Acousto-optic Filter Research Articles

Design and testing of a multi-electrode apodized acousto-optic filter for arbitrary polarized light.

We present an original acousto-optic tunable filter that is able to filter visible light from 400 to 650 nm and is designed to interact simultaneously with two polarizations. The filter shows an adjustable optical bandwidth and apodization capabilities. These features make it suitable for practical spectroscopic applications. Experimental validation is also presented.

Design of a broadband acousto-optic filter using bulk acoustic wave beam steering with an interdigital transducer

Acousto-optic tunable filters are photonics devices whose operation is fundamentally based on crystal acoustics. One of the phenomena used to improve performances of bulk acousto-optic devices is acoustic beam steering. Here we present a complete analytical model, which covers bulk acoustic wave beam steering in crystals with phased-array excitation and Bragg phase matching of anisotropic acousto-optic diffraction. An interdigital phased-array transducer is proposed for laterally excited bulk acoustic wave generation enabling the effect of beam steering. The design of a paratellurite-based acousto-optic filter with extended transmitted optical wavelength range from 0.4 to 1.1μm is discussed and supported by numerical simulations.

All-fiber broadband spectral acousto-optic modulation of a tubular-lattice hollow-core optical fiber.

We demonstrate a broadband acousto-optic notch filter based on a tubular-lattice hollow-core fiber for the first time to our knowledge. The guided optical modes are modulated by acoustically induced dynamic long-period gratings along the fiber. The device is fabricated employing a short interaction length (7.7 cm) and low drive voltages (10 V). Modulated spectral bands with 20 nm half-width and maximum depths greater than 60% are achieved. The resonant notch wavelength is tuned from 743 to 1355 nm (612 nm span) by changing the frequency of the electrical signal. The results indicate a broader tuning range compared to previous studies using standard and hollow-core fibers. It further reveals unique properties for reconfigurable spectral filters and fiber lasers, pointing to the fast switching and highly efficient modulation of all-fiber photonic devices.

Acousto-Optic Transfer Function Control by a Phased-Array Piezoelectric Transducer

We present analysis and numerical simulations of the acousto-optic spatial filter (AOSF) transfer function under the condition of dual-transducer operation and phase control. Based on these simulations, the AOSF crystal configuration is optimized for operation in the near-infrared wavelength region from 0.7 to 1.0 μm. We demonstrate that ultrasonic phase control can provide efficient tuning of the transfer function, which is independent of conventional frequency control. Thus, the application of phase control coupled with frequency control can reduce the transfer function asymmetry that is inherent to anisotropic Bragg diffraction in uniaxial crystals.

Independent multicolored bottle-beam generation using acousto-optic spatial shaping of a femtosecond laser beam.

We report on the development of a tunable spectral and spatial frequency shaping system for ultrashort laser pulses using acousto-optic filters. The system enables the creation of arbitrary axially symmetric multi-wavelength field configurations in the Ti:sapphire laser emission range near 800 nm and controlling them at a multi-kilohertz rate. We experimentally demonstrate independent generation of two-colored annular intensity distributions from a single femtosecond laser beam and a bottle beam having the hollow cylindrical volume with the aspect ratio of 9:1. This laser beam shaping system can be useful in creating advanced setups for an optical control of cold atoms.

C + L band ultra-widely tunable SOA hybrid fiber laser based on acousto-optic add-drop filter induced by polarization conversion

Scientific and industrial applications have put forward new requirements for the wavelength tuning range of continuous-wave lasers. Herein, the constructed filter based on the acoustically-induced polarization conversion in single-mode fiber (SMF) covers the wavelength from 1464.2–1640.8 nm, and the tuning range is about ∼176.6 nm. The lasing wavelength is managed by the acousto-optical interaction in the SMF. Moreover, the gain spectrum of semiconductor optical amplifier (SOA) is flat and wide, which can achieve a wide range of laser tuning. Hence, we propose and demonstrate an ultra-widely tuning range ring cavity C + L tunable SOA hybrid fiber laser. Eventually, the tuning channel of the fiber laser is from 1527.3–1613 nm with the tuning range of about ∼85.7 nm, and its tuning range is determined by the gain bandwidth of SOA. The measured output power and center wavelength variations of the proposed fiber laser over a long-term observation are less than 0.11 mW and 0.44 nm, respectively. This work opens up a new way to expand the tuning range of other types of lasers.

Selection and Justification of Optimal Spectral Wavelengths for Control of Methane Emission from an Advanced Nanosatellite

The choice and justification of optimal wavelengths of spectral radiance density registration for the task of monitoring methane emissions in the atmosphere from the orbit of an artificial satellite by a passive optical sensor in the spectral region near 1.65 μm have been performed. Mathematical modelling of the spectra recorded by the optical sensor for tropical and subarctic atmospheric models of the Earth’s atmosphere, different widths of the spectral function of the acousto-optic spectrometer, and different solar zenith angle were performed. It is assumed that methane emissions in the Earth’s atmosphere will be monitored from a promising nanosatellite (weighing less than 6 kg) using an acousto-optic spectrometer, which uses two narrow (0.1 nm and 0.5 nm) wavelengths of radiation registration to implement a differential method of absorption spectroscopy based on acousto-optic filtering. A criterion for selecting optimal wavelengths for monitoring the integral methane content in the atmosphere from the orbit of the artificial satellite is proposed. The values of the central wavelengths of optimal wavelengths for recording the energy brightness of scattered radiation for the width of the spectral function of the acousto-optic spectrometer in range (0.0, 0.1, 0.2 and 0.5) nm have been obtained. It is shown that the choice of the optimal pair of wavelengths is determined by the width of the spectral function of the sensor, depends insignificantly on the model of the Earth’s atmosphere (tropical or subarctic model) and does not depend on the solar zenith angle in range (0–80) angular degree.

Advanced control of an acousto-optic filter performance through arbitrary RF signals.

The proposed study consists of using the capabilities of an arbitrary waveform generator to create a specific radio frequency (RF) signal to be applied to an acousto-optic tunable filter based on quasicollinear interaction. By controlling the duration and shape of the acoustic pulse, the filter transmission function can be changed in terms of selectivity and side-lobe levels. The measurement results show good agreement with the simulation model based on the fast Fourier transform. In particular, the Hamming window has been found to be a good option for suppressing side lobes in the transmission function, even when using high acoustic powers.

Color Reproduction by Multi-Wavelength Bragg Diffraction of White Light.

Accurate color reproduction is highly important in multiple industrial, biomedical and scientific applications. Versatile and tunable light sources with high color-rendering quality are very much in demand. In this study, we demonstrate the feasibility of multi-wavelength Bragg diffraction of light for this task. Tuning the frequencies and amplitudes of bulk acoustic waves in the birefringent crystal demonstrates high precision in setting the number, wavelengths and intensities of the monochromatic components necessary to reproduce a specific color assigned according to its coordinates in the CIE XYZ 1931 space. We assembled a setup based on multi-bandpass acousto-optic (AO) filtration of white light and verified the reproduced color balance in multiple experiments. The proposed approach delivers almost full coverage of the CIE XYZ 1931 space and facilitates building compact color reproduction systems (CRSs) for various purposes.

Acousto-optic reconfigurable filter based on vector mode fusion in dispersion-compensating fiber.

An acousto-optic reconfigurable filter (AORF) is proposed and demonstrated based on vector mode fusion in dispersion-compensating fiber (DCF). With multiple acoustic driving frequencies, the resonance peaks of different vector modes in the same scalar mode group can be effectively fused into a single peak, which is utilized to obtain arbitrary reconfiguration of the proposed filter. In the experiment, the bandwidth of the AORF can be electrically tuned from 5 nm to 18 nm with superposition of different driving frequencies. The multi-wavelength filtering is further demonstrated by increasing the interval of the multiple driving frequencies. The bandpass/band-rejection can also be electrically reconfigured by setting the combination of driving frequencies. The proposed AORF gains the feature of reconfigurable filtering types, fast and wide tunability, and zero frequency shift, which is advantageous for high-speed optical communication networks, tunable lasers, fast optical spectrum analyzing and microwave photonics signal processing.

The design of a flexible RF generator for driving Acousto-Optical devices in space applications

Acousto-Optical (AO) devices are not only used for filtering purposes in spectral imaging, but also in optical communications and spatial tracking systems. Some AO devices with space applications are AO Tunable Filters (AOTFs), Modulators (AOMs), Deflectors (AODs) and Frequency Shifters (AOFSs). Though these device’s applications differ, they are all controlled with Radio-Frequency (RF) signals. These signals are converted by a transducer into an acoustic wave, which propagates inside the AO device. The interaction between the incoming light and the acoustic waves inside the birefringent AO material creates multiple output beams. This interaction can result in filtering, modulation, deflection or frequency shifting, depending on the AO device in question. This research focuses on the design of a flexible, uniform RF generator, applicable to all AO devices in the space applications domain. The RF output maximizes the performance of the AO device, while the use of components available in space qualified grades eases integration with future space missions. Its design is a key step towards a miniaturized, space qualified, general-purpose RF generator. This research presents schematics, design and preliminary component test results.

IDENTIFICATION OF CONCENTRATIONS OF SUBSTANCES IN THE OPEN ATMOSPHERE WITH THE PROCESSED MEASUREMENT OF THE FOURIER SPECTRAL RADIOMETER

The theoretical justification was carried out and the method of operational remote non-sampling gas analysis of the atmosphere in emergency situations was selected. As a result of the analysis of the methods of non-sampling determination of substances in the open atmosphere, it was established that the most appropriate use of optical spectral methods of monitoring the composition of the atmosphere, based on the registration and subsequent analysis of the electromagnetic radiation of the object of observation. To determine the most effective method of remote monitoring, an analytical classification of all possible optical spectral methods of remote monitoring was compiled: lidar methods, acousto-optic filters, correlation spectrometers, interference light filters with reconfiguration, use of the Fabry-Pere interferometer. It has been established that to solve the problem of non-sampling analysis of substances in the open atmosphere, it is most appropriate to use an infrared Fourier spectral radiometer of medium spectral resolution, which works in passive mode in the window of atmospheric transparency of 7-14 μm and allows obtaining experimental information in real time. Algorithms for the numerical solution of the problem of identification of substances and restoration of their concentrations based on infrared spectra of medium resolution in the region of 7…14 μm obtained on an IR Fourier spectroradiometer have been developed. Algorithms make it possible to identify substances by the unit spectrum of an open atmospheric track. Instead of considering a multidimensional system of linear equations, it is proposed to solve one or several one-dimensional problems. The transition to a one-dimensional problem is carried out by methods of correlation analysis. The influence of noise in the original spectrum on errors in solving the problem of determining concentrations was studied.

Fast interrogation wavelength tuning for all-optical photoacoustic imaging.

Optical detection of ultrasound for photoacoustic imaging provides a large bandwidth and high sensitivity at high acoustic frequencies. Therefore, higher spatial resolutions can be achieved using Fabry-Pérot cavity sensors than conventional piezoelectric detection. However, fabrication constraints during the deposition of the sensing polymer layer require precise control of the interrogation beam wavelength to provide optimal sensitivity. This is commonly achieved by employing slowly tunable narrowband lasers as interrogation sources, hence limiting the acquisition speed. We propose instead to use a broadband source and a fast-tunable acousto-optic filter to adjust the interrogation wavelength at each pixel within a few microseconds. We demonstrate the validity of this approach by performing photoacoustic imaging with a highly inhomogeneous Fabry-Pérot sensor.

RGB color camera for dynamical measurements of high temperature distribution on a surface of the heated solid

In this report we describe a fast 3-color method of the measurement of temperature distributions on a surface of a heated solid using a RGB color camera with a high frame rate (100 images per second). Statistical error the RGB method is not high, and do not exceed around 5.5% which is surprising taking in to account the number of the measurements at each pixel. Comparison of the results of the temperature measurements on a tungsten plate heated by infra-red laser radiation and conducted with this technique and those obtained with the acousto-optical tunable filter technique demonstrate that error of the temperature measured by 3-color method is only two times as high as that of the tandem acousto-optic filter technique method.

Colour Reproduction by System Consisting of a Radiation Source and Acousto-optic Tuneable Filter

Multiple-window acousto-optic filtering of wide-band (white) light allows to control chromaticity of radiation in the visible range, which allows to use a system consisting of a source of radiation and an acousto-optic filter for colour reproduction problems, for instance, in colorimetry, psychophysiology, spectral analysis, biomedicine and other applied and research areas requiring radiation chromaticity control. The article describes the problem of reproduction of a necessary colour when using two or three spectral components (narrow bands), evaluates the factors affecting accuracy of colour reproduction and demonstrates the capability to reach errors below the chromatic threshold. Also it demonstrates the capabilities of reproduction of colours close to chromaticity locus in the multi-frequency mode of an acousto-optic filter.

Hybrid endoscope with television and multispectral image processing for the internal organs cancer early diagnostics

Endoscopes are widely used to diagnose various internal diseases. The most advanced endoscopes contain miniature television cameras built into the probe which provide high spatial resolution. However, traditional television cameras significantly distort the spectral composition of the transmitted image because in the RGB system they reproduce only limited width spectral lines. However, the full optical spectrum of an image of an internal organ under examination provides a broader range of information that can be crucial in cancer diagnostics. Thus, the spectral resolution of the image plays an important role. Multispectral processing has been suggested for efficient detection of both spatial and spectral information simultaneously. As a result, a series of monochrome sub-images can be obtained, each of which corresponds to a selected resolved spectral interval. Acousto-optical tunable filters are used as a selective element allowing real-time extraction of sub-images. However, an increase in spectral resolution is accompanied by a decrease in spatial resolution. A new hybrid endoscope was suggested to solve this problem. It implements the method of combined application of a TV camera and a multispectral processing device. It is shown that the suggested method of hybrid endoscopy makes it possible to ensure high resolutions: spatial resolution with a camera and spectral resolution (no less than a hundred of resolvable intervals) due to multispectral processing. The resulting optical images reveal a clear outline of the neoplasm area. The results of multispectral processing diagnose the malignant nature of the neoplasm. A schematic diagram of the proposed endoscope is presented, and the principles of control are shown. The characteristics of the television and multispectral images are determined. An estimation of the linearity characteristics of the acousto-optical filters shows the possibility of transmitting the maximum amount of information due to the linearity of the signal transformation in the filter sections. Implementation of the proposed method in the design of a hybrid endoscope allowed obtaining the most complete spectral information about neoplasms and providing an opportunity of diagnostics of cancer of internal organs at earlier stages of development.

Spectral-Polarization Systems of Three-Dimensional Computer Vision Based on Acousto-Optic Filtering

Development of two-channel acousto-optic spectral devices with enhanced functionality is described. Approaches to development of multifunctional acousto-optic devices capable to register different types of optical information (spatial distribution, spectra, polarisation characteristics) are discussed. Opportunities to register multi-dimensional complementary data based on Bragg light diffraction on ultrasonic waves in a uniaxial crystal are analysed. It is demonstrated that such approach may be developed on the basis of implementation of simultaneous diffraction of two light beams in planes rotated relative to each other by a small angle. Such enhancement of the acousto-optic filter operational concept allows to go beyond the classic constraints imposed on their characteristics.

Experimental Study of the Influence of the Transmission Function of Acousto-Optic Tuneable Filter on the Characteristics of the Interference Pattern in Off-Axis Digital Holography

Acousto-optic tuneable filters (AOTFs) are used in digital holography (DH) to obtain interference images at different wavelengths, which expands the possibilities of studying technical and biological objects. However, the width of the spectral bands selected by AOTF is quite large, which can lead to a decrease in the size of the high-contrast region of the interference pattern in off-axis DH schemes and reduce the quality of the obtained holograms. In this work, the effect of the geometry of acousto-optic (AO) interaction and the power of the driving signal on the spectral transmission function of the AOTF, the visibility of the interference pattern, and the width of the effective field of view (FOV) is experimentally studied. For this, a setup with a broadband radiation source, Mach-Zehnder interferometer, and a spectrometer was used. The interference patterns were recorded and the transmission spectra were measured for several values of the angle of incidence on the entrance face of the AOTF in the frequency tuning range corresponding to the visible spectrum. We evaluated the dependence of the coherence length on the rotation angle of the AOTF and the central transmission wavelength and compared the results obtained by the interference method and those calculated from the spectra measured by the spectrometer. It is shown that when the AOTF is rotated through the angles from 5° to –15° relative to the wide-aperture AO interaction geometry, it is possible to increase the coherence length and the width of the effective FOV by a factor of 2.5. It was established that the width of the FOV can significantly decrease with an increase in the driving power. The results obtained can be used to certify the AOTF, optimize the characteristics of DH setups with AOTF, and determine the optimal parameters of their operation.

Methods for Calculating the Shape of the Spectral Transmission Window of an Acousto-Optic Filter

The problem of calculation of acousto-optic filter characteristics with control of the number of volumetric diffraction gratings and their structure is taken into consideration. Special-case calculation methods are described including those in collinear geometry. It is demonstrated that there is an approach allowing to perform detailed calculation of spectral characteristics in wide-aperture diffraction geometry used for problems of spectral analysis of object images. It allows to divide the entire luminous flux into separate beams with the partial transmission function of each of them being described by a simple path integral. This method allows for determining the impact of acoustic wave modulation on the filter spectral transmission function.

Spectral Endoscopy: Technical Means and Methods

The article describes the current state of spectral endoscopic methods. Requirements to technical means are formulated and methods of their implementation are presented. Classification of spectral endoscopic devices is presented, prospects of application of tuneable acousto-optic filters as spectral elements in spectral endoscopy are noted. Examples of practical application of spectral endoscopy methods are presented for non-destructive testing and medical diagnostics.