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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.

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.

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.

For optical losses reducing at fiber optic systems creation the angular distribution of the output radiation of birefringent optical fibers with elliptical stress cladding with 4, 12, 16 and 18 mol. % GeO2 is measured. Obtained data make it possible to predict losses at joining of optical fibers with elliptical stress cladding and different concentrations of GeO2 with each other, as well as standard telecom fibers, such as SMF-28, other birefringent optical fibers and integrated optics components.