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Detection of a novel mechanism of acousto-optic modulation of incoherent light.

Jarrett CW, Caskey CF, Gore JC - PLoS ONE (2014)

Bottom Line: This pattern differs from previous reports of acousto-optical interactions that produce diffraction effects that rely on phase shifts and changes in light directions caused by the acoustic modulation.Moreover, previous studies of acousto-optic interactions have mainly reported the effects of sound on coherent light sources via photon tagging, and/or the production of diffraction phenomena from phase effects that give rise to discrete sidebands.These effects potentially provide a novel method for visualizing sound fields and may assist the interpretation of other hybrid imaging methods.

View Article: PubMed Central - PubMed

Affiliation: Vanderbilt University Institute of Imaging Science, Vanderbilt University, Nashville, Tennessee, United States of America; Program in Chemical and Physical Biology, Vanderbilt University, Nashville, Tennessee, United States of America.

ABSTRACT
A novel form of acoustic modulation of light from an incoherent source has been detected in water as well as in turbid media. We demonstrate that patterns of modulated light intensity appear to propagate as the optical shadow of the density variations caused by ultrasound within an illuminated ultrasonic focal zone. This pattern differs from previous reports of acousto-optical interactions that produce diffraction effects that rely on phase shifts and changes in light directions caused by the acoustic modulation. Moreover, previous studies of acousto-optic interactions have mainly reported the effects of sound on coherent light sources via photon tagging, and/or the production of diffraction phenomena from phase effects that give rise to discrete sidebands. We aimed to assess whether the effects of ultrasound modulation of the intensity of light from an incoherent light source could be detected directly, and how the acoustically modulated (AOM) light signal depended on experimental parameters. Our observations suggest that ultrasound at moderate intensities can induce sufficiently large density variations within a uniform medium to cause measurable modulation of the intensity of an incoherent light source by absorption. Light passing through a region of high intensity ultrasound then produces a pattern that is the projection of the density variations within the region of their interaction. The patterns exhibit distinct maxima and minima that are observed at locations much different from those predicted by Raman-Nath, Bragg, or other diffraction theory. The observed patterns scaled appropriately with the geometrical magnification and sound wavelength. We conclude that these observed patterns are simple projections of the ultrasound induced density changes which cause spatial and temporal variations of the optical absorption within the illuminated sound field. These effects potentially provide a novel method for visualizing sound fields and may assist the interpretation of other hybrid imaging methods.

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Related in: MedlinePlus

Experimental Apparatus.US: ultrasound transducer; LED: light-emitting diode; OW: optical window; WT: water tank; PMT: photomultiplier tube; MS: motion stage; I–V: transimpedance amplifier; LIA: lock-in amplifier; OSC: oscilloscope; 3DS: three axis motion stage; PC: LabVIEW system control and data acquisition computer.
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pone-0104268-g001: Experimental Apparatus.US: ultrasound transducer; LED: light-emitting diode; OW: optical window; WT: water tank; PMT: photomultiplier tube; MS: motion stage; I–V: transimpedance amplifier; LIA: lock-in amplifier; OSC: oscilloscope; 3DS: three axis motion stage; PC: LabVIEW system control and data acquisition computer.

Mentions: Figure 1 shows the experimental system that was designed and built to be able to measure both optical and acousto-optical signals. A water tank was constructed with an opening for a transmitting ultrasound transducer (directed along the x-axis) and an orthogonal optical window (directed along the y-axis) centered at approximately 38.1 mm distance along the transducer (x) axis, and offset by 103 mm (along y). A waterproof LED light source (Super Bright LEDs, RL5-R8030, 630 nm) was attached to a three-dimensional translation stage and inserted into the water tank. The LED was positioned 10 mm along the y-axis from the ultrasound beam axis and directed towards the optical window. A focused circular ultrasound transducer (Olympus Panametrics V314, 1 MHz center frequency, 19.05 mm element size and 38.1 mm focal length, or a Valpey Fischer, IL0206HP, 2.25 MHz center frequency, 19.05 mm element size and 50.8 mm focal length) was placed such that the axial propagation of the ultrasonic beam (along x) was perpendicular to the principal direction (along y) of the LED light. The ultrasound focus of the 1 MHz transducer was located directly in front of the center of the optical window. A function generator (Agilent Technologies, 33500B) supplied a continuous wave, 1 MHz sinusoidal signal to an RF amplifier (Amplifier Research, 200 L) to drive the ultrasound transducer at a selected voltage (0–60 volts peak to peak) to achieve a corresponding ultrasound focal zone peak negative pressure of 0–60 kPa. The voltage to pressure conversion was calibrated and verified using a hydrophone (Onda HNC-0200). A photomultiplier tube (PMT) (Hamamatsu, H5783-20) and long, narrow sampling slit (0.75 mm width) were mounted to a translation stage at the optical window. The slit was positioned so that the light signal reaching the PMT was integrated vertically across the slit at the center of the PMT surface. The translation stages allowed the two dimensional movement of the PMT and slit to scan the pattern of LED light directed towards the PMT. The PMT signal was then passed through a trans-impedance amplifier (Hamamatsu, C6438) and amplified 20 dB and then input into a lock-in amplifier (Stanford Research Systems, SR844). The signal entering the lock-in amplifier was amplified a further 20 dB. The lock-in amplifier measured only the modulated light signal from the PMT that matches the frequency of the reference signal, which was the same as the ultrasound frequency. To record the entire incident light signal (modulated and unmodulated) reaching the PMT for a reference level, the output from the PMT could bypass the lock-in amplifier and be recorded directly on a recording oscilloscope (Hewlett Packard, 54503A). Both the lock-in amplifier and oscilloscope measured the signal amplitude (v) of the modulated input signal received from the PMT. However, we report all findings as the intensity of the modulated input signal (v2) or the squared signal amplitude. A computer with LabVIEW software was used to control all stage movements and data acquisition from the lock-in amplifier and/or the oscilloscope.


Detection of a novel mechanism of acousto-optic modulation of incoherent light.

Jarrett CW, Caskey CF, Gore JC - PLoS ONE (2014)

Experimental Apparatus.US: ultrasound transducer; LED: light-emitting diode; OW: optical window; WT: water tank; PMT: photomultiplier tube; MS: motion stage; I–V: transimpedance amplifier; LIA: lock-in amplifier; OSC: oscilloscope; 3DS: three axis motion stage; PC: LabVIEW system control and data acquisition computer.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4126715&req=5

pone-0104268-g001: Experimental Apparatus.US: ultrasound transducer; LED: light-emitting diode; OW: optical window; WT: water tank; PMT: photomultiplier tube; MS: motion stage; I–V: transimpedance amplifier; LIA: lock-in amplifier; OSC: oscilloscope; 3DS: three axis motion stage; PC: LabVIEW system control and data acquisition computer.
Mentions: Figure 1 shows the experimental system that was designed and built to be able to measure both optical and acousto-optical signals. A water tank was constructed with an opening for a transmitting ultrasound transducer (directed along the x-axis) and an orthogonal optical window (directed along the y-axis) centered at approximately 38.1 mm distance along the transducer (x) axis, and offset by 103 mm (along y). A waterproof LED light source (Super Bright LEDs, RL5-R8030, 630 nm) was attached to a three-dimensional translation stage and inserted into the water tank. The LED was positioned 10 mm along the y-axis from the ultrasound beam axis and directed towards the optical window. A focused circular ultrasound transducer (Olympus Panametrics V314, 1 MHz center frequency, 19.05 mm element size and 38.1 mm focal length, or a Valpey Fischer, IL0206HP, 2.25 MHz center frequency, 19.05 mm element size and 50.8 mm focal length) was placed such that the axial propagation of the ultrasonic beam (along x) was perpendicular to the principal direction (along y) of the LED light. The ultrasound focus of the 1 MHz transducer was located directly in front of the center of the optical window. A function generator (Agilent Technologies, 33500B) supplied a continuous wave, 1 MHz sinusoidal signal to an RF amplifier (Amplifier Research, 200 L) to drive the ultrasound transducer at a selected voltage (0–60 volts peak to peak) to achieve a corresponding ultrasound focal zone peak negative pressure of 0–60 kPa. The voltage to pressure conversion was calibrated and verified using a hydrophone (Onda HNC-0200). A photomultiplier tube (PMT) (Hamamatsu, H5783-20) and long, narrow sampling slit (0.75 mm width) were mounted to a translation stage at the optical window. The slit was positioned so that the light signal reaching the PMT was integrated vertically across the slit at the center of the PMT surface. The translation stages allowed the two dimensional movement of the PMT and slit to scan the pattern of LED light directed towards the PMT. The PMT signal was then passed through a trans-impedance amplifier (Hamamatsu, C6438) and amplified 20 dB and then input into a lock-in amplifier (Stanford Research Systems, SR844). The signal entering the lock-in amplifier was amplified a further 20 dB. The lock-in amplifier measured only the modulated light signal from the PMT that matches the frequency of the reference signal, which was the same as the ultrasound frequency. To record the entire incident light signal (modulated and unmodulated) reaching the PMT for a reference level, the output from the PMT could bypass the lock-in amplifier and be recorded directly on a recording oscilloscope (Hewlett Packard, 54503A). Both the lock-in amplifier and oscilloscope measured the signal amplitude (v) of the modulated input signal received from the PMT. However, we report all findings as the intensity of the modulated input signal (v2) or the squared signal amplitude. A computer with LabVIEW software was used to control all stage movements and data acquisition from the lock-in amplifier and/or the oscilloscope.

Bottom Line: This pattern differs from previous reports of acousto-optical interactions that produce diffraction effects that rely on phase shifts and changes in light directions caused by the acoustic modulation.Moreover, previous studies of acousto-optic interactions have mainly reported the effects of sound on coherent light sources via photon tagging, and/or the production of diffraction phenomena from phase effects that give rise to discrete sidebands.These effects potentially provide a novel method for visualizing sound fields and may assist the interpretation of other hybrid imaging methods.

View Article: PubMed Central - PubMed

Affiliation: Vanderbilt University Institute of Imaging Science, Vanderbilt University, Nashville, Tennessee, United States of America; Program in Chemical and Physical Biology, Vanderbilt University, Nashville, Tennessee, United States of America.

ABSTRACT
A novel form of acoustic modulation of light from an incoherent source has been detected in water as well as in turbid media. We demonstrate that patterns of modulated light intensity appear to propagate as the optical shadow of the density variations caused by ultrasound within an illuminated ultrasonic focal zone. This pattern differs from previous reports of acousto-optical interactions that produce diffraction effects that rely on phase shifts and changes in light directions caused by the acoustic modulation. Moreover, previous studies of acousto-optic interactions have mainly reported the effects of sound on coherent light sources via photon tagging, and/or the production of diffraction phenomena from phase effects that give rise to discrete sidebands. We aimed to assess whether the effects of ultrasound modulation of the intensity of light from an incoherent light source could be detected directly, and how the acoustically modulated (AOM) light signal depended on experimental parameters. Our observations suggest that ultrasound at moderate intensities can induce sufficiently large density variations within a uniform medium to cause measurable modulation of the intensity of an incoherent light source by absorption. Light passing through a region of high intensity ultrasound then produces a pattern that is the projection of the density variations within the region of their interaction. The patterns exhibit distinct maxima and minima that are observed at locations much different from those predicted by Raman-Nath, Bragg, or other diffraction theory. The observed patterns scaled appropriately with the geometrical magnification and sound wavelength. We conclude that these observed patterns are simple projections of the ultrasound induced density changes which cause spatial and temporal variations of the optical absorption within the illuminated sound field. These effects potentially provide a novel method for visualizing sound fields and may assist the interpretation of other hybrid imaging methods.

Show MeSH
Related in: MedlinePlus