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Early detection of potentially unreliable areas in optical fibers makes it possible to detect degradation of optical fibers of fiber-optic communication lines at an early stage. Early diagnostics of the physical state of optical fibers located in the laid optical cables of telecommunication systems is an important topical task. The paper presents adaptive algorithms that allow one to determine the maximum of the Mandelstam–Brillouin scattering spectrum (Brillouin frequency shift), and then the degree of tension of optical fibers. The process of determining the Brillouin frequency shift, whose values at wavelengths of laser radiation used in telecommunication systems belong to the microwave range, can be significantly accelerated if adaptive algorithms for obtaining and processing data are implemented due to a special choice of the step for the scanning frequency and the time of accumulation of measurement results. The adaptive algorithms considered in this paper make it possible to speed up the process of obtaining output results in Brillouin reflectometers by ignoring readings that do not significantly affect the final characteristics. The construction of approximate graphs of the distribution of the spectrum and tension along the length of the fiber allows the trained user of the Brillouin reflectometer to stop the analysis process in order to make corrections to the measuring process (selection of the scanning range in frequency, change in the scanning step in frequency, choice of the accuracy of the presentation of output results, change in spatial resolution, etc.), which also speeds up the testing of the selected fiber. The measurement process can also be accelerated by adaptively changing the number of averages. With a database of measured Mandelstam–Brillouin scattering characteristics of optical fibers of various types and manufacturers, the speed of obtaining tension plots can also be increased. Since the approximate value of the Brillouin frequency shift is calculated already at the initial steps of the measurement process, preliminary dependences of the tension distribution along the length of the fiber will be constructed rather quickly. [ABSTRACT FROM AUTHOR]

The problem of information protection against unauthorized access is considered and the results of a comparative analysis of methods for extracting information from an optical fiber are presented. A method for detecting the formation of channels of information leakage, which is performed in various ways, is proposed for the successful struggle against unauthorized access. Among the analyzed methods for forming information leakage channels in a fiber-optic communication line, the simplest method involves bending the optical fiber to disturb the conditions for the total internal reflection. In this case, the monitoring system must track the introduced losses made in order to detect fiber bends. [ABSTRACT FROM AUTHOR]

A module for recording single-photon pulses is described. It is able to reliably operate in the mode of single-photon counting at a wavelength of 1550 nm (the minimum attenuation in quartz fibers). For this purpose, unique avalanche photodetectors (APDs) based on InGaAs/InP heterostructures with the three-stage cooling to temperatures of -50°C via Peltier elements are used during reception. Test measurements of the level of noise counts, including both dark pulses and afterpulses, have been performed using the developed testing system. The dependences of the recorded noise counts on the duration of gate pulses have been obtained. The nonlinear character of these dependences is evidence of the difference in the contributions of the afterpulsing effect and the dark pulses of the APDs to the noise counts. At short durations of the gate pulses, the major contribution to the noise counts is made by dark pulses of the APDs. [ABSTRACT FROM AUTHOR]

The design and results of studies of a fiber-optic-link-based transmission system for transmitting analog signals of nanosecond duration using an external modulation of radiation are presented. Experimental studies that were performed at the Dukhov All-Russia Research Institute of Automatics demonstrated a method for determining the transmission function of an intensity modulator, which is based on the Mach-Zehnder interferometer, simultaneously with the signal transmission. The dynamic range of the system with one intensity modulator is ~70 when recording a single-shot process. The dynamic range of the transmission system with two modulators that form two information channels exceeds 200 with a bandwidth that exceeds 4 GHz and a signal-to-noise ratio of at least 5 for the minimum recorded signal. [ABSTRACT FROM AUTHOR]