The presence of different modulators within a cavity often leads to increased losses and is bound by temporal limitations. Therefore, it is reasonable to investigate alternative optical approaches to modulate and control radiation parameters. We propose an optical method for laser radiation modulation based on the use of an excited-state absorption (ESA) transition in a laser active medium under the action of resonant radiation from an external control source. The special features of this method are demonstrated by numerical simulations based on a generalized laser model with the minimum number of parameters. A comparison with the data obtained by detailed simulation of the generation dynamics of a holmium fiber laser demonstrated the matching of the key physical features. The obtained data allow us to understand the limitations of such an approach for different types of lasers. The experimental results confirm the successful application of the proposed method in a holmium fiber laser under exposure by the external pulsed control radiation at lambda(e) 1.6 mu m, corresponding to the ESA transition I-5(7)-> I-5(5) in holmium-doped fiber.

Исследована возможность оптической стабилизации коэффициента усиления эрбиевого волоконного усилителя с удаленной накачкой (ROPA) и распределенного ВКР-усилителя. Основой рассматриваемых систем оптической стабилизации является активный спектрально-селективный резонатор, генерирующий излучение на одном из не используемых спектральных каналов усилителя. Показано, что для ROPA предложенная система дает возможность поддерживать коэффициент усиления постоянным при изменении температуры, числа и мощности каналов. Для распределенного ВКР-усилителя обеспечивается постоянный коэффициент усиления при изменении числа каналов, постоянная выходная мощность при изменении потерь в линии.

We demonstrate the threshold increase of Modulation Instability in phase-sensitive optical time-domain reflectometer by implementing 10 GHz-bandwidth optical filter.

In this Letter, we report a high-sensitive singleended distributed acoustic sensor based on a phase-sensitive OTDR with the 130 km operating distancealong standard single mode telecom fiber. This sensing range is achieved by incorporating a single erbium-doped fiber segment and remotely pumping it using same sensing fiber at a distance of 98 km from the near end.

We have demonstrated optical switching of generation in the Holmium-doped fiber laser. The generation switching from continuous-wave to pulsed regime was performed by external pulsed radiation with a wavelength of $\approx 1.6 \mu \mathrm{m}$ due to excited state absorption from ${ }^{5} I_{7}$ level.

We study statistics of signal power spectrum ripple in a chain of erbium-doped fiber amplifiers (EDFAs). It is found that allowance for the statistical properties of EDFA gain ripple can markedly increase the accuracy of estimating the magnitude of spectrum ripple of a multichannel signal (by 4 dB in a chain of five EDFAs). By analyzing the simulation results for various configurations of fiber-optic communication lines (FOCLs), an optimal equalizer spacing period is estimated. It is shown that the signal quality deterioration in a FOCL caused by the EDFA gain ripple depends on the number of spans between devices flattening the signal spectrum, and on statistical characteristics of gain spectrum ripple of the used EDFAs; however it is practically independent of the number of spectrum equalizing sections, signal parameters, and span length. The simulation results are used to construct a model for calculating the penalty from accumulated EDFA ripple. An algorithm is proposed for estimating the penalty, applicable for an arbitrary FOCL configuration with known statistics of EDFA accumulation ripple.

В работе методом численного моделирования исследуется устойчивость работы алгоритмов цифровой обработки сигналов (ЦОС) когерентного приемника к быстрому вращению поляризации, вызванному ударом молнии в оптический кабель в грозотросе (ОКГТ). Установлено, что скорость изменения состояния поляризации (СП) является наиболее важным параметром, определяющим рост коэффициента ошибок (BER) в приемнике. Детальный анализ работы алгоритмов ЦОС в условиях быстрого вращения СП показал, что причиной роста коэффициента ошибок является ограниченное быстродействие блоков пересинхронизации и адаптивной фильтрации.

Приведен краткий обзор применений распределенных акустических сенсоров для решения геофизических и сейсмометрических задач. Получены теоретические оценки и экспериментальные измерения уровня собственных шумов в распределенных акустических сенсорах, в том числе на субгерцевых частотах. Проведено численное моделирование, позволяющее количественно оценить мощность низкочастотного шума (связанного с медленным изменением температуры зондируемого волокна) в сигнале распределенных акустических сенсоров. Полученные результаты дополнены теоретическими оценками спектральной мощности сигнала от удаленного землетрясения, они демонстрируют важность учета температурных эффектов в волокне при планировании экспериментов, связанных с регистрацией слабых сейсмических событий .

An optical gain clamping system for forward remote optically pumped amplifier (F-ROPA) was experimentally studied. To implement gain clamping, spectrally selective optical feedback within one channel is created. When the unsaturated gain exceeds the losses in the feedback circuit, oscillation occurs with gain clamped at a level exactly compensating for the losses in the feedback circuit. It is established that the ROPA gain in the optical clamping mode changes by no more than 0.17 dB when the pump laser power varies in the range from 21.5 to 29 dBm, 0.43 dB when the input signal power modulates from 2 to 10 dBm, 0.24 dB when the ambient temperature changes in the range from –40 to +50°C. It is shown that optical gain clamping does not lead to distortion of the gain spectrum, and the gain can be controlled with high accuracy by changing losses in the feedback cavity. The results obtained indicate the prospects of using optical gain clamped F-ROPAs in optical communication lines with long spans.

— The operating range of a distributed acoustic sensor based on a phase-sensitive optical time-domain reflectometer has been increased using an erbium-doped fiber amplifier with remote forward pumping. It is shown that by incorporating a single segment of erbium-doped fiber at a distance of 70 km and pumping it from the front end by a 500-mW laser at a wavelength of 1480 nm over the sensing fiber, it is possible to increase the operating range of the reflectometer by 45 km and, thereby, obtain the total operating range as large as 120 km along a standard single-mode fiber.

This work is a review of the fundamental noise processes affecting the operation of phase-sensitive coherent reflectometers. The presented material can be used for educational purposes. We present the results of measurements using artificially modified single mode fibers with arrays of artificial scatterers distributed along the length of the fiber. Their advantages compared to conventional fibers are demonstrated and analyzed. The relationship between the phase noise of a probing laser and the phase noise in the reflectometer signal is shown.

Исследована возможность увеличения дальности работы распределенного датчика "Дунай" разработки компании "Т8 Сенсор" на базе фазочувствительного когерентного импульсного рефлектометра путем установки в волоконно-оптическую линию эрбиевого волоконного усилителя с удаленной накачкой. Проведен анализ зависимостей мощности обратного рассеяния и отношения сигнал-шум фототока от расстояния вдоль волокна. Показано, что установка в линию эрбиевого волоконного усилителя с удаленной накачкой позволяет увеличить дальность работы датчика на 45 км (с 75 до 120 км).

A new method to measure laser phase noise and frequency stability based on the phase-sensitive optical time-domain reflectometry is proposed. In this method, the laser under test is utilized in a phase-sensitive optical time-domain reflectometer, which employs phase-modulated dual pulses and acts as an optical frequency discriminator: laser frequency fluctuations are deduced from the analysis of the reflectometer data corresponded to phase fluctuations along the vibration-damped and thermally insulated fiber spool. The measurement results were validated by comparison with direct optical heterodyning of the tested and more coherent reference lasers. The use of dual pulses generated by an acousto-optic modulator makes it easy to adjust the time delay during measurements, which distinguishes favorably the proposed method from standard optical frequency discriminators. The method is suitable for testing highly coherent lasers and qualifying their parameters such as linear drift rate, random frequency walk rate, white frequency noise (which is directly related to laser instantaneous linewidth), and flicker noise level.

A technique for measuring the interchannel distortions arising in fiber-optic communication lines with QAM/CD and IM/DID channels and a way to describe these distortions in the form of additive noise are proposed. Measurements based on the proposed technique are performed in a wide range of conditions, and an empirical model is elaborated to estimate the deterioration of coherent signal quality because of the effect of IM/DID channels.

— We consider technologies that allow the throughput of fiber-optic transmission systems (FOTSs) to be increased. The need for this is due to a growth in the volume of newly available information, virtualization of network applications, development of cloud services, and consolidation and construction of new data centers. Dense wavelength division multiplexing (DWDM) and introduction of coherent detection with digital signal processing have made it possible to most significantly increase the FOTS capacity. The main directions of FOTS development are the improvement of active equipment and optical fiber lines. This paper is devoted to the trends in the development of active FOTS equipment.

Absorption of continuous wave and pulsed resonance radiation at a wavelength of 1.6 µm at the upward transition from the excited state $^{5}{\mathrm{I}}_{7}$ of holmium (Ho 3+ ) ions in silica-based fiber has been studied. Analysis of experimental and simulated data of the absorption dynamics for the case of pulsed pumping of Ho-doped fiber allowed us to determine the previously unknown value of the absorption cross-section at the transition $^{5}{\mathrm{I}}_{7}{ \to }^{5}{\mathrm{I}}_{5}$ as 1.9×10 –21 cm 2 . It allowed us to correctly simulate the relaxation process of the $^{5}{\mathrm{I}}_{7}$ level population averaged over the fiber length in the presence of stationary resonance radiation. The effect of pulsed resonance radiation with different peak powers (10 and 100 W) and pulse duration of 1 μ s on the stationary active medium was considered. The mutual influence of the Ho-doped fiber characteristics and the resonance radiation at a wavelength of 1.6 μm was found. This allows the resonance radiation to be used as an attenuator or modulator of laser radiation at the $^{5}{\mathrm{I}}_{8}{ \to }^{5}{\mathrm{I}}_{7}$ transition in Ho-doped fibers.

A brief review of the applications of distributed acoustic sensors for solving geophysical and seismometric problems is given. Theoretical estimates and experimental measurements of the level of self-noise in distributed acoustic sensors, including those at subhertz frequencies, are obtained. Numerical modeling has been carried out, which makes it possible to quantify the power of low-frequency noise (associated with a slow temperature change of the probed fiber) in the signal of distributed acoustic sensors. The obtained results are supplemented by theoretical estimates of the power spectrum of a signal from a distant earthquake; they demonstrate the importance of taking into account temperature effects in the fiber when planning experiments related to the registration of weak seismic events.

A numerical experiment for light passing through the section of the optical ground wire during a lightning strike events was carried out. The “isotropy effect” for areas in state of polarization rotation speed time profiles was discovered and explained. This effect consists in the fact that time profiles shapes of anisotropic and isotropic fibers practically coincide in short zones at the beginning and the end of lightning current rapid increase interval (lightning front), but it differ significantly in the middle part. It is shown that such a behavior of time profile is determined by the smallness of the length of interaction of light with the region of rapid change of the magnetic field at the beginning and the end of the lightning front, while the effect of the magnetic field on short sections of the anisotropic fiber practically coincides with the effect on sections of isotropic fiber of the same length. Using the model of light propagation in a fiber with a random distribution of linear birefringence, located in the magnetic field of lightning, a formula for calculating the matrix rotation speed of the polarization state for an extended fiber is obtained.