The technique for estimation of an integrated-optical modulator electro-optical response without optical measurements was developed and tested for a lithium niobate modulator. The electro-optic response prediction was based on experimentally measured propagation characteristics of high-frequency coplanar electrodes using numerical simulation results of optical waveguides. The presented technique can be used to perform rapid tests on a wafer in production of integrated optical modulators.

A method for fast estimation of electrooptic frequency response of integrated optical traveling wave modulators is presented. It is based on measurement of Sparameters of modulator electrodes and results of simulation of optical waveguide of the modulator only and does not need direct light coupling. The presented approach has been successfully demonstrated for experimentally fabricated modulators on thin-film lithium niobate substrates. The correspondence between estimated and directly measured frequency dependencies of electrooptical response of modulators has been shown.

A high-frequency phase modulator based on a multimode optical waveguide made of thin-film lithium niobate was designed and studied. The modulator was fabricated in a topology with metal electrodes placed near the edges of the waveguide to suppress higher-order waveguide modes and provide quasi-single-mode light propagation. Efficient modulation with a voltage-length product $V_{\pi} L \approx 4 \mathrm{~V} \cdot \mathrm{cm}$ and a bandwidth of more than 30 GHz have been successfully demonstrated.

The results of investigation of modal filtering and providing quasi single-mode operation regime of an inherently multimode optical waveguide phase modulator based on thin-film lithium niobate (TFLN) are presented. It was shown that decrease of interelectrode gap can induce different propagation losses to optical modes of an multimode waveguide. It was also shown that decrease of interelectrode gap and narrowing distance between edges of electrodes and the waveguide can provide high-order mode suppression at the level of 60 dB/cm.

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.

It is shown that laser modification of a nanometer titanium film can be used for precise control of the loss level in optical waveguides on lithium niobate substrates. Theoretical estimates of the change in linear losses at the level of 0.95 dB/mm were confirmed by experiments on local laser modification using radiation at a wavelength of 976 nm with a threshold intensity of 1 kW/mm 2 . The proposed method can be effectively used for precision adjustment of the characteristics of integrated optical devices, such as the modulation contrast of Mach–Zehnder modulators.

The optical tuning of the power splitting ratio in an integrated optical Y -splitter fabricated on the substrate of a photorefractive lithium niobate crystal (LiNbO 3 ) has been studied. The areas of the Y -splitter that are most sensitive to local external illumination are determined. Tuning of the splitting ratio by 2% was experimentally demonstrated, which can be effectively used to increase the extinction ratio in Mach–Zehnder modulators.

A method for precise optical loss adjustment by laser modification of a thin titanium film on the surface of lithium niobate optical waveguides was developed. Precise balancing of Mach-Zehnder modulator was demonstrated and an increase of the extinction ratio from 30 to 48 dB was achieved.

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.

Superconducting nanowire single-photon detector on the lithium niobate substrate was demonstrated in integrated optical configuration. New waveguides configurations for improvement of quantum efficiency are discussed.

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.

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.

A method for photorefractive trimming of lithium niobate integrated optical modulators was developed. An improvement in the extinction ratio in a Mach-Zehnder (MZ) modulator by photorefractive precision adjustment of its couplers was achieved. An increase in the extinction ratio by 18 dB (from 30 to 48 dB) was demonstrated.

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.