Integrated optical modulators based on lithium niobate Ti-indiffused waveguides (Ti:LiNbO3) are used extensively for broadband devices that have a high coupling efficiency with optical fibers at a 1.5 & mu;m wavelength window. However, the ferroelectric nature of lithium niobate crystal results in some undesirable effects causing parameters fluctuations. For example, the pyroelectric effect leads to thermal dependent DC-drift and excess optical losses. Therefore, to develop effective compensation methods, it is important to understand the origin of instability. In the interelectrode gap of phase modulators we found needle-like domain defects with switched polarization along a surface layer of the monodomain X-cut lithium niobate substrate. Based on domain nucleation theory and polarization switching behavior, the spontaneous nature of domain nucleation caused by the pyroelectric field was shown and described for the interelectrode gap of the modulator along electrode edges. Considering the corona effect, models of domain nucleation and growth are proposed, and the morphology of needle-like domains is describes based on numerical simulation of the pyroelectric field distribution in the crystal volume. Experimental studies of industrial optical phase modulators characterize the shape and size of domains not only in the interelectrode gap, but also along the boundary of external electrodes proving the pyroelectric type of domain nucleation. Several methods were used for domain visualization-selective chemical etching, piezoresponse force microscopy, and electron scanning microscopy. The present work provides a theoretical and practical basis for future research on stabilization of the parameters for optical phase modulators.

Subject of study. Residual amplitude modulation in diffused -channel Ti : LiNbO 3 integrated optical waveguides is studied. Aim of study. The aim is the suppression of residual amplitude modulation in waveguides with a LiNbO 3 substrate by modifying the waveguide geometry. Method. The waveguide geometry modification involves adding a bend in the input and output sections to form angles of 3.47 degrees and 1.43 degrees between the propagation direction near the phase modulator and near the input and output ports, respectively. These angles do not exceed the maximum optical divergence angle permitted by the numerical aperture of the optical fiber; however, the likelihood of optical radiation propagating from the input fiber to any of the output fibers through the substrate of the integrated optical circuit, bypassing the optical waveguide, must be reduced. Main results. The density of residual amplitude modulation values shown in the boxplot is higher when the waveguides for the input and output sections of the optical integrated circuit are bent. Thus, the interquartile range of the residual amplitude modulation values decreased by almost a factor of 2 from 0.077% to 0.043%, and the difference between the maximum and the minimum values was also nearly halved, from 0.161% to 0.095%. Practical significance. These results can be used to improve the characteristics of fiber-optic gyroscopes and other devices containing a multifunctional optical integrated circuit based on Ti : LiNbO 3 . (c) 2024 Optica Publishing Group

This work investigates a lithium niobate X-cut birefringence modulator with a titanium diffusion channel waveguide. The wave voltage of the modulator depends on the growth and processing conditions of the lithium niobate crystal and on the technology of waveguide formation. The tolerance for determining the length of the electrodes and the gap between them exceeds 1 %. In this regard, the calculated values of the wave voltage can differ significantly, and an experimental measurement of the wave voltage is required for practical use. The authors present an experimental refinement for the value of the wave voltage of the modulator and perform a comparison of the value with the theoretical one. In the experiment, the wave voltage was determined using a scanning Michelson interferometer. It is shown that the experimentally measured value of the wave voltage diverges from the calculated one by more than 26 %. This difference is based on the assumption that the vector of the electric field inside the crystal is directed perpendicular to the axis of propagation of optical radiation, and the magnitude of the electric field does not change over the depth of the crystal. In this case, the overlap integrals of the ordinary and extraordinary waves are equal. In real modulators with a channel waveguide formed by titanium diffusion technology, these assumptions are not fulfilled. The refractive index of lithium niobate and the electro-optical coefficient may vary for different crystal samples, depending on the conditions of their growth, processing and waveguide formation technology. The results of the work can find application in the field of interferometric measuring devices, in which a birefringence modulator is used, since the value of the wave voltage is necessary for the design of control electronics.

Simulation results and experimental data on the operation of an inductively heated sensor of flow velocity, using fiber Bragg gratings as sensitive elements, are presented. The principle of operation of the sensor is based on the cross-correlation method. In the course of the experiment, it was possible to detect the transit time of the heat pulse between the temperature sensors from 0.2 to 0.5 s, which corresponds to the flow velocity in the range of 0.1–0.5 m/s.

The paper presents simulating and experimental results of flowmeter operation with induction heating, where fiber Bragg gratings were used as sensitive elements of temperature sensors. The operating principle of flowmeter based on cross-correlation method. During the experiment, it was detected time response of heat pulse from 0.2 s to 0.5 s that corresponds to a flow velocity range 0.1-0.5 m/s.

Subject of research. The paper presents the study of effect that external cavity length with the fiber Bragg grating has on the spectrum of Vertical Cavity Surface Emitting Laser (VCSEL) with a central emission wavelength of 1554 nm. Fiber Bragg grating reflection coefficient was 95 % and full width at half maximum (FWHM) was 0.095 nm. Method. An interrogator was used for emission central wavelength measurement over time. We performed the study of the standard deviation and drift of the VCSEL central wavelength, both with and without an external cavity. The studied external cavity length varied from 1040 to 30 mm by decreasing the fiber length. Main results. As a result of the study, the dependence of the VCSEL spectral characteristics on the external cavity length was obtained. It was also shown that the VCSEL center wavelength variation was reduced by more than an order of magnitude, so 3σ standard deviation was 0.17 pm using an external cavity 1040 mm long, in contrast to 8 pm for the case without an external cavity. We showed the possibility of reducing the amplitude of the central wavelength drift to one-fourth, from 2 to 0.5 pm, with the use of an external cavity. Practical relevance. The study proves to be useful when creating fiber-optic distributed sensors of physical quantities with affordable cost, if the design requires the use of a highly coherent emission source.

We report about the study of Ti diffused LiNbO3 x-cut electro-optic modulator (EOM). This paper describes a study of mechanical resonances induced by applied voltage in EOM and their influence on the EOM functioning. The method for reduction of mechanical resonances influence is presented. Described method can be used for noise reduction of fiber-optic interferometric sensors and fiber-optic gyroscope in particular.

Functioning of closed-loop FOG is presented. Block diagram of FOG control system is given, and sources of nonlinear distortions of its components are described. A method to analyze the closed-loop FOG control system is proposed for adjustment of PID controller that does not require complex and high precision test equipment. Control target function is set, and coefficients of PID controller are selected optimal for this target function and certain FOG sample.

An investigation has been performed on the distributed H-parameter in an anisotropic optical fiber with an elliptical strained shell in a multilayer coil of a fiberoptic gyroscope. Each fiber length is ~50 m and there are five layers. Studies have been carried out using three variants of coils. In the first variant, the layers were permeated by epoxy; in the second coil, the layers were permeated with silicon-based material; and, in the third variant, the coils were wound up without impregnation (using dry winding). The different influence of compressive mechanical stresses on the distributed H-parameter of the studied fiber has been observed depending on the variant of coil impregnation.

The work offers the method for a study of parasitic refractive index drift (RID) of electro-optical modulator (EOM) based on the lithium niobate (NL) crystal under the action of applied electric voltage. As a result there is proposed a method of compensation EOM's inertial behavior. Described method can be used for improving the accuracy of fiber-optic interferometric sensors and of fiber-optic gyroscope in particular.

An experimental setup for observation of a scattering interference pattern from optical fibers is assembled. The dependence of scattering interference pattern from the anisotropic optical fiber with 4mol% of GeO2 in core on the stress applying ellipse position is studied experimentally. The anisotropic optical fiber with 4mol% of GeO2 in core was used for this study. The mathematical modeling and experimental data are in a good agreement. On its basis some recommendations for less time-consuming stressing ellipse orientation in specialty fibers used in fiber-optic interferometric sensors determination are made.

This paper presents comparative results of the recording of type-I Bragg gratings in birefringent optical fibers with an elliptical stress cladding with standard and increased concentrations of germanium dioxide for various orientations of the birefringence axes. The experimental results showed that it is more efficient to record Bragg gratings in the single-pulse regime when the slow axis of the birefringent optical fiber with an elliptical stress cladding is perpendicular to the fringes of the interference pattern. (C) 2017 Optical Society of America

The investigation is devoted to residual amplitude modulation (RAM) of phase electro-optic modulator, which guides are made in LiNbO3 crystal by Ti diffusion technology. An analysis is presented that shows influence of RAM on the signal of fiber-optic gyroscope. The RAM compensation method is offered.

The paper deals with optical scheme for research of polarization state transformation at the junction of anisotropic waveguides. It consists of a light source, polarization controller, multifunctional integrated optical scheme (MIOS), single-mode fiber for input and output of optical radiation in MIOS and the polarization scanning Michelson interferometer. Optical radiation from the source of the plant comes through the polarization controller in one of the MIOS ports. Further, in one of the opposite ports the radiation is received by different fibers, polished at the angles of 19.5˚, 10.5˚ and 0˚. After that, the optical radiation gets into polarization Michelson interferometer. With that, the picture visibility is analyzed at different displacement of one arm upon which the value has been determined in the polarization conversion point connections. At the course of work it was obtained that the polarization state conversion at a splicing point rises with the slant angle deviation from its optimal value. Anisotropic waveguides splicing is one of the main tasks during fabrication of any fiber-optic sensor with integrated optical elements. The results of this work are of great interest for the wide range of specialists in the optical waveguides application field.

Subject of Research. The paper presents the results of experimental comparative analysis of homodyne demodulation algorithms based on differential cross multiplying method and on arctangent method under the same conditions. The dependencies of parameters for the output signals on the optical radiation intensity are studied for the considered demodulation algorithms. Method. The prototype of single fiber optic phase interferometric sensor has been used for experimental comparison of signal demodulation algorithms. Main Results. We have found that homodyne demodulation based on arctangent method provides greater (by 7 dB at average) signal-to-noise ratio of output signals over the frequency band of acoustic impact from 100 Hz to 500 Hz as compared to differential cross multiplying algorithms. We have demonstrated that no change in the output signal amplitude occurs for the studied range of values of the optical pulses amplitudes. Obtained results indicate that the homodyne demodulation based on arctangent method is most suitable for application in the phase fiber-optic sensors. It provides higher repeatability of their characteristics than the differential cross multiplying algorithm. Practical Significance. Algorithms of interferometric signals demodulation are widely used in phase fiber-optic sensors. Improvement of their characteristics has a positive effect on the performance of such sensors.

This paper discusses the optical and electrical characteristics of the RayCan RC32 laser diode, with a vertical cavity that emits at a wavelength of 1550 nm, as a function of various operating regimes. The possibility of using laser diodes with a vertical cavity in fiber-optic interferometric phase sensors is considered. An experimental method of obtaining the coherence function of a laser diode with a vertical cavity is also used to obtain the dependences of the variation of the central wavelength of the radiation and the width of the spectral components when the laser is directly modulated in current at various frequencies and with various inverse duty cycles.

Subject of Research. We have investigated the effect of anisotropic optical fibers winding with an elliptical sheath subjecting to stress on the H-parameter invariance degree. This type of optical fiber is used in the manufacture of fiber loop in fiber-optic gyroscopes. Method of Research. The method of research is based on the application of Michelson polarization scanning interferometer as a measuring device. Superluminescent diode with a central wavelength of 1575 nm and a half-width of the spectrum equal to 45 nm is used as a radiation source. The studies were carried out with anisotropic optical fiber with 50 m long elliptical sheath subjecting to stress. The fiber was wound with one layer turn to turn on the coil with a diameter of 18 cm, which is used in the design of fiber-optic gyroscope. The tension force of the optical fiber was controlled during winding on a special machine. Main Results. It was found that at the increase of tension force from 0.05 N to 0.8 H the value of H-parameter increases from 7×10-6 1/m up to 178×10-6 1/m, respectively; i.e. the coupling coefficient of orthogonal modes in the test fiber is being increased. Thus, it is necessary to consider the longitudinal tension force of fiber in the design and manufacture of the fiber-optic sensors of high accuracy class: the less the fiber winding power, the higher invariance degree of distributed H-parameter. The longitudinal tension force of anisotropic optical fiber with elliptical sheath subjecting to stress equal to 0.2 N is recommended in the process of designing fiber-optic gyroscopes. Practical Relevance. The proposed method of Michelson scanning interferometer is usable in the production process for quality determination of the optical fiber winding: no local defects, value controlling of fiber H-parameter.