Diffraction phase microscopy with white light

We present white light diffraction phase microscopy (wDPM) as a quantitative phase imaging method that combines the single shot measurement benefit associated with off-axis methods, high temporal phase stability associated with common path geometries, and high spatial phase sensitivity due to the white light illumination. We propose a spatiotemporal filtering method that pushes the limit of the pathlength sensitivity to the subangstrom level at practical spatial and temporal bandwidths. We illustrate the utility of wDPM with measurements on red blood cell morphology and HeLa cell growth over 18 hours.     

In-line reference-delayed digital holography using a low-coherence light source

We present a holographic imaging device with a low-coherence light source that uses the reflection of the objective lens as reference illumination. This results in a simple setup and allows applications to microscopy with only small modifications of the setup for aberration measurements. In addition, it opens the prospects to in vivo ophthalmic imaging. We present in vitro experiments using a resolution test target to quantify the system performance. We demonstrate that we can achieve diffraction-limited resolution and show the possibility of aberration correction. We also present preliminary results using a scattering sample.     

Bessel beam spectral-domain high-resolution optical coherence tomography with micro-optic axicon providing extended focusing range

Endoscopic imaging in tubular structures, such as the tracheobronchial tree, could benefit from imaging optics with an extended depth of focus (DOF) to accommodate the varying sizes of tubular structures across patients and along the tree within the same patient. Yet the extended DOF needs to be accomplished without sacrificing resolution while maintaining sufficient sensitivity and speed of imaging. In this Letter, we report on the measured resolution and sensitivity achieved with a custom-made micro-optic axicon lens designed to theoretically achieve an 8 mm DOF. A measured invariant resolution of approximately 8 microm is demonstrated across a 4 mm measured DOF using the micro-optic axicon while achieving an invariant sensitivity of approximately 80 dB with a 25 mW input power. Double-pass Bessel beam spectral-domain optical coherence tomography with an axicon micro-optic lens (i.e., <1 mm in diameter) is, for the first time to our knowledge, demonstrated in a biological sample demonstrating invariant resolution and signal-to-noise ratio across a 4 mm measured DOF, which is compared to Gaussian beam imaging.     

Multimode diode laser correlation spectroscopy using gas-filled porous materials for pathlength enhancement

A novel pathlength enhancement approach has been applied in multimode diode laser correlation spectroscopy measurements of gas concentrations. Reference and sample gas cells made of porous polystyrene foam and alumina ceramic, respectively, were employed in proof-of-principle measurements of molecular oxygen. Equivalent pathlengths of 164 and 52?cm were obtained for the reference and sample cells with physical pathlengths of 4 and 1?cm, respectively. With a measurement time of 60?s, a physical-pathlength-integrated sensitivity of 15?ppm?m was achieved with an enhancement of more than one order of magnitude compared with an open-air setup. Practical application aspects were investigated including gas cell size and light alignment tolerance. This generic approach for pathlength enhancement provides advantages in terms of compactness and robustness.     

Confocal diffraction phase microscopy of live cells

We present a new quantitative phase microscopy technique, confocal diffraction phase microscopy, which provides quantitative phase measurements from localized sites on a sample with high sensitivity. The technique combines common-path interferometry with confocal microscopy in a transmission geometry. The capability of the technique for static imaging is demonstrated by imaging polystyrene microspheres and live HT29 cells, while dynamic imaging is demonstrated by quantifying the nanometer scale fluctuations of red blood cell membranes.     

Measurement of ocular axial length using full-range spectral-domain low-coherence interferometry

We demonstrate a system for measuring the ocular axial length(AL) with high sensitivity and high speed using spectral-domain low-coherence interferometry(SD-LCI). To address the limit in measuring such a large range by using SD-LCI, we propose a full-range method to recognize the positive and negative depths. The reference arm length is changed synchronously with the shift of the focal point of the probing beam. The system provides a composite depth range that is sufficient to cover the whole eye. We demonstrate the performance of the presented system by measuring the ALs of five volunteers. This system can provide the A-scan ocular biometric assessment of the corneal thickness and AL in 0.1 s.     

Fourier phase microscopy for investigation of biological structures and dynamics

By use of the Fourier decomposition of a low-coherence optical image field into two spatial components that can be controllably shifted in phase with respect to each other, a new high-transverse-resolution quantitative-phase microscope has been developed. The technique transforms a typical optical microscope into a quantitative-phase microscope, with high accuracy and a path-length sensitivity of lambda/5500, which is stable over several hours. The results obtained on epithelial and red blood cells demonstrate the potential of this instrument for quantitative investigation of the structure and dynamics associated with biological systems without sample preparation.     

Phase-resolved correlation and its application to analysis of low-coherence interferograms

A new signal-processing technique is proposed that involves a phase-resolved correlation method that one can use to determine the phase distribution of low-coherence interferograms. This method improves the sensitivity and resolution of low-coherence interferometers. The depth structure of an aluminum oxide-coated aluminum mirror was determined by use of a low-coherence interferometer with this method. Three signal peaks were successfully extracted from a noisy interferogram.     

Nonlinear phase dispersion spectroscopy

Nonlinear phase dispersion spectroscopy is introduced as a means to retrieve wideband, high spectral resolution profiles of the wavelength-dependent real part of the refractive index. The method is based on detecting dispersion effects imparted to a light field with low coherence transmitted through a thin sample and detected interferometrically in the spectral domain. The same sampled signal is also processed to yield quantitative phase maps and spectral information regarding the total attenuation coefficient using spectral-domain phase microscopy and spectroscopic optical coherence tomography (SOCT), respectively. Proof-of-concept experiments using fluorescent and nonfluorescent polystyrene beads and another using a red blood cell demonstrate the ability of the method to quantify various absorptive/dispersive features. The increased sensitivity of this method, novel to our knowledge, is compared to intensity-based spectroscopy (e.g., SOCT), and potential applications are discussed.     

A comparison of DIC and grid measurements for processing spalling tests with the VFM and an 80-kpixel ultra-high speed camera

During the last decades, the spalling technique has been more and more used to characterize the tensile strength of geomaterials at high-strain-rates. In 2012, a new processing technique was proposed by Pierron and Forquin [1] to measure the stress level and apparent Young’s modulus in a concrete sample by means of an ultra-high speed camera, a grid bonded onto the sample and the Virtual Fields Method. However the possible benefit to use the DIC (Digital Image Correlation) technique instead of the grid method has not been investigated. In the present work, spalling experiments were performed on two aluminum alloy samples with HPV1 (Shimadzu) ultra-high speed camera providing 1?Mfps maximum recording frequency and about 80?kpixel spatial resolution. A grid with 1?mm pitch was bonded onto the first sample whereas a speckle pattern was covering the second sample for DIC measurements. Both methods were evaluated in terms of displacement and acceleration measurements by comparing the experimental data to laser interferometer measurements. In addition, the stress and strain levels in a given cross-section were compared to the experimental data provided by a strain gage glued on each sample. The measurements allow discussing the benefit of each (grid and DIC) technique to obtain the stress-strain relationship in the case of using an 80-kpixel ultra-high speed camera.     

Optical detection of indocyanine green encapsulated biocompatible poly (lactic-co-glycolic) acid nanoparticles with photothermal optical coherence tomography

We describe a functional imaging paradigm that uses photothermal optical coherence tomography (PT-OCT) to detect indocyanine green (ICG)-encapsulated biocompatible poly(lactic-co-glycolic) acid (PLGA) nanoparticles embedded in highly scattering tissue phantoms with high resolution and sensitivity. The ICG-loaded PLGA nanoparticles were fabricated using a modified emulsification solvent diffusion method. With a 20 kHz axial scan rate, PT-OCT based on spectral-domain interferometric configuration at 1310 nm was used to detect phase changes induced by a 808 nm photothermal excitation of ICG-encapsulated PLGA nanoparticles. An algorithm based on Fourier transform analysis of differential phase of the spectral interferogram was developed for detecting the depth resolved localized photothermal signal. Excellent contrast difference was observed with PT-OCT between phantoms containing different concentrations of ICG-encapsulated PLGA nanoparticles. This technique has the potential to provide simultaneous structural and molecular-targeted imaging with excellent signal-to-noise for various clinical applications.     

Inverse scattering problem for optical coherence tomography

We deal with the imaging problem of determining the internal structure of a body from backscattered laser light and low-coherence interferometry. Specifically, using the interference fringes that result when the backscattering of low-coherence light is made to interfere with the reference beam, we obtain maps detailing the values of the refractive index within the sample. Our approach accounts fully for the statistical nature of the coherence phenomenon; the numerical experiments that we present, which show image reconstructions of high quality, were obtained under noise floors exceeding those present for various experimental setups reported in the literature.     

Heterodyne measurement of Wigner distributions for classical optical fields

We demonstrate a two-window heterodyne method for measuring the x-p cross correlation, ??(*)(x)? (p)?, of an optical field ? for transverse position x and transverse momentum p. This scheme permits independent control of the x and p resolution. A simple linear transform of the x-p correlation function yields the Wigner phase-space distribution. This technique is useful for both coherent and low-coherence light sources and may permit new biological imaging techniques based on transverse coherence measurement with time gating. We point out an interesting analogy between x-p correlation measurements for classical-wave and quantum fields.     

Spectral-domain optical coherence phase microscopy for quantitative phase-contrast imaging

We describe a novel microscopy technique for quantitative phase-contrast imaging of a transparent specimen. The technique is based on depth-resolved phase information provided by common path spectral-domain optical coherence tomography and can measure minute phase variations caused by changes in refractive index and thickness inside the specimen. We demonstrate subnanometer level path-length sensitivity and present images obtained on reflection from a known phase object and human epithelial cheek cells.     

Optical Doppler tomographic imaging of fluid flow velocity in highly scattering media

An optical Doppler tomography (ODT) system that permits imaging of fluid flow velocity in highly scattering media is described. ODT combines Doppler velocimetry with the high spatial resolution of low-coherence optical interferometry to measure fluid flow velocity at discrete spatial locations. Tomographic imaging of particle flow velocity within a circular conduit submerged 1 mm below the surface in a highly scattering phantom of Intralipid is demonstrated.     

Phase-dispersion optical tomography

We report on phase-dispersion optical tomography, a new imaging technique based on phase measurements using low-coherence interferometry. The technique simultaneously probes the target with fundamental and second-harmonic light and interferometrically measures the relative phase shift of the backscattered light fields. This phase change can arise either from reflection at an interface within a sample or from bulk refraction. We show that this highly sensitive (~5 degrees ) phase technique can complement optical coherence tomography, which measures electric field amplitude, by revealing otherwise undetectable dispersive variations in the sample.     

A novel speckle pattern—Adaptive digital image correlation approach with robust strain calculation

Digital image correlation (DIC) has seen widespread acceptance and usage as a non-contact method for the determination of full-field displacements and strains in experimental mechanics. The advances of imaging hardware in the last decades led to high resolution and speed cameras being more affordable than in the past making large amounts of data image available for typical DIC experimental scenarios. The work presented in this paper is aimed at maximizing both the accuracy and speed of DIC methods when employed with such images. A low-level framework for speckle image partitioning which replaces regularly shaped blocks with image-adaptive cells in the displacement calculation is introduced. The Newton-Raphson DIC method is modified to use the image pixels of the cells and to perform adaptive regularization to increase the spatial consistency of the displacements. Furthermore, a novel robust framework for strain calculation based also on the Newton-Raphson algorithm is introduced. The proposed methods are evaluated in five experimental scenarios, out of which four use numerically deformed images and one uses real experimental data. Results indicate that, as the desired strain density increases, significant computational gains can be obtained while maintaining or improving accuracy and rigid-body rotation sensitivity.     

Noncontact photoacoustic imaging achieved by using a low-coherence interferometer as the acoustic detector

We report on a noncontact photoacoustic imaging (PAI) technique in which a low-coherence interferometer [(LCI), optical coherence tomography (OCT) hardware] is utilized as the acoustic detector. A synchronization approach is used to lock the LCI system at its highly sensitive region for photoacoustic detection. The technique is experimentally verified by the imaging of a scattering phantom embedded with hairs and the blood vessels within a mouse ear in vitro. The system's axial and lateral resolutions are evaluated at 60 and 30?μm, respectively. The experimental results indicate that PAI in a noncontact detection mode is possible with high resolution and high bandwidth. The proposed approach lends itself to a natural integration of PAI with OCT, rather than a combination of two separate and independent systems.     

In vivo bidirectional color Doppler flow imaging of picoliter blood volumes using optical coherence tomography

We describe a novel optical system for bidirectional color Doppler imaging of flow in biological tissues with micrometer-scale resolution and demonstrate its use for in vivo imaging of blood flow in an animal model. Our technique, color Doppler optical coherence tomography (CDOCT), performs spatially localized optical Doppler velocimetry by use of scanning low-coherence interferometry. CDOCT is an extension of optical coherence tomography (OCT), employing coherent signal-acquisition electronics and joint time-frequency analysis algorithms to perform flow imaging simultaneous with conventional OCT imaging. Cross-sectional maps of blood flow velocity with <50-microm spatial resolution and <0.6-mm/s velocity precision were obtained through intact skin in living hamster subdermal tissue. This technology has several potential medical applications.     

Dispersion-based optical coherence tomography OCT measurement of mixture concentrations

We present a low-coherence technique for the measurement of concentrations in aqueous solutions. It is based on a second-order dispersion measurement using the phase of the Fourier transform of the interferogram term obtained with an unbalanced Michelson interferometer. We demonstrate this approach by concentration measurements of aqueous mixtures of NaCl and glucose solutions. Results show errors between a few percent for high concentrations, up to 31% for low concentrations. The application of such a technique in OCT might require ultrahigh depth resolution and/or other scattered photon rejection techniques.