Wide-aperture TeO₂ AOTF at low temperatures: operation and survival

Analysis of Phase Mismatch for Mercurous Bromide-Based Non-Collinear AOTF Design in Spectral Imaging Applications

The spectral and spatial characteristics of Acousto-Optic Tunable Filters (AOTFs), such as a tuning curve, spectral resolution, angular aperture, and diffraction efficiency, are determined by the device's acousto-optic crystal configuration and piezoelectric transducer. For high-throughput spectral imaging applications, it is essential to enlarge the spectral bandwidth and angular aperture during the design phase of AOTFs. Thus, phase mismatch due to incident angle or wavelength was studied analytically using phase diagrams in this paper. Additionally, a performance parameter analysis model was established based on the use of mercurous bromide crystals for large angular aperture AOTF device design, and the impact of crystal and transducer design parameters on the spectral bandwidth and angular aperture was evaluated. This also experimentally validates the diffraction capability of AOTFs made from mercurous bromide crystal, which possess a broad spectral transmission ability ranging from visible to long-wave infrared.

Angular-Spectral Characteristics of Acousto-Optic Tunable Filters Based on Mercurous Halide Crystals

The angular and spectral properties crucial for the functionality of acousto-optic (AO) devices are determined by phase-matching geometries in AO interactions. In applications such as spectral imagers based on acousto-optic tunable filters (AOTFs), systematic throughput is constrained by the angle separating diffracted and transmitted light. This research introduces an analytical model that elucidates the angular-spectral properties of diffracted beams in mercurous halide crystals. These crystals possess a wide transmissive spectral range, from visible light to long-wave infrared light. The study computes and confirms correlations between the separation angle and parameters including incident angle, polarization, acoustic angle, and crystal birefringence. Experimental validation conducted on mercurous halide and tellurium dioxide crystals demonstrates that higher birefringence in crystals significantly amplifies the separation angle, augmenting the device's performance. The study contributes to the development of devices with large separation angles during the design phase, enhancing systematic throughput in spectral imaging applications.

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.

Characterization and alignment of acousto-optic devices using digital tailored RF waveforms

Acousto-optic (AO) devices are used in a variety of light control and processing applications in different environments, including laboratory conditions and the field environment, as well as on air- and space-borne platforms. Accurate alignment and calibration of AO deflectors and tunable filters is crucial for correct operation and to achieve specified performances of those devices. We describe a simple routine to test and align AO devices with a programmable RF waveform generator. Using specially tailored RF waveforms as test signals enables single-shot measurement of an AO device's diffraction efficiency as a function of the applied RF signal frequency and amplitude. MATLAB code is available to implement the testing procedure and application demonstrations for an AO deflector and two types of AO tunable filters.

Initial In-Flight Spectral Calibration of the Near-Infrared Spectra Acquired by the MarSCoDe Onboard the Zhurong Rover

The Zhurong rover successfully landed in southern Utopia Planitia as part of the Tianwen-1 mission on 15 May 2021. One of the objectives of the Mars Surface Composition Detector (MarSCoDe) onboard the rover is to investigate mineral compositions on the Martian surface by measuring the near-infrared reflectance spectra. Before conducting spectral interpretation, in-flight data calibration is a crucial step due to the significant differences between the laboratory and the Martian environment. The work of the MarSCoDe spectrometer is based on an acousto-optic tunable filter (AOTF). The temperature variation of the AOTF could induce wavelength offset, making mineral identification uncertain. We first analyzed the viewing geometry of the spectral measurements of the calibration targets on the Mars rover according to its attitude to identify the anomalous data. The wavelength offsets were then determined by fitting the absorption positions of CO2 at ~1400 and ~2000 nm, representing the primary composition of the Martian atmosphere. The results showed 2–8 nm wavelength offsets, which correlated well with AOTF temperatures. The artifacts were removed in the wavelength-corrected reflectance spectra, which is critical to identify the material types on Mars, especially water-related minerals.

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.

FEM Analysis of Various Multilayer Structures for CMOS Compatible Wearable Acousto-Optic Devices

Lately, wearable applications featuring photonic on-chip sensors are on the rise. Among many ways of controlling and/or modulating, the acousto-optic technique is seen to be a popular technique. This paper undertakes the study of different multilayer structures that can be fabricated for realizing an acousto-optic device, the objective being to obtain a high acousto-optic figure of merit (AOFM). By varying the thicknesses of the layers of these materials, several properties are discussed. The study shows that the multilayer thin film structure-based devices can give a high value of electromechanical coupling coefficient (k2) and a high AOFM as compared to the bulk piezoelectric/optical materials. The study is conducted to find the optimal normalised thickness of the multilayer structures with a material possessing the best optical and piezoelectric properties for fabricating acousto-optic devices. Based on simulations and studies of SAW propagation characteristics such as the electromechanical coupling coefficient (k2) and phase velocity (v), the acousto-optic figure of merit is calculated. The maximum value of the acousto-optic figure of merit achieved is higher than the AOFM of all the individual materials used in these layer structures. The suggested SAW device has potential application in wearable and small footprint acousto-optic devices and gives better results than those made with bulk piezoelectric materials.

Adaptation of the polarimetric multi-spectral Aerosol Limb Imager for high altitude aircraft and satellite observations

An elegant breadboard prototype of the Aerosol Limb Imager (ALI) has been developed to meet key performance parameters that will meet requirements for the retrieval of aerosol from the upper troposphere and stratosphere from limb scattered sunlight radiance measurements. Similar in concept to previous high altitude balloon-based generations, this instrument pairs a liquid crystal polarization rotator with an acousto-optic tunable filter to capture polarimetric multi-spectral images of the atmospheric limb. This design improves the vertical resolution, signal-to-noise ratio, and athermalization, all of which will facilitate observation from a moving high altitude aircraft platform, which provides a platform analogous to the spatially varying measurements that would be made from a satellite. Finally, a preliminary design is presented for a satellite-based generation of ALI.

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.

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.

Measurement and Correction Model for Temperature Dependence of an Acousto-Optic Tunable Filter (AOTF) Infrared Spectrometer for Lunar Surface Detection

The photoelectric response characteristics of an infrared spectrometer based on an acousto-optic tunable filter (AOTF) are greatly affected by the temperature change of the radio frequency power amplifier and shortwave infrared detector. If calibration is not conducted, that will affect the quantitative level of the data. This paper puts forward a measurement and correction method for the temperature characteristics of an AOTF infrared spectrometer which is used in lunar surface detection and sets up a temperature characteristics correction model. This model was applied to an AOTF infrared spectrometer mounted on China's unmanned lunar rover. Laboratory tests show that the temperature causes an instrument variation of ∼20% when the temperature is between -20 ℃ and + 55 ℃, but this model reduces this variation to < 6%. Evaluating data from the lunar surface also verifies the effectiveness of this method.

Examination of the temperature influence on phase matching frequency in tunable acousto-optic filters

The temperature effect on the acousto-optic (AO) phase matching condition was examined both theoretically and experimentally on an example of wide-angle acousto-optic filter fabricated from tellurium dioxide crystal. It was shown that the AO crystal temperature variation changes the acoustic wave velocity that is involved into the AO interaction and shifts the phase matching frequency of AO diffraction. The AO phase matching frequency shift temperature coefficient was introduced, characterizing the magnitude of the frequency shift. The examination of frequency shift magnitude was carried for the optical wavelength band from 440 nm to 1.52 µm. It was shown that the temperature coefficient decreases with increasing the optical wavelength. A method was introduced that makes it possible to calculate the temperature shift coefficients for the real AO devices in a wide range of optical wavelengths. The coefficients obtained with the proposed method are in good agreement with the experimental data. Ultrasound attenuation was also examined in the given AO cell. Attenuation caused by the acoustic power absorption is the main mechanism leading to the appearance of the inhomogeneous temperature distribution inside the AO cell during the operation.

Acousto-optic tunable filter spectrometers in space missions [Invited]

Spectrometers employing acousto-optic tunable filters (AOTFs) rapidly gain popularity in space, and in particular on interplanetary missions. They allow for reducing volume, mass, and complexity of the instrumentation. To date, space operations of 11 AOTF spectrometers are reported in the literature. They were used for analyzing ocean color, greenhouse gases, atmospheres of Mars and Venus, and for lunar mineralogy. More instruments for the Moon, Mars, and asteroid mineralogy are in flight, awaiting launch, or in the state of advanced development. The AOTFs are used in point (pencil-beam) spectrometers for selecting echelle diffraction orders, or in hyper-spectral imagers and microscopes. We review the AOTF-employing devices flown in space or ready to set off. The paper considers basic principles of the AOTF and science applications of the AOTF spectrometers, and describes developed instruments in some detail. We also address some advanced developments for future missions and plans. In addition, we discuss lessons learned during instrument design, build, calibration, and exploitation, and advantages and limitations in implementing the AOTF-based systems in space instrumentation.

Infrared Spectrometer for ExoMars: A Mast-Mounted Instrument for the Rover

ISEM (Infrared Spectrometer for ExoMars) is a pencil-beam infrared spectrometer that will measure reflected solar radiation in the near infrared range for context assessment of the surface mineralogy in the vicinity of the ExoMars rover. The instrument will be accommodated on the mast of the rover and will be operated together with the panoramic camera (PanCam), high-resolution camera (HRC). ISEM will study the mineralogical and petrographic composition of the martian surface in the vicinity of the rover, and in combination with the other remote sensing instruments, it will aid in the selection of potential targets for close-up investigations and drilling sites. Of particular scientific interest are water-bearing minerals, such as phyllosilicates, sulfates, carbonates, and minerals indicative of astrobiological potential, such as borates, nitrates, and ammonium-bearing minerals. The instrument has an ∼1° field of view and covers the spectral range between 1.15 and 3.30 μm with a spectral resolution varying from 3.3 nm at 1.15 μm to 28 nm at 3.30 μm. The ISEM optical head is mounted on the mast, and its electronics box is located inside the rover's body. The spectrometer uses an acousto-optic tunable filter and a Peltier-cooled InAs detector. The mass of ISEM is 1.74 kg, including the electronics and harness. The science objectives of the experiment, the instrument design, and operational scenarios are described. Key Words: ExoMars-ISEM-Mars-Surface-Mineralogy-Spectroscopy-AOTF-Infrared. Astrobiology 17, 542-564.