Optical Time-domain Reflectometry Trace Research Articles

Mitigation of the parallel-path effect for reliable monitoring of a passive optical network using standard optical time domain reflectometry

Optical access networks evolve towards higher bit-rates, which is driven by a growing number of fiber-to-the-home users and by bandwidth-demanding mobile transport. Maintenance requirements of such networks call for very effective and efficient supervision of the fiber infrastructure. Moreover, since the fiber is also known for its sensing capabilities, the outside distribution plant of the access network has been recently considered for providing sensing services co-existing with telecom services. To enable reliable fiber monitoring and sensing, the detrimental impact of the parallel-path effect in point-to-multipoint access networks needs to be addressed. This paper introduces the concept of serialization of an optical time domain reflectometry trace of a point-to-multipoint passive optical network, which radically reduces the parallel-path effect impact on fiber anomaly detection and measurement. The concept is experimentally proven in a representative access network architecture.

Optical Fiber Fault Detection and Localization in a Noisy OTDR Trace Based on Denoising Convolutional Autoencoder and Bidirectional Long Short-Term Memory

Optical time-domain reflectometry (OTDR) has been widely used for characterizing fiber optical links and for detecting and locating fiber faults. OTDR traces are prone to be distorted by different kinds of noise, causing blurring of the backscattered signals, and thereby leading to a misleading interpretation and a more cumbersome event detection task. To address this problem, a novel method combining a denoising convolutional autoencoder (DCAE) and a bidirectional long short-term memory (BiLSTM) is proposed, whereby the former is used for noise removal of OTDR signals and the latter for fault detection, localization, and diagnosis with the denoised signal as input. The proposed approach is applied to noisy OTDR signals of different levels of input SNR ranging from -5 dB to 15 dB. The experimental results demonstrate that: (i) the DCAE is efficient in denoising the OTDR traces and it outperforms other deep learning techniques and the conventional denoising methods; and (ii) the BiLSTM achieves a high detection and diagnostic accuracy of 96.7% with an improvement of 13.74% compared to the performance of the same model trained with noisy OTDR signals.

Reduction of modal evolution fluctuation in 2-LP mode optical time domain reflectometry.

This paper describes theoretical and experimental investigations of an optical time domain reflectometry (OTDR) that conducts measurements in the 2-LP mode region of the fiber under test. Rayleigh backscattering amplitude coefficients from the HE11 mode to higher-order vector modes are analyzed to clarify the evolution in the state of polarization of the backscattered vector modes with respect to that of the probe HE11 mode. Based on the analysis, we then propose a technique for reducing amplitude fluctuations in the OTDR traces attributed to intensity distribution evolution of the LP11 mode. A proof-of-concept demonstration is performed on optical fibers in the laboratory and field environments, and the measured results confirm its usefulness.

Distributed optical fiber vibration sensor based on Sagnac interference in conjunction with OTDR.

A real-time distributed optical fiber vibration sensing prototype based on the Sagnac interference in conjunction with the optical time domain reflectometry (OTDR) was developed. The sensing mechanism for single- and multi-points vibrations along the sensing fiber was analyzed theoretically and demonstrated experimentally. The experimental results show excellent agreement with the theoretical models. It is verified that single-point vibration induces a significantly abrupt and monotonous power change in the corresponding position of OTDR trace. As to multi-points vibrations, the detection of the following vibration is influenced by all previous ones. However, if the distance between the adjacent two vibrations is larger than half of the input optical pulse width, abrupt power changes induced by them are separate and still monotonous. A time-shifting differential module was developed and carried out to convert vibration-induced power changes to pulses. Consequently, vibrations can be located accurately by measuring peak or valley positions of the vibration-induced pulses. It is demonstrated that when the width and peak power of input optical pulse are set to 1 μs and 35 mW, respectively, the position error is less than ± 0.5 m in a sensing range of more than 16 km, with the spatial resolution of ~110 m.

Coherent optical time domain reflectometry by logarithmic detection and timed random frequency hopping

A scheme using timed random frequency hopping and signal logarithmic mean method is proposed and experimentally demonstrated in a coherent optical time domain reflectometry (OTDR) system to reduce the fading noise of the OTDR trace and simplify the signal processing procedure. The timed random frequency hopping is realized by randomly changing the driving current of the laser at certain time points. By this method, the fading noise of OTDR trace can be reduced to be 1/5 of that without using it. Also, a radio frequency power detector (RFPD), whose output voltage has linear relationship with the input logarithmic RF power, is used to extract the power of the RF signals from the balanced photodetector. Then, a data acquisition card directly captures and adds the digital voltage signals from the RFPD to reduce the fading noise and improve the measurement dynamic range. Compared with synchronous and asynchronous frequency hopping scheme, the proposed method is of high efficiency.

Optical Time-Domain Reflectometry Simulations of Passive Optical Networks: A Linear Time-Invariant System Approach for Arbitrary Pulses

We present a comprehensive mathematical formalism, together with its numerical implementation, for optical time-domain reflectometry (OTDR) simulations with the ability to produce OTDR traces of passive optical point-to-multipoint networks (PONs). According to our knowledge, the proposed OTDR simulator is the first one with this feature, thus progressing beyond the existing state of the art of OTDR simulations. In our approach, the optical fiber network under test is treated as a linear time-invariant single-input/single-output system, which allows us to include the effects of the OTDR pulse shape. Furthermore, the limitations of current OTDR equipment such as (nonlinear) power saturation of the detector, limited dynamic range due to OTDR noise, and the finite bandwidth of the detector, are also incorporated into the simulation model. Our simulation results are experimentally verified with OTDR measurements, and we show an excellent agreement between the simulated traces and the measured traces. Especially, we demonstrate that the simulations properly handle PONs, that may contain a cascade of splitters, providing an important tool for telecom operators to investigate cost-effective centralized monitoring techniques in fiber-to-the-home networks.

Feasibility of Centralized Passive Optical Network Monitoring Using Passive Optical Network-Tuned Optical Time-Domain Reflectometry

The increasing deployment of passive optical networks implies an equally increasing need for automatic monitoring systems. This article evaluates the feasibility of obtaining reliable information on fault position and loss by direct inspection of the optical time-domain reflectometry trace and via measurements of end-fiber reflectivity devices. Detailed calculations considering parameters often neglected, such as non-uniformity of splitters, fiber bending losses, as well as different Rayleigh backscattering factors and fiber attenuation coefficients, will be provided and experimentally verified. The limits and reliability of automated solutions for optical time-domain reflectometry passive optical network monitoring will be discussed.

Determination of back-scatter coefficient from third-order Rayleigh effect in a Raman amplifier

We propose a precise measurement technique for the Rayleigh back-scattering coefficient based on a single-ended optical time-domain reflectometry trace analysis. By comparing the relative strength of the Rayleigh components-Rayleigh scattering and triple Rayleigh scattering-we show that it is possible to avoid the uncertainties associated with previous measurement techniques, achieving less than 0.1-dB error in the determination of the Rayleigh coefficient. Application of this technique to the noise characterization of the fiber Raman amplifiers will be provided at the end.

Extended optical fiber line testing system using new eight-channel L/U-band crossed optical waveguide coupler for L-band WDM transmission

This paper describes the system design for an extended optical fiber line testing system that uses a new L/U-band crossed optical waveguide coupler and a fiber Bragg grating filter for L-band wavelength-division multiplexing transmission. We describe the reflection characteristic required for optical filters located in central offices in order to suppress the ghost signal caused by multireflection in the optical time-domain reflectometry (OTDR) trace. We design and evaluate an eight-channel crossed optical waveguide coupler with a new thin dielectric film filter that separates a 1650-nm test light from the L-band communication light, and confirm that there was no degradation caused by multireflections in the OTDR trace. We also demonstrate the in-service line monitoring of a 10-Gb/s L-band transmission with no degradation in the transmission quality.

Optical time domain reflectometry using an M-ary FSK probe and coherent detection

This paper proposes a new approach to enhancing the performance of optical time domain reflectometry (OTDR). This approach launches a probe signal modulated in the M-ary FSK format into a test fiber and detects the backscattering by coherent detection, which offers excellent frequency discrimination, followed by simple signal processing to recover the fiber impulse response. Fault location experiments verify the theoretical prediction of that the proposed approach reduces the measurement time to 1/M without sacrificing measurable dynamic range or spatial resolution. They also show that the approach is effective in reducing fluctuations in the OTDR trace. It is experimentally confirmed that OTDR with the proposed approach is feasible for long haul transmission systems.

Stochastic amplitude fluctuation in coherent optimal time domain reflectometry and its reduction technique

AbstractWhen the coherent detection technique is employed in optical time domain reflectometry (OTDR) (C‐OTDR), enhancing the dynamic range is expected to be an effective means of improving the minimum detectable signal level to that of shot noise. In C‐OTDR, however, there are fluctuations in the OTDR trace, and the problem is the degradation of the resolution of the loss measurements. The main reasons for such amplitude fluctuations are: (1) scattering light‐power jags due to the interference between Rayleigh backscattered lights; (2) variations of the heterodyne detection efficiency due to rotations of Rayleigh backscattering polarization; and (3) variations of the detection efficiency due to phase fluctuations of Rayleigh backscattering light.In this paper, such fluctuations are evaluated stochastically, and the techniques for their reduction are proposed. With regard to cause (1), the amplitude fluctuations can be reduced by integration over the frequency of light, while with regard to cause (2), the improvement can be achieved by polarization diversity detection or integration over the polarization. In case (3), phase diversity detection or phase integration is effective in reducing the amplitude fluctuations. Experimental investigations with regard to cause (1) are conducted in detail, and it is shown that the amplitude fluctuations can be reduced by about 3 to 7 times as a result of integration in the frequency domain compared to those without such integration.

Effects of trap levels in single-photon optical time-domain reflectometry: evaluation and correction

Single photon detectors used in optical time-domain reflectometry (OTDR) suffer from carrier trapping phenomena. It is shown that trapping effects can result in a severe OTDR trace distortion, and that accurate measurements of trap releases open the way to the correction of the OTDR data. A simple procedure for offline correction of the experimental traces is presented. Its applicability range is analyzed, using a Monte Carlo program which simulates the actual OTDR experiments. A great improvement of signal-to-noise ratio in single-photon OTDR is expected by exploiting the technique. Requirements for the electronics to be used for these measurements are discussed. >

Optical time-domain reflectometry (OTDR) of diameter modulations in single mode fibers

The influence of several types of diameter variations on the attenuation and on the optical time-domain reflectometry (OTDR) traces of specially prepared depressed cladding SM (single-mode) fibers is experimentally investigated. Depressed-cladding SM fibers have been prepared by the low-pressure plasma-induced chemical vapor deposition (PCVD) process. The core consists of GeO/sub 2/-doped SiO/sub 2/ and the cladding region of fluorine-doped silica. During drawing, the diameter of the fiber is controlled within +or-0.3 mu m for slow perturbations by adjusting the drawing speed (Anritsu M501A). Variations of the core diameter have been accomplished by changing the set point of the fiber diameter control. Core diameter changes give rise to steps or deviations in slope of the OTDR power trace of about 0.3 dB/ mu m. No indication of additional attenuation due to axial fiber parameter variations has been found up to core diameter changes of 12 mu m/m, despite severe distortions of the OTDR traces. >