One-dimensional ultrasound receive array using spectrally encoded optical detection

An ultrafast Ti:Sapphire laser and a Sagnac interferometer are combined for optical detection of ultrasound. Distinct spatial positions are probed simultaneously by different wavelengths within the broadband laser. Ultrasonic signals from each probe position are derived from the spectrum of the reflected light. The same single-mode fiber delivers incident and reflected light. A one-dimensional receive array is demonstrated by measuring the acoustic field of a spherically focused piezoelectric transducer. This is a promising form of parallel detection for miniaturized high-frequency ultrasonic arrays.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71131/2/APPLAB-85-24-6045-1.pd

Time reversal three-dimensional imaging using single-cycle terahertz pulses

We demonstrate three-dimensional imaging using single-cycle terahertz electromagnetic pulses. Reflection-mode imaging is performed with a photoconductive transmitter and receiver and a reconstruction algorithm based on time reversal. A two-dimensional array is synthesized from ten concentric ring annular arrays with numerical apertures ranging from 0.27 to 0.43. The system clearly distinguishes image planes separated by 1.5 mm and achieves a −6 dB lateral resolution of 1.1 mm. In terms of the illuminating terahertz power spectrum, the lateral resolution is 38% and 81% of the peak and mean wavelengths, respectively. © 2004 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69881/2/APPLAB-84-12-2196-1.pd

Coded excitation of broadband terahertz using optical rectification in poled lithium niobate

We demonstrate coded excitation of broadband terahertz for imaging applications. The encoded transmitter uses optical rectification of femtosecond laser pulses in poled lithium niobate patterned with a 53-bit53-bit binary phase code. The terahertz wave forms are detected by electro-optic sampling in zinc telluride. A digital pulse compression filter decodes the binary wave forms, producing broadband pulses at 1.0 THz1.0THz. A two-dimensional imaging experiment shows comparable performance between the encoded transmitter and a zinc telluride emitter.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87834/2/251105_1.pd

High-frequency ultrasound array element using thermoelastic expansion in an elastomeric film

The thermoelastic effect was used to produce high-frequency, broadband ultrasound in water. A pulsed diode laser, followed by an erbium-doped fiber amplifier, was focused onto a light-absorbing film deposited on a glass substrate. Conversion efficiency was improved by over 20 dB using an elastomeric film instead of a more commonly used metallic one. Radiation pattern measurements show that considerable energy is radiated at +/−45° for frequencies beyond 50 MHz. These results show that the thermoelastic effect can be used to produce phased arrays for high-frequency ultrasound imaging. © 2001 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69937/2/APPLAB-79-4-548-1.pd

Optoacoustic imaging using thin polymer étalon

Optical detection of ultrasound is a promising technique for high frequency imaging arrays. Detection resolution approaches the optical resolution, which can be on the order of the optical wavelength. We describe here an optical technique for ultrasound detection based on a thin (10 μm)(10μm) Fabry–Perot étalon optimized for high resolution imaging. The signal to noise ratio (SNR) approaches that of an ideal piezoelectric transducer over a 100 MHz100MHz bandwidth. Array functionality is demonstrated by scanning a probe beam along a line. Thermoelastic excitation was applied to generate acoustic waves in a test phantom containing a single “pointlike” source. An image of the source was reconstructed using signals acquired from the étalon detector array.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87845/2/134102_1.pd

Imaging nanostructures with coherent phonon pulses

We demonstrate submicron resolution imaging using picosecond acoustic phonon pulses. High-frequency acoustic pulses are generated by impulsive thermoelastic excitation of a patterned 15-nm15-nm-thick metal film on a crystalline substrate using ultrafast optical pulses. The spatiotemporal diffracted acoustic strain field is measured on the opposite side of the substrate, and this field is used in a time-reversal algorithm to reconstruct the object. The image resolution is characterized using lithographically defined 1-micron1-micron-period Al structures on Si. Straightforward technical improvements should lead to resolution approaching 45 nm45nm, extending the resolution of acoustic microscopy into the nanoscale regime.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71146/2/APPLAB-84-25-5180-1.pd

Thermoelastic generation of ultrasound for high frequency phased arrays.

High frequency two-dimensional phased arrays for ultrasound microscopy are difficult to construct using piezoelectric technology. A promising alternative involves optical generation of ultrasound, where an array element is defined by the size and location of a laser beam. Optical generation of ultrasound is possible through a variety of mechanisms, the most common being the thermoelastic effect. Optical absorption of a laser pulse rapidly heats a localized volume, where thermal expansion launches an acoustic wave containing frequencies corresponding to the envelope of the absorbed optical pulse. Acoustic pulse shaping is therefore possible by suitable modulation of the laser pulse. An erbium doped fiber amplifier (EDFA) was built to amplify pulses from a diode laser to peak powers of several watts, sufficient to generate ultrasound detectable with conventional transducers. Preliminary experiments used a chromium film deposited on a glass substrate as the light absorbing structure. This structure is found to generate ultrasound with a bandwidth comparable to the excitation laser. This is very promising, but the poor SNR of the measured signals call for an improvement in the optical to acoustical conversion efficiency. Efficiency can be improved by choosing materials with high thermal expansion coefficients. An increase in conversion efficiency of nearly 20 dB was obtained using an optical absorbing layer consisting of a mixture of polydimethylsiloxane (PDMS) and carbon black spin coated onto a glass microscope slide. Radiation pattern measurements using high frequency piezoelectric transducers show that the thermoelastic element is small enough for a 75 MHz array. However, measurements with an optoacoustic detection array show that the radiated acoustic field is corrupted by leaky Rayleigh waves excited along the PDMS/glass interface. Replacing the glass with a clear PDMS substrate eliminates this problem to produce a smooth radiation pattern. These results show that a 75 MHz 2-D transmitting array can be produced using thermoelastic generation of ultrasound in an all PDMS structure.Ph.D.Applied SciencesBiomedical engineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/123138/2/3068834.pd

Near-infrared multispectral photoacoustic microscopy using a graded-index fiber amplifier

We demonstrate optical resolution photoacoustic microscopy (OR-PAM) of lipid-rich tissue using a multi-wavelength pulsed laser based on nonlinear fiber optics. 1047 nm laser pulses are converted to 1098, 1153, 1215, and 1270 nm pulses via stimulated Raman scattering in a graded-index multimode fiber. Multispectral PAM of a lipid phantom is demonstrated with our low-cost and simple technique