Acousto-optic Filter Research Articles

Application of hyperspectral imaging for analysis of embryonic development of lower vertebrates

The task of analyzing the characteristics of the embryonic development of lower vertebrates using non-invasive optical methods is considered. We propose to apply hyperspectral imaging for segmentation of embryo zones with different physical and chemical properties. This approach allows one to obtain information on the spatial distribution of optical spectral properties of the observed object. An experimental setup based on a microscope coupled with a spectral imaging add-on module utilizing a tunable acousto-optic filter has been created to implement this approach. Hyperspectral data arrays are formed as a series of spectral narrowband images obtained by successive tuning of the acousto-optic filter within the working spectral range. A series of experiments was conducted in which the development of embryos of the Misgurnus fossilis loach during their transition from the 32nd to the 33rd developmental stage was observed using the described experimental setup. Characteristic spatial regions corresponding to certain parts of the loach embryo were identified. The time dependences characterizing the change in the spectral properties of the selected regions were obtained. It is shown that joint processing of a series of spectral images obtained in different spectral bands within the working spectral range of an acousto-optical spectral imaging module allows one to effectively identify differences in the kinetics of the optical transmission spectral density for various regions of the axial mesoderm.

Compensation of spectral image shift in AOTF-based system

Recently, it was shown that the use of a tandem acousto-optic tuneable filter (TAOTF) in combination with a camera allows one to measure the temperature distribution at the focus of a laser beam on an object’s surface. The TAOTF advantage is a reduction of aberrations and spatial distortions. Unfortunately, a TAOTF significantly reduces the intensity of the transmitted light beam, requires longer exposure time and often hampers temperature measurements below 1200 K. A single-crystal AOTF is free from this drawback and allows increasing the intensity of the incident radiation to the camera sensor. The paper presents experimental data for comparing measurements of the temperature distribution at the laser focus, obtained using single-crystal AOTF and TAOTF. A strong chromatic drift caused by AOTF-based system was compensated using the digital processing of the spectral images.

A review of non-polar liquids as materials for bulk acousto-optic devices operating with terahertz radiation

In this paper, the acoustic and optical properties of a number of liquids transparent in the terahertz range are systematized. The acousto-optic quality is calculated for the quasi-orthogonal and collinear geometry of the acousto-optic interaction. The parameters of acousto-optic deflectors and filters of terahertz radiation are determined. The diagrams linking the resolution for the filter and the maximum number of resolvable elements for the deflector with the diffraction efficiency are given.

Highly multiplexed Bragg gratings for large field of view gas sensing in planetary atmospheres.

Detecting and monitoring gas species is an important part of remote sensing because the state of the environment can be retrieved from the state of the gas species. This can be used to track temperature and pressure structures in the atmosphere for weather predictions, or monitor the air quality. Discriminating different species is easier at higher spectral resolution when the spectral lines are clearly resolved. The need to do this at high spatial resolution and over large fields of view leads to a trade-off between spectral and spatial resolution and spectral bandwidth. We propose to use a highly multiplexed Bragg grating that can optically combine the relevant information from the spectrum without the need to disperse the whole spectrum. This allows us to circumvent the spatial and spectral trade-off and therefore substantially increase the field of view compared to conventional hyperspectral imagers. A dynamic implementation based on acousto-optical filters that can be adapted on the fly is discussed as an easy and flexible way to create the multiplexed gratings. We describe the details of multiplexed Bragg gratings and show that we can retrieve the spatial distribution of individual species abundances in gas mixtures, and we show that we can even do this for the atmospheres of exoplanets orbiting far-away stars.

Broadband tuning of a long-cavity all-fiber mode-locked thulium-doped fiber laser using an acousto-optic bandpass filter

A long-cavity passively mode-locked thulium-doped all-fiber laser is reported incorporating a tapered acousto-optic tunable bandpass filter (AOTBF). The operation of the AOTBF relies on the intermodal coupling between core and cladding modes when a flexural acoustic wave propagates along an 80μm tapered fiber. The filter works in transmission and exhibits a 3dB bandwidth of 9.02nm with an insertion loss of 3.4dB. The laser supports ultrashort pulse generation at a low repetition rate of 784.93kHz. Optical pulses with 2.43nm of optical bandwidth and 2.1ps pulse duration were obtained in a broad tuning range from 1824.77 to 1905.16nm.

Hyper-spectrometer Based on an Acousto-optic Tuneable Filters for UAVS

The problem of creating a hyper spectral optoelectronic system for observing natural and artificial objects by means of unmanned aerial vehicles (UAV) is considered. The structure and composition of the system that solves this problem are described. It is based on acousto-optic filters. The results of laboratory testing of the hyper spectrometer are presented.

Collinear Acousto-Optic Filtration With Electronically Adjustable Transmission Function

An acousto-optic system containing collinear acousto-optic filter fabricated from calcium molybdate crystal and a positive electronic feedback was examined. The feedback signal is formed due to the optical heterodyning effect that appears for the special geometry of polarizer and analyzer polarization planes mutual orientation between which the collinear filter is located. It is shown that the feedback circuit electrical parameters variation enable controlling the spectral characteristics of the acousto-optic collinear filtration, resulting in enhancing the maximal spectral resolution and transmission function side lobes suppression.

Applications of the frequency locking effect in acousto-optic systems for control of optical radiation spectral composition

A spectral optoelectronic system combining a collinear acousto-optic (AO) cell and positive electronic feedback is examined. The operation of this system in the generation mode and possibilities of practical application of the locking effect of the system’s self-oscillations for controlling the optical radiation mode composition of semiconductor lasers are studied. The design and test results of the new AO device, which can be treated as a time-domain optical demultiplexer, are described. This demultiplexer also applies the frequency locking effect. The demultiplexer model presented achieves the following characteristics: the wavelength spacing between channels is less than 0.6 nm; the channel passband width is 0.4 nm; the crosstalk attenuation between adjacent channels is more than 42 dB; insertion loss is less than 2 dB; and the optical tuning range of the demultiplexer corresponds to the AO filter tuning range, which is not less than one octave.

Frequency-domain two-dimensional infrared spectroscopy using an acousto-optic programmable dispersive filter.

We report on multidimensional spectroscopy in the mid-infrared, performed using a calomel-based acousto-optic programmable dispersive filter. Although the associated spectral resolution is currently not as good as what has been previously reported using time-domain interferometers or other pulse-shaping technologies, the extreme compactness of the calomel crystal allows a straightforward implementation in a pre-existing pump-probe setup. Furthermore, the frequency-domain approach allows easy measurement of 2D slices of the multidimensional spectrum associated with a given pump frequency. We demonstrate our method with the measurement of the mid-infrared spectrum of carboxy-hemoglobin in three spectro-temporal dimensions.

Measurement of the temperature distribution on the surface of the laser heated specimen in a diamond anvil cell system by the tandem imaging acousto-optical filter

ABSTRACTWe demonstrate that combining the laser heating (LH) system in a diamond anvil cell (DAC) with a tandem acousto-optical tunable filter (TAOTF) allows measurement of the temperature distribution (TD) under infrared (IR, 1064 nm) LH of a specimen under high pressure in a DAC. The main component of the system is a TAOTF synchronized with a video camera. The system allows TD mapping within a temperature range of approximately 1000–2000 K on the surface of a laser-heated object (Fe plates) at high pressure in a DAC with a spatial resolution 2 µm by fitting the actual signal to Planck’s equation at each point.

Femtosecond supercontinuum generation in scattering media

We report our results on the effect of random multiple scattering on supercontinuum (SC) generation by intense light pulses. Spectral measurements are performed on the SC generated when femtosecond laser pulses propagate in water containing randomly suspended microspheres. We observed that the SC emission in the presence of scatterers is significantly suppressed in the blue side of the spectrum in comparison to that observed in neat water. We attribute this to the dissipation of incident energy, strong scattering of the shorter wavelengths, and stretching of the pulse temporally. This reasoning is supported by our experiments in neat water where we alter the shape of the incident femtosecond pulse by means of an acousto-optic programmable dispersive filter, and find similar suppression in the blue side with the lengthening of the input pulse. We believe our study will help in understanding the physics governing applications that involve transmission of high-intensity ultrashort pulses through long distances in scattering media like the atmosphere and sea water.

Broadband Frequency Synthesizer for Acoustooptics

A broadband digital frequency synthesizer for acoustooptical filters has been designed and studied. The range of synthesized frequencies has been expanded as compared to synthesizers based on phase locking. The interface for control of the proposed design has been developed. Using the user can enter frequency values by himself. A prototype of a dual-channel frequency synthesizer with possibility of independent operation of channels has been produced.

Theory of Acousto-Optic Filtering of Radiation in the Multifrequency Near-Field Zone of a Plane Piezoelectric Transducer

Theory of Acousto-Optic Filtering of Radiation in the Multifrequency Near-Field Zone of a Plane Piezoelectric Transducer

Increase of an Output Optical Signal of an Acousto-Optic Monochromator upon Frequency Modulation of a Control Signal

An angular aperture of an acousto-optical filter upon a linear frequency-modulated (chirp) control signal has been experimentally studied for the first time. The results of studying the spectral and angular dependences of the spectral transmission function of a filter upon a frequency modulation have shown the possibility of an efficient increase of the output optical signal. The value of the optical signal was increased by 2000 times using the selected design of the acousto-optical filter. This makes it possible to obtain a gain in the measurement time or in the value of the detected optical signal. Such an increase of the output signal upon a chirp control signal is provided by the increase of the angular aperture and broadening of the spectral transmission bandwidth together with the required proportional increase of the control signal power.

Application of Multiband Acoustooptical Filtering of Radiation to Solution of the Object Remote Identification Problems by the Method of Programmed Linear Separation of Multispectral Optical Signals in the Case of Reflection of a Wideband Light from Objects with A Priori Known Reflection Coefficients

A new approach to solution of the problem of remote determination of spectrally complex optical signals and identification on this base of two objects out of a set of possible spatially inseparable objects located in one small ground pixel is considered. Computer simulation is performed; it is shown that the results of measurements of the mean powers of filtered signals at the outputs of two (or more) selected reference multiband acoustooptical filters (MAOFs) can ensure subsequent programmed (digital) separation of spectrally complex optical signals and identification of the objects corresponding to these signals. By the example of an arbitrarily chosen group of ten types of objects and three types of the background, computer verification of the new method is carried out with the use of realizable parameters of modern MAOFs. It has been found that, in conditions of the actual accuracy of setting and/or measuring of the energy of the main components of the problem (~1%), this method can provide unambiguous correct decision about the type of the object located on part of a pixel with a probability of ~99.9%.

Dispersion control for temporal contrast optimization

We investigate the temporal contrast of the Light Wave Synthesizer 20 (LWS-20): a powerful, few-cycle source based on the optical parametric synthesizer principle. Saturation effects in the RF amplifier driving the acousto-optic programmable dispersive filter (AOPDF) were found to degrade the coherent contrast for non-monotonic group delay corrections. We subsequently present a new dispersion scheme and design a novel transmission grism-based stretcher optimized for LWS-20. The resulting temporal contrast of the amplified, compressed output pulses is improved by 2-4 orders of magnitude compared to the former design.

Spectrally tunable illumination system based on acousto-optic diffraction of light

Spectrally tunable illumination is widely used for spectral imaging, spectroscopy, spectral-domain optical coherence tomography and other applications. Most of swept-wavelength light sources are based on mechanical change of narrow-band filters or on a supercontinuum laser coupled with a tunable filter. Such commercially available sources suffer either from a limited number of spectral channels or from a spatial noise caused by parasitic interferences. Here, we describe an alternative solution, which is free from the mentioned disadvantages and provides speckle-free uniform illumination. It is based on acousto-optic (AO) diffraction of a wide-band light. AO tunable filters (AOTFs) are fully software-controlled solid-state devices and do not require mechanical scanning. The selected wavelength of light is determined by the period of the volume phase grating induced in the crystal by an ultrasound wave, i.e. by the frequency of the electric signal applied to the piezo-transducer that excites ultrasound wave. Due to high spectral resolution, small entrance pupil diameter and small angular aperture of AOTFs, its utilisation together with wide-band light sources is problematic even with a specific optical coupling to optimize the light-energy and aberration parameters of the filtered light. In this paper, we show that efficiency of this approach may be radically improved by acoustic frequency modulation.

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.

Polarizer-Free Acousto-Optic Monochromators

Monochromators based on a noncollinear acousto-optic filter without external polarizers have been studied. As an element to select an operating diffracted light beam, (1) a spatial filter-telescope and (2) an output plane of a filter crystal rotated in a diffraction plane have been used. In the latter case, dispersion of the acousto-optic filter crystal is compensated using a correcting prism made from the same crystal and placed behind an output filter plane along a path of a filtered light beam. As a result, the transmission coefficient of the monochromator is increased upon an effective compensation of the angular drift of the filtered light beam, the monochromator design is simplified, and its sizes are decreased.

Noncollinear Acousto-Optic Filtering of Polychromatic Bessel Light Beams in Uniaxial Crystals

We have studied the features of noncollinear acousto-optic filtering of quasi-nondiffracting Bessel light beams in uniaxial crystals. We have obtained coupled wave equations for the vector amplitudes of an incident ordinary (o-type) Bessel beam and an extraordinary (e-type) diffracted beam for different modes. We have found an expression for the diffraction efficiency as a function of the parameters of interacting ordinary and extraordinary Bessel beams, and also as a function of the values of the overlap integrals. Using the overlap integral method, we have shown that regardless of the mode order for the Bessel light beam, under transverse phase matching conditions for ordinary and extraordinary diffracted Bessel beams in the optical spectral range 0.4–1.1 μm we can achieve a filter transmission bandwidth of ~0.6 nm in paratellurite crystals .