Subject of study. This paper introduces an original method for calculating the dynamic range of a coherent optical time-domain reflectometer. Using this method, the upper theoretical limit of the operating range of such a reflectometer can be calculated by considering the influence of the local oscillator (LO) power and the noise level of a selected balanced photodetector. Aim of study. The aim was to determine the dependence of the dynamic range of a coherent reflectometer on the optical power of the LO at a given noise level of the balanced photodetector. In addition, the limitations of this dependence in real circuits were evaluated. Method. The dynamic range was calculated for two cases: (i) for a given optical power of the LO signal and (ii) for the ideal theoretical maximum sensitivity of the balanced photodetector. The calculation results were verified based on an operating prototype of a coherent optical reflectometer; the parameters of the prototype were used in the calculations. Main results. The dependence of the dynamic range of the coherent reflectometer on the LO power was determined. For a pulse duration of 3 mu s, an output optical power of 10 dBm, a reflectometer circuit loss of 11.5 dB, and an LO power of 225.9 mu W, the maximum dynamic range value was 35.4 dB, which was obtained by averaging over 215 reflectograms. Both excess and insufficient LO optical powers reduced the dynamic range of the coherent reflectometer. Practical significance. The proposed calculation method is suitable for designing coherent reflectometers with a specified dynamic range and operating distance and for assessing the potential for further increasing the dynamic range. (c) 2026 Optica Publishing Group. All rights, including for text and data mining (TDM), Artificial Intelligence (AI) training, and similar technologies, are reserved.

A BSTRACT This article explores phase errors created by low-pass filter in interferometric signals which are processed by In-Phase and quadrature demodulation algorithm (IQ-demodulation). These errors were calculated using the analytical method and were compared with mathematical modeling, which uses pre-calculated parameters: phase, sampling period, infinitesimal parameters. In this paper, we show that phase errors calculated with analytical method clearly correlate with mathematical modeling errors. This work made it possible to calculate corrections to the demodulated phase, which, in turn, made it possible to refine the phase calculation step in IQ demodulation algorithm using the arctangent function. The resulting formulas describing the correction to the demodulated phase will increase the accuracy of the quadrature method, which is used to process signals from interferometric devices of various types, such as: reflectometers, geophysical seismic systems, interferometric radiometry, etc.

In this paper, we study the errors of the homodyne demodulation method based on arctangent function solutions (PGCATAN) which are associated with the use of a low-pass filter (LPF) in this signal phase demodulation algorithm. The method of demodulation of an interference signal by PGC-ATAN method is investigated in order to detect and consider in more detail the errors at the filtering stage (the article considers the moving average method), and corrections to the signal are analytically calculated, taking into account the error introduced by the low-pass filter. We obtained formulas for calculating corrections to the signals S1(t), S2(t), S3(t), S4(t) which received by filtering the original signal multiplied by the reference oscillator signal, the calculations were compared with the results of mathematical modeling of the interference signal processing by the PGC-ATAN method. The demodulation of the signal, taking into account the corrections, showed that, in general, the effect on the signal phase is small at a low heating rate, however, for high-speed processes, the error can lead to serious distortions of the desired signal phase. These calculated corrections for processed interference signal will improve the demodulation method based on the calculations of the arc tangent function and more accurately calculate the desired phase of the signal.

Subject of study. Afiber-optic temperature sensor, based on a Fabry-Perot interferometer obtained bywelding optical fibers, is developed and characterized. Aim of study. The aim is to develop and characterize a temperature sensor based on a fiber-optic Fabry-Perot interferometer obtained by arc welding and evaluate its temperature-sensing performance. Method. The design of the temperature sensor is implemented based on a Fabry-Perot interferometer obtained by welding optical fibers. The reflecting mirrors of the sensor comprise layers of thin films of titanium dioxide. The interferometer interrogation method is based on the use of a vertically emitting laser operating in the pulsed generation mode. The principle of registering the phase shift between radiation reflected from the interferometer mirrors with changes in temperature is based on auxiliary modulation of the laser radiation wavelength due to a periodic change in the pulse duration. Main results. During the temperature evaluation of the developed sensor, the phase characteristics of the Fabry-Perot interferometer were obtained in the temperature range of C20 degrees C to C380 degrees C. A shift in the operating point was registered with changes in the ambient temperature. The results of the experiment indicated that an increase in the ambient temperature leads to an increase in phase difference between the radiation reflected from the first and second mirrors of the interferometer. Results suggest that the phase sensitivity to temperature change is 0.96, 1.68, and 2.35 rad/degrees C at distances between mirrors corresponding to 5, 8, and 11mm, respectively. Practical significance. The proposed fabrication method for the Fabry-Perot interferometer provides sufficient opportunities for the formation of a resonator with different lengths and also makes it possible to exclude the use of additional materials in its design. Hence, the interferometer exhibits small weight and size parameters, and thus, it can be used in a wide range of applications, including as a sensitive element of a temperature sensor. The sensor interrogation method employed in the study eliminates the need for costly spectral measuring instruments such as optical spectrumanalyzers and interrogators. (c) 2024 Optica Publishing Group

Subject of investigation.In this paper, we analyzed in detail the thermal mode of the operation of a vertical-cavity surface-emitting laser (VCSEL) used in the experiment.Method.As a part of the work, we carried out a theoretical study of the recurrence relation describing the change in the VCSEL wavelength under the action of specially selected modulation current pulses.The high speed of the device working is determined by the optical demodulation scheme, which is based on using a phase-modulated carrier (the demodulation method used is arctangent demodulation: Phase Generated Carrier (PGC-ATAN)).Main results.Formulas were obtained that determine the frequency, phase, and modulation depth which leads to calculation of the principle of change in the modulated VCSEL wavelength at sampling points.Comparison with experimental data showed that the obtained formulas allow one to choose the optimal thermal mode of VCSEL operation and reliably calculate the characteristics of the modulation process in terms of the carrier phase.Practical significance.The obtained formulas make it possible to calculate the exact characteristics of the modulation process, and the precisely calculated phase of the modulation source.As a result, one can compensate it more effectively when demodulating phase of the interferometer.

An embodiment of a fiber-optic temperature sensor based on a Fabry-Perot interferometer and a scheme for interrogating an experimental sample of the sensor are proposed. The proposed solution makes it possible not to use expensive spectral measuring devices (spectrum analyzer, interrogator). The region of free dispersion and the phase sensitivity of the developed Fabry-Perot interferometer were determined in the temperature range from 20 °C to 590 °C. The accuracy of measuring the ambient temperature is calculated. The long-term stability of the measuring setup at room temperature has been evaluated. The phase shift of the Fabry-Perot interferometer with temperature change was registered. The design of the Fabry-Perot interferometer is implemented using reflective thin-film multilayer structures obtained by stage-by-stage electron-beam deposition in vacuum on polished end cleavages of an optical fiber. The interferometer interrogation method is based on the use of a vertical-cavity surface-emitting laser (VCSEL) operating in a pulsed mode. The principle of registering the phase shift of the interferometer with a change in temperature is based on the use of auxiliary modulation of laser radiation along the wavelength due to modulation (periodic change) of the duration of optical pulses. Auxiliary modulation makes it possible to obtain additional harmonic components in the interferometer signal, which are further used in homodyne demodulation to restore the interferometer phase shift signal proportional to the change in the optical path difference between the interferometer mirrors. The design of the high-temperature sensor is based on a Fabry-Perot interferometer the reflecting mirrors of which are five alternating layers of thin films of TiO2 and Al2O3. Based on the results of the temperature experiment, it was concluded that an increase in the ambient temperature leads to a decrease in the free dispersion region of the Fabry-Perot interferometer. The conclusion made is consistent with the theoretical data. According to the results of the experiment, it is shown that the phase sensitivity of the interferometer to temperature changes is 0.94 rad/K. The accuracy of temperature measurements at the 3σ level was 0.017 K. The results of the study may be of great importance in creating systems for monitoring temperatures above 300 °C. The use of such an interferometer makes it possible to carry out high-precision relative temperature measurements.

In this paper, we presented results of influence of the dynamic range in fiber-optic streamer signal processing circuit on the quality of recorded seismograms. By contrast with the existing hydroacoustic systems, which based on piezoceramic transducers, dynamic range limitations in a fiber-optic towed streamer do not lead to clipping of acoustic signals, but to complex nonlinear distortions that affect both the amplitude and phase frequency characteristics of the recorded signals. Therefore, main task of this work is to assess the distortion of seismograms obtained from acoustic signals recorded under limited dynamic range conditions of fiber-optic towed seismic streamer. To solve this problem, seismic signals obtained during field tests for various types of seismic streamers in the water area of Kola Bay are used. The recorded acoustic signals in the form of digital readings, converted into radians, taking into account the known sensitivity coefficient of hydrophones of the towed seismic streamer, are converted into digital reports of optical interference signals. Interference signals in digital form are equivalent to real optical signals recorded by the receiving path in the signal processing unit of the fiber optic streamer. The digital form of the recorded acoustic signals makes it possible to amplify them by multiplying them by a given gain factor, simulating various energy levels of an acoustic source. Further, these signals served as input data in the mathematical model of the signal processing circuit of the fiber-optic towed streamer, which takes into account the fundamental limitations of the dynamic range due to the finite sampling frequency of the interference signal, the fixed frequency of the auxiliary phase modulation, and the finite bandwidth of the low-pass filters used. Thus, it is possible to simulate the process of recording acoustic signals of a fiber-optic towed seismic streamer both without distortion and under conditions of limited dynamic range. As a result, signals from the output of the processing circuit model are used to construct seismograms of the same shelf area with different levels of acoustic amplification using the reflected wave method. The results demonstrate that the limitations of the dynamic range of signal processing circuit of fiber-optic towed seismic streamer have a significant impact on the quality of seismograms, on reducing the signals detailing, and also on decreasing the amplitudes of the recorded waves (in the presented data, the amplitude decreases by 5 times). The quality of seismograms drops significantly in areas of sharp transition between layers with different densities, which generate the most distinct and strong reflected seismic vibrations. The results obtained are of great practical importance, since they allow evaluating the effect of complex nonlinear distortions of acoustic signals under the conditions of limited dynamic range of the signal processing circuit of a fiber-optic towed seismic streamer on the received seismograms. These results are presented for the first time due to the lack of world analogues of the developed fiber-optic towed seismic streamer. In addition, taking into account the known sensitivity of the fiber-optic hydrophones of the streamer, the constructed model of the signal processing path allows choosing the optimal energy of the acoustic source for seismic exploration with the most efficient use of the dynamic range of the fiber-optic streamer.

The paper proposes a version of realization for a fiber optic vibration sensor. The sensor possesses a wide range of operation frequencies with such interrogation devices as optical power meters. In comparison to spectral measuring complexes, the sensor imposes lower requirements on operating conditions. The authors investigated a sensing head based on a fiber SMF-MMF-SMF structure and a fiber Bragg grating inscribed in this structure. The external vibration frequency applied to the tested sensor structure is obtained using the Fourier transform of the signal received from the photodetector. The structure designed in this study with a fiber Bragg grating inscribed in it can be used as a sensing head of a fiber optic vibration sensor. It is demonstrated that the sensor based on the developed sensitive structure is able to obtain the external vibration frequency in the range of 20-9 000 Hz with accuracy up to 1 %. The research results are essential for monitoring systems for the state of structural elements of buildings and structures. The implementation of the vibration sensor in the format of a fiber optic device allows overcoming the limitations of piezoelectric sensors, providing high noise immunity and resistance to harsh environmental effects.

One of the significant factors affecting the operation of fiber-optic towed streamers is towing noise, which is caused by the varying speed of the towing vessel and the resistance of the buoys and the floating anchor of the streamer during towing. In this study, a method for compensating the towing noise of a fiber-optic streamer is proposed. The proposed method entails independent measurement of the towing noise using an additional interferometer and subtraction of its signal from the signals of the fiber-optic hydrophones of the streamer. The experimental results show that the towing noise can be reduced to 70% in the frequency range of 0–40 Hz.

The optical source phase noise is one of the main factors influencing the noise performance of fiber-optic interferometric measurement systems. The phase noise is directly proportional to the optical path length difference of the interferometer and wavelength instability of the optical source. Conventional phase noise suppression techniques involve the use of highly stable optical sources and reduction of path length differences of sensing interferometers. However, such methods are not always appropriate for all interferometric configurations. Alternative phase noise suppression techniques suggest the use of an auxiliary reference interferometer for independent measurements of the phase noise. In this case, the phase noise signal can be directly subtracted from the sensor signal. But equal path length differences are required for all interferometers in the measurement system. In this paper, an adaptive phase noise cancellation technique for fiber-optic interferometric sensors is presented. The proposed technique is based on the real-time evaluation of optical path length differences for all interferometers in the measurement system. This approach doesn't require identical path length differences and takes into account their variations caused by environmental conditions. Thus, the optical source phase noise can be independently measured by the reference interferometer and subtracted from the sensor signal with the required attenuation. The results of experimental investigations of the proposed technique for two types of interferometric schemes are presented. It is shown that the proposed technique demonstrates the phase noise reduction up to 30 dB in the frequency range up to 500 Hz.

Fiber-optic interferometric sensor arrays are very attractive for many applications. One of the common used interrogation approaches for fiber-optic sensor arrays is the path matched differential interferometry (PMDI). It allows to interrogate a large number of multiplexed fiber-optic sensors by using an auxiliary compensation interferometer (CIF). This approach with using the phase modulator allows to implement the phase-generated carrier demodulation for multiplexed fiber-optic sensors. However, this approach has the significant disadvantage, which is that environmental vibroacoustic impacts on the CIF produce undesirable phase signals from the sensor array. It leads to increasing the noise floor level of the measuring system. In this paper the development methodology and results of the experimental investigation of the passive vibroacoustic isolation system for the CIF in the PMDI-based fiber-optic interferometric sensor are presented. The proposed isolation system is implemented by using acoustic absorbing layers and the special mechanical suspension system for the CIF mounted in the single rack unit. It is shown that the proposed approach for the CIF isolation allows to reduce its sensitivity to vibroacoustic impacts by up to 30 dB in the frequency range up to 500 Hz. It is also demonstrated that potting of the CIF fiber-optic components reduces its isolation efficiency by an average value of 8.3 dB in the considered frequency range, that is not applicable for interferometric sensors. The presented methodology and obtained results might be used in PMDI-based fiber-optic interferometric sensors to improve their performance characteristics.

The results of theoretical calculations and experimental studies of the characteristics of an acoustic test signal when using an acoustic enclosure of the “open-shield” type are presented. The enrichment of the test acoustic signal spectrum due to the use of the developed enclosure was up to +30 dB in the frequency range of up to 500 Hz and from +4 to +20 dB in the frequency range of 1500–5000 Hz in comparison with the loudspeaker without an enclosure. The effect of room characteristics on the acoustic signal at the measurement point was studied and a method for compensation of this influence was proposed. As a result of applying it, the nonuniformity of the amplitude–frequency response of the acoustic signal at the measurement point decreased from 7 to 1.5 dB and the slopes of its spectral characteristic were eliminated.

The dependence of the noise density in a signal of a fiber-optic towed streamer in a frequency band of 10 Hz–1 kHz on the towing speed in a range of 1 to 5 knots, on the coefficient of relative elongation of its elastic section, its type, and on the type of attachment of the elastic section to the towed body have been studied. In addition, the dependence of the tensile load on the studied streamer on the towing speed is obtained. The experimental data obtained during field tests on the streamer noise level and their analysis are presented. The reduction of the towing noise level when using an elastic section occurred by a factor of up to 3 in a frequency range of 180 to 600 Hz depending on the type of elastic section when attaching it to a strength member of the towed body, and up to 2 in the entire studied frequency range.

Subject of Research. Fiber optic measurement systems are widely used in various industries. Most of these systems are fiber optic devices for detecting physical quantities. The development and creation of a measurement system for determination of the location and bends of extended objects in 3D space are actual at the moment. We propose the implementation of the sensitive part of the fiber optic measurement system based on seven single-mode fibers with an array of fiber Bragg gratings fixed to each other. Method. The method for determining the position and bends of extended objects in space is based on finding the level of axial deformation (compression, tension) of the fiber Bragg gratings during bending, and provides the calculation of the curvature direction and magnitude. Further reconstruction of the curve in space is realized by solving a system of differential equations with given initial conditions containing Frenet-Serre formulas. Main Results. The paper presents experiment results on writing fiber Bragg grating arrays into single-mode optical fibers, taking into account the features of the optical scheme and the source spectrum. The design and prototype of the fiber optic measurement system are developed. The results on the reconstruction of the shape based on experimental data are obtained. Practical Relevance. A special feature of this system is the fiber Bragg grating arrays formed in a wide range of wavelengths. Each Bragg grating corresponds to its own reflection wavelength, providing a sensitive part 1 m long. The geometry of the structure is based on seven fiber-optic armored microcables twisted with a given lay length during manufacturing, and provides the measurement of the kink value and direction.

A path matched differential interferometry (PMDI) is one of the most common architectures for fiber-optic sensor arrays. It allows to interrogate many fiber-optic sensors time-multiplexed at a single fiber by using an auxiliary compensation interferometer (CIF). The CIF is typically based on an unbalanced Michelson or Mach-Zehnder interferometer. Therefore, it may be affected by different acoustic and vibration environmental noises. Any vibroacoustic impacts on the CIF might produce undesirable noises in the sensor signals and the noise performance of the measuring system will degrade. An environmental noise cancellation technique for the CIF in fiber-optic PMDI-based sensor arrays is presented in this paper. The proposed approach is based on the separate interrogation of the CIF by the additional fiber laser together with using polarization-division multiplexing. The noise cancellation is performing by the subtraction of the independent CIF's signal from measured signals from the sensors array. In this paper, the main parameters of the noise cancellation system, which are critical for its performance, are discussed in detail. The experimental efficiency estimation of the proposed technique was performed in situ under open water conditions. It was shown, that for the measured signal with the standard deviation of 0.059 rad and the environmental impact on the CIF with the standard deviation of 0.61 rad the noise cancellation efficiency was -24.9 dB. The proposed technique might be easily integrated to large-scale PMDI-based fiber-optic sensors arrays and it's compatible with PMDI-based Fiber Bragg Grating arrays in combination with the wavelength division multiplexing.

The results of an experimental test of the effectiveness of a method for compensating the influence of background noise on the operation of a fiber-optic compensative interferometer incorporated in a towable hydroacoustic cable assembly are presented herein. The principle of compensation is the detection of a separate phase signal induced by the noise on the interferometer's arms and its subsequent subtraction from the interferometer's output phase signal. An additional reference sensor that is isolated from external influence is introduced in the measuring system design to achieve the compensation. The effectiveness of noise canceling upon implementation of the suggested method is −14.73dB. This result can significantly reduce the noise level of measuring systems based on fiber-optic phase sensors and interferometers under real operating conditions.

Theoretical calculations and an experimental study of the degree of decrease in the acoustic sensitivity of an optical fiber in the frequency range of 20–20 000 Hz inside the cables of special design were carried out. A substantial decrease in acoustic sensitivity has been achieved, that is, more than –29 dB with respect to the standard single-mode SMF-28 fiber in a polymer shell. This result can significantly increase the threshold sensitivity of measuring systems based on fiber optic interferometers.

Fiber optic components, such as fiber optic interferometers, fiber sensors and fiber lasers have high level of sensitivity to environmental acoustic and vibration noise due to elasto-optical effect and fiber elongation under pressure. For most applications this effect is undesirable and it should be reduced for proper operation of fiber optic system. The proposed method for reducing the fiber components environmental noise sensitivity level is based on the implementation of two-layer protective construction. The experimental test confirmed its effectiveness of undesirable impact suppression at the level from -3.2 to -19 dB relatively to the acoustic sensitivity level of bare optical fiber sample. The effectiveness appeared to be directly proportional to the acoustic pressure level. This fact allows us to implement proposed construction as an acoustic shock absorber or an acoustic filter with external impact level dependent transfert characteristic for sensitive fiber optic components.

Abstract The influence of low-frequency acoustic vibrations on optical signals in integrated optic lithium niobate modulators is considered. It is shown that low-frequency acoustic vibrations can significantly contribute to the modulator transfer function. Characteristics of the acoustic vibrations have been observed. It is shown that these vibrations correspond to excitation of standing Lamb waves. Methods for acoustic resonance suppression have been suggested and verified.

Methods for acoustic desensitization of fiber optic interferometer were studied as applied to the compensation interferometer (CIF) of a fiber optic phase sensor system. Materials with high mechanical loss factor were used for the fiber optic interferometer rack case. Also case acoustic conditioning measures and vibration isolation system based on steel springs were developed. Different types of fiber coating for acoustic desensitization were investigated. The best implementation provided up to 30 times (-29.8 dB) suppression of CIF undesirable acoustic sensitivity level.