In this letter, we present the results of the study of the pyroelectric effect in Ti:LiNbO3 titanium diffusion waveguides formed on an X-cut substrate, which are the basis for the creation of phase modulators. This letter presents an analysis of the contribution of pyroelectric phase drift, its magnitude, and the relaxation time characteristics of the charges causing the drift. The measurements were carried out interferometrically in the circuit of a fiber-optic Mach-Zehnder interferometer. The final part of the experimental study shows the results of significant suppression of the pyroelectric effect in the modulator. Moreover, reductive annealing of the crystal in hydrogen-nitrogen medium does not deteriorate the optical transmission properties of the waveguide. The prospective application of this method in the fabrication of Ti:LiNbO3 phase modulators is also evaluated.

Integrated FMCW LiDARs and their application in Advanced Driver Assistance Systems besides photonic integration involve meeting the vital requirements for accuracy, range detection and object velocity. Automotive LiDARs need to provide a high detection probability and ranging accuracy of moving objects at distances up to hundreds of meters. Precision control of the modulation waveform keeping a high degree of coherency is the pre-requisition for reliable ranging. We have separated the terms of frequency linearization and stabilization, which are responsible for the linearity of the detected response, accuracy, and detection probability of a LiDAR. The required phase-locked loop bandwidth maximum of 500 kHz for the laser control system is specified for a 300-meter measurement range. Requirements for a laser modulation waveform linearity for FMCW LiDAR could be simplified by introducing the scaling factor that is strictly determined by the residual deviation function. Additionally, the practical aspects of laser linewidth and frequency noise density spectrum distribution influence on the detection probability are demonstrated. As a result, optimized criteria for laser swept source performance are proposed, which can be used for the development of long-range detection systems. Finally, the practical results of frequency chirp characterization and ranging performance over 100 meters are demonstrated.

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.

This paper aims to consider fabrication of single-mode polarization-maintaining (PM) optical fibers with elliptical core, doped with 20 mol. % GeO2 and to study their optical properties and radiation resistance. The optical fiber preform was fabricated by chemical vapor deposition method. The low-oxygen central piece of the core in the preform was removed by high-temperature chemical gas-phase etching. The single-mode optical fibers 125 mu m in diameter, with 65 mu m thick polymer protection coating were drawn. The results of measurements of the optical properties at a wavelength of 1.3-1.55 mu m confirmed that the highly-doped GeO2 optical fiber wave guides have a low optical loss and high radiation resistance owing to gas phase etching. Polarization-maintaining factor (h-parameter) of asymptotic to 8.10(-6) m(-1) was achieved. As to the radiation resistance, our single-mode PM optical fiber with elliptical core appears to be quite competitive to the known PANDA PM optical fibers with a core made of pure silica glass.

The aim of this work was to develop a fabrication process of single-mode polarization-maintaining germanosilicate optical fibers with elliptical core (GOFEC) doped with 20 mol % GeO2. The fiber preform was fabricated by MCVD method. The low-oxygen central piece of the core in the preform was removed by high-temperature chemical gas-phase etching. The influence of gamma-radiation with a dose rate of 1 Gy/s on the optical losses in GOFEC at a wavelength of 1550 nm and at 25 degrees C and -60 degrees C were investigated. Their radiation resistance at -60 degrees C is proved to surpass that of the existing analogous fibers. It has been found that the radiation-induced attenuation in such highly doped GeO2 single-mode fibers is higher than that in multimode fibers. In contrast to multimode fibers, radiation resistance in single-mode fibers depends on the material of the core and cladding. As to the radiation resistance, these GOFEC are comparable with the fiber of PANDA type with a pure silica glass core.

The paper presents the analysis of nucleating kinetics and growing of switched domains in the surface layer of monodomain lithium niobate X-cut crystal in the interelectrode gap of integrated optical phase modulators. The work proposes the morphology model of domains growing along the boundary of surface electrodes in X-cut phase modulators. The mechanism of spontaneous needle-like domain growing as a result of the electric field induced by the pyroelectric effect at temperature changing of the crystal was theoretically substantiated. The Comsol Multiphysics cross-platform was used for the numerical estimation of the pyroelectric field in the interelectrode gap. The needle-like domain structures were studied experimentally at industrial samples of integrated optical phase modulators based on Ti:LiNbO3 waveguides. The experimental research of the form and size of domains was performed with the anisotropic etching method by HF solution and followed by visual analysis. For non-destructive testing, the authors used scanning electron microscopy and piezo-response force microscopy. For the first time, the morphology of needle-like domains occurring in the interelectrode gap of phase modulators based on lithium niobate was experimentally studied. The results showed the theoretical and numerical model of domain growing that involves the pyroelectric nature of the electric field. It was demonstrated that along the electrode boundary, the needle-like domains grow up to 20 μm long at normal conditions and achieve 30 μm after the thermal shock by cooling at ∆T = – 125 °С. The discovered switched domains in the interelectrode gap can affect electro-optical characteristics of integrated optical phase modulators with the lithium niobate base and should be taken into account in the future design of electrode topology and modulator usage.

The research deals with the topical problem of stabilizing the optical parameters of a single-mode vertical-cavity surfaceemitting laser. A solution to the problem is proposed by forming an external cavity based on a Bragg grating induced into an optical polarization-maintaining fiber. In this experimental research, the authors estimated the contribution of an external cavity to the relative intensity noise of a vertical-cavity surface-emitting laser. The stability of the laser generation with a varying phase of back reflections at various pump currents is investigated. The relative intensity noise of laser radiation in the pump current range of 1.1–6.3 mA is estimated using a photodetector with a bandwidth of 8.5 MHz. The phase change of the back reflections was carried out by shifting the optical fiber end with a 100 nm step. As a result of the research, it was obtained that the vertical-cavity surface-emitting laser with such an external cavity shows a stable emission in the current range of 1.8–3.2 mA. However, one can observe instability with constant switching between adjacent longitudinal modes outside this range. The most stable laser operation mode with an external cavity against a change in the phase of back reflections is registered at a pump current of 2.78 mA. Utilizing an external cavity with the fiber Bragg grating made it possible to reduce the relative intensity noise from 7.2∙10–10 1/Hz to 1.5∙10–11 1/Hz in the currents range 1.86–3.2 mA. The study can be useful in applications that require an optical radiation coherent source with a low relative intensity noise level (such as coherent optical communication or fiber-optic sensors).

Subject of Research. The paper considers the effect of a relative pressure change on the H-parameter value in a birefringent optical fiber with ESC-4 elliptical straining cladding and in Sagnac coil with 140 mm diameter and 700 m length. We analyze the effect of a relative pressure change on the phase shift in Sagnac interferometer. Method. The broadband interferometry method was used for the H-parameter estimation. Michelson polarizing scanning interferometer with a broadband source with a 1560 nm central wavelength and a 45 nm bandwidth was applied. The relative pressure was varied in a special pressure chamber in the range from –80 kPa to +300 kPa. Main Results. It is found out that when the relative pressure changes, random phase jumps occur in Sagnac interferometer. Such random phase jumps are not observed under normal conditions. It is also shown that the effect of increased relative pressure on Sagnac phase shift is more pronounced than the effect of reduced relative pressure; the frequency of phase jumps is greater in case of increased relative pressure. Practical Relevance. Analysis of research results is focused on the problem of high-quality winding and laying of birefringent optical fiber in Sagnac coil, which will improve the accuracy characteristics of fiberoptic sensors based on Sagnac interferometers.

Subject of study. The paper presents the study of temperature value effect on the h-parameter in birefringence optical fibers for different samples: fibers with an elliptical tension cladding with a double acrylate coating (250 μm in diameter), Bow-Tie fibers with a double acrylate coating (170 μm in diameter). Method. The research method is based on wideband interferometry application. We used polarization scanning Michelson interferometer with the central wavelength of the optical source equal to 1575 nm and the width equal to 45 nm. The temperature was changed by thermostat up to +70 °С, and by “dry” ice down to –70 °С. Main results. It was found that the value of the h-parameter increases from ~10-7 1/m at +22 °С to ~10-3–10-4 1/m at –70 °С in the negative temperature region, while in the positive temperature region h-parameter variation is insignificant for all fiber samples. Practical relevance. The scanning Michelson interferometer can find wide application in birefringent optical fiber production for its quality analysis: the absence of local defects, control of the fiber h-parameter value and beat length. Understanding of the temperature effect on the h-parameter value in birefringent fibers used in creation of fiber-optic sensors, will expand the performance characteristics of the developed device, as well as improve its accuracy parameters.

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.

Subject of Research. The paper presents the results of the laser diode module optical system alignment, which includes three steps: the laser diode radiation collimation, the laser beams multiplexing from three sources and the radiation input into the optical fiber. The laser diode module optical system with a fiber output is realized as a stepped configuration of sources position with a height shift of laser diodes relative to each other by 1.6 mm. Semiconductor laser diodes with Fabry-Perot resonator and peak generation wavelength of 1020 nm are used as radiation sources. The core diameter of the output multimode quartz optical fiber is equal to 400 μm and the numerical aperture is NA 0.22. Method. The method of spatial multiplexing for laser beams from three laser diodes in a continuous-wave mode was implemented. The residual divergence and deviations control of the optical axes in each channel were carried out by measuring the laser beam profiles in two sections with the beam profile meter displaced strictly along the laser beams propagation axis by 100 mm. The radiation input efficiency into the optical fiber was determined by measuring the radiation power before entering the laser beam and at the output from the optical fiber. Main Results. The maximum output power of the laser diode module prototype is 19.65 W. The loss reduction is achieved owing to the application of anti-reflective coatings on the lenses and a highly reflective coating on the mirrors, taking into account the spectral composition of the radiation and the angle of incidence of the laser beams. Practical Relevance. The implemented assembly method can be used to manufacture higher output power laser diode modules, including more than three laser diodes without reduction in effectiveness. The developed micro-optical component positioning test bench allows for high-precision alignment of lenses and mirrors, and optical fiber coupling of optoelectronic devices.

Subject of Research. The paper presents the results of the laser diode module optical system alignment, which includes three steps: the laser diode radiation collimation, the laser beams multiplexing from three sources and the radiation input into the optical fiber. The laser diode module optical system with a fiber output is realized as a stepped configuration of sources position with a height shift of laser diodes relative to each other by 1.6 mm. Semiconductor laser diodes with Fabry-Perot resonator and peak generation wavelength of 1020 nm are used as radiation sources. The core diameter of the output multimode quartz optical fiber is equal to 400 μm and the numerical aperture is NA 0.22. Method. The method of spatial multiplexing for laser beams from three laser diodes in a continuous-wave mode was implemented. The residual divergence and deviations control of the optical axes in each channel were carried out by measuring the laser beam profiles in two sections with the beam profile meter displaced strictly along the laser beams propagation axis by 100 mm. The radiation input efficiency into the optical fiber was determined by measuring the radiation power before entering the laser beam and at the output from the optical fiber. Main Results. The maximum output power of the laser diode module prototype is 19.65 W. The loss reduction is achieved owing to the application of anti-reflective coatings on the lenses and a highly reflective coating on the mirrors, taking into account the spectral composition of the radiation and the angle of incidence of the laser beams. Practical Relevance. The implemented assembly method can be used to manufacture higher output power laser diode modules, including more than three laser diodes without reduction in effectiveness. The developed micro-optical component positioning test bench allows for high-precision alignment of lenses and mirrors, and optical fiber coupling of optoelectronic devices.

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 paper presents the results of experimental investigations of the fiber optic hydrophone array consisting of six sensors, placed in one thin sensitive cable. Sensors were formed by pairs of Bragg gratings spaced 1.5 m apart and recorded in a birefringent optical fiber with the elliptical stressed coating. To form an extended sensor array the optical fiber was additionally covered with a silicone material RTV655 and protective coatings. Experimental investigations of the array showed that fiber-optic sensors pressure sensitivity increases as the acoustic frequency decreases at average value from -169.4 dB re rad/uPa at 495 Hz to -143.7 dB re rad/uPa at 40 Hz. The minimum detectable pressure was at average value from 53 mPa/root Hz at 495 Hz to 8.3 mPa/root Hz at 40 Hz. The obtained results might be used for developing and producing long thin hydroacoustic arrays for geophysical investigations and other hydroacoustic applications. (C) 2017 Elsevier Inc. All rights reserved.

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.

A study of the orthogonal polarization modes crosstalk changes in the point of different mechanical actions (pressure force) in the polarization-maintaining fiber with straining elliptical cladding is presented. It was found that by increasing of the pressure force the polarization extinction ratio increases nonlinearly. Also revealed the dependence of the extinction coefficient and the angle between vector of the mechanical action and polarization axes of the test fiber, which leads to change the extinction coefficient variable from -57 dB to -25 dB under the pressure force of 0.7 N. Also it was found that the cross angle of the fiber axes doesn't influence on the extinction ratio value of the mechanical induced polarization crosstalk.

Integrated optical birefringent waveguides are widely used in the field of phase interferometric fiber optic sensors. In sensors integrated optical waveguides are coupled to birefringent fiber waveguides. Due to different optical properties polarization conversion occurs at waveguides coupling point which may cause parasite interference to the signal of fiber optic sensors.

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.