As a PhD graduate of The Institute of Optics in 1968, Jim Wyant made important technical contributions to the field in academia and industry. His thesis on topics in holography would augur decades of work in interferometric techniques that have been essential in applications from astronomy to computer technology. His journey from a farm in Ohio, to Case Institute of Technology to the University of Rochester, to the Itek Corporation in Boston and of course to the University of Arizona spanned the country and left an indelible print on the history of optics and The Institute itself. Throughout many years, even as he committed time to the University of Arizona, the OSA and the companies he founded, he continued to support The Institute of Optics through our summer short course program, through advice, and through generous support of professorships and programs at the University of Rochester.
KEYWORDS: Long wavelength infrared, Biological and chemical sensing, Short wave infrared radiation, Spectroscopy, Standoff detection, Solitons, Detection and tracking algorithms
The use of optical frequency combs (OFCs) for multi-heterodyne spectroscopy has enabled unprecedented measurement capabilities for spectroscopic sensing, including rapid acquisition speed, high resolution and high sensitivity1,2. Development of field deployable OFC sources that are widely tunable in the important chemical fingerprint region in the long-wavelength infrared (LWIR) is a major research challenge. In this paper, we report our recent efforts towards developing LWIR comb source for SILMARILS (Standoff ILluminator for Measuring Absorbance and Reflectance Infrared Light Signatures) program by IARPA. LGS has developed fiber optic sources producing spectral combs in the SWIR (1.52 to 1.56 μm and 1.7 to 2.0 μm) and in the LWIR (7.7 to 12.1 μm) regions. The spectral combs in the LWIR are generated by difference-frequency mixing one OFC centered around 1.54 μm with another OFC, whose center wavelength is tunable between 1.7 and 2.0 μm, in a nonlinear optical crystal. Average power of the generated LWIR is 1.2-12 mW and its instantaneous spectral breadth of the combs is > 80 cm-1, sufficiently broad to cover multiple molecular absorption peaks. We demonstrate standoff sensing of chemical targets having concentration as low as 12 μg/cm2 by measuring LWIR transflectance spectra using the comb source.
It is well known that patient-specific ocular aberrations limit imaging resolution in the human retina. Previously,
hardware adaptive optics (HAO) has been employed to measure and correct these aberrations to acquire high-resolution
images of various retinal structures. While the resulting aberration-corrected images are of great clinical importance,
clinical use of HAO has not been widespread due to the cost and complexity of these systems. We present a technique
termed computational adaptive optics (CAO) for aberration correction in the living human retina without the use of
hardware adaptive optics components. In CAO, complex interferometric data acquired using optical coherence
tomography (OCT) is manipulated in post-processing to adjust the phase of the optical wavefront. In this way, the
aberrated wavefront can be corrected. We summarize recent results in this technology for retinal imaging, including
aberration-corrected imaging in multiple retinal layers and practical considerations such as phase stability and image
optimization.
We demonstrate high-resolution imaging of the living human retina by computationally correcting highorder ocular aberrations. These corrections are performed post-acquisition and without the need for a deformable mirror or wavefront sensor that are commonly employed in hardware adaptive optics (HAO) systems. With the introduction of HAO to ophthalmic imaging, high-resolution near diffraction-limited imaging of the living human retina has become possible. The combination of a deformable mirror, wavefront sensor, and supporting hardware/software, though, can more than double the cost of the underlying imaging modality, in addition to significantly increasing the system complexity and sensitivity to misalignment. Optical coherence tomography (OCT) allows 3-D imaging in addition to naturally providing the complex optical field of backscattered light. This is unlike a scanning laser ophthalmoscope which measures only the intensity of the backscattered light. Previously, our group has demonstrated the utility of a technique called computational adaptive optics (CAO) which utilizes the complex field measured with OCT to computationally correct for optical aberrations in a manner similar to HAO. Until now, CAO has been applied to exvivo imaging and invivo skin imaging. Here, we demonstrate invivo imaging of cone photoreceptors using CAO. Additional practical considerations such as imaging speed, and stability are discussed.
High-resolution tomography is of great importance to many areas of biomedical imaging, but with it comes several apparent tradeoffs such as a narrowing depth-of-field and increasing optical aberrations. Overcoming these challenges has attracted many hardware and computational solutions. Hardware solutions, though, can become bulky or expensive and computational approaches can require high computing power or large processing times. This study demonstrates memory efficient implementations of interferometric synthetic aperture microscopy (ISAM) and computational adaptive optics (CAO) – two computational approaches for overcoming the depthof- field limitation and the effect of optical aberrations in optical coherence tomography (OCT). Traditionally requiring lengthy post processing, here we report implementations of ISAM and CAO on a single GPU for real-time in vivo imaging. Real-time, camera-limited ISAM processing enabled reliable acquisition of stable data for in vivo imaging, and CAO processing on the same GPU is shown to quickly correct static aberrations. These algorithmic advances hold the promise for high-resolution volumetric imaging in time-sensitive situations as well as enabling aberrationfree cellular-level volumetric tomography.
Breast-conserving surgery is a frequent option for women with stage I and II breast cancer, and with radiation treatment,
can be as effective as a mastectomy. However, adequate margin detection remains a challenge, and too often additional
surgeries are required. Optical coherence tomography (OCT) provides a potential method for real-time, high-resolution
imaging of breast tissue during surgery. Intra-operative OCT imaging of excised breast tissues has been previously
demonstrated by several groups. In this study, a novel handheld surgical probe-based OCT system is introduced, which
was used by the surgeon to image in vivo, within the tumor cavity, and immediately following tumor removal in order to
detect the presence of any remaining cancer. Following resection, study investigators imaged the excised tissue with the
same probe for comparison. We present OCT images obtained from over 15 patients during lumpectomy and
mastectomy surgeries. Images were compared to post-operative histopathology for diagnosis. OCT images with micron
scale resolution show areas of heterogeneity and disorganized features indicative of malignancy, compared to more
uniform regions of normal tissue. Video-rate acquisition shows the inside of the tumor cavity as the surgeon sweeps the
probe along the walls of the surgical cavity. This demonstrates the potential of OCT for real-time assessment of surgical
tumor margins and for reducing the unacceptably high re-operation rate for breast cancer patients.
The transition of optical coherence tomography (OCT) technology from the lab environment towards the more challenging clinical and point-of-care settings is continuing at a rapid pace. On one hand this translation opens new opportunities and avenues for growth, while on the other hand it also presents a new set of challenges and constraints under which OCT systems have to operate. OCT systems in the clinical environment are not only required to be user friendly and easy to operate, but should also be portable, have a smaller form factor coupled with low cost and reduced power consumption. Digital signal processors (DSP) are in a unique position to satisfy the computational requirements for OCT at a much lower cost and power consumption compared to the existing platforms such as CPU and graphics processing units (GPUs). In this work, we describe the implementation of optical coherence tomography (OCT) and interferometric synthetic aperture microscopy (ISAM) processing on a floating point multi-core DSP (C6678, Texas Instruments). ISAM is a computationally intensive data processing technique that is based on the re-sampling of the Fourier space of the data to yield spatially invariant transverse resolution in OCT. Preliminary results indicate that 2DISAM processing at 70,000 A-lines/sec and OCT at 180,000 A-lines/sec can be achieved with the current implementation using available DSP hardware.
Interferometric synthetic aperture microscopy (ISAM) reconstructs the scattering potential of a sample with spatially
invariant resolution, based on the incident beam profile, the beam scan pattern, the physical model of light sample
interaction, and subsequent light collection by the system. In practice, aberrations may influence the beam profile,
particularly at higher NA, when ISAM is expected to provide maximum benefit over optical coherence microscopy.
Thus it is of interest to determine the effects of aberrations on ISAM reconstructions. In this paper we present the
forward model incorporating the effects of aberrations, which forms the basis for aberration correction in ISAM.
Simulations and experimental results show that when operating far from focus, modest amounts of spherical
aberration can introduce artifacts to the point-spread function, even at relatively low NA ~ 0.1-0.2. Further work will
investigate computational methods to correct the effects of aberrations, i.e. to perform virtual adaptive optics.
Interferometric Synthetic Aperture Microscopy (ISAM)1 is an optical microscopy computed-imaging
technique for measuring the optical properties of three-dimensional structures and biological tissues.
In this work, the principle of ISAM is reviewed, and its application to imaging tissue properties in
various scanning geometries and instrument configurations is explored. The practicality of ISAM
is demonstrated by imaging a rat heart and muscle using a real-time implementation of ISAM in
conjunction with a clinical cart Optical Coherence Tomography instrument.
We have developed and implemented a system which can acquire, process, and display the inverse scattering solution for
optical coherence tomography (OCT) in real-time at frame rates of 2.25 fps for 512 X 1024 images. Frames which
previously required 60 s, now take under 500 ms, an improvement in processing speed by a factor of over 120 times. An
efficient routine was designed which requires two interpolations of the columns, one one-dimensional real-to-complex
fast Fourier transform (FFT) of the columns, and two two-dimensional FFTs. The limits to speed are now reliant on the
parallelizability of the processing hardware. Our system provides quantitatively meaningful structural information from
previously indistinguishable scattering intensities and provides proof of feasibility for future real-time systems.
In high-numerical-aperture optical coherence tomography, the depth-of-field is usually quite short and therefore
the focus is scanned through the object to form a well-resolved image of the entire volume. However, this may
be inconvenient for in vivo scanning when precision placement is not easily achieved between the object and the
focusing objective. We show that by scanning the illumination wavelength, and using novel inverse scattering
methods on the detected interferograms, features outside of the focus can be resolved and therefore the focus
does not need to be scanned.
Diffraction tomography (DT) is an established imaging technique for reconstructing the complex-valued refractive index distribution of a weakly scattering 3D sample. Due to experimental difficulties associated with the direct measurement of the phase of an optical wavefield, the effectiveness of DT for optical imaging applications
has been limited. A theory of intensity diffraction tomography (I-DT) has been proposed to circumvent this phase retrieval problem. In this work, we review the features of I-DT reconstruction theory that are relevant to optical microscopy. Computer-simulation studies are conducted to investigate the performance of reconstruction
algorithms for a proposed I-DT microscope. The effects of data noise are assessed, and statistically optimal reconstruction strategies that employ multiple detector planes are proposed.
Combining light scatting spectroscopy (LSS) and spectroscopic optical coherence tomography (LS-SOCT) can provide a wealth of information. A theoretical model for analyzing the scattering in SOCT is proposed based on plane wave decomposition. Based on the model, we discuss the possibility of matching the physical characteristics of scatterers with observed spectroscopic signals. Many complicating factors are considered including the effects of scatterer size, incident light polarization, interference between the fields scattered from closely adjacent scatterers, and the numerical aperture of the OCT system. We found the modulation of the spectrum of the incident light by scattering of a plane wave from a single sphere is a good indicator of particle size and composition. We demonstrate that measuring wavelength-dependent scattering in SOCT can be used for particle sizing and contrast enhancement by differentiating cells in 3D cell culture.
An object structure can be better resolved in optical coherence tomography by using inverse scattering theory, which
takes into account the finite beam width and focusing. Specifically, we show experiments where scatterers are
resolved outside of the confocal region such that resolution is uniform to the focused region. Numerical simulations
demonstrate the effectiveness of this technique. When the algorithm is applied to experimentally-acquired OCT data,
the transverse resolution outside of the confocal parameter is improved, extending the apparent confocal parameter
range. The experimental results validate improvement for capabilities of OCT to perform high-resolution cross-sectional
imaging.
Often in imaging systems, the bandpass of the system is not uniform. In temporal coherence imaging methods such as optical coherence tomography, one would like to achieve the most spatially confined impulse response possible with a given source spectrum, minimizing sidelobes that blur adjacent features together. Typically the spectrum of the source is controlled in order to remove sidelobes from the measured interferogram. However, the measured interferogram is not necessarily the best estimate of the scattering density of the object. In this work, a sidelobe supression method is proposed and demonstrated to achieve low sidelobes even with highly nonuniform, non-Gaussian spectra.
Projected index computed tomography (PICT) is a newly developed technique that uses the measured optical path length from multiple optical coherence tomography (OCT) scans through a semitransparent sample to computationally reconstruct tomographic images based on the spatial variations of the refractive index. Since the index of refraction of most samples is not constant, a depth-wise distortion due to the varying index of the sample is evident. Using a highly reflective reference surface placed behind the sample, optical path length measurements yield an aggregate index value for each beampath through the sample. Rotating the sample allows a data set to be formed for multiple beam angles. These data can be understood as the projections of the object index, i.e. the Radon tranform of the index of the object. Using filtered backprojection algorithms set of projection data were used to reconstruct PICT images. The resulting images are free from the spatial distortions found in standard OCT. Experimental results show that PICT images correspond well with the dimensional characteristics of specific samples.
KEYWORDS: Near field, Inverse scattering, Scattering, Image resolution, 3D image processing, Signal to noise ratio, Microscopy, Inverse scattering problem, Near field optics, Spatial resolution
In classical microscopy and inverse scattering, images are made from far field measurements. It will be shown that the standard Abbe or Rayleigh resolution limits arise as a result of assumptions which do not hold in the near field. Existing technologies such as TIRM and PSTM that exploit these properties of the near field produce two dimensional images which exhibit structure on a subwavelength scale. These images have proven difficult to interpret when the sample presents variations in structure and optical properties simultaneously in three dimensions. We present the analytic singular value decomposition of the linearized scattering operator. This representation leads to a robust inversion algorithm for the inverse scattering problem in the near zone. Numerical implementation of the algorithm is demonstrated using simulated noisy data to produce tomographic images of a three dimensional scatterer.
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