Generation of dual and quad-optical frequency combs in the injected radiation free mode-locked frequency-shifted feedback laser.

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.

Examination of an acoustic field longitudinal power distribution in quasicollinear acousto-optic cells.

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.

Quasi-Collinear AOTF Spectral Transmission Under Temperature Gradients Aroused by Ultrasound Power Absotption.

Quasi-collinear geometry is a special configuration of acousto-optic (AO) diffraction that provides an extremely large AO interaction length for achieving extremely high spectral resolution of AO tunable filters (AOTFs). Large AO interaction length also makes it possible to implement multifrequency diffraction in quasi-collinear AOTFs, which has found multiple applications in modern optoelectronics. The most widespread of them being ultrashort laser pulse shaping when the pulse shape and spectral composition is controlled by the spectral composition of the ultrasound pulse aroused in the AO crystal. The operation of quasi-collinear AOTFs is accompanied by the appearance of the temperature gradients in the AO device mainly due to the acoustic power absorption by the material. We experimentally assessed the influence of these gradients on the AOTF spectral characteristics. A theoretical model based on the Raman-Nath equations was proposed, which allows to consider the influence of ultrasound attenuation and temperature gradients on the AOTF transmission. This model is valid for transmission simulations both in single- and modulated-frequency AOTF operation modes. The study includes the effects of acoustic wave attenuation and AO phase matching shift caused by inhomogeneous crystal temperature along the optical beam path. The compensation strategy based on ultrasound frequency and magnitude adjustment is proposed for minimizing the effect of temperature gradients and acoustic field attenuation on AOTF spectral transmission for broadband operation in ultrashort laser pulse shaping.

Tunable acousto-optic optical frequency combs.

Examination of various issues related to the generation and application of optical frequency combs (OFCs) is an important branch of modern optoelectronics. Some of the proposed OFC generation methods apply acousto-optic (AO) devices. The AO devices are used either as the element devoted to the OFC phase stabilization or they play the role of an optical radiation frequency shifting element in the frequency-shifting loop (FSL) scheme. The results of two experiments related to the application of AO cells in the FSL scheme are represented in this paper. The first experiment confirms the previously proposed effect of AO mismatch influence on all the OFC characteristics. The second experiment shows the possibility of tunable AO dual-comb downconversion with a single AO device.

An Electrical Method for Acoustic Resonance Frequency Adjustment in Standing-Wave Acousto-Optic Devices.

An acousto-optic (AO) device can be configured to operate in a standing wave (SW) mode. The effectiveness of an SW AO device surpasses the effectiveness of a conventional, running-wave one, when the driving signal frequency meets the SW condition. This SW frequency depends primarily on the acoustic wave velocity and the crystal length. While promising significant benefits, this situation imposes serious restrictions on the AO device performance. In this study, we examine the SW frequency dependency of such a device on its electrical impedance matching circuit (EIMC) parameters and configuration both theoretically and experimentally. Our analysis is aimed at this effect utilization for the purpose of expanding of the standing-wave AO devices applicability domain.

Acousto-Optic Cells with Phased-Array Transducers and Their Application in Systems of Optical Information Processing.

This paper presents the results of theoretical and experimental studies of anisotropic acousto-optic interaction in a spatially periodical acoustic field created by a phased-array transducer with antiphase excitation of adjacent sections. In this case, contrary to the nonsectioned transducer, light diffraction is absent when the optical beam falls on the phased-array cell at the Bragg angle. However, the diffraction takes place at some other angles (called "optimal" here), which are situated on the opposite sides to the Bragg angle. Our calculations show that the diffraction efficiency can reach 100% at these optimal angles in spite of a noticeable acousto-optic phase mismatch. This kind of acousto-optic interaction possesses a number of interesting regularities which can be useful for designing acousto-optic devices of a new type. Our experiments were performed with a paratellurite (TeO2) cell in which a shear acoustic mode was excited at a 9∘ angle to the crystal plane (001). The piezoelectric transducer had to nine antiphase sections. The efficiency of electric to acoustic power conversion was 99% at the maximum frequency response, and the ultrasound excitation band extended from 70 to 160 MHz. The experiments have confirmed basic results of the theoretical analysis.

Optimization of piezotransducer dimensions for quasicollinear paratellurite AOTF.

The method of quasicollinear acousto-optic tunable filters (AOTFs) piezoelectric transducer dimensions optimization is presented. The AOTFs with large interaction length apply the bulk acoustic wave (BAW) reflection from the input optical facet. Optimization is based on spectral approach to simulation of the BAW field in anisotropic media and Raman-Nath equations numerically solved for the inhomogeneous acoustic field. It was found that variation of the transducer dimensions can minimize RF power consumption of the AOTF. Comparison of the optimized transducer dimensions with those commonly used in quasicollinear paratellurite AOTFs showed that it is possible to improve the AOTF energy efficiency in about 2 times. It was shown that acoustic field simulation results obtained for one AOTF geometry can be applied to the other geometries but for equivalent frequency providing the same ultrasound beam ray spectrum width. It was also shown that when choosing the quasicollinear AOTF with reflection geometry, one should not rely only on the AO figure of merit value, since the energy efficiency of such AOTF type will be determined by the product of the AO figure of merit and the AOTF efficiency, which takes into account the change in the acoustic beam width in the reflection process. The results aim at improving the design of AOTFs for ultrashort laser pulse shaping.

Shear acoustic wave attenuation influence on acousto-optic diffraction in tellurium dioxide crystal.

The slow shear acoustic wave attenuation in a tellurium dioxide crystal (11¯0) plane is examined. The measurements were carried out for two crystal cutoff angles: 7° and 10.5° from the [110] axis. The ultrasound attenuation was examined in the frequency range of 80 to 300 MHz by the acousto-optic (AO) method. The attenuation coefficients were determined for both acoustic wave propagation directions for the frequency range of 200 to 300 MHz. The influence of ultrasound attenuation on the AO cell transmission functions was examined for the 10.5° AO cell both theoretically and experimentally. It is shown that acoustic wave absorption affects mainly the AO diffraction efficiency. The attenuation influence on the AO cell transmission function shape and its half-width is not significant.

Possibilities of wide-angle tellurium dioxide acousto-optic cell application for the optical frequency comb generation.

The development of the optical frequency comb (OFC) generation and practical application methods is one of the most important and rapidly developing areas of the modern optical electronics. One of the comb types is acousto-optical (AO) OFCs. This type of OFCs is obtained by the multiple passage of an optical signal through a closed loop containing an acousto-optic frequency shifter (AOFS). Despite the fact that AO OFCs have been studied quite intensively lately, the published papers did not focus on the influence of the main element, the AO cell used as AOFS, parameters on the characteristics of the obtained optical comb, primarily on the comb spectral width, number of spectral components and its envelope shape. In this paper, we perform a theoretical analysis of all possiblities in paratellurite crystal wide-angle AO diffraction geometries in order to determine the most suitable for the application as AOFS in a frequency shifting loop.

Control of Acousto-Optic Mode Locker by Means of Electronic Matching Circuit.

An acousto-optic mode locker (AOML) operating in a standing wave mode has a fixed working frequency, defined by the length of the crystal, which makes the device prone to temperature variations. In this study, we examine the effect of the electrical matching circuit parameters on the acoustic resonances and AO diffraction in the AOML both theoretically and experimentally. Acoustical and electrical models of the AOML are introduced. We outline the ways of utilizing this effect for compensation of ill thermal influence. Our analysis allows us to broaden the temperature and driving power domains of an AOML's reliable operation without a heat sink or a complicated temperature stabilization system.

Compact acousto-optic imaging spectro-polarimeter for mineralogical investigations in the near infrared.

Spectral imaging in the near infrared is a promising method for mineralogy analysis, in particular well-suited for airless celestial objects or those with faint atmospheres. Additional information about structure and composition of minerals can be obtained using spectral polarimetry with high spatial resolution. We report design and performance of laboratory prototype for a compact near infrared acousto-optic imaging spectro-polarimeter, which may be implemented for remote or close-up analysis of planetary surfaces. The prototype features telecentric optics, apochromatic design over the bandwidth of 0.8-1.75 µm, and simultaneous imaging of two orthogonal linear polarizations of the same scene with a single FPA detector. When validating the scheme, reflectance spectra of several minerals were measured with the spectral resolution of 100 cm-1 (10 nm passband at 1 µm). When imaging samples, the spatial resolution of 0.6 mm at the target distance of one meter was reached. It corresponds to 100 by 100 diffraction-limited elements resolved at the focal plane array (FPA) for each of the two light polarizations. A similar prototype is also being designed for the spectral range from 1.7 to 3.5 µm. This type of the spectro-polarimeter is considered as a potential reconnaissance and analysis tool for future planetary or moon landers and rovers.

Development of a space-borne spectrometer to monitor atmospheric ozone.

A new compact satellite spectrometer dedicated to monitoring terrestrial atmospheric ozone (ozonometer) is in preparation for the Russian Geophysics Program. Four instruments at four satellites (Ionosphere) are intended to monitor the total ozone content by measuring spectra of scattered solar radiation in nadir. The spectrometer is based on the Rowland scheme with a concave holographic diffraction grating. It covers the near UV and visible range of the spectrum, 300-500 nm, with a spectral resolution of ∼0.3 nm. At present, a qualification model has been manufactured and tested. We introduce the description of the instrument and the results of laboratory and ground-based atmospheric calibrations. The ozone amount retrieved from atmospheric measurements using the differential optical absorption spectroscopy (DOAS) method is in good agreement with that measured by the collocated Brewer spectrophotometer and ozone monitoring instrument on board the Aura satellite.

Collinear diffraction of divergent optical beams in acousto-optic crystals.

Peculiarities of collinear acousto-optic interaction of a strongly divergent optical beam are examined theoretically by examples of two crystals widely used in acousto-optics: calcium molybdate (CaMoO(4)) and paratellurite (TeO(2)). These materials demonstrate essentially different diffraction characteristics because of peculiar features of optical and acousto-optic anisotropy in these crystals. The dependence of the integral diffraction efficiency and the transmission band of collinear acousto-optic filters on the optical beam divergence and acoustic power is studied. It is shown that with increasing light divergence these characteristics of the filter are worsened according to the same law, and the product of the relative bandwidth and the diffraction efficiency remains constant and independent of the optical wavelength and the acousto-optic interaction length.