of the low signal-to-noise ratio of the Φ-OTDR signal, a variational modal decomposition (VMD) is proposed. Signal processing algorithm combined with singular spectrum analysis. First, the equalization optimizer (EO) optimizes the VMD. The VMD decomposes the preprocessed signal into a multi-layer eigenmode function (IMF), selects the IMF component according to the correlation coefficient, and after the SSA noise reduction process, the signal Reconstruction to realize the noise reduction processing of the Φ-OTDR signal. Through experiments, it is proved that the relative mean square error (RMSE) of the method in this paper is lower than that of EO-VMD, and the noise reduction performance is better.

Distributed Humidity Sensing in Concrete Based on Polymer Optical Fiber

We present a preliminary investigation on distributed humidity monitoring during the drying process of concrete based on an embedded polymer optical fiber (POF). The water dissipated into the POF changes several properties of the fiber such as refractive index, scattering coefficient and attenuation factor, which eventually alters the Rayleigh backscattered light. The optical time domain reflectometer (OTDR) technique is performed to acquire the backscattered signal at the wavelengths 650 nm and 500 nm, respectively. Experimental results show that the received signal increases at 650 nm while the fiber attenuation factor clearly increases at 500 nm, as the concrete dries out. In the hygroscopic range, the information retrieved from the signal change at 650 nm agrees well with the measurement result of the electrical humidity sensors also embedded in the concrete sample.

Extending OTDR Distance Span by External Front-End Optical Preamplifier

Optical time-domain reflectometer (OTDR) is used to characterize fiber optic links by identifying and localizing various refractive and reflective events such as breaks, splices, and connectors, and measuring insertion/return loss and fiber length. Essentially, OTDR inserts a pulsed signal into the fiber, from which a small portion that is commonly referred to as Rayleigh backscatter, is continuously reflected back with appropriate delays of the reflections expressed as the power loss versus distance, by conveniently scaling the time axis. Specifically, for long-distance events visibility and measurement accuracy, the crucial OTDR attribute is dynamic range, which determines how far downstream the fiber can the strongest transmitted optical pulse reach. As many older-generation but still operable OTDR units have insufficient dynamic range to test the far-end of longer fibers, we propose a simple and cost-effective solution to reactivate such an OTDR by inserting a low-noise high-gain optical preamplifier in front of it to lower the noise figure and thereby the noise floor. Accordingly, we developed an appropriate dynamic range and distance span extension model which provided the exemplar prediction values of 30 dB and 75 km, respectively, for the fiber under test at 1550 nm. These values were found to closely match the dynamic range and distance span extensions obtained for the same values of the relevant parameters of interest by the preliminary practical OTDR measurements conducted with the front-end EDFA optical amplifier, relative to the measurements with the OTDR alone. This preliminary verifies that the proposed concept enables a significantly longer distance span than the OTDR alone. We believe that the preliminary results reported here could serve as a hint and a framework for a more comprehensive test strategy in terms of both test diversification and repeating rate, which can be implemented in a network operator environment or professional lab.

Characterization and Application of Solitons in a High Nonlinearity Optical Link with Dense Wavelength Division Multiplexing (DWDM) and Ultra-Dense Wavelength Division Multiplexing (UDWDM) Technics

The constantly growing needs in terms of speed and bandwidth, especially all services using data has become greedy. Today optical fiber is the medium of choice for many advantages over others channels. although the optical fiber has taken the label of transmission channels, we won’t be able to do without its nonlinear character and dispersions, which are part of the major problems during optical transmission. In this article, we are presenting the use of optical solitons in the fiber optic link, with a particular way; we have considered two kind of links, the underground one and the overhead one, in the first link, we have applied optical soliton in order to just take into account the optical Kerr effect. And for the overhead link, we can transmit for short distance by using the return to zero modulation’s format and the use of optical Erbium Doped Fiber Amplifiers (EDFA). The solitons which are the impulses, capable of propagating in the nonlinear optical fiber and dispersive without any distortion; to justify our hypothesis, we compare a dense wavelength division multiplexing (DWDM) without soliton and the dense wavelength division multiplexing with solitons; we notice that, the link with soliton is better than the first because the pulse keeps its shape from the source up to the destination, its eye diagram is opened which means there is no much noises. After the simulations in Matlab and optisystem software, we went to Congo telecom, to do our test with the optical time-domain reflectometer (OTDR), for both installations. From that study while transmitting over 100km it is better to use optical solitons with the high speed.

Secure Software for Rail Circuits in the Form of Distributed Sensors

In order to improve the safety of train traffic, we propose to introduce into practice a new type of rail circuits, fiber-optic rail circuits. Fiber optic cable is very sensitive to external shocks, vibration and deformation, which can play a crucial role in the detection of mechanical damage of rails and wheel sets and also to improve the positioning of the rolling stock. The branches of the fiber optic cable not only serve as a conductor of information, but also serve as a sensor, as they can perceive vibration. An OTDR (Optical Time Domain Reflectometer) analyzes the backscattered light signal to determine the shape of the physical impact that caused the bending. From the time between the emission of the light signal and the receipt of the backscattered signal, the fault location is calculated. The authors offer the model of the system, which will check the security problems of the trains and the rail circuits. The software of the corresponding system is presented, using the simulation techniques. The authors present the pseudo code of the software. They also offer the testing environment for the software.