Thomas Berer

Three-dimensional photoacoustic imaging using fiber-based line detectors

For photoacoustic imaging, usually point-like detectors are used. As a special sensing technology for photoacoustic imaging, integrating detectors have been investigated that integrate the acoustic pressure over an area or line that is larger than the imaged object. Different kinds of optical fiber-based detectors are compared regarding their sensitivity and resolution in three-dimensional photoacoustic tomography. In the same type of interferometer, polymer optical fibers yielded much higher sensitivity than glass fibers. Fabry-Perot glass-fiber interferometers in turn gave higher sensitivity than Mach-Zehnder-type interferometers. Regarding imaging resolution, the single-mode glass fiber showed the best performance. Last, three-dimensional images of phantoms and insects using a glass-fiber-based Fabry-Perot interferometer as integrating line detector are presented.

Remote photoacoustic imaging on solid material using a two-wave mixing interferometer

We report on remote and contactless photoacoustic imaging (PAI) for the inspection of solid materials using a two-wave mixing interferometer. In this Letter, a semitransparent sample was excited with picosecond laser pulses. The local absorption of the electromagnetic radiation led to generation of broadband ultrasonic waves inside the sample. Ultrasonic waves arriving at the sample surface were detected utilizing a two-wave mixing interferometer. After data acquisition, the initial pressure distribution was reconstructed using a Fourier space synthetic aperture technique algorithm. We show the potential of PAI for the inspection of semitransparent solid materials.

Non-contact photoacoustic imaging using a fiber based interferometer with optical amplification.

In photoacoustic imaging the ultrasonic signals are usually detected by contacting transducers. For some applications contact with the tissue should be avoided. As alternatives to contacting transducers interferometric means can be used to acquire photoacoustic signals remotely. In this paper we report on non-contact three and two dimensional photoacoustic imaging using an optical fiber-based Mach-Zehnder interferometer. A detection beam is transmitted through an optical fiber network onto the surface of the specimen. Back reflected light is collected and coupled into the same optical fiber. To achieve a high signal/noise ratio the reflected light is amplified by means of optical amplification with an erbium doped fiber amplifier before demodulation. After data acquisition the initial pressure distribution is reconstructed by a Fourier domain reconstruction algorithm. We present remote photoacoustic imaging of a tissue mimicking phantom and on chicken skin.

Characterization of broadband fiber optic line detectors for photoacoustic tomography

The frequency response of fiber optic line detectors is investigated in the presented paper. An analytical model based on oblique scattering of elastic waves is used to calculate the frequency dependent acousto-optical transfer functions of bare glass optical and polymer optical fibers. From the transfer functions the transient response of fibers detectors to photoacoustically excited spherical sources is derived. Photoacoustic tomography is simulated by calculating the temporal response of arrays of fiber optic line detectors and subsequent image reconstruction. The results show that the choice of the fiber material is of significant importance and influences the quality of imaging.

Photoacoustic imaging using an adaptive interferometer with a photorefractive crystal

In this paper we report on remote three-dimensional photoacoustic imaging without the need for a coupling agent or other aids (e.g. detector foils) by utilizing a two-wave mixing interferometer (TWMI). In this technique the beams coming from sample and reference are brought to interfere in a Bi₁₂SiO₂₀ photorefractive crystal (PRC). Such a setup allows the measurement of ultrasonic displacements on rough sample surfaces. After data acquisition the initial pressure distribution is reconstructed by a Fourier domain synthetic aperture focusing technique (FSAFT). We present three-dimensional imaging of a hair ribbon phantom, on biological samples with embedded artificial blood vessels or pig bristles, and measurements on a human forearm in-vivo.

Two-photon absorption-induced photoacoustic imaging of Rhodamine B dyed polyethylene spheres using a femtosecond laser

In the present paper we demonstrate the possibility to image dyed solids, i.e. Rhodamine B dyed polyethylene spheres, by means of two-photon absorption-induced photoacoustic scanning microscopy. A two-photon luminescence image is recorded simultaneously with the photoacoustic image and we show that location and size of the photoacoustic and luminescence image match. In the experiments photoacoustic signals and luminescence signals are generated by pulses from a femtosecond laser. Photoacoustic signals are acquired with a hydrophone; luminescence signals with a spectrometer or an avalanche photo diode. In addition we derive the expected dependencies between excitation intensity and photoacoustic signal for single-photon absorption, two-photon absorption and for the combination of both. In order to verify our setup and evaluation method the theoretical predictions are compared with experimental results for liquid and solid specimens, i.e. a carbon fiber, Rhodamine B solution, silicon, and Rhodamine B dyed microspheres. The results suggest that the photoacoustic signals from the Rhodamine B dyed microspheres do indeed stem from two-photon absorption.

Complex band structures of two dimensional phononic crystals: Analysis by the finite element method

In this work, the calculation of complex band structures of two-dimensional bulk phononic crystals (2DPCs) is discussed by the finite element method. A modification of the classical ω(k) approach—calculating the unknown frequencies for a real wave number—is modified to a k(ω) solution, which allows the evaluation of complex wave numbers for real frequencies. The dispersion relation of a 2DPC in a square lattice is presented and it is shown that the problem reduces to a polynomial eigenvalue problem with quadratic and quartic eigenvalue problems in the Γ−X−M−Γ directions. The developed method is applied for solid-vacuum PCs made of isotropic materials consisting cylindrical holes. Complex dispersion diagrams are calculated with various Poissons ratios and the mode shapes of the propagating and the evanescent modes are presented. The significance of the complex bands is discussed.

Multimodal noncontact photoacoustic and optical coherence tomography imaging using wavelength-division multiplexing

Abstract. We present multimodal noncontact photoacoustic (PA) and optical coherence tomography (OCT) imaging. PA signals are acquired remotely on the surface of a specimen with a Mach-Zehnder interferometer. The interferometer is realized in a fiber-optic network using a fiber laser at 1550 nm as the source. In the same fiber-optic network, a spectral-domain OCT system is implemented. The OCT system utilizes a supercontinuum light source at 1310 nm and a spectrometer with an InGaAs line array detector. Light from the fiber laser and the OCT source is multiplexed into one fiber using a wavelength-division multiplexer; the same objective is used for both imaging modalities. Reflected light is spectrally demultiplexed and guided to the respective imaging systems. We demonstrate two-dimensional and three-dimensional imaging on a tissue-mimicking sample and a chicken skin phantom. The same fiber network and same optical components are used for PA and OCT imaging, and the obtained images are intrinsically coregistered.

Focusing and subwavelength imaging of surface acoustic waves in a solid-air phononic crystal

Focusing and subwavelength imaging of surface acoustic waves (SAWs) through a phononic crystal flat lens are discussed in the presented work. Experimental and numerical wave fields are obtained in the time-domain by an optical technique and by numerical simulations. Spatial distributions of the acoustic field are accessed using a temporal Fourier transform. The revealed focusing of the elastic waves in the first band of the crystal is governed by the concave equifrequency contour of the leaky-Rayleigh wave. The spatial distributions of the experimental and numerical acoustic fields also unfold subwavelength imaging of SAWs. Numerical simulations show that the imaging quality can be improved by embedding the flat lens into a medium with higher wave velocity.

Numerical modeling of thermoelastic generation of ultrasound by laser irradiation in the coupled thermoelasticity

Highlights ► Numerical modeling of ultrasound in the coupled thermoelasticity is discussed. ► Thermoelastic generation of ultrasound by laser irradiation is modeled. ► The influence of the coupling in the generalized thermoelasticity is investigated. ► For ultra high frequency waves (∼100 GHz) strong attenuation is observed. ► Laser-generated wave fields are presented in isotropic and hexagonal media.