Abstract—A two-channel high-frequency acousto-optic (AO) modulator-splitter has been developed, which splits optical radiation into two equal optical channels and modulates the intensities of the output channels at double the sound frequency. The modulator makes it possible to simply change the polarization of the output beams. A model of a modulator made of a TeO2 single crystal was used to modulate and split radiation with a wavelength of 0.63 × 10–4 cm. Intensity modulation of the output channels at a frequency of ~200 MHz was achieved.

A method of acousto-optical (AO) Bragg diffraction is proposed that provides the amplitude modulation of optical radiation at a doubled acoustic frequency. The method is based on the double transmission of the light through the AO modulator made of a gyrotropic crystal and is experimentally tested by the example of the modulation of light with a wavelength of 0.63 mu m, controlled by the paratellurite AO cell.

Three acousto-optic materials, ТеО2, SiO2, and LiNbO3, widely used in production of acousto-optic cells for pulse modulation of the multicolor optical radiation, have been compared. It is demonstrated that the ТеО2 acousto-optic cell has an undoubted advantage in terms of the electric power consumption, first of all, due to its high acousto-optic figure of merit, while SiO2 and LiNbO3 cells are preferable from the viewpoint of the lowest distortions in transformation of electrical pulses into optical signals. Main theoretical conclusions have been confirmed by experiments on modulation of the multicolor Ar laser radiation.

A method for the optical beam production with the rotating polarization vector based on the interference of two beams with the circular polarizations is proposed. The frequency shift between beams is implemented by means of acousto-optic (AO) diffraction. The method is used for the amplitude light modulation with the frequency nf where f is acoustic frequency and n is integer. AO modulators are fabricated from paratellurite crystal. Modulators allow modulating the optical radiation with wavelength of 0.63 mcm at the quadruple frequency of the acoustic wave. The modulation frequency achieves 180 MHz. Keywords: acousto-optic diffraction, Bragg regime, frequency shift, rotating polarization vector.

Methods for the calculation of the acousto-optic (AO) transfer functions forming as a result of superposition of two AO fields with similar and orthogonal light polarizations are proposed. Specific calculations are made on the basis of the parameters widely used in practice with uni-axial gyrotropic crystal paratellurite. It is revealed that the resulting field is generally very inhomogeneous but contains domains with two-dimensional behavior properties. These domains can be used for two-dimensional image edge enhancement based on Fourier processing. Theoretical conclusions are verified by means of the optical image Fourier processing with the spatial frequency AO paratellurite filters.

During the last decade, pulsed laser radiation has gained interest by the material processing industry and the medical sector. For a growing set of applications (laser drilling, laser marking, laser surgery, semiconductor doping profiling, micro-structuring, layer deposition, etc.) it is advantageous to use pulsed laser radiation instead of continuous wave (CW) illumination as time limited exposure often results in reduced collateral damage and more precise processing (Phipps, 2007). In laser ablation for example, one aims to put an intense laser pulse on the surface of a target material in an as short time as possible. This short exposure time, which limits thermal diffusion inside the material, together with a carefully selected wavelength with a minimal absorption depth is required to ensure energy deposition in a small volume of the target material. Hence, for laser pulses which meet the ablation requirements, one can evaporate material in a very controlled fashion. Different methods exists as Q-switching, mode-locking and cavity dumping for achieving the required pulse characteristics for laser ablation. Laser pulses span an enormously large parameter space in terms of wavelength, repetition rate, pulse duration and pulse energy, further referenced as pulse-parameters. Each of these pulse-parameters can be optimized for a given application and target material. Besides its dependence on the temporal characteristics of pulsed laser radiation, for some applications, the processing quality is also strongly dependent on the transverse laser beam profile. Consequently, there is a growing interest in detecting the spatio-temporal behavior of laser pulses. In this chapter we briefly describe the most common infrared detector principles for measuring laser pulses and point out their respective advantages and disadvantages with respect to different pulse-parameters. Next, we show that Seebeck-effect based thermo-electric photodetectors can be designed to cover a relatively broad range of pulse-parameters (Stiens, 2006). Further, we discuss the working principle and operation regimes of the thermo-electric photo detector and explain the corresponding theoretical background in detail. Experimental results concerning short laser pulse induced thermo-voltages in n-GaAs are presented. This chapter is also concerned with the possibility of using the thermo-electric effect to measure the spatio-temporal behavior of laser pulses by means of linear focal plane arrays (LFPA). Certain related issues will be highlighted such as

Here, we demonstrate a novel Terahertz (THz) wave characterization technique and modulation devices developed at ETRO-IR (VUB) that facilitate the detection of Chemical Biological Radiation and Nuclear (CBRN) hazards. First, a novel transient radar technique is presented for a Non-Destructive Testing (NDT) of a multilayered dielectric object. A transient reflected electromagnetic wave is captured with femtosecond time resolution, so it can be able to reconstruct both the dielectric and thickness information from multitude of scattered waves from different dielectric interfaces in a multilayered object. A blind reconstruction algorithm does not require any priori knowledge of the object under test. Proof-of-principle is demonstrated experimentally at 10 GHz on a three-layered object, featuring subwavelength depth resolution. Second, amplitude and phase modulators are proposed based on hollow metal pipe waveguide loaded with single layer graphene on a dielectric substrate. It is demonstrated that the modes propagation constants can be substantially modulated by the graphene Fermi level modulation especially near the regions of strong mode modifications. Solutions of wave propagation equations are given and design parameters are optimized for implementing an efficient Terahertz modulator devices. Fast 3D object scanning and detection of CBRN hazards can be implemented in THz by means of transient radar with phased antenna array with graphene phase shifting modulators.

The dispersion equation for waveguide modes of rectangular metal waveguides containing a dielectric buffer/graphene/air gap layered structure has been derived in the approximation of perfectly conducting metal waveguide walls. Dispersion dependences of TE0m modes for sub-terahertz frequencies were analysed against buffer layer thickness and Fermi energy level of graphene electrons. For the first time, it is demonstrated that the waveguide modes can be substantially modulated by tuning the electron Fermi energy level in graphene, especially near the regions of strong waveguide mode modifications. This enables the potential for future designs of amplitude and phase modulators for sub-THz waves in metal waveguides filled with an integrated graphene structure.

Sub-terahertz (THz) frequency is attracting extensive interests for a wide range of applications. Electronically tunable phase shifters as one of the key building blocks in sub-THz systems, especially waveguide based phase shifters, encounter difficulties in both design and fabrication. In this paper, an electronically tunable rectangular metallic waveguide with graphene integrated operating in sub-THz frequency is proposed and its phase modulation properties are investigated. It shows that a maximum 34-degree linear phase shift is obtained at 315 GHz with bandwidth more than 50 GHz, and the reflection and maximum insertion loss are lower than -15 dB and 3.5 dB in this frequency range, respectively. The phase modulation feature can be applied to design of graphene based waveguide phase shifters and other graphene based waveguide phase modulators.

Предложена и экспериментально подтверждена возможность формирования двумерного контура оптического изображения одновременно в двух дифракционных порядках в процессе его фурье-об-работки с использованием трехкратной брэгговской дифракции. Развита новая модель трехмерного акустооптического взаимодействия в гиротропном кристалле, рассчитаны передаточные функции. Теоретические результаты обработки изображения подтверждены экспериментально с использованием трехкратной брэгговской дифракции, происходящей в кристалле TeO2.

The possibility of formation of the edge of a 2D optical image in two diffraction orders by means of Fourier processing with the use of triple Bragg diffraction is proposed and confirmed experimentally. A new model of 3D acousto-optic interaction in a gyrotropic crystal is developed, and transfer functions are calculated. The theoretical results of image processing are confirmed experimentally using triple Bragg diffraction in a TeO2 crystal.

GSolver software is used to optimize the parameters of a GaAs-based layers structure for modulating the reflectivity of light. This structure can be used in an Integrated Mirror Optical Switch (IMOS) for the Q-switching technology. A system of low doped GaAs and highly doped AlGaAs structure is built on a binary diffraction grating composed of germanium and gold. The diffraction efficiency is determined with and without the existence of free carriers in the highly doped layer. The impact of the sheet charge density at the interface of the heterostructure is considered in determining of the diffraction efficiency. At the end of the study, the structure parameters and thicknesses are determined for a high sensitive device.

We have studied acousto-optic Bragg diffraction of multicolour radiation, generated by an Ar laser in the blue-green region of the spectrum, on an acoustic wave propagating in crystalline quartz. It is shown that crystalline quartz significantly exceeds commonly used paratellurite in terms of phase matching of optical beams with a single acoustic wave. We have performed experiments on pulse modulation of Ar-laser radiation. It is shown that distortions introduced into optical pulses are substantially less when use is made of a quartz crystal rather than paratellurite.

Fundamentals of the mode method including orthogonality conditions between different modes and derivations of coupled equations systems for unknown expansion coefficients are given in the paper for the case of 2D isotropic structures. Using the mode method, several concrete problems of ultrasonic waves diffraction in non-homogeneous layered structures are presented in the paper such as: Scholte-Stoneley wave excitation and conversion at the edge of a liquid loaded plate, interaction of Lamb waves with delaminations in plates coated by highly absorbing materials and nonlinear modulation of Lamb modes by clapping delaminations.

The underlying working principle of detecting impulsive stimulated scattering signals in a differential configuration of heterodyne diffraction detection is unraveled by involving optical scattering theory. The feasibility of the method for the thermoelastic characterization of coating-substrate systems is demonstrated on the basis of simulated data containing typical levels of noise. Besides the classical analysis of the photoacoustic part of the signals, which involves fitting surface acoustic wave dispersion curves, the photothermal part of the signals is analyzed by introducing thermal wave dispersion curves to represent and interpret their grating wavelength dependence. The intrinsic possibilities and limitations of both inverse problems are quantified by making use of least and most squares analysis.

In this paper we focus on the dispersion characteristics of TM and TE waveguide modes and the reflectivity of plane waves, incident on the four-layer structure consisting of air/graphene monolayer/dielectric buffer layer/metal substrate. The TM waveguide modes split up into two branches for small frequencies, one of the branches (cutoff waveguide branch) undergoes cutoff at a certain cutoff buffer thicknesses. There is no splitting of TE waveguide modes. However, these modes can be converted into short-range waves for smaller buffer thickness with subsequent modes cutoff depending on frequency and graphene electron concentration. Reflection coefficients of the TM polarized incident electromagnetic waves from air on the multilayer structure vanish in the vicinity of cutoff buffer thicknesses. It is demonstrated that waveguide mode propagation constants and reflectivity in the multilayer structure can be considerably influenced by the presence of a graphene layer in the vicinity of the cutoff thicknesses of the waveguide modes.

TM surface plasmon (SP) characteristics of a four-layer structure, consisting of air as the superstrate, a monolayer of graphene, a dielectric buffer layer and metal as the substrate are analyzed at sub-THz frequencies. TM SPs in such case are represented by metal-like and graphene-like branches. For small frequencies the metal-like plasmon splits up into two branches depending on the graphene electron concentration; one of the branches goes into cutoff at the point where the branch features Brewster-type characteristics. Graphene-like plasmon modes are converted into short-range modes for small buffer thicknesses. Brewster-type SP modes can be effectively modulated in the vicinity of their cutoff thicknesses by means of the graphene electron concentration.