Optoelectronics, Instrumentation and Data Processing (v.51, #6)

Results of a detailed study of a gas-discharge He–Ne laser with a wavelength of 1.52 µm for telecommunications systems are reported. Some important parameters of the laser are measured, such as the laser beam divergence (M 2), the laser power as a function of temperature, laser power stability, noise characteristics of laser radiation, etc. A possibility of laser microminiaturization is mentioned, which is of interest for its application under conditions of intense electromagnetic interference.
Keywords: laser; gain factor; wavelength of 1.52 µm; Gaussian beam; mode composition

A simulation model for laser diode modulation by multichannel 64-QAM and 256-QAM TV signals is developed. The signal/noise (S/N) ratio is used as a criterion for evaluating the quality of TV signals. The effect of nonlinearity of the light-current characteristic on the S/N ratio is studied. Simulation results are presented in the form of a dependence of the maximum number of transmitted channels on the optical modulation depth with certain requirements for S/N. The simulation model is implemented in the object-oriented programming language Delphi 7.
Keywords: clipping; signal/noise ratio; subcarrier TV channel

Long-distance fiber-optic quantum key distribution using superconducting detectors by V. L. Kurochkin; A. V. Zverev; Yu. V. Kurochkin; I. I. Ryabtsev; I. G. Neizvestnyi; R. V. Ozhegov; G. N. Gol’tsman; P. A. Larionov (548-552).
This paper presents the results of experimental studies on quantum key distribution in optical fiber using superconducting detectors. Key generation was obtained on an experimental setup based on a self-compensation optical circuit with an optical fiber length of 101.1 km. It was first shown that photon polarization encoding can be used for quantum key distribution in optical fiber over a distance in excess of 300 km.
Keywords: quantum cryptography; single photon detectors

Active mode-locking in semiconductor lasers by V. F. Zakhar’yash; A. V. Kashirskii; V. M. Klement’ev (553-559).
This paper presents the results of experimental studies of active mode-locking in an external-cavity semiconductor laser. Two methods of obtaining active mode-locking regimes have been studied: by modulation from an external generator with a frequency close to the intermode frequency of the external laser cavity and by laser current modulation at the frequency of intermode beats of the external laser cavity through an amplification feedback loop. In the first case, a stability of intermode beats of 10-14 is experimentally achieved, and in the second case, it is shown that semiconductor laser current modulation by an amplified intermode beat signal from the output of a photodetector provides a stable mode-locking regime also with mode frequency tuning, i.e., a mode-locking regime involving intermode frequency tuning with optical cavity tuning. The effects that accompany mode locking, in particular, the pulling effect and the stability of the mode-locking regime, and possibility of obtaining highly stable intermode beats by suppressing the amplitude noise component in the modulating signal have been studied. A phenomenological study of the pulling and phase locking of the intermode frequency is attempted.
Keywords: active mode locking; semiconductor laser; external cavity

The shape of the diffractive structure of a deflecting kinoform element designed for focusing high-power laser beams with a wavelength of 10.6 µm is studied. Formulas are derived for calculating light losses induced by tilting of the incident beam, diffraction, and technological blurring of the steep slope of the structure. It is demonstrated that the height of the structure varies from the minimum to the maximum value depending on the azimuthal angle. It is found that light loss induced by shadowing due to oblique incidence of the beam is rather low (0.6–1.3%), and that caused by diffraction is smaller by an order of magnitude and can be neglected. The light losses induced by technological blurring of the slope and by deviation of the structure height from the design value (in the case of violation of the azimuthal dependence) can exceed 10%.
Keywords: deflecting focusing element; diffractive structure; efficiency; light loss

Adaptive compensation of atmospheric distortions of multichannel laser radiation by O. L. Antipov; F. Yu. Kanev; N. A. Makenova; V. P. Lukin; E. I. Tsyro (568-572).
This paper presents the results of modeling the propagation of multichannel radiation under free diffraction conditions and discusses features inherent in this type of beams under these conditions. In addition, the effect of distortions arising in a turbulent atmosphere on radiation quality is considered, and the efficiency of adaptive compensation of atmospheric turbulence is analyzed.
Keywords: multichannel optical systems; atmospheric turbulence; phase conjugation

Natural oscillations of a spherical interface between a gas and a liquid in a bubble are registered. A possibility of measuring the geometric parameters of stationary and moving particles of the disperse phase by a laser Doppler anemometer is demonstrated. A method for simultaneous determination of the size and velocity of a bubble or a droplet in a two-phase flow is developed. The mean sizes of a group of bubbles settled on a ruler are compared: the results are obtained by two independent methods, i.e., by analyzing the image and by processing the Doppler signal containing information about the natural oscillations of the spherical interface between the media. A possibility of using a laser Doppler anemometer for simultaneous measurements of the velocity and size of bubbles or droplets in a two-phase flow is confirmed.
Keywords: optical diagnostics; laser Doppler systems; gas bubbles in a liquid; natural oscillations of a spherical interface; gas–liquid flows

Theoretical analysis of the energy evolution in dissipative soliton fiber lasers by A. S. Skidin; I. A. Yarutkina; O. V. Shtyrina; M. P. Fedoruk (577-581).
Theoretical results on the energy evolution in ring and linear cavities of dissipative soliton fiber lasers are analyzed. The correctness of the theoretical results is confirmed by mathematical modeling.
Keywords: laser theory; mathematical modeling; linear cavity; ring cavity

Rapid method for evaluating the efficiency of excitation energy transfer between ytterbium and erbium ions in an active fiber preform by A. S. Smirnov; K. P. Latkin; Ya. E. Sadovnikova; E. G. Leksina; A. S. Kurkov (582-586).
A rapid method is proposed to evaluate the efficiency of excitation energy transfer between ytterbium and erbium ions from measured relative luminescence intensity in different spectral regions in an active fiber preform, taking into account the concentration of active ions and their spectroscopic constants.
Keywords: erbium; ytterbium; erbium–ytterbium fiber; excitation energy transfer efficiency

This paper presents a method for calculating the cross-wind speed at the entrance aperture of an adaptive system from the coordinates of the energy centers of gravity of the focal spots measured by a Shack–Hartmann wavefront sensor. The range of applicability of the method are determined for data obtained in an experiment on an atmospheric path depending on the intensity of turbulent distortions and the optical parameters of the sensor.
Keywords: Shack–Hartmann wavefront sensor; centroid coordinates; turbulent distortions; wind speed

The process of interactive modeling of geometric shapes defined by mathematical functions is studied. A method of interactive modeling of functionally defined objects without preliminary triangulation is proposed. This allows more accurate definition of three-dimensional shapes and simplifies the modeling system. A method adapted for graphic processing units is used for rapid visualization of objects.
Keywords: interactive modeling; geometric objects; perturbation functions

The current status of the list of elements of uncooled infrared (IR) imaging arrays based on thermal insulation of their sensors and on accumulation of heat in the element volume during the time necessary for taking one frame is considered. Specific features of operation and the structure of resistive microbolometers and pyroelectric elements are described, as well as typical parameters of these elements. Prospects of improving the parameters of thermally insulated sensors of megapixel arrays are considered. It is noted that the main factor limiting further improvement of the parameters of pixels in IR imaging arrays and increasing the size of the latter is the value of the thermal resistance between the sensor and the substrate. At the modern level of technology development, the thermal resistance has actually reached the limiting value. Another physical principle of operation of IR detectors is required for a further increase in the sensitivity of sensors, a decrease in their response time, and an increase in the size of uncooled megapixel IR arrays; this principle should not include the necessity of thermal insulation of IR detectors. The operation principle based on using pyrotechnical effects will be considered in the second part of the paper.
Keywords: IR engineering; IR detectors; thermal insulation; microbolometers; pyroelectricity; thermal resistance; megapixel array; IR imaging system

An experimental method of fabrication of a sensor based on a metal–dielectric structure (Al + ZnS) and optimization of its characteristics is described. The coefficient of light reflection (p-polarization) from the aluminum layer is studied as a function of the layer thickness for different angles of incidence at the wavelength of 532 nm. Based on calculations, which are qualitatively consistent with experimental results, a structure consisting of matched layers of aluminum and zinc sulfide is fabricated; this structure has a higher angular resolution than the aluminum film with no dielectric coating. The detection limit of angular measurements by the sensor based on this structure is estimated as 2.6 · 10-5 RIU (refraction index units).
Keywords: frustrated total internal reflection; thin metal film; metal–dielectric interference structure; refractive index sensorics

A method for calculating the distance characteristic of optoelectronic systems is proposed which provides increased accuracy in comparison with known techniques. Theoretical results are compared with experimental data.
Keywords: optoelectronic system; calculation of parameters; distance characteristic; ray tracing

Algorithms for improving the resolution of a sequence of images distorted by applicative noise manifested in the form of randomly shaped local regions of occlusion are synthesized and analyzed. The basis is the implementation of an optimal linear filtering algorithm in block form, which is synthesized with account for possible emergence of false observations in individual elements of the analyzed images. A method for incorporating the results of independent segmentation of each individual image into the recursive filtering process is proposed, which enhances the quality of the final processing.
Keywords: image processing; segmentation; applicative noise; super resolution; the Kalman filter

This paper describes the structure and control algorithms of a controller of a high-voltage source with an output voltage of 60 kV and a power of up to 30 kW. The source is designed to be used as part of a power unit of an electron-beam welding device developed at the Budker Institute of Nuclear Physics of the Siberian Branch of the Russian Academy of Sciences (BINP SB RAS). High-quality welds require a stable electron beam energy. Herewith, a standard operation mode is quick and deep modulation of the electron current. The problem of achieving high-quality control and limiting the transient distortions of output voltage of the source is solved. The error of establishing and stabilizing the output voltage is 0.1%, which allows obtaining high-quality welding. Transient distortions at 100% modulation of the load current do not exceed the value of ±1%, which, along with fast reaction of the source to a load breakdown (energy released during breakdown is smaller than 15 J), protects the welded parts and elements of the gun from being damaged by an electron beam.
Keywords: electron beam welding; high-voltage source; transient process; voltage stabilization; adaptive feedback

Nonpositional analog-digital conversion methods by A. A. Kozhevnikov; K. P. Bespalov (638-642).
This paper describes the analysis of analog-digital converters operating in positional and nonpositional number systems. A new method for analog-digital conversion on the basis of the features of a modular number system is proposed.
Keywords: analog-digital converter; residue number system; aliasing