An Automated Instrument for Reflectometry Study of the Pyroelectric Effect in Proton-Exchange Channel Waveguides Based on Lithium Niobate

Belokrylov M.E., Kozlov A.A., Karnaushkin P.V., Konstantinov Y.A., Ponomarev R.S., Turov A.T.

The paper considers experimental technological regimes at different stages of integrated optical chips creation for modern applications: optoelectronics, fiber-optic sensors, microfluidics. The considered experimental regimes demonstrate not only qualitative, but also quantitative efficiency: at the stage of the ridge waveguides formation, there is a possibility of achieving high etching rates of lithium niobate - up to 950 nanometers per minute in fluorine-containing plasma with an increased consumption of plasma gases without forced thermostating of the sample. Using scanning electron microscope, structural changes in the surface have been demonstrated: clustering of surface etching foci, indicating the uniformity of etching at the macroscale, and changes in the thickness and structure of the near-surface defect layer, indicating the possibility of its complete removal by the plasma-chemical method. The method can be applied both for the manufacturing of ridge waveguides and optical elements based on them, and for microfluidics elements. Then, the efficiency of the waveguides and optical fibers interfacing method has been confirmed with frequency domain reflectometry. The algorithmic details of the interfacing by the modified Newton's method are also described. The modified Newton’s method allowed to simplify the alignment process by performing coarse alignment based on reflections from the far end face of the waveguide, speed up the alignment process at fine mode, and fully automate the alignment of the waveguide and optical fiber at any initial displacements. All the proposed technological process modernization leads to the improvement of technological characteristics and the individual stages production time reduction.

Feng W., Wang M., Jia H., Xie K., Tu G.

In recent years, Optical Frequency Domain Reflectometry (OFDR) has been able to realize strain measurement with high sensitivity and high spatial resolution (SR) along the sensing fiber. Compared with the conventional cross-correlation method, OFDR based on phase-measuring method has the potential to obtain strain information with better SR. However, the accuracy of this method is severely affected by coherent fading. In this paper, a method combining multi-frequency detection and nearest neighbor analysis is proposed to solve this problem. By comparing the sub-phase curves in different wavelength regions, we successfully eliminate the interference of fading noise and obtain the undistorted phase signal. Finally, in the case of an effective wavelength scanning range of 3.6 nm (corresponding to the ranging SR of 0.22 mm), we realized the deformation measurement with standard deviation of 0.015 μm and SR of 1.1 mm, as well as the strain measurement with standard deviation of 0.55 μϵ and SR of 5.6 cm. The experimental results also show that compared with the conventional multi-frequency average method, the proposed scheme can achieve similar phase measurement accuracy under less frequency division, so as to obtain better SR in the same scanning range.

Gorshkov B.G., Yüksel K., Fotiadi A.A., Wuilpart M., Korobko D.A., Zhirnov A.A., Stepanov K.V., Turov A.T., Konstantinov Y.A., Lobach I.A.

This work presents a detailed review of the development of distributed acoustic sensors (DAS) and their newest scientific applications. It covers most areas of human activities, such as the engineering, material, and humanitarian sciences, geophysics, culture, biology, and applied mechanics. It also provides the theoretical basis for most well-known DAS techniques and unveils the features that characterize each particular group of applications. After providing a summary of research achievements, the paper develops an initial perspective of the future work and determines the most promising DAS technologies that should be improved.

Ruzhitskaya D.D., Zhluktova I.V., Petrov M.A., Zaitsev K.A., Acheva P.P., Zunikov N.A., Shilko A.V., Aktas D., Johlinger F., Trefilov D.O., Ponosova A.A., Kamynin V.A., Makarov V.V.

Quantum communication protocols are considered secure provided that all devices included in the system are fully characterized, and side channels are closed. However, as a result of laser radiation exposure, it is possible to change quantum communication systems components’ characteristics. This leads to vulnerabilities appearing in the quantum key distribution system. In this paper, we consider the effect of pulsed laser radiation on fiber-optic isolators used in quantum communication systems. Isolators protect the source of the system from attacking optical radiation coming from the “eavesdropping” side via the quantum channel. Lowering the isolation factor can bring the entire system out of a secure state. This gives an eavesdropper access to information about the secret key. The scenario of the most probable attack to the source of the quantum key distribution system via a pulsed laser was simulated. The experimental setup provided exposure of fiber isolators with pulsed laser radiation at a wavelength of 1064 nm (within the transparency window of the isolators) with a mean power up to 840 mW in four different pulse generation modes. The isolation factor and throughput of tested samples were monitored using a laser diode with a wavelength of 1550 nm and average power of 10.5 mW. Spectrally selective splitters were used to separate the lasers. It is shown that the isolation factor (isolator attenuation in the direction from the quantum channel to the system) at a wavelength of 1550 nm decreases from the initial value of 59.1 dB to 24.5 dB. The throughput (in the direction from the system to the quantum channel) at the same wavelength decreases from 0.6 dB to 1.2–12.3 dB or remains the same, depending on the acting pulsed laser radiation parameters. Temperature monitoring showed that the temperature of the isolator body changes insignificantly when exposed to pulsed radiation. The obtained effects of changing the isolation coefficient and throughput can be explained by the presence of nonlinear effects in the magneto-optical crystal of the isolator. The results of the work can be applicable in the practical evaluation of quantum communication systems security, in particular, the security evaluation of quantum key distribution systems. The results can be used to prepare the standards for certification procedures for assessing the security of quantum communication systems. The work gives recommendations for enhancing signal source unit security in quantum communication systems. Namely, as countermeasures to protect against the effects of pulsed laser radiation, it is proposed to set optical fuses with a limited threshold power, detectors for monitoring input optical radiation power, and narrow-band optical filters at the entrance of the quantum communication system.

Hoshino K., Saito D., Zan M.S., Tanaka Y.

We experimentally demonstrate a fiber optic distributed strain sensor based on slope assisted Brillouin optical time domain analysis (SA-BOTDA) using virtual Brillouin gain spectrum (BGS) generated by multi-frequency light. Generally, strain along an optical fiber can be measured by using Brillouin scattering, because BGS has a linear dependence on the fiber strain. While conventional Brillouin fiber optic strain sensors sweep the probe light frequency to measure the BGS, it limits the measurement time and requires precise control of probe frequency. In contrast, the recently proposed method of SA-BOTDA measures the strain in real time from the power change of fixed frequency probe light that has been amplified by Brillouin amplification. However, because the conventional SA-BOTDA has a nonlinear response to the strain, it has to compensate the nonlinearity or only use BGS with relatively narrow range of linear slope region. In the last study, we proposed virtual BGS that has a relatively wide linear slope region and confirmed the principle. In this paper, we experimentally generate virtual BGS that has more than 100 MHz of linearslope region by using 12-GHz spacing five-frequency pump and probe. The strain distribution along the fiber is successfully measured in real time.

Ponomarev R., Konstantinov Y., Belokrylov M., Lobach I., Shevtsov D.

This work is devoted to the study of the pyroelectric effect on the properties of optical waveguides formed in a lithium niobate crystal by proton exchange. In the present work, we studied the cessation effect of the radiation channeling during thermocycling of Y-splitters samples. We examined the spectral dependence of optical losses on the wavelength using an optical spectrum analyzer. The results demonstrate that in the range of 1530–1570 nm, all wavelengths are suppressed equally. The optical frequency domain reflectometry shows that the increase of optical losses is observed along the entire waveguide, but not only at the Y-splitting point, as supposed earlier.

Karnaushkin P.V., Konstantinov Y.A.

A method for aligning the waveguide of a photonic integrated circuit and an optical fiber according to reflections from the far end of the waveguide, which was obtained using the optical reflectometry method in the frequency domain, is presented. The reflectometer was designed on the basis of a Michelson interferometer. The measuring arm of the interferometer was a line formed by the optical fiber and the channel waveguide of a photonic integrated circuit (PIC). The PIC with proton-exchange channel LiNbO3 waveguides was polished at an angle of 10°, while the tip with an optical fiber was polished at an angle of 15°. In this study, experiments on the alignment of the optical fiber and the waveguide were performed and analyzed. It is shown that the amplitude of a signal reflected from the far end of the waveguide was determined by the size of the longitudinal and lateral displacements between the fiber and waveguide. The maximum amplitude of the peak was 16 dB. During the experiments, it was found that the accuracy of the alignment method was 4, 1, and 1 μm along the X, Y, and Z axes, respectively.

Matveenko V., Kosheleva N., Serovaev G., Fedorov A.

The results of strain measuring experiments, with the help of rosettes consisting of fiber Bragg grating sensors (FBG) embedded at the manufacturing stage in a polymer composite material are considered in this paper. The samples were made by the direct pressing method from fiberglass prepregs. A cross-shaped sample was tested under loading conditions corresponding to a complex stress state. A variant of strain calculations based on experimental data is discussed. The calculations were performed under the assumption of a uniaxial stress state in an optical fiber embedded in the material. The obtained results provide a reasonable explanation of the absence in the conducted experiment of two peaks in the reflected optical spectrum, the presence of which follows from the known theoretical principles. The experimental result with two peaks in the reflected optical spectrum was obtained for the same sample under a different loading scheme. The proposed variant of the numerical model of the experiment and the results of numerical simulation made for FBG rosettes embedded in the material allowed to estimate error in the strain values calculated on the assumption of the uniaxial stress state in the optical fiber and in the presence of two peaks in the reflected optical spectrum.

Gabdulkhakov I.M., Morozov O.G., Kuznetsov A.A., Burdin A.V., Tiwari M.

A new structure of a quantum key distribution system with double orthogonal spectral polarization and frequency coding based on the tandem amplitude-phase modulation of an optical carrier and linearly chirped fiber Bragg gratings with phase π-shift supporting birefringence is proposed.

Feng T., Zhou J., Shang Y., Chen X., Yao X.S.

We correct an inaccurate statement on the birefringence measurement using the PA-OFDR in our previous publication [Opt. Express28, 31253 (2020)10.1364/OE.405682].

Rao Y., Wang Z., Wu H., Ran Z., Han B.

Phase-sensitive optical time domain reflectometry (Ф-OTDR) is an effective way to detect vibrations and acoustic waves with high sensitivity, by interrogating coherent Rayleigh backscattering light in sensing fiber. In particular, fiber-optic distributed acoustic sensing (DAS) based on the Ф-OTDR with phase demodulation has been extensively studied and widely used in intrusion detection, borehole seismic acquisition, structure health monitoring, etc., in recent years, with superior advantages such as long sensing range, fast response speed, wide sensing bandwidth, low operation cost and long service lifetime. Significant advances in research and development (R&D) of Ф-OTDR have been made since 2014. In this review, we present a historical review of Ф-OTDR and then summarize the recent progress of Ф-OTDR in the Fiber Optics Research Center (FORC) at University of Electronic Science and Technology of China (UESTC), which is the first group to carry out R&D of Ф-OTDR and invent ultra-sensitive DAS (uDAS) seismometer in China which is elected as one of the ten most significant technology advances of PetroChina in 2019. It can be seen that the Ф-OTDR/DAS technology is currently under its rapid development stage and would reach its climax in the next 5 years.

Gorlov N.I., Bogachkov I.V.

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.

Yuksel K., Jason J., Kocal E.B., Sainz M.L., Wuilpart M.

In this paper, we discuss the operation principles, sensing mechanism, challenges and application areas of FBG-assisted phase-sensitive optical time-domain reflectometry. A special emphasis is given to the interrogation of fiber Bragg grating arrays for vibration sensing application. Results obtained by different research groups are compared in terms of performance characteristics and future perspectives. Recent progress obtained through our research collaboration are also presented. In particular, the detrimental spectral shadowing effect and multiple reflection crosstalk are analysed and mitigation techniques are proposed.

Шевцов Д., Константинов Ю., Белокрылов М., Пономарев Р.

Эмпирическими и имитационными методами исследованы процессы изменения фазы излучения в интегрально-оптических модуляторах на подложке из ниобата лития, а также волоконных световодах, состыкованных с ними. Для проведения исследования использованы интегрально-оптические фазовые модуляторы на подложке из ниобата лития, демонстрирующие эффект прекращения каналирования фундаментальной моды волновода на резких температурных градиентах при нагреве из области низких температур, и волоконные световоды со спровоцированными температурными деформациями. Построена компьютерная модель процесса.

Ponomarev R.S., Shevtsov D.I., Karnaushkin P.V.

It is shown that the termination of the channeling of the fundamental radiation mode in the waveguide can be observed upon heating of an optical integrated circuit based on proton exchange channel waveguides formed in a lithium niobate single crystal. This process is reversible, but restoration of waveguide performance takes tens of minutes. The effect of the waveguide disappearance is observed upon rapid heating (5 K/min) from a low temperature (minus 40 °C). This effect can lead to a temporary failure of navigation systems using fiber optic gyroscopes with modulators based on a lithium niobate crystal.

Konstantinov Y.A., Turov A.T., Latkin K.P., Claude D., Azanova I.S.

This work is devoted to the scientific and technical aspects of individual stages of active optical fibers preforms’ optical-geometric parameters metrological control. The concept of a system presented makes it possible to carry out a study of a rare earth element distribution in the preform of an active optical fiber and to monitor geometric parameters, and also to study the evolution of the refractive index profile along the length of the sample at a qualitative level. As far as it is known, it is the first description of the preform optical, geometric, and luminescent properties measurement within a single automated laboratory bench. Also, the novelty of the approach lies in the fact that the study of the refractive index profile variation along the length of the preform is, for the first time, conducted using the “dry” method, that is, without immersing the sample in synthetic oil, which makes the process less labor-intensive and safer.

Belokrylov M.E., Claude D., Konstantinov Y.A., Karnaushkin P.V., Ovchinnikov K.A., Krishtop V.V., Gilev D.G., Barkov F.L., Ponomarev R.S.

Simple measures to improve the signal-to-noise ratio of optical frequency domain reflectometer (OFDR) readings are described. After applying a two-stage optical amplification of the backscattered signal, as well as eliminating the source of spurious reflections, it was possible to increase the signal-to-noise ratio of the frequency domain trace from 8 to 19 dB. This technique can be applied in fiber optic sensors and metrology of fiber optic and integrated optical elements.

Белокрылов М.Е., Клод Д., Константинов Ю.А., Карнаушкин П.В., Овчинников К.А., Криштоп В.В., Гилев Д.Г., Барков Ф.Л., Пономарев Р.С.

Turov A.T., Konstantinov Y.A., Barkov F.L., Korobko D.A., Zolotovskii I.O., Lopez-Mercado C.A., Fotiadi A.A.

Moving differential and dynamic window moving averaging are simple and well-known signal processing algorithms. However, the most common methods of obtaining sufficient signal-to-noise ratios in distributed acoustic sensing use expensive and precise equipment such as laser sources, photoreceivers, etc., and neural network postprocessing, which results in an unacceptable price of an acoustic monitoring system for potential customers. This paper presents the distributed fiber-optic acoustic sensors data processing and noise suppression techniques applied both to raw data (spatial and temporal amplitude distributions) and to spectra obtained after the Fourier transform. The performance of algorithms’ individual parts in processing distributed acoustic sensor’s data obtained in laboratory conditions for an optical fiber subjected to various dynamic impact events is studied. A comparative analysis of these parts’ efficiency was carried out, and for each type of impact event, the most beneficial combinations were identified. The feasibility of existing noise reduction techniques performance improvement is proposed and tested. Presented algorithms are undemanding for computation resources and provide the signal-to-noise ratio enhancement of up to 13.1 dB. Thus, they can be useful in areas requiring the distributed acoustic monitoring systems’ cost reduction as maintaining acceptable performance while allowing the use of cheaper hardware.