An experimental study of the acoustic pulse duration influence on the spatial resolution of ultrasound beam structure visualization in paratellurite crystal quasi-collinear acousto-optic cell (AOC) is presented. Detailed spatial distributions of the acoustic power are obtained over the entire volume of the AOC in the (1-10) plane and in the orthogonal plane using time-resolved acousto-optic (AO) diffraction of a narrow laser beam. It is shown that applying short acoustic pulses with a duration of about $1 \mu \mathrm{s}$ provides high-quality images of the acoustic power distribution and enables visualization of small-scale inhomogeneities within the $\mathbf{A O}$ interaction region. Based on the experimental data, the impact of inhomogeneous acoustic beam structure, acoustic power absorption in paratellurite, and temperature gradients on the shape of the quasi-collinear AO cell transmission function is analyzed. The obtained results are important for optimizing quasi-collinear AO filters and dispersion delay lines used for multi-channel spectral filtering and femtosecond laser pulse shaping.

The acoustic and acousto-optic (AO) properties of the Ga10Ge15Te75 glass, which is promising for creation of AO devices operating with the near and middle infrared optical radiation, are studied. In this paper, the experimental results for the longitudinal and shear acoustic waves phase velocities and the AO figure of merit (M2) in case of isotropic diffraction by these waves are presented. All measurements are carried out for the 3.39 mu m optical wavelength. The obtained M2 magnitudes turned out to be close to 1000 & sdot;10-15s3/kg that significantly exceeds the typical values observed in germanium (Ge) crystal - the basic material for middle infrared AO devices fabrication. The experimentally determined longitudinal acoustic wave attenuation coefficient for 100 MHz ultrasound frequency turns out to be 1.32 +/- 0.05 cm-1. Finally, the acoustic and AO properties of telluride based glasses with the close chemical composition, namely Ga10Ge15Te75, Si25Te75 and Ge25Se15Te60 glasses, are compared.

We present the results of an optoelectronic oscillator based on collinear acousto-optic (AO) filter characterization. The system includes hybrid feedback and operates above the self-excitation threshold. Thus, it does not require the RF generator to arouse ultrasound in the AO cell. The rise time and delay time dependencies on the optoelectronic feedback parameters were defined. It was found that by tuning the feedback gain it is possible to vary the system time characteristics in a fairly wide range. The examination of the feedback loop phase shift influence on the system operation made it possible to discover the existence of multistability, manifested in an abrupt change of the feedback loop RF signal frequency and, consequently, the optical radiation intensity at the AO cell output. The multistability existence was explained in terms of the phase balance condition.

In this paper, an optoelectronic generator consisting of a collinear acousto-optic (AO) filter and a hybrid feedback loop connecting the filter optical output and its piezoelectric transducer is examined theoretically and experimentally. Analysis of signal propagation through the system made it possible to obtain expressions for the phase and amplitude balance conditions and to explain the previously experimentally discovered operation multistability. It manifests as the dependence of the AO generator operating frequency, output radiation intensity, and the collinear AO diffraction transfer functions shape on the electrical signal phase shift magnitude and delay time introduced by the hybrid feedback loop.

The frequency-shifted feedback (FSF) laser is an optoelectronic system with an optical feedback loop with a characteristic feature that consists of a broadband optical emission spectrum that does not contain modes. Nevertheless, this system may operate in the so-called mode-locking regime, when the output optical signal is a sequence of short optical pulses with a repetition rate determined by the delay time in the feedback loop. In the frequency domain such a sequence forms the optical frequency comb with line spacing defined by time delay magnitude. It was shown recently that the sequential frequency down-conversion technique allows us to obtain tunable dual- and quad-combs when the FSF laser is seeded only with optical amplifier spontaneous emission. We propose to apply these combs to fulfill measurements associated with time delay variation, such as distance ranging and optical material refraction coefficient measurements.

The theoretical and experimental study of acoustic anisotropy effect on the structure of the acoustic field in tellurium dioxide crystal quasi-collinear acousto-optic (AO) cells is fulfilled. The obtained results reveal potential improvements of acousto-optic dispersion delay lines in the laser pulse shaping applications. The optical characteristics such as spectral resolution, AO interaction phase matching frequency, possible optical pulse compression rate of the studied quasi-collinear AO device were also examined and compared with theoretically predicted.

The divergence of a diffracting light beam affects the characteristics of acousto-optic (AO) interaction. This is due to the side components of the optical beam spatial spectrum propagating in the crystal along the directions different from the beam axis. The divergence causes the AO diffraction efficiency decrease and the distortion of the AO device transmission function — its bandwidth increases and the side lobes are smoothed. We suggest the method to compensate the optical beam effect on the collinear AO diffraction transmission functions shape based on optoelectronic feedback circuit application connecting the AO cell optical output and its piezoelectric transducer. It is shown that such method makes it possible to narrow the transmission function passband and increase the spectral contrast of the system. It is found out that feedback does not affect the AO diffraction transfer functions. Thus, the introduction of feedback makes it possible to increase the spectral resolution of AO devices without compromising the AO device angular aperture.

The reflection of acoustic waves in anisotropic media has found important applications in acousto-optics (AO). It may be realized both with changing the type of acoustic mode and without it. The latter case is applied for the realization of the quasi-collinear AO diffraction geometry for which the incident light and ultrasound wave group velocity vectors are collinear. The acoustic wave involved into the AO interaction exists due to the reflection from the AO cell input optical face. Quasi-collinear configuration of AO interaction enables achieving exceptionally high spectral resolution and diffraction efficiency, therefore, it is applied for the laser pulse shaping. However, the application of AO tunable filters for such purposes requires high acoustic power values, which induce the temperature gradients in the AO crystal. Temperature variations influence the stiffness constants of the AO crystal, impacting the characteristics of incident and reflected acoustic waves, affecting the acoustic wave reflection condition and consequently, the AO diffraction characteristics. This study presents the theoretical and experimental examination of the temperature influence on the acoustic beam reflection in anisotropic crystals on the example of tellurium dioxide crystal. This paper is supported by the Russian Science Foundation, grant 23-12-00057.

In this paper we continue to examine various types of optical frequency combs (OFCs) obtained with frequency down-conversion method in a frequency-shifted feedback laser operating in mode-locking regime without an external optical seeding. We propose to apply these combs to solve several problems associated with the feedback loop length and, consequently, time delay variations - for example, distance measurements, determination of the optical materials refractive index value and dispersion. The advantages of OFCs application obtained by the sequential frequency down-conversion method (dual- and quad-combs) in comparison with the initial OFCs, the spectral interval between the lines of which is determined only by the delay time in the feedback circuit, have been demonstrated. The sensitivity of the examined system to the optical path variation in the feedback loop is determined.

We present the results of an experimental investigation of the lithium based biaxial crystals' acoustic acousto-optic properties: lithium selenogallate (LiGaSe2), lithium indium selenide (LiInSe2) and lithium indate (LiInS2). The velocities of longitudinal and shear acoustic waves propagating along the crystal axes determined for the first time. We have also calculated the values of the six elastic constants c ii (i = 1-6). In paper we present the result of the experimental investigation of the acousto-optic figure of merit. We measured the acousto-optic figure of merit for the first time in the case of diffraction by the longitudinal acoustic waves. Using the obtained experimental data we have computed the values of nine photoelastic coefficients j = 1-3) for each investigated lithium based biaxial crystal.

Quasi-collinear geometry is a special configuration of acousto-optic (AO) diffraction that applies the acoustic wave reflection from the AO cell input optical face and provides an extremely large interaction length for achieving abnormally high spectral resolution of AO tunable filters. As a result, it becomes possible to implement the multifrequency diffraction which has found important applications for laser pulse shaping. The operation of quasi-collinear AO devices in the multifrequency diffraction regimen is accompanied by the appearance of the longitudinal and transverse temperature gradients in the crystal, mainly due to the acoustic power absorption. Temperature changes the AO cell material stiffness moduli, affecting the characteristics of the incident and reflected acoustic waves (propagation velocities and walk-off angles), and the reflection condition in general. On the example of paratellurite crystal is shown that the AO cell heating near the reflecting facet leads to a deviation of the reflected acoustic beam propagation direction from that specified during the AO cell manufacturing. The deviation magnitude depends on the reflection geometry choice and, in the paratellurite, may exceed several degrees, which adversely affects the AO diffraction characteristics, reducing the AO interaction efficiency and distorting the transmission function shape. The reflected beam deviation may be compensated by means of choosing the angle between the AO cell reflecting face and the piezoelectric transducer face, taking into account the operating AO device thermal regimen.

The results of a multistability operation observation in the acousto-optic (AO) system with feedback circuit (FC), functioning above the self-excitation threshold in the absence of an external RF generator are presented. Feedback loop connects the optical output and the piezoelectric transducer of the AO cell. The ultrasound frequency in the AO cell and the corresponding frequency of the radio frequency signal in the FC are determined by the collinear AO diffraction phase matching condition for the chosen wavelength of incident optical radiation. It was found out that the presence of multistability depends on the magnitude of the electrical signal phase shift introduced by the phase shifter and the delay time in the FC. Both these parameters variation may influence the RF signal frequency and magnitude.

Generation of dual-combs in frequency shifted feedback laser seeded only with optical amplifier spontaneous emission and containing single frequency shifting loop with single acousto-optic tunable filter as the frequency shifter is demonstrated. It is also shown that it is possible to obtain a pair of dual-combs simultaneously and to implement further down-conversion achieving the frequency spacing between the spectral components of the resulting optical comb equal to the frequency difference of these dual-combs and amounting to several tens of kHz. The OFC with 40 kHz frequency spacing containing more than 5000 spectral lines was observed. It is demonstrated that the generated dual-combs may be applied for optical materials refraction coefficient measurements.

The investigation of quasicollinear acousto-optic (AO) interaction implemented by reflecting an acoustic wave from the input optical face of an AO cell is fulfilled. AO cells on the base of three mercury halide crystals: chloride, bromide, and mercury iodide were examined theoretically. The parameters of acoustic beam reflection realization were calculated – the angle between the transducer and the optical input facet was determined and the acoustic beam widening in the reflection process was studied. The optical characteristics – spectral resolution, AO interaction phase matching frequency, possible optical pulse compression rate of the quasicollinear AO devices were also examined and compared with those obtained for tellurium dioxide crystal.

The operation of an optoelectronic generator with collinear geometry of acousto-optic interaction is studied, in which the feedback signal is formed due to the effect of optical heterodyning. The process of establishing different types of oscillations depending on the feedback coefficient and the phase mismatch between interacting optical and acoustic beams is considered. The issues of possible application of such systems are discussed.

The presence of light beams divergence affects the characteristics of acousto-optic (AO) diffraction. It leads to a decrease in the AO interaction efficiency and transformation of the AO devices transmission function - its bandwidth increases and the side lobes are smoothed. In this paper we propose to compensate the light divergence effect on the collinear AO diffraction transmission functions shape applying an optoelectronic feedback circuit connecting the AO cell optical output and its piezoelectric transducer. It is shown that the introduction of feedback makes it possible to narrow the passband and increase the spectral contrast of the transmission function. Unfortunately, feedback does not help to increase the maximal possible AO diffraction efficiency as it is determined by the optical beam divergence, since the feedback does not affect the AO device transfer function

The quasicollinear geometry of acousto-optic (AO) diffraction is notable as makes it possible to achieve an extremely high AO interaction length and, consequently, an anomalously high spectral resolution for AO devices. This geometry is especially convenient for the implementation of multifrequency AO diffraction, which has found wide application for solving the problems related to the laser pulse shaping. Since acoustic beams propagate over long distances in quasicollinear AO devices, and optical radiation spectral components diffract in the acoustic field in different parts of the AO crystal, accurate calculation of the characteristics of such devices requires knowing the distribution of the acoustic field amplitude inside the AO cell. The acoustic beam structure is affected by several factors in quasicollinear AO cells: the dimensions of the piezoelectric transducer, the geometry of acoustic wave propagation in the AO cell, acoustic anisotropy and the acoustic energy absorption along the chosen direction in the crystalline material used. In this paper, we propose a generic method to measure the acoustic beam power distribution along the direction of its propagation in the quasicollinear AO cell in the presence of ultrasound power absorption and media acoustic absorption. The measurements were carried out for the ultrasound frequency range from 72 to 176 MHz, for the case when the wave vector of the acoustic beam is directed at an angle of 1.58∘ to the [110] axis in the (11¯0) plane of the paratellurite crystal. The ultrasound attenuation coefficients were obtained for frequency interval between 87 and 176 MHz and their linear dependence on ultrasound frequency was confirmed.

The results of an optoelectronic system—frequency-shifted feedback (FSF) laser experimental examination are presented. The considered FSF laser is seeded only with optical amplifier spontaneous emission (ASE) and operates in the mode-locked regime, whereby the output radiation is sequence of short pulses with a repetition rate determined by the delay time in its optical feedback circuit. In the frequency domain, the spectrum of such a pulse sequence is an optical frequency comb (OFC). These OFCs we call initial. We consider the possibility of tunable acousto-optic (AO) dual and quad-comb frequency spacing downconversion in the FSF laser seeded with ASE and operating in the mode-locked regime. The examined system applies a single frequency shifting loop with single AO tunable filter as the frequency shifter that is fed with several radio frequency signals simultaneously. The initial OFCs with frequency spacing of about 6.5 MHz may be obtained in the wide spectral range and their width, envelope shape and position in the optical spectrum may be tuned. The dual-combs are obtained with a pair of initial OFCs aroused by two various ultrasound waves in the acousto-optic tunable filter (AOTF). The dual-combs frequency spacing is determined by the frequency difference of the signals applied to the AOTF piezoelectric transducer and can be tuned simply. The quad-combs are obtained with three initial OFCs, forming a pair of dual-combs, appearing when three ultrasound frequencies feed the AOTF transducer. The quad-combs frequency spacing is defined by the difference between the frequency spacing of dual-combs. Quad-combs with more than 5000 spectral lines and tunable frequency spacing are observed. The successive frequency downconversion gives the possibility to reduce the OFC frequency spacing form several MHz for initial OFC to tens of kHz for quad-combs.