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 presents a mathematical model of the sensitive element of a refractometric fiber-optic sensor the principle of operation of which is based on the phenomenon of surface plasmon resonance. The sensing element design is a sequential connection of a multimode fiber (MMF), a single-mode fiber (SMF), and a multimode fiber forming an MMF-SMF-MMF structure. The SMF site is coated with a thin film of gold. To model the element, the approach used in calculating the classical Kretschmann configuration for volumetric optical structures was applied. The refractive index of the fiber is calculated based on the Sellmeyer equation, and the refractive index of the gold is determined using the Drude model. The simulation results are compared with experimentally obtained transmission spectra of fabricated samples of sensing elements. For approbation of the model, the sensing elements of fiber-optic sensors with the following parameters are made: core diameter of multimode fiber 62.5 μm, core diameter of singlemode fiber 9 μm, coating SMF-segment with 50 nm gold film. Transmission spectra of fiber-optic sensor sensing elements in aqueous glucose solutions of various concentrations were obtained. It is demonstrated that the proposed model describes well the experimentally obtained transmission spectra of sensitive elements based on MMF-SMF-MMF structures in the region of surface plasmon resonance. The proposed model can be used to optimize the design of the sensitive element of refractometric fiber-optic sensors in order to increase the sensitivity. The proposed model implies its use in the development of an algorithm for interrogation of sensing elements based on fiber MMF-SMF-MMF structures.

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 effect of gamma-irradiation of the fluorine-doped silica depressed cladding graded-index fibre (DCGIF) has been researched. New mechanism of the non-bridging oxygen hole centers (NBOHC) defect formation, conditioned by different mobility of diffusion counter flows of fluorine and oxygen at the core-cladding interface, has been proposed. Using silica tubes with 0.02 wt% of OH groups and hydrogen in free state in MCVD-technology of DCGIF fabrication completely eliminates the NBOHC absorption at the wavelength of 630 nm. However, gamma-irradiation of such fibres results in anomalously high attenuation level in the visible range of optical spectrum. It was established that irradiation of DCGIF leads to light scattering which is probably due to the appearance of optical inhomogeneities in the glass matrix.

The paper deals with the problems of excimer laser pulse energy measurements. The goal of the current work is to research and develop the construction and characteristics of laser energy meter based on the photoelectric films of indium-tin oxide. Photoelectric voltage linearly depends on the laser pulse energy. We propose the construction and electric scheme of the energy meter. Comparison of the measurement results of photoelectric energy meter and pyroelectric energy meter is carried out. It is shown that the measurement error of photoelectric energy meter does not exceed the one for pyroelectric energy meter. Also it is found that photoelectric energy meter has several advantages: mechanical shock has no influence on the energy meter results, energy meter requires no zero level calibration, sensor temperature dependence is less than for pyroelectric meter. We measure the work energy diapason for the meter sensor element. It is shown that the surface degradation occurs under the irradiation with energy density equal to 78 mJ/cm2.

The paper presents the results of writing of type I and high-performance type II fiber Bragg gratings in birefringent optical fiber with an elliptical stress cladding by a single 20 ns pulse of KrF excimer laser (248 nm). The gratings’ efficiency produced by a single pulse was up to 100%. Experimental results on visualization of these gratings are presented.