We present experimental results of continuous wavelength measurements for narrow linewidth laser in C -band. Designed setup is based on a phi-OTDR scheme and can be added as a module of DAS system for laser frequency drift measurement and compensation. Wavemeter uses multi-base Fizeau interferometers with passive or active thermal stabilization. They are realized on weak fiber Bragg gratings (wFBGs) inscribed in single fiber line located in compact thermostat. Achieved accuracy of setup was about 100 fm with 1 kHz discretization frequency. Possible improvements can allow reaching about 10 fm accuracy with more than 10 kHz sampling frequency.

We report on a study of distributed fiber-optic temperature sensor based on Raman scattering, in which an ultrashort-pulse fiber laser is used as a source of optical probing pulses. Together with the short pulse duration and pulse repetition rate decimation scheme using an acousto-optical modulator, we were able to achieve experimentally 0.3 m spatial resolution over more than 8 km long fiber-under-test.

Accurate laser spot-size measurements are important in many scientific and industrial applications: the knife-edge technique is commonly used to measure the laser beam dimension due to the relative ease, low cost, and reliability of the measurement. We propose an original use of a numerical method to evaluate the standard uncertainty of the measured laser spot-size in knife-edge measurements and we apply it to the analysis of experimental data from two different laser sources. The method allows obtaining the sensitivities of the spot-size uncertainty to the uncertainties of relevant input quantities such as the measured optical power and the knife-edge displacement. Since the indirect measurement of laser spot-size by the knife-edge technique is non-linear, analytical calculation of the uncertainty sensitivities is not possible and they are evaluated numerically based on experimental data and specific input noise/uncertainty conditions. By our analysis, calculation of specific sensitivities for two different experimental conditions is done and then used to estimate the standard uncertainty of the measured parameter: the standard radius of a Gaussian-beam laser spot. More generally, the method proposed allows evaluating the effects of different uncertainty contributions on the laser spot-size uncertainty estimation. This can then suggest proper design of the measurement experiment with the goal of reducing the final uncertainty. Accurate measurements, and of known and tailorable accuracy, of the laser parameters are of interest in different industrial applications where precise beam shaping is important.

The paper presents the application of a phase-sensitive optical time-domain reflectometer (phi-OTDR) in the field of urban infrastructure monitoring. In particular, the branched structure of the urban network of telecommunication wells. The encountered tasks and difficulties are described. The possibilities of usage are substantiated, and the numerical values of the event quality classification algorithms applied to experimental data are calculated using machine learning methods. Among the considered methods, the best results were shown by convolutional neural networks, with a probability of correct classification as high as 98.55%.

Distributed optical fiber sensors are important for continuous remote monitoring of large infrastructures, such as gas and oil pipelines, civil controlled perimeters, dams, roads, railroads, and also telecommunication networks. We present the theoretical study and practical implementation of a phase-sensitive distributed fiber sensor, capable of real-time monitoring of an urban area telecommunication network. The traditional data processing is here enhanced by CNN machine learning algorithms providing recognition accuracy >98 % for different perturbation or intrusion processes in the inspection wells located along the fiber network. In addition to this specific application to a telecommunication network, other applications of this fiber sensor are possible -and also already implemented- for gas&oil pipelines, railroads, power plants, and transportation and industrial sites in general.

The influence of temperature fluctuations of the reference section of an optical fiber as part of the instrumental part on the absolute measurement error of a distributed fiber-optical temperature sensor was performed. The design of the reference section with active temperature control with high stability as part of the instrumental part of the sensor is proposed and experimentally investigated. The efficiency of using active temperature control to improve the repeatability of measurements and reduce the measurement error has been experimentally demonstrated.

We demonstrated a fiber optic distributed acoustic sensor based on a double Sagnac interferometer, using two wavelengths separated by CWDM modules. A mathematical model of signal formation principle, based on a shift in two signals analysis, was described and substantiated mathematically. The dependence of the sensor sensitivity on a disturbance coordinate and frequency was found and simulated, and helped determine a low sensitivity zone length and provided sensor scheme optimization. A data processing algorithm without filtering, appropriate even in case of a high system noise level, was described. An experimental study of the distributed fiber optic sensor based on a Sagnac interferometer with two wavelengths divided countering loops was carried out. An accuracy of 24 m was achieved for 25.4 km SMF sensing fiber without phase unwrapping.

We report on a study of a new scheme for acoustic sensitivity measurements of fiber optic cables. It uses a Fizeau interferometer scheme with weak fiber Bragg gratings as a mirrors. It's advantage is the absence of interferometer reference arm which exclude noise influence on it and on other parts of scheme.

In this study, an experimental study of the burning rate of solid fuel in a model solid propellant rocket motor (SRM) E-5-0 was conducted using a non-invasive control method with fiber-optic sensors (FOSs). Three sensors based on the Mach–Zehnder interferometer (MZI), fixed on the SRM E-5-0, recorded the vibration signal during the entire cycle of solid fuel burning. The results showed that, when using MZI sensors, the non-invasive control of solid fuel burnout is made possible both by recording the time of arrival of the combustion front to the sensor and by analyzing the peaks on the spectrogram of the recorded FOS signal. The main mode of acoustic vibrations of the chamber of the model SRM is longitudinal, and it changes with time, depending on the chamber length. Longitudinal modes of the combustion chamber were detected by MZI only after the combustion front passed its fixing point, and the microphone was unable to register them at all. The results showed that the combustion rate was practically constant after the first second, which was confirmed by the graph of the pressure versus time at the nozzle exit.

We report on a scalable chirped-pulse Er-doped all-fiber laser, passively mode-locked by single-wall carbon nitride nanotubes. The average output power is ~15 mW, which corresponds to a peak power of ~77 W, and pulse energy of ~1.9 nJ and was achieved using a single amplification stage. We observed chirped-pulse generation with a duration of ~24.6 ps at a relatively low repetition rate of ~7.9 MHz, with a signal-to-noise ratio of ~69 dB. To characterize the short-term stability of the obtained regime, we have measured the relative intensity noise of the laser, which is <−107 dBc/Hz in the range of 3 Hz–1000 kHz. It should be noted that the standard deviation of root mean square of average power does not exceed a magnitude of 0.9% for 3 h of measurement.

We present a theoretical and experimental study in which we increased the sensitivity of a phase-sensitive optical time-domain reflectometer (phi-OTDR). This was achieved by constructing coils in the sensor cable, which increased the total amplitude of the impact on the fiber. We demonstrate this theoretically using the example of a phase-sensitive reflectometer model and practically in testing grounds with a buried nearby conventional sensor and a sensor with coils. The sensitivity increased 2.2 times. We detected 95% of events when using coils, versus 20% when using a straight cable. The suggested method does not require any modifications to the device.

We propose a new scheme of a fiber optic distributed acoustic sensor based on two same-length loops of a Sagnac interferometer formed with wavelength division multiplexing, which makes the sensor compact, easy to mount along a perimeter, and both suitable when ring and straight line cable laying application is required. We also describe a dual Sagnac interferometer based sensor operation principle, which allows a disturbance localization by estimation of a time delay, occurring between signals from two loops. An issue of assembling in a single cable an interferometer based sensor systems is studied as an approach of a phase difference occurring between clockwise and counter-clockwise light directions in a Sagnac interferometer loop. A scheme for investigation of the light phase incursion due to disturbance in two directions in the loop is proposed and mathematically substantiated. A potential of implementing the proposed sensor based on Sagnac interferometer with wavelength division multiplexing in a single cable has been proven experimentally.

We have studied optical comb peculiarities of a high-energy chirped-pulse Er-doped all-fiber laser. A relatively low repetition rate of ~ 7.9 MHz with signal-to-noise ratio ~ 69 dB and a pulse width of 24.6 ps was achieved. To characterize the short-term stability of the obtained regime we have measured the relative intensity noise of the laser, which is <; -107 dBc/Hz in the range of 3 Hz -1 000 kHz. The output average power is ~15 mW which corresponds to ~ 77 W peak power and ~1.9 nJ pulse energy.

We perform the first results of experiments aimed at the design of a high-spatial-resolution distributed temperature sensing system (DTS) based on an ultra-short pulse mode-locked fiber laser. We have developed an experimental prototype of DTS with the spatial resolution of ~ 0.5 m, sensor length of 3 m and ±1 °C temperature measurement error.

Weak fiber Bragg gratings (WFBGs) in a phase-sensitive optical time-domain reflectometer (phi-OTDR) sensor offer opportunities to significantly improve the signal-to-noise ratio (SNR) and sensitivity of the device. Here, we demonstrate the process of the signal and noise components' formation in the device reflectograms for a Rayleigh scattering phi-OTDR and a WFBG-based OTDR. We theoretically calculated the increase in SNR when using the same optical and electrical components under the same external impacts for both setups. The obtained values are confirmed on experimental installations, demonstrating an improvement in the SNR by about 19 dB at frequencies of 20, 100, and 400 Hz. In this way, the minimum recorded impact (at the threshold SNR = 10) can be reduced from 100 nm per 20 m of fiber to less than 5 nm per 20 m of fiber sensor.

The method of a spatial resolution improvement for common phi-OTDR system is proposed. The suggested method allows to decrease the size of acquisition points in the fiber sensor due to the special arrangement of weak Fiber Bragg Gratings (wFBG). The scheme of phi-OTDR doesn't need any changes, moreover it does not possess the limitation of an acousto-optic modulator switching frequency.

We present a chirped pulse erbium-doped all-fiber hybrid mode-locked laser as a source of high-energy probe pulses in a Raman distributed temperature sensor with high spatial resolution.

In recent times, phase-sensitive optical time domain reflectometers became widely common for monitoring of extended objects and providing necessary measurements in optical telecommunications. In this paper we present our investigations and results in the development and characterization of a new type of laser for phase-sensitive optical time domain reflectometry.

The testing of an all-fiber erbium ultrashort pulsed laser in a distributed fiber temperature sensor as a source of probing pulses has been performed. Among the prospects of such an approach are an improved signal–noise ratio in the receiving system and a better spatial resolution of the temperature sensor. The experiments have revealed the factors that limit the effective length of the fiber temperature sensor, such as a high pulse repetition frequency and intrinsic laser noises. As a result of the performed work, the distributed fiber optical temperature sensor with a near-room-temperature resolution of ~1.5 K, an effective length of ~3 m, and a spatial resolution of ~10 cm has been developed.

The influence of the laser frequency drift on the operation of phase-sensitive optical time domain reflectometry (φ-OTDR) systems is considered. Theoretical results based on a new numerical φ-OTDR model demonstrating the influence of the laser frequency instability on a signal are reported. This model is verified based on experimental data. It has been used to calculate the signal-to-noise ratio (SNR) of the system for different parameters of the laser source stability. As a result, quantitative requirements for lasers used in φ-OTDR systems are formulated.