Broadband quantum noise source based on the shot noise detection of a balanced photodetector is demonstrated. Precise electro-optical tuning of the balanced photodetector circuit was carried out by an integrated optical beam splitter in the form of the dual output Mach-Zehnder interferometer on a lithium niobate substrate. The classical component of the detected noise related to the relative intensity noise of a laser diode was suppressed by more than 15 dB. At the maximum laser power of 100 mW, power spectral density of detected shot noise was 12 dB higher than the level of technical noise of the measuring system in the frequency band above 3 GHz.

Topology of a hybrid waveguide device, which performs an effective transformation of a standard gradient titanium in-diffused waveguide mode to a hybrid waveguide mode, is considered. With its help a rather large optical mode with size optimal for coupling with standard single-mode fibers can be converted to a mode with a smaller size. Two the most perspective materials for hybrid waveguide fabrication were considered: silicon and titanium dioxide. The theoretical analysis has shown that transformation efficiency of more than 99% is achievable for waveguide devices based on titanium dioxide with contact lithography resolution.

Представлены оригинальные разработки в области интегральной оптики на подложках ниобата, начиная с традиционных высокочастотных модуляторов для систем квантовой криптографии и заканчивая демонстрацией отдельных компонентов квантовых фотонных интегральных схем.

Впервые продемонстрирована работа квантового генератора шума в полосе не менее 4 ГГц и с превышением квантовых шумов над классическими на величину 12–13 дБ. Впервые основной элемент такого генератора – ​оптический светоделитель с электрически-­управляемым коэффициентом деления – ​выполнен в виде интегрально-­оптического интерферометра Маха-­Цендера на подложке из ниобата лития.

For the first time, a significant effect of passive (i. e., without additional amplification) optical feedback on the number of spectral components of the output signal of a phase microwave integrated optical modulator has been demonstrated. The conditions are revealed under which the number of additional spectral components increases to 2 × 20 pieces, and the spectral range covered by them reaches up to 69 GHz. It is proposed to use this effect to increase the transmission rate of the quantum key, as well as to expand the functionality of optical communication systems using based on DWDM standards. electro- optical oscillator [8]: a system consisting of an electro- optical modulator, a beam splitter that removes part of the radiation through a light filter to a photodiode, an electronic amplification and filtering system, and a microwave divider that returns a part of the pro-cessed signal from the photodiode to the electronic input of the modulator [9]. Even in the original idea with the selection of one frequency on the photodetector after the RF amplifier on the modulator, higher- order harmonics can naturally arise. When several side frequencies return to the electronic feedback, the system modulates each of them. This significantly increases the number of spectral peaks.

Optical tuning of the splitting ratio in an integrated optical Y-branch on lithium niobate (LiNbO3) substrate was investigated. Regions of an Y-branch susceptible to the external illumination were defined. Tuning of the splitting ratio in the range of 2% was experimentally demonstrated, which can be effectively used for improvement of Mach-Zehnder modulators extinction ratio.

The influence of standing acoustic waves and parasitic traveling surface acoustic waves on instabilities of optical signals at the output of an integrated acousto-optic frequency shift modulators is considered. The method for signal stabilization is suggested and verified.

The availability of thin titanium film laser modification for precise loss control in lithium niobate optical waveguides was demonstrated. A simple ray model of the interaction between a nanosized metal cover on the top surface of a Ti-indiffused channel waveguide and orthogonal optical waveguide modes with changes in optical losses up to 0.95 and 1.05 dB/mm for TE- and TM-polarized modes respectively in case of 5 nm titanium film covering was suggested. The consistency between obtained theoretical losses and results of optical film modification by a 976 nm laser beam with the threshold intensity up to 1 kW/mm2 was demonstrated. The suggested method was applied for precise balance of optical field intensities in arms of Mach-Zehnder optical modulator; an associated increase of the extinction ratio from 30 to 48 dB was achieved.

Впервые продемонстрировано существенное влияние пассивной (т. е. без дополнительного усиления) оптической обратной связи на количество спектральных компонент выходного сигнала фазового СВЧ интегрально-­оптического модулятора. Выявлены условия, при которых количество дополнительных спектральных компонент увеличивается вплоть до 2 × 20 шт, а перекрываемый ими спектральный диапазон достигает 69 ГГц. Предложено использовать данный эффект для увеличения скорости передачи квантового ключа, а также для расширения функциональных возможностей оптических систем связи, использующих спектральное уплотнение (DWDM).

The possibility of correcting the power coupling ratio of an integrated optical directional X-coupler upon excitation of a local photorefractive response in the substrate material (LiNbO3) has been analyzed. The X-coupler operation has been numerically simulated, and the regions of its maximum sensitivity to photorefraction have been determined. The correspondence between the calculated and experimental data has been obtained. The maximum correction value of the coupling ratio is 1–2%. The photorefractive correction of the coupling ratio has been used to increase the extinction ratio of a Mach–Zehnder modulator to 47 dB.

We have studied the excitation, propagation, and interaction of surface and pseudosurface acoustic waves in X -cut lithium niobate substrate. Resonance excitation frequencies, propagation velocities, and dispersion characteristics for the acoustic waves of each type are determined. It is established that the electrode material influences the efficiency of excitation and interaction between the acoustic waves of various types. Requirements for the material and configuration of electrodes in integrated acousto-optic devices are determined.

AbstractThe possibility of correcting the power coupling ratio of an integrated optical directional X-coupler upon excitation of a local photorefractive response in the substrate material (LiNbO_3) has been analyzed. The X-coupler operation has been numerically simulated, and the regions of its maximum sensitivity to photorefraction have been determined. The correspondence between the calculated and experimental data has been obtained. The maximum correction value of the coupling ratio is 1–2%. The photorefractive correction of the coupling ratio has been used to increase the extinction ratio of a Mach–Zehnder modulator to 47 dB.

Исследованы процессы возбуждения и воздействия акустических колебаний на оптические сигналы в интегральных электрооптических модуляторах на подложках ниобата лития. Определены резонансные частоты возбуждения колебательных мод подложки. Показано, что акустические колебания могут вносить значительный вклад в передаточную характеристику модулятора на частотах до нескольких сотен мегагерц, что должно учитываться при использовании модуляторов на низких частотах, например, в составе волоконно-оптических датчиков. DOI: 10.21883/PJTF.2017.21.45166.16559

The processes of the excitation of acoustic vibrations and their influence on the optical signals in integrated electro-optic modulators on substrates of lithium niobate have been investigated. The resonant frequencies of the excitation of substrate vibrational modes have been determined. It has been shown that acoustic vibrations can make a significant contribution to the modulator transfer characteristic at frequencies up to several hundred megahertz, which should be taken into account when using the modulators at low frequencies, for example, as a part of fiber-optic sensors.

The influence of technological parameters of the process of thermal diffusion of titanium in lithium niobate substrates on the polarization-dependent losses of obtained optical waveguide has been theoretically studied. It is established that the anisotropy of refractive index variation leads to different conditions of polarization eigenmode cutoff that can be used for separating extraordinary and quenching ordinary polarization modes. Experiments with titanium-diffused waveguides showed the possibility of extraordinary polarization mode separation above 40 dB in the C-range (1530–1565 nm) of telecommunication wavelengths.

A method to increase the transmission efficiency of an analog fiber-optic line using a shift of the working point of an external modulator toward minimum transmission is proposed. The method is based on an increase in the signal contrast owing to the suppression of the constant component and the application of an erbium-fiber amplifier in the near-saturation mode. An increase in the transmission (up to 5 dB) in comparison with the transmission corresponding to the quadrature working point is demonstrated. An increase in the nonlinear distortion factor due to the shift of the modulator from the quadrature working point is analyzed. An increase in the nonlinear distortions under the conditions for the maximum transmission is no greater than 0.5%. A theoretical model is developed to describe the experimental effects. The theoretical results are in good agreement with the experimental data.

The use of a saturable absorber for increasing the extinction ratio at external modulation of optical signals is considered. An erbium doped fiber was used as the saturable absorber in the experiments. A considerable increase in the static extinction ratio (up to 50 dB) was demonstrated. A rather long erbium doped fiber relaxation time (about 10 ms) was a limiting factor in the case of pulse modulation. Ways of overcoming this drawback are discussed.

A paradigm has been developed according to which an additional FM signal transfer channel is formed in a standard optical fiber transmission line without violating the main AM channel operation. Using the proposed approach, an RS-232 interface signal has been experimentally transferred via a standard intraobject 100-Mbit Ethernet line based on an SMF-28 single-mode optical fiber.

The working frequency band of an integral electro-optical modulator (EOM) significantly depends on the specific electric conductivity of the material of electrodes. In the case of optimum matching between velocities of the light wave and modulating microwave, the frequency dependence of the EOM response is determined by losses in electrodes related to the skin effect. In this case, the passage from traditional gold to silver (having a higher specific conductivity) provides for a 1.4-fold increase in the EOM bandwidth.

Dependence of the optical properties of a microstructured (holey) fiber with a silica glass core on the refractive index of a liquid that fills the channels in the cladding has been studied. Optimum conditions ensuring a single-mode regime, high degree of the interaction of radiation with the liquid, and low losses are determined for the fiber with a silica glass core and a microcapillary structure geometry parameter of d /Λ = 0.9.