We have developed an apparatus for fast and non-contact assessment of photoacoustic signals from tissue-simulating media. The apparatus was based on electronic speckle pattern interferometry (ESPI), which in our case featured a Mach- Zehnder interferometer, a 532-nm probe laser, and a double exposure CMOS camera to record the holographic speckles that originated from the surface of a tissue-mimicking phantom. The double exposure camera enabled high speed recording of the speckle patterns at MHz timescales. The speckle patterns were reconstructed into phase and out-of-plane displacement maps of the phantom surface. Experiments were performed with an agarose phantom that contained a 1 cm diameter embedded spherical absorber 2 cm below the detection surface. Exposure of the phantom to a pulsed laser at 1064 nm resulted in photoacoustic waves from the absorber that were detectable at the surface of the phantom. Repeated laser exposure with increasing delay times between the two camera exposures enabled spatial-temporal sampling of the displacement maps. The results show the apparatus could identify the position and size of the photoacoustic source relative to the detection surface. Future work will investigate reconstruction of photoacoustic images from the recorded displacement maps.
In this paper, we introduce an electronic speckle pattern interferometry (ESPI) method for rapid assessment of transient deformations on an opaque object. The method records the change in speckle patterns over time, which relate to the change in phase of the reflected light. The system was capable of high-speed recordings enabled by a camera capable of double exposure and external triggering. Experiments were performed on an opaque PDMS phantom to track rapid surface movements from a piezoelectric acoustic driver located at the back of phantom. Acoustic pulses of different period and amplitude were tested. In each double exposure recording cycle, the image pair were digitally subtracted to reveal the change in the speckle pattern, which represented the change in phase of the object beam relative to the reference. We developed custom software to process the data, including an algorithm to unwrap the phase maps. Experiments revealed an ultimate sensitivity to displacements of approximately 1 nm for signals ranging in period from 50 μs to 200 μs. Future work will examine the capabilities of the system with respect to surfaces with different optical absorption and scattering characteristics.
KEYWORDS: Holograms, Digital holography, 3D image reconstruction, Holographic interferometry, 3D visualizations, Algorithm development, Fringe analysis, Reflection, 3D metrology, Sensors
Our group has developed a digital holographic interferometry camera based on an off-axis dual-beam setup. A single hologram acquired with the camera represents the phase and intensity distributions of light reflecting off the surface of an object. Our goal was to develop an algorithm that converted the acquired hologram into a 3D representation of the object. First, to determine the phase excursion of the object, a discrete Fresnel transform reconstruction was applied to a high-pass-filtered version of the hologram. The reconstructed phase map contained both the spatial carrier fringes due to the off-axis geometry of our setup and the phase information related to the object’s surface. Next, we developed a reliable 2D spatial carrier fringe removal technique that was capable of separating out the object’s phase information from the carrier fringes. Last, the object phase map was calibrated to convert the phase information to height information based on the geometrical parameters of the system. The system was evaluated using a silicon-etched lateral-axial resolution target based on a USAF design. The system achieved 33 nanometer axial resolution and sub-pixel lateral resolution over a wide field of view of more than 10 mm x 10 mm. The algorithm will enable the digital holographic camera to be used for non-destructive testing of surface morphology at nanometer scales. With repeated acquisitions, the algorithm has the potential to estimate nanometer-scale dynamic vibrations of an object’s surface.
We report on the development of a holographic camera capable of measuring nanometer-scale surface features. The system is based on a modified off-axis Mach-Zehnder interferometer and was optimized to provide high-quality interference patterns. Fast imaging was implemented with a time-gated intensified CCD camera directly facing the surface of the object. By increasing the intensifier gain, holographic images with good contrast could be captured within 1 ns. We tested the ability of the camera to measure nanometer-scale height differences using a patterned USAF target. The depth resolution of the camera was estimated to be better than 10 nm. We also found that both the object-CCD distance and the angle between the object and reference beams had significant impact on the quality of the reconstructed surface profiles. Potential applications of the camera include measurement of tissue surface displacements for non-contact photoacoustic imaging.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.