Doppler-free optoacoustic spectroscopy

The first observation of Doppler-free optoacoustic spectroscopy is reported. As a first example the P (193) line of the 11-0 band of the B←X transition of ¹²⁷I₂ is used. The output of cw single mode dye laser is split into two equal intensity beams chopped at frequencies ω₁ and ω₂. The nonlinear compoment of the optoacoustic signal at the frequency (ω₁ + ω₂) is detected and Doppler-free resolution is obtained. Comparing the Doppler-free optoacoustic and fluorescence spectra of iodine measured under similar conditions, good agreement is found. Since optoacoustic and fluorescence methods complement each other, this opens up new possibilities for weakly or nonfluorescing molecules.     

Control of tissue mechanics upon the repetitive-pulse laser heating of cartilage

The viscoelastic properties of laser-irradiated cartilages are studied with optoacoustic methods upon the thermal excitation of mechanical oscillations by repetitive-pulse laser radiation. The effect of laser power, pulse duration, repetition rate, and irradiation time on the shape of the optoacoustic signal is analyzed. It is demonstrated that the optoacoustic response of the cartilage to the repetitive-pulse radiation of a fiber laser depends on the softening of the tissue upon the variation in its shape. Under repetitive-pulse laser irradiation, the optoacoustic response of the cartilage depends on the mechanical characteristics of the biotissue (elastic modulus, hydraulic permeability, and thickness). A simple model that makes it possible to estimate the contribution of the viscoelastic properties to the formation of the optoacoustic response at various laser repetition rates is proposed.     

液体光击穿阈值的研究

运用声学基础理论,讨论了激光入射到液体中,激光能量与声信号强度间的变化关系.发现激光能量与声信号强度之间存在着对数线形关系.根据液体中光声信号的连续性,得到了一种求取液体光击穿阈值的新方法.     

Doppler-free optoacoustic spectroscopy of O-18 methanol

The Doppler-free optoacoustic spectrum of the oxygen-18 species of methanol has been investigated with a CO₂ laser. Seventeen coincidences have been found between infrared absorptions of CH¹⁸₃OH and CO₂ laser lines. The strongest feature seen, pumped by the 10R(20) CO₂ laser line, has two plausible alternative identifications, determined from a parallel high-resolution spectroscopic study of the C-O stretching band. Further experimental tests are suggested to resolve the assignment, and predicted far-infrared laser frequencies are given for the two possibilities.     

Optoacoustic Laser Stark spectroscopy and FIR laser action on CH3Br using a waveguide CO2 laser

The optoacoustic spectrum of CH₃Br around 10 m band lines of a tunable cw waveguide CO₂ laser is investigated. Several new infrared absorptions are observed and most of the correspond ing molecular transitions are assigned. Far infra red laser action is reported by pumping with the same CO₂ laser: pump offsets are given using the Transferred Lambs dip (TLD) technique. A new FIR laser emission is obtained and assigned. An optoacoustic Laser Stark spectroscopy technique is used to investigate off resonance infrared tran sitions.     

Nonlinear optoacoustic transformation in the system of dielectric substrate/submicron metal coating/liquid

The optoacoustic method has been shown to be an accurate technique for the measurement of the properties of submicron metal coatings deposited on a dielectric substrate, i.e., mirrors. The method has been previously theoretically described in terms of a linear model of optoacoustic transformation in a system substrate/coating/liquid. The goal of the present work was to determine the limits at which the linear model is still applicable. The modification of the laser induced acoustic signal profiles and transfer functions of optoacoustic transformation versus the laser fluence was studied for two liquids: ethanol and water.     

Use of IR Absorption Laser Spectroscopy at Nuclear Fuel Cycle Plants: Problems and Prospects (Review)

The main stages of the nuclear fuel cycle from extraction of uranium ores to disposal of radioactive waste resulting from the processing of spent nuclear fuel were briefly analyzed. A list of the most probable radioactive substances and toxic chemicals that can be part of emergency emissions at each stage of the nuclear fuel cycle was composed. The basic physical principles of local and remote IR absorption laser technologies for detecting radioactive substances and toxic chemicals in the atmosphere for solving some unique problems of environmental monitoring were considered. The analytical potential of the currently most effective laser technologies for atmospheric monitoring at nuclear fuel cycle plants based on the achievements of diode laser spectroscopy, cavity ringdown laser spectroscopy, and optoacoustic laser spectroscopy using diode and quantum-cascade lasers was discussed. Current trends in the development of laser technologies for atmospheric monitoring in different IR spectral ranges were analyzed.     

Application of the optoacoustic effect in optically pumped submillimeter lasers

The dependence of the optoacoustic signal generated in an optically pumped submillimetre laser system on gas pressure and modulation frequency has been studied experimentally and the results accounted for theoretically. With care an extracavity optoacoustic cell containing the laser gas diluted with air can be used to stablize the pump laser frequency to maximum absorption but this does not always optimise laser output.     

时域光声谱技术及其在生物组织检测中的应用

当强度调制的光束照射于吸收物质,周期性热流使周围的介质热胀冷缩而激发声波,这种将光能转化为声能的现象称为光声效应。基于光声效应的时域光声谱技术将光学和声学有机地结合,为生物组织的无损检测技术提供了新的检测手段。该技术能够实现类似光学技术的高对比度和近似于声学技术的高精度和穿透深度,在生物医学检测中具有广阔的应用前景。文章介绍时域光声谱技术的原理及其在生物组织成分检测和层析成像检测中的应用。     

Shock-wave generation during dry laser cleaning of particles

Generation of shock waves during a dry laser-cleaning process was examined with the probe-beam-deflection technique. Our experimental set-up allows measurement of a density gradient and a time of flight of the generated optoacoustic waves. With analysis of the measured signals we observed supersonic propagation velocities, density profiles matching the shock-wave profile, and reasonable agreement between the shock-wave theory and measured data. Furthermore, we inspected the mechanisms for the generation of these optoacoustic waves. For the clean surface a rapid thermoelastic surface expansion is the main generation mechanism, creating only weak shock waves; however, during the dry laser cleaning the ejected contaminants generate relatively strong shock waves. This difference is easily observed with our experimental set-up, thus enabling the on-line monitoring of the laser-cleaning process. PACS 79.20.Ds; 81.65.Cf; 43.25.+y     

Acoustic Monitoring of the Sea Medium Variability: Experimental Testing of New Methods

Two new methods of acoustic monitoring of the sea medium are tested in a shelf region. One of the methods is the difference-pulse technique in which the variability of the medium is characterized by the difference in the energies of the sequences of arrivals of two multiray signals. The other method is the self-sustained oscillator technique in which the information on the changes that occur in the water temperature, the velocity of currents, and other parameters of the medium is obtained from the frequency deviation of a self-sustained oscillator that has a feedback loop closed through the underwater sound channel.     

Real-time optodynamic monitoring of pulsed laser decoating rate

This paper deals with a laser-cleaning process for removing a layer of paint. The effects of each individual laser pulse were monitored by a three-probe system. Ultrasonic signals from the rear surface of the substrate were obtained using an arm compensated Michelson interferometer, the optoacoustic waves in the surrounding air were detected using a laser beam deflection probe and the shape of the growing crater was measured using a laser anamorph profilometer. Two different cleaning lasers were used: an Excimer and a Nd:YAG laser. Typical parameters of the optodynamic signals versus the laser-pulse number are presented. A linear correlation between the quantity of ablated material and the integral of the ultrasonic signals was found together with an exponential relationship between the time-of-flight of the optoacoustic signals and the decoating rate.     

Reconstruction of Optical Energy Deposition for Backward Optoacoustic Imaging

Visualizing optical properties, such as the optical absorption coefficient, helps us to obtain structural information of biological tissues. In this paper, we present an efficient reconstruction algorithm for optical energy deposition in backward optoacoustic imaging. Note that econstruction of optical energy deposition is the first step to imaging the optical absorption coefficient distribution. This algorithm is derived from the optoacoustic wave equations with line focusing, in which the focusing techniques were utilized to reduce the reconstruction problem from three dimensions (3-D) to one dimension (1-D). Simulations and experiments were conducted to verify efficacy of this algorithm. In the simulations, optoacoustic signals were generated based on the solution of the optoacoustic wave equations. In the experiments, a 3-D backward mode optoacoustic imaging system was built. The system consisted of a Nd YAG laser for optical irradiation and an acoustic detection system with a broadband hydrophone. A phantom was used to illustrate validity of the proposed algorithm. The results show that optical energy deposition can be efficiently reconstructed in both simulations and experiments.     

Methods of optoacoustic diagnostics of biological tissues

Acoustical Physics - Laser optoacoustic diagnostics is based on thermoelastic excitation of ultrasonic signals in a medium due to absorption of pulsed laser radiation. The pressure profile of such...     

All-optical probing of the nonlinear acoustics of a crack

Experiments with an all-optical method for the study of the nonlinear acoustics of cracks in solids are reported. Nonlinear acoustic waves are initiated by the absorption of radiation from a pair of laser beams intensity modulated at two different frequencies. The detection of acoustic waves at mixed frequencies, absent in the frequency spectrum of the heating lasers, by optical interferometry or deflectometry provides unambiguous evidence of the elastic nonlinearity of the crack. The high contrast in crack imaging achieved by remote optical monitoring of the nonlinear acoustic processes is due to the strong dependence of the efficiency of optoacoustic conversion on the state of the crack. The highest acoustic nonlinearity is observed in the transitional state of the crack, which is intermediate between the open and the closed ones.     

Optoacoustic detection of Doppler-free two-photon resonances in the v2 bands of NH3

A new approach to infrared two-photon spectroscopy is described. An optoacoustic cell operating at low pressure is used to detect Doppler-free transitions excited by a conventional and a waveguide CO₂ laser. Beams from the two lasers are passed through the sample in opposite directions and an electric field tunes the transitions into resonance by molecular Stark effect. A few examples are discussed to illustrate the features and the sensitivity of the method.     

Sensitive interferometric detection of ultrasound for minimally invasive clinical imaging applications

Miniaturized optical detectors of ultrasound represent a promising alternative to piezoelectric technology and may enable new minimally invasive clinical applications, particularly in the field of optoacoustic imaging. However, the use of such detectors has so far been limited to controlled lab environments, and has not been demonstrated in the presence of mechanical disturbances, common in clinical imaging scenarios. Additionally, detection sensitivity has been inherently limited by laser noise, which hindered the use of sensing elements such as optical fibers, which exhibit a weak response to ultrasound. In this work, coherence‐restored pulse interferometry (CRPI) is introduced – a new paradigm for interferometric sensing in which shot‐noise limited sensitivity may be achieved alongside robust operation. CRPI is implemented with a fiber‐based resonator, demonstrating over an order of magnitude higher sensitivity than that of conventional 15 MHz intravascular ultrasound probes. The performance of the optical detector is showcased in a miniaturized all‐optical optoacoustic imaging catheter.     

Optical and optoacoustic measurements of the absorption properties of spherical gold nanoparticles within a highly scattering medium

In the present paper the requirements for optical parameter characterization of absorbing materials located within a highly scattering medium has been addressed. The measurement scheme incorporates the optoacoustic technique where a single acoustic transducer is used to detect ultrasonic transients generated from laser irradiation. The absorbing medium is based on different concentrations of spherical gold nanoparticles (SGNP’s), these are currently being considered as non-toxic targeted optical contrast agents for both medical imaging and cancer therapeutics. In this paper we present results which demonstrate the two main advantages the optoacoustic technique has over other measurement schemes. These are the possibility to obtain information on the position and dimensions of absorbing bodies using a time of flight analysis (TOF) and secondly, the higher sensitivity of the optoacoustics compared to optical transmission techniques. The former advantage is of particular interest for imaging applications and the latter for detection and characterization of absorbing materials surrounded by high levels of high scattering mediums. We present for the first time the characterization of SGNP within a highly scattering medium. To further demonstrate the feasibility of the optoacoustic technique, the scattering coefficient of the surrounding medium has also been characterized.     

Frequency Stabilization of Waveguide CO2 Laser by a Digital Technique

In this work we present a simple technique for laser frequency stabilization, based on Digital signal processing. The technique is used to stabilize a waveguide CO₂ laser of wide tunability by using three kinds of reference signals: the CO₂ laser ouptut power, an infrared absorption optoacoustic signal and the output power of a Far-Infrared optically pumped molecular laser.     

Spectroscopy of the excited C-O stretching Q branch of 13CD3OH: Measurement and assignment of new FIR laser lines

The Q branch of the C-O stretching fundamental band of ¹³CD₃OH has been investigated. Starting from a high resolution (4 × 10⁻³cm⁻¹) infrared Fourier transform spectrum and using a waveguide CO₂ laser of 300 MHz tunability and an acoustooptic modulator for an extension of ±90 MHz, 31 new FIR laser lines have been observed. The related absorptions have been measured by means of optoacoustic detection. The frequency of one new FIR laser line was also measured. Eight tentative assignments are proposed for the IR absorption and FIR laser emissions.