Recent trends in bioacoustophotonics and biothermophotonics of tissues are presented. The presentation is centered on the development of well-known frequency-domain photothermal and photoacoustic techniques to address issues associated with diffuse photon density waves during optical excitation of turbid media, both in hard tissues (teeth) and soft tissues. These methods have concrete advantages over the conventional pulsed-laser counterparts. In Part I we present biothermophotonic principles and applications to the detection of the carious state in human teeth as embodied by laser photothermal radiometry supported by modulated luminescence. The emphasis is on the abilities of these techniques to approach important problems such as the diagnosis of occlusal pits and fissures and interproximal lesions between teeth which normally go undetected by x-ray radiographs. In Part II we present theoretical and experimental results in frequency-domain bioacoustophotonics of turbid media, such as soft tissues, and we describe the development of sensitive sub-surface imaging methodologies which hold the promise for sensitive diagnostics of cancerous lesions in e.g. a human breast. Results using tissue phantoms and ex-vivo specimens are discussed and the current level of subsurface lesion sensitivity compared to state-of-the-art pulsed photoacoustic techniques is examined. In summary, advances in coupled frequency-domain diffuse-photon-density-wave and thermal or thermoelastic responses of turbid media constitute new trends in bioacoustophotonics and biothermophotonics promising for their signal quality and high dynamic range.

The possibility of application of natural photosynthetic pigments and their derivatives in photodynamic therapy and photodynamic diagnosis has been investigated. The triplet states of dyes are highly photochemically active, therefore, the dyes exhibiting high efficiency of triplet states generation are usually efficient photosensitizers in photodynamic therapy. Generation of triplet states has been estimated by means of the laser induced optoacoustic spectroscopy. The type of the photoreaction was established by comparing the yield of triplet generation with and without the oxygen presence. The intersystem crossing transition from excited singlet state S₁ to triplet state T₁ is competing with fluorescence emission, therefore, the dyes with efficient triplet generation usually are not suitable for diagnosis which requires a high yield of fluorescence. The chlorophyll-like pigments of prospective applications in medicine have been studied. Preliminary selection of dye-sensitizers has been made on the basis of investigation of the same dyes in model systems (solutions) and their complexes with macromolecules. The final study has been performed for resting (healthy) and stimulated (cancerous cells models) lymphocytes. Of course, the properties of sensitizers in cells and in models are not the same, but the measurements have enabled a selection (from a set of similar molecules) of the most promising dyes, tested later in the cells. The results obtained for chlorophyll-like pigments are compared with those obtained for the synthetic dyes (porphyrins, stilbazolium merocyanines and phthalocyanines) earlier investigated in our laboratory and with literature data.

Thermal lens microscope (TLM) is our original sensitive detector for non-fluorescent molecules in microspace. The principle is based on absorption of light followed by photothermal process. TLM has been successfully applied tosensitive detection on microchip, and TLM enabled various applications combined with microchip technologies. We are now developing HPLC microchips as one of the important separation techniques for analysis and synthesis. For HPLC microchip systems, direct and sensitive UV detection on microchip becomes key technology. Therefore, we extended applicability of TLM from visible to UV light absorbing samples by pulse UV laser excitation (UV-TLM). Quasi- continuous wave (QCW) method was applied for lock-in amplifier detection. By applying UV-TLM for biomolecules separation and detection, about two orders of higher sensitivity was achieved compared with UV spectrophotometer. For synthesis on microchip, recognition and detection of chiral samples become important in pharmaceutical field. Therefore, function of TLM was extended for selective detection of chiral samples by utilizing polarization modulation of excitation beam and resultant circular dichroism of sample (CD-TLM). The chirality of samples was detected selectively on microchip with two orders higher sensitivity than CD spectrophotometer. Finally, we explained the instrumentation using fiber optics and micro lens technology for achieving a miniaturized practical device.

The size distribution and total amount of the particles in paper pulp is vital information for the paper manufacturer in optimising process control and maintaining a high product quality. There is a further need for improving the on-line measurement methods to measure these parameters. It is known that fibre and fines fractions in the pulp have different optical and acoustic properties. In this study, we simultaneously use laser radar and laser generation of acoustic waves to further study optical and acoustic parameters, such as optical time delay, acoustic speed and attenuation. A near infrared pulse laser is used to illuminate the pulp suspensions and the time-of-flight of scattering photons is recorded; and at the same time, a high energy pulsed laser is applied to produce an acoustic wave. The acoustic wave propagates through the pulp suspensions and an acoustic transducer is used to detect the signals from which the attenuation and acoustic speed are determined. The results show that these combined optical techniques can potentially determine the content of fibres and fine particles simultaneously.

Wide gap II-VI semiconducting mixed crystals are extensively studied during the past few years as they are promising candidates for applications in construction photo and electrooptical devices operating in blue-green and UV spectral region. The high degree of covalent bonding of Be chalcogenides leads to increase of their lattice rigidity. For this reason, mixing of Be chalcogenides with other wide gap II-VI binary compounds would increase the resistance of the optoelectronic structure to defect generation and propagation. The photoacoustic spectroscopy has been developed to investigate the thermal and optical properties of semiconductors since it is very sensitive and complementary method to the absorption and photoluminescence spectroscopy. The modified Jackson - Amer model is used to interpret the obtained spectra for the piezoelectric photothermal technique. From the amplitude spectra of the piezoelectric signal, the method enables computation of the optical absorption coefficient spectra and estimation of the energy gap values of the investigated samples. It also enables the determination of the thermal diffusivity values of the samples from the analysis of the piezoelectric phase signal. In special cases, the multi-layer model, developed very recently, can be applied for the interpretation of experimental spectra. The influence of the annealing process of II-VI samples in cation vapor on the amplitude photoacoustic spectra in the saturation region is shown and discussed. The values of the parameter η (efficiency of the nonradiative recombination processes) for both, as grown and annealed crystals were determined and discussed.

This paper presents different frequency photoacoustic characteristics both amplitude and phase used for the purpose of determination of different physical parameters of materials and structures. Because of the huge number of applications one can say that they are a real source of information in photoacoustics. The applications presented in this paper are limited to two methods of detection of a photoacoustic signal: a microphone and piezoelectric one.

Based on Fick's second law the problem of drug diffusion into a membrane was solved. Spatial and time dependent distribution of the drug in the membrane was described analytically. Comparison of obtained solution with experimental results from infrared attenuated total reflectance studies of dithranol diffusion into the dodecanol-collodion membrane confirmed correctness of the theory. Based on a model describing the concentration of diffusing matter it was possible to calculate numerically the photoacoustic signal arising when the membrane was illuminated by modulated light. Detailed numerical analysis showed that photoacoustic spectroscopy (PAS) can be used in experimental investigation of diffusion, but some limitations connected with optical parameters of diffusing matter and the membrane must be taken into account. Proposed mathematical model was used for interpretation of experimental data from "classical" PAS measurement in UV range and step-scan Fourier-transform infrared PAS. Experiments were carried out for diffusion of: dithranol, ketoconazole and methoxalen. Using a multiparameter fitting procedure one has determined diffusion coefficient of enumerated drugs in dodecanol-collodion membrane. Obtained results confirmed usability of PAS technique for studies of diffusion processes.

We have demonstrated electron diffraction from a standing light wave. More recently, we have also demonstrated the onset of Bragg scattering for electrons by a thick standing wave of light. Here, we investigate the use of thick Bragg crystals for the electron analogue of acousto-optical modulators. In atom optics it has been shown both theoretically and experimentally that effects analogous to acousto-optical modulation can be achieved for atoms. Based on this approach we estimate the experimental parameters needed for electrons. It is perhaps surprising that atomic and electron beams with poorly defined energy can be used for diffraction and interferometry. It may also be surprising that we consider modulation at frequencies much smaller than the frequency width of the electron beam as being useful. This contrasts the usual optical application of AOM's as a frequency shifter. For white light interferometers, the use of such modulation promises a greatly improved signal to noise by providing a beatnote in the interference signal at the modulation frequency.

Theory of collinear acousto-optic interaction in anisotropic media has been considered. On the base of the statement that anisotropic diffraction of light by ultrasound originates from the acoustically induced changes of optical indicatrix in an anisotropic medium, the qualitative and quantitative analysis of the collinear AO interaction has been carried out. The collinear diffraction has been examined in the medium subdivided into a number of thin layers. In each of these layers, the axes of the indicatrix are rotated relatively to the neighboring ones. It has been shown that propagation of light through such system may be considered as propagation through a number of birefringent plates, rotated relatively to each other. The analytical expressions for the intensities of the incident and diffracted light have been derived. It has been shown that the diffraction efficiency depends on the angle of rotation of the axes and on birefringence of the crystal. In the case of a big number of the layers, the calculations have been carried out with the help of Jones calculus and the efficiencies of the collinear diffraction in the most commonly used acousto-optic materials such as paratellurite, α-quartz and tellurium crystals, have been calculated.

Propagation and reflection of plane acoustic waves in crystals of similar tetragonal symmetry but different grade of elastic anisotropy is examined. The reflection of the waves from a homogeneous and flat boundary separating a crystal and the vacuum in the case of glancing acoustic incidence is investigated. As much as two elastic waves may be reflected from the crystal surface. It is predicted that in the materials with a strong anisotropy of elastic properties, the energy flow of one of the reflected waves propagates in a quasi-back direction with respect to the incident energy flow. It is found that the energy flows of the incident and the back reflected waves are separated in space by the angle as narrow as a few degrees. Analysis proves that the relative intensity of the unusually reflected wave may be close to a unit. A decrease in the grade of the anisotropy in a crystal results in vanishing of the backward reflected wave so that its energy fraction is diminishing down to zero. A regular trend is predicted that the effect of the unusual reflection increases with the growth of the grade of the elastic anisotropy.

A quantitative method of evaluation of phase mismatches and calculation of intensities of diffracted light is described in the paper. The analysis is carried out for the case of the acousto-optic interaction taking place in an acoustically anisotropic medium at Bragg angle of light incidence. An example of the calculation is provided for the particular regime of light diffraction in the single crystal of TeO₂ possessing the walkoff angle between directions of acoustic phase and group velocities as large as 74 degrees.

We consider some properties of the collinear acousto-optical interaction and principles of the binary encoded digital modulation of light based on exploiting collinear three-wave coupled states. The features of linearized dispersion relations are successfully used to show that the parametric coupling of light waves, being accomplished by the acoustic wave, modifies the dispersion relations for both the light waves and shapes two branches of nonlinear dispersion, whose curvatures are opposite in sign. The analytical model of shaping acousto-optical three-wave weakly coupled states is presented. The exact analytical solution allows deriving the localization condition for coupled states as well as simulating both spatio-frequency and phase distributions. The analysis developed was tested experimentally using the collinear Bragg regime of acousto-optical interaction in uniaxial single crystal. The experiments carried out make it possible to observe the dynamics of reshaping acousto-optic three-wave weakly coupled states under variations in the acoustic pulse width and the frequency mismatch. Applying the technique of localizing the coupled states to the problem of binary modulation of light beam, the conversion of multi-bit electronic signals into binary encoded optical pulse trains is demonstrated.

The paper is devoted to analysis of regular trends and peculiarities of optical image processing by means of tunable acousto-optic filters. One of the goals of the investigation was to determine basic reasons limiting spatial resolution in acousto-optically processed images. As found, the limitations originate from laws of Bragg acousto-optic interactions in birefringent crystals. A case of the wide angle geometry of light diffraction was examined for the single crystal of paratellurite. It is shown that optical quality of the processed images depend on such interaction characteristics as mutual directions of propagation of optic and acoustic waves in the crystal, on dimensions of piezoelectric transducers launching acoustic waves in TeO₂, on optical wavelength and also on linear and angle apertures of optical beams. It is proved in the paper that the spatial resolution during the acousto-optic imaging depends on all these factors in a very complicated manner so that selection of an optimal cut of the birefringent crystal occurs not simple and evident. A trade-off between the spatial resolution and the optical throughput of the imaging device at various optical wavelengths is required. A detailed consideration of the problem becomes necessary depending on a particular problem to be solved by the acousto-optic filter. General methods to design the acousto-optic imaging devices operating in the ultraviolet, visible and infrared regions of spectrum are also discussed in the presentation.

The acousto-optic method of phase object visualization is generalized to the case when an optical wave as well as being spatially phase-modulated, has also amplitude spatial modulation. A special geometry of acousto-optic interaction is proposed for separation of the phase modulation and visualization of the optical wavefront. Basic characteristics of the visualizing system are calculated. It is shown that in the optimal case the distribution of light intensity in the visualized image is proportional to the phase gradient in the acousto-optic interaction plane. The resolution and the contrast of the visualized image are determined primarily by the divergence angle of the acoustic beam. Some examples of computer simulation are presented for illustration of potentialities of this method.

MATLAB is a widely accepted software tool routinely used in engineering, but not so much in applied optics and especially in acousto-optics. In this talk, we first explore the use of MATLAB to solve some well-known examples in acousto-optics such as Bragg diffraction and Raman-Nath diffraction. After establishing the correctness of the MATLAB approach, we then apply it to investigate image processing using acousto-optics and complete power transfer into the second and third Bragg order.

All acoustooptic (AO) devices are based on the inherent nonlinear dependence of the light diffraction efficiency as a function the acoustical wave intensity. In AO devices using a single frequency acoustic wave it is easy to be taken into account to describe all functional properties and limits of the first order diffraction. But in many practical devices using multi-frequency sounds like AO signal processors or multipoint AO deflectors the description are a very complicated, because of a very strong frequency intermodulations. These effects give functional disadvantages in the AO implementations- the decrease of the first order diffraction efficiency maximum to much less than 100%, the appearance of spurious orders and etc. In this work there were developed the theoretical arguments for the possibility to have the appropriate signal dynamic predistortions to have a serious change of mentioned intermodulations and to obtain some advances in limitations of many existing AO devices. The proposal uses the new technique by authors for synthesis and programmable dynamic changes of all RF signals performing the preliminary electronic signal treatment to cancel a certain diffraction modes. The experimental verifications with AO modulators based on TeO₂ crystal have been performed. With use of the analogue third order polynomial RF synthesizer there were obtained a good suppression up to 10-15 dB of two-tone second-order intermodulations in area of second order diffraction and two-tone third order spurious modes in the first order diffraction area, in accordance with proper theoretical calculations. There was also verified new technique providing maximal optical power in multi beams laser diagram. In TeO₂ AO modulators the growth of the efficiency of two-beam He-Ne laser about 14% was obtained.

We present an optimized design of an acousto-optic tunable filter (AOTF) using a phased-array transducer for a spectrally-multiplexed ultrafast pulse-shaping RF beamformer application. The momentum-space interaction geometry is used to optimize an AOTF using acoustic beam-steering techniques in combination with acoustic anisotropy in order to linearly map the applied RF frequency to the filtered output optical frequency. The appropriate crystal orientation and phased-array transducer design are determined to linearize the RF to optical frequency mapping even in the presence of optical dispersion of the birefringence. After optimizing the phased-array transducer, acoustic anisotropy, and optical anisotropic diffraction geometry, the designed AOTF will compensate for the birefringent dispersion of TeO₂ to give a linear modulation of RF frequencies onto the corresponding optical frequencies. This linearized frequency mapped AOTF is required for a squint-compensated, wavelength-multiplexed, optically processed RF imager.

Production of high- performance and low-cost new devices to be used in space applications is strongly required due to the remarkable development of innovative technologies in the last few years. Guided-wave optoelectronics technologies, including integrated optics, acousto-optics and electro-optics can provide some significant benefits to the space applications. In particular, they can overcome the intrinsic limits of the conventional technologies improving also the cost/performance figures, and enabling new services. Earth observation, telecommunications, radar surveillance and navigation control are the main space areas where guided-wave devices can contribute significantly. In this paper, after some general considerations on the potential of optoelectronics for space, on the use of acousto-optic guided-wave devices, a brief description of the acousto-optic interaction is given. Some functional devices reported in literature having significant potential impact in space applications are described with the aim of highlighting the main features of the acousto-optic technology. The performance limits of guided-wave devices for space applications are also shortly discussed.

Sensing from space requires the development of hyperspectral imagers operating in the two atmospheric window regions (3-5 μm and 8-12 μm) in the infrared region. At the Army Research Laboratory we are developing hyperspectral imagers operating in these two regions based on using acousto-optic tunable filters (AOTFs). Research is going on in the growth of new acousto-optic materials-mercurous halides (Hg₂Cl₂ and Hg₂Br₂) and tellurium (Te) for development of AOTF cells. We have developed a hyperspectral imager using a tellurium dioxide (TeO₂) AOTF with a liquid-nitrogen-cooled indium antimonide (InSb) 256×256 focal plane array (FPA) that covers 2-4.5 μm region and another imager using a thallium arsenic selenide (TAS) AOTF with liquid-nitrogen-cooled mercury cadmium telluride (HgCdTe) 256×256 FPA to cover 7.8-9.8 μm region. Each of these imagers has been integrated on a tripod-mountable platform with an appropriate optical train. Control of RF electronics, and image acquisition and storage operations are carried out under full computer-control with a user-friendly interface. We will discuss the progress in material development and present imaging results.

AOTF-based Spectral Imaging System for microscopic sample analysis in visible and NIR is described, characteristic features and some potential applications are discussed. Advantages of double AOTF configuration are analyzed

Information capacity of acousto-optic devices depends both on their resolving power and number of recognized gray scale levels. The least is defined by the device dynamic range. At the same time, the resolving power depends on the number of gray scale levels. Hence, a rather complicated interconnection exists between the device dynamic range and its information capacity. This interconnection is the subject to study in the present paper. The close interconnection but not the similar one takes place between the amount of transmitted information and the number of gray scale levels of the processed signal. The study of these interconnections has been performed proceeding from the information criterion of the acousto-optic device resolving power. It has been shown that the optimal number of gray scale levels exists under which the maximum amount of information can be transmitted by an acousto-optic device. This optimal number depends on the admissible probability of failure as well as on the level of noise. The obtained and discussed experimental data are well corresponded to the calculations made proceeding from the device resolving power information criterion.

The paper presents studies on influence of the gas laser cavity shape on an acoustic wave created in the cavity. Results of investigations on an RF excited CO₂ slab-waveguide laser are shown. As is demonstrated, rapid changes of a laser plasma pressure appear in the laser gas mixture as a consequence of the pulsed RF discharge in the laser. The pressure variations create an acoustic wave propagated in the laser chamber, and involve changes of the refractive index of an excited plasma. As a result, the frequency of the optical wave emitted by the laser changes - a "line hoppings" effect appears. In the case of the slab-waveguide laser an acoustic wave propagates in a closed space - the laser reservoir, that is a special kind of an acoustic resonator. As known, a material from which cavity is made, a shape of the walls and their mutual position are significant for a wave propagation. In the experiment, the walls of the chamber are made of aluminum, so it is a very reflecting area. More, the walls are parallel that is an advantageous condition for creating standing waves. The aluminum wedges were used in the experiment to change the geometry of the reservoir. The influence of pulse duration time on the acoustic signal is investigated.

This paper presents a new portable instrument called Autonomous Tunable Filtering System (ATFS), developed for highly customisable imaging spectrometry in the VIS-NIR range. The ATFS instrument consists of an Acousto-Optic Tunable Filter (AOTF), an optical system, a Radio Frequency (RF) driver based on a Direct Digital Synthesiser (DDS) and control software. The ATFS can be attached to a variety of high-performance monochrome cameras. The system works as a spectral bandpass filter whose wavelength can be selected between 400nm and 1000nm and whose bandwidth can be adjusted between 4nm and 50nm. The filter can be tuned electronically at a very high speed and accuracy, thanks to the DDS versatility. The control software synchronises the camera with the RF generation and implements a smart auto-exposure algorithm that maximises the dynamic range of the instrument for each band. The software can take a set of spectral images sequentially and save them in ENVI® multispectral format or as multiple TIFF images. The system has been validated using a reference point spectrometer. An optional acquisition procedure has been developed, based on the acquisition of dark and white Spectralon® reference images, in order to use the system in applications involving quantitative (reflectance) measurements. Procedures have been established in order to fully calibrate the instrument. The system has been demonstrated in a real world application, which uses the ATFS to map the leaf chlorophyll content from multispectral reflectance images.

The subject of the paper is related to analysis of Bragg regimes of light diffraction by ultrasound that may be observed in optically biaxial media. The regimes of acousto-optic diffraction characterized by change of optic mode during the interaction process as well as the diffraction with conservation of optic polarization are discussed in the presentation. Dependences of Bragg incidence angle on acoustic frequency are examined for various interaction geometries including the cases of symmetrical and non-symmetrical wave vector diagrams. It is shown that many of the analyzed cases of Bragg acousto-optic diffraction may be applied for development of acousto-optic devices such as modulators, deflectors and tunable filters. The sufficient part of the calculations was carried out for the promising biaxial material double lead molybdate (Pb₂MoO₅) possessing a considerably high acousto-optic figure of merit. Wavelength dispersion of directions of optical axes in the material as well as the dispersion of directions of basic indicatrix axes has been taken into account during the analysis.

Three- and four-wave spatial Bragg solitary waves in the form of weakly coupled states, originating with one- and two-phonon non-collinear scattering of light in anisotropic medium, are considered. The space-frequency distributions of their optical components are investigated both theoretically and experimentally.

Acousto-optic tunable filters (AOTF) represent a very strong tool for study the features of different surfaces, especially if these features depend on the spectral dispersion of the light reflection indicatrix form and position. The typical sample of the problem connected with the surface features definition is the problem of recognition of the surface relief which is used for different objects genuineness. The surface relief can represent a sum of a number of diffraction gratings, oriented in different directions and having different spacings. If the studied surface is illuminated by light of variable wavelength, the reflected light varies correspondingly its intensity and direction. The variable wavelength of light can be provided by AOTF whereas spatial and temporal distribution of the reflected light can be registered by 2-D CCD array. The AOTF which is used for the surface study, gas been manufactured using acousto-optic Bragg cell based on tellurium dioxide single crystal. The interaction configuration in the cell corresponds to a non-collinear AOTF. The luminous flux at the AOTF output can be characterized by the controlled central wavelength, controlled spectral selectivity, and controlled form of AOTF spectral characteristic. The data measured by means of CCD array, are compared with the sample data using special software.

This paper is devoted to experimental investigation of regular trends of acoustic waves propagation and glancing reflections in XY plane of TeO₂ single crystal. The unusual "backward" reflection of the acoustic waves from a side border of the crystal when the phase velocity of sound is directed along the border has been observed. All experimental results were in good agreement with the predictions of theory, related to the magnitudes of the phase and group velocities of ultrasound as well as to the acoustic energy walkoff angles in the crystal.

The basic factors causing the deformations of optical spectrum recorded by the AOTF-based spectrometers are discussed. It is shown that the spectrum restoration being an inverse ill-posed problem can be formulated in correct form. Some approaches are developed for spectra correction (partial restoration) and examples of correction are presented.

We consider physical principles of realizing the Bragg regime of two- and three-fold scattering of light in optically anisotropic crystals in specially elaborated cases, when direct transitions between all the orders of scattering are allowed and, moreover, the probabilities of these transitions can be controlled. The exact and closed analytical models for describing these cases are developed. The performed analysis reveals an optical nonlinearity governed by an external electronic signal, and computer simulations illustrate the obtained results. Possible applications lie in the fields of controlling light with light and all-optical logic-based switching. Here, in particular, all-optical multi-bit digital registers are presented.

High sensitivity and selectivity of gas/vapour detection are achieved employing registration of laser photoacoustic spectra. The lasers are usually operated in the continuous-wave (CW) single-frequency mode. The tuning range of the single CW laser system is not sufficient to cover spectral bands of variety of gases/vapours of interest. The optical parametric oscillator (OPO) systems are more preferential for multi component laser analyzers allowing the simultaneous measurement of different gases or pollutants. Pumped by the same 7 ns duration pulse of Nd:YAG laser and its harmonics, two OPO systems were tested. One system generates in the 0.7-1.9 μm range and covers overtones of stretching vibrations and combination vibrations of hydrogen atoms in the analyte molecule. Other system generating in the 5-11 μm range covers vibrations of molecular characteristic groups ("fingerprints"). Photoacoustic spectra of nitro compound vapours, e.g. nitromethane, nitroethane, nitropropane, nitrobenzene and nitrotoluene, also spectra of methane and water vapour were measured and compared to simulated spectra derived with the aid of HITRAN data base and to the literature spectral data. Photoacoustic detection thresholds are evaluated from the ratios of measured signal to the registration noise.