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

Adriana Lipovac,Vlatko Lipovac,Vedran Batoš,Mirza Hamza

doi:10.3390/s21217303

Abstract

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.

Highlights

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The below tables contain the BER/Block Error Ratio (BLER) entries as the overpesimistic “worst‐case scenario“ values obtained by applying the well‐known BER expression for the M‐QAM signal transmission over the additive white Gaussian noise (AWGN) channel, as a function of the ratio of the energy Eb of a bit to noise spectral density N0, is [10]:
From the optical time-domain reflectometer (OTDR) trace, the actual SNR value at the far end, is read and substituted into (21) abstracting the actual BER by the AWGN‐only based one

Summary

Introduction

Considering test and measurement equipment used during installation and maintenance of contemporary lightwave transmission systems, optical time-domain reflectometer (OTDR) is still an outstanding fault detection and localization tool enabling simple and integral characterisation of a variety of refractive and reflective events on a fiber link, such as splices, connectors and breaks, with according measurements of insertion/return loss [1,2,3,4], Figure 1.Basically, OTDR transmits a high-power laser light pulse into the fiber, and detects reflections from various structural irregularities along the fiber (backscatter), describing them in the power-loss-versus-distance format, i.e., expressing the time axis in distance units by inherently taking into account the speed of light in the fiber [5,6,7,8].The main functional OTDR entities are: microprocessor, pulse trigger and generator, laser diode, optical directional coupler (ODC), detector, analog-to-digital converter (ADC) and display, Figure 1.The test is initiated by the microprocessor executing the according instructions and sending the control signals to the pulse generator to trigger the laser. OTDR transmits a high-power laser light pulse into the fiber, and detects reflections from various structural irregularities along the fiber (backscatter), describing them in the power-loss-versus-distance format, i.e., expressing the time axis in distance units by inherently taking into account the speed of light in the fiber [5,6,7,8]. The main functional OTDR entities are: microprocessor, pulse trigger and generator, laser diode, optical directional coupler (ODC), detector, analog-to-digital converter (ADC) and display, Figure 1. The test is initiated by the microprocessor executing the according instructions and sending the control signals to the pulse generator to trigger the laser. The outgoing series of laser light pulses transverses the ODC on their way into the fiber under test, whereas the reverse backscatter incoming signal is channeled by the ODC towards the detector—most often the avalanche photodiode (APD). The received signal proceeds through the ADC to the microprocessor for final analysis and display, which includes statistical averaging to improve the signal-to-noise ratio (SNR), and enhance the Sensors 2021, 21, x FOR PEER REVIEWOTDR trace by displaying average power of a number of sampling points, rather tha2notfh1e2 instantaneous values, Figure 2

Objectives

Results

Conclusion