High dynamic range coherent OTDR for fault location in optical amplifier systems

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.

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.

The technique of attenuation measurement using an optical time domain reflectometer (OTDR) was introduced in 1976. Over the years the features, size, and performances of OTDRs have steadily improved. Today the OTDR is a well-established, mature, and ubiquitous test set. The success of the OTDR is due to its unique ability to characterize key parameters of an optical- fiber link, such as the fiber attenuation coefficient, fiber length, and splice loss and location. These measurements can be made having access to one end of the fiber only. Despite a widespread reliance upon OTDR measurements, instrument performance, specification, and calibration remain unstandardized. Standardized OTDR calibration procedures are still missing even for most fundamental parameters. This lack of standardization confuses the OTDR users community when selecting an OTDR, interpreting measurement results, and comparing measurements made with different OTDRs. In 1988 the International Electrotechnical Commission (IEC) Technical Committee 86 (TC 86) Working group 4 (WG 4): Metrology, began work on OTDR calibration techniques concerning attenuation and distance measurements. As one of the basic laboratories of the Bureau National de Metrologie (BNM) the Laboratoire Central des Industries Electriques (LCIE) is designated to work in the field of fiber optics. As a national representative it develops and/or sets new methods for standardization in collaboration with national and international partners. This paper presents the techniques related to research leading to a French proposal for IEC/TC 86/WG 4 OTDR calibration. The methods worked out by the French group are passive techniques. Their aim is to permit OTDR calibration in order to reduce resulting effects of uncertainties arising from fiber characteristics. Two complementary methods are presented: (1) A coupler method proposed by the Centre National d'Etudes des Telecommunications (CNET). (2) A cavity method proposed by the Centre d'Etudes et de Recherches de Toulouse (CERT).© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Pencil-lead-break transient detecting is demonstrated by phase- optical time domain reflectometer based on coherent detecting and data processing. By using coherent detecting technology, combined with Erbium-doped Optical Fiber Amplifier and high power load Acousto-optical Modulators (AOM), we improves signal detecting sensitive and Signal-noise-ratio by 2∼3dB coherent detection advantage over direct detection of phase-OTDR. Location of pencil-lead-break transient signal was detected with higher spatial resolution in time domain by averaged traces transiently. Using varying power with time of the location, frequency information can be obtained by fast Fourier transforms (FFT) analyzer. With loop geometry fiber, as short as 0.25cm fiber can give effective signal over 12-km long single-mode fiber and as high as 50 kHz frequency component can be detected.

Two methods employed during fusion splicing as well as two methods for calculating the average splice loss per fiber-span are presented. The two splicing methods described are the telemetry and optical time domain ref lectometer (OTDR) methods. The cut-back and OTDR measuring methods for calculating the average splice loss per fiber-span are also described. A statistical model relating the various measuring uncertainties with the sample size uncertainties is used to predict the population parameters from the sample statistics. Laboratory results are also included to analyze the important parameters needed for splice optimization. The accuracy of splice loss measurements obtained using OTDRs is studied in the laboratory and is used to analyze field data. Our results show that there is no significant difference in the quality of the telemetry splicing method as compared to the OTDR splicing method. In addition, the average splice loss per fiber-span calculated from cut-back measurements was found to yield a distribution that is only 80% as accurate as the one derived from the OTDR measurements.

This work is concerned with research of Haar wavelet threshold sensitivity for detection of non-reflective events corresponding to optical fiber fusion splices on fragments of fiber optic link traces measured by optical time domain reflectometer (OTDR). We performed test series contained 40 step-by-step repeatable splicing of the same couple of standard singlemode fibers of ITU-T Rec. G.652 spools with length 2.2 km and 4.2 km and outer termination by conventional singlemode FC/UPC pigtails for OTDR connection and following bidirectional trace measurement. Results of insertion loss estimation and accomplished by OTDR software automated detection of non-reflective event corresponding to fusion splice of described optical fibers as well as ability of its visual manual localization on trace were recorded. The first 30 splices were realized by fusion splicer conventional program for jointing standard singlemode optical fibers of ITU-T Rec. ITU-T G.652, while the next 10 splices were done under “attenuator” mode with preset increased insertion loss. Described link traces were measured by OTDR operating in a standard mode with high spatial resolution under pulse width 20 ns and appropriate dynamic range with far-end OSNR 6.5 dB at wavelength 1550 nm. As a result 80 traces were analyzed in both automated and manual modes with fusion splice insertion loss varied from 0.002 dB to 0.554 dB. At the next stage we applied continues Haar wavelet transform for detection non-reflective event on measured traces of lab fiber optic link containing described one fusion splice with total length about 6.4 km. According to results, wavelet transform of described link trace provides detection of extremely low loss non-reflective events with OTDR reading down to own error corresponding to 0.020…0.030 dB range under improved scaling over normalized spatial diagram of wavelet coefficients.

Chapter 15 - Considerations when selecting an OTDR

In recent years, the demand for safe coalbed methane pipelines has dramatically increased in our society. With the superior advantages of large-scale monitoring, low cost, high sensitivity, quick response and immunity to electromagnetic interference, distributed optical fiber sensors provide an appropriate solution for large scale pipeline safety monitoring. Among them, Brillouin optical time domain reflectometer (BOTDR) and phase sensitive optical time domain reflectometer (Φ-OTDR) are fast developing scientific techniques and attract a significant amount of research attention. A multi-parameter measurement scheme based on BOTDR and Φ-OTDR is designed to monitor the strain and vibration conditions of the pipeline, respectively. In the BOTDR system, the pulse probe light modulated by an acoustic optical modulator (AOM) is launched into the sensing optical fibers and the beat frequency signal is obtained through coherent detection. Then, the Brillouin gain spectrum is figured out via heterodyne frequency sweeping. Ultimately the curve of Brillouin frequency shift versus sensing fiber length is plotted to realize the distributed strain measurement. In the Φ-OTDR system, light pulses from a laser with narrow linewidth and low frequency drift are gated into the fiber through an AOM, then amplified by erbium-doped fiber amplifier (EDFA), and finally coupled into the sensing fiber via a circulator. The Rayleigh backscattered light are monitored with a photodector (PD). The vibration position can be detected by subtracting the post-vibration Φ-OTDR trace from an earlier stored trace. The experimental results demonstrated that over a fiber sensing length more than 10 km, the accuracy of Brillouin frequency shift at the far end of the sensing fiber was ±0.88 MHz, which corresponds to the strain accuracy of 40 µe, and a vibration measurement with a positional accuracy less than 20m is also achieved, which can satisfy the practical requirements and shows considerable application potential for coalbed methane pipelines safety monitoring.

An effective measurement method for investigating the transmission characteristics of an optical fiber segment using automated backscattering techniques is described in this paper. Three different Optical Time Domain Reflectometers (OTDRs) are employed in the manual and automated modes of operation to characterize several single mode fibers. Length dependent optical loss measurements are made on several precision-wound fiber optic bobbins (containing many layers) by setting the distance markers at the start and end locations which bound the desired fiber segment. Discrepancies are observed in the data acquired using the manual vs the automated procedure. Apparent round-off errors and inaccurate measurements are observed when operating the OTDR in the automated mode due to the elimination of the operator's observations and adjustments. Algorithms are written to automatically frame the fiber segment of interest by monitoring the actual OTDR marker settings and adjusting the marker setting request(s) when the fiber segment boundaries are violated by the OTDR. This novel framing technique ensures consistent and precise measurements. The technique can provide assistance in using OTDRs in the automated mode of operation.

This paper introduces a novel pulse-compression optical time domain reflectometer (OTDR) by utilizing linear frequency modulation (LFM) pulse-compression technology. The working principle of the pulse-compression OTDR is demonstrated. The spatial resolution is determined by the sweeping range of the pulsed LFM instead of the pulse width, which solves the dilemma of spatial resolution and measurement range in the conventional OTDR. A preliminary experiment of the pulse-compression OTDR is performed, providing 55 cm spatial resolution and 5.4 km measurement range under 2 μs pulse, 221 MHz LFM sweeping range, and 100 kHz linewidth laser diode. It is expectable to achieve a spatial resolution of less than 1 cm and dozens-kilometer measurement range by tens of GHz LFM sweeping ranges via microwave photonics.

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.

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.

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.