Measuring large optical transmission matrices of disordered media.

We report a measurement of the large optical transmission matrix (TM) of a complex turbid medium. The TM is acquired using polarization-sensitive, full-field interferometric microscopy equipped with a rotating galvanometer mirror. It is represented with respect to input and output bases of optical modes, which correspond to plane wave components of the respective illumination and transmitted waves. The modes are sampled so finely in angular spectrum space that their number exceeds the total number of resolvable modes for the illuminated area of the sample. As such, we investigate the singular value spectrum of the TM in order to detect evidence of open transmission channels, predicted by random-matrix theory. Our results comport with theoretical expectations, given the experimental limitations of the system. We consider the impact of these limitations on the usefulness of transmission matrices in optical measurements.

Synthetic Fourier transform light scattering.

We present synthetic Fourier transform light scattering, a method for measuring extended angle-resolved light scattering (ARLS) from individual microscopic samples. By measuring the light fields scattered from the sample plane and numerically synthesizing them in Fourier space, the angle range of the ARLS patterns is extended up to twice the numerical aperture of the imaging system with unprecedented sensitivity and precision. Extended ARLS patterns of individual microscopic polystyrene beads, healthy human red blood cells (RBCs), and Plasmodium falciparum-parasitized RBCs are presented.

Digital optical phase conjugation for delivering two-dimensional images through turbid media.

Optical transmission through complex media such as biological tissue is fundamentally limited by multiple light scattering. Precise control of the optical wavefield potentially holds the key to advancing a broad range of light-based techniques and applications for imaging or optical delivery. We present a simple and robust digital optical phase conjugation (DOPC) implementation for suppressing multiple light scattering. Utilizing wavefront shaping via a spatial light modulator (SLM), we demonstrate its turbidity-suppression capability by reconstructing the image of a complex two-dimensional wide-field target through a highly scattering medium. Employing an interferometer with a Sagnac-like ring design, we successfully overcome the challenging alignment and wavefront-matching constraints in DOPC, reflecting the requirement that the forward- and reverse-propagation paths through the turbid medium be identical. By measuring the output response to digital distortion of the SLM write pattern, we validate the sub-wavelength sensitivity of the system.

Measurement of the time-resolved reflection matrix for enhancing light energy delivery into a scattering medium.

Multiple scatterings occurring in a turbid medium attenuate the intensity of propagating waves. Here, we propose a method to efficiently deliver light energy to the desired target depth in a scattering medium. We measure the time-resolved reflection matrix of a scattering medium using coherent time-gated detection. From this matrix, we derive and experimentally implement an incident wave pattern that optimizes the detected signal corresponding to a specific arrival time. This leads to enhanced light delivery at the target depth. The proposed method will lay a foundation for efficient phototherapy and deep-tissue in vivo imaging in the near future.

Single-shot quantitative dispersion phase microscopy.

We present an imaging modality capable of providing high-speed optical dispersion measurements of live cells. The technique permits wide-field measurement of the optical phase shifts introduced by a sample for multiple discrete wavelengths in a single image capture. Utilizing spatial modulation and the wavelength dependence of the interference-fringe spacing, average refractive index as a function of wavelength is obtained, yielding optical dispersion measurements of the sample under observation. Because of its simple and low-cost design, the technique can be readily integrated into a standard microscope to collect additional diagnostic information about biological cells.

Optical measurement of biomechanical properties of individual erythrocytes from a sickle cell patient.

Sickle cell disease (SCD) is characterized by the abnormal deformation of red blood cells (RBCs) in the deoxygenated condition, as their elongated shape leads to compromised circulation. The pathophysiology of SCD is influenced by both the biomechanical properties of RBCs and their hemodynamic properties in the microvasculature. A major challenge in the study of SCD involves accurate characterization of the biomechanical properties of individual RBCs with minimum sample perturbation. Here we report the biomechanical properties of individual RBCs from a SCD patient using a non-invasive laser interferometric technique. We optically measure the dynamic membrane fluctuations of RBCs. The measurements are analyzed with a previously validated membrane model to retrieve key mechanical properties of the cells: bending modulus; shear modulus; area expansion modulus; and cytoplasmic viscosity. We find that high cytoplasmic viscosity at ambient oxygen concentration is principally responsible for the significantly decreased dynamic membrane fluctuations in RBCs with SCD, and that the mechanical properties of the membrane cortex of irreversibly sickled cells (ISCs) are different from those of the other types of RBCs in SCD.

Near-Common-Path Self-Reference Quantitative Phase Microscopy.

We report a near-common-path self-reference quantitative phase microscope, wherein a quantitative phase image is formed through the off-axis interference of the sample wave with a 180-degree rotated version of itself. A pair of dove prisms, oriented in different directions, is used to effect the relative transformation between the beams. Our technique features a simple optical design that requires no maintenance, rendering it accessible to nonspecialists in the field of optics. Additionally, its variable magnification and low-noise phase measurement capabilities make it suitable for high-accuracy imaging of biological samples of different sizes.

Speckle reduction in optical coherence tomography images using tissue viscoelasticity.

We present a technique to reduce speckle in optical coherence tomography images of soft tissues. An average is formed over a set of B-scans that have been decorrelated by viscoelastic creep strain. The necessary correction for the deformation-induced spatial distortions between B-scans is achieved through geometrical co-registration using an affine transformation. Speckle reduction by up to a factor of 1.65 is shown in images of tissue-mimicking soft fibrin phantoms and excised human lymph node tissue with no observable loss of spatial resolution.

Three-dimensional depth-resolved and extended-resolution micro-particle characterization by holographic light scattering spectroscopy.

Fourier-holographic light scattering spectroscopy is applied to record complex angular scattering spectra of two- and three-dimensional samples over a wide field of view. We introduce a computational depth sectioning technique and, for the first time, demonstrate that a single-exposure hologram can generate a quantitative, three-dimensional map of particle sizes and locations over several cubic millimeters with micrometer resolution. Such spatially resolved maps of particle sizes are generated by Mie-inversion and could not be ascertained from the directly reconstructed intensity-distribution images. We also demonstrate synthesis of multiple angular scattering intensity spectra to increase the angular range and improve size detection sensitivity.

Speckle reduction in optical coherence tomography by strain compounding.

We present a speckle reduction technique for optical coherence tomography based on strain compounding. Decorrelation is introduced between B-scans by altering the sample's strain. A theoretical description of the technique, based on a transfer-function formalism, and experimental results on silicone phantoms are presented. Nearly complete decorrelation between successive B-scan speckle patterns was observed for a variation in strain of 0.045. Strain compounding by averaging nine B-scans, with 0.003 strain increments between them, resulted in a 1.5-fold reduction in speckle contrast ratio.

Detection of multiple scattering in optical coherence tomography by speckle correlation of angle-dependent B-scans.

Angular diversity is a successful speckle-reduction technique in optical coherence tomography (OCT). We employ angle-dependent detection for a different purpose: to distinguish the singly backscattered and multiply scattered signal components. Single backscattering is highly correlated over a large range of detection angles; multiple scattering rapidly decorrelates as the angle is varied. Theoretical justification is provided using a linear-systems description of the OCT imaging process; detection of multiple scattering is corroborated experimentally.

Coherent aperture-synthesis, wide-field, high-resolution holographic microscopy of biological tissue.

We show for the first time, to our knowledge, high-resolution wide-field images of biological samples recorded using coherent aperture-synthesis Fourier holography. To achieve this, we combined off-axis plane-wave polarized illumination with an axial sample rotation and polarization-sensitive collection of backscattered light. We synthesized 180 Fourier holograms using an efficient postdetection phase-matching correlation scheme. The result was an annular spatial frequency-space synthetic aperture (NA=0.93) with an effective area 25 times larger than that due to a single hologram. A high-resolution high-contrast microscopic reconstruction of biological tissue was computed over a sample area of 9 mm(2) from holograms acquired at 34 mm working distance.

In vivo dynamic optical coherence elastography using a ring actuator.

We present a novel sample arm arrangement for dynamic optical coherence elastography based on excitation by a ring actuator. The actuator enables coincident excitation and imaging to be performed on a sample, facilitating in vivo operation. Sub-micrometer vibrations in the audio frequency range were coupled to samples that were imaged using optical coherence tomography. The resulting vibration amplitude and microstrain maps are presented for bilayer silicone phantoms and multiple skin sites on a human subject. Contrast based on the differing elastic properties is shown, notably between the epidermis and dermis. The results constitute the first demonstration of a practical means of performing in vivo dynamic optical coherence elastography on a human subject.

High-resolution, wide-field object reconstruction with synthetic aperture Fourier holographic optical microscopy.

We utilize synthetic-aperture Fourier holographic microscopy to resolve micrometer-scale microstructure over millimeter-scale fields of view. Multiple holograms are recorded, each registering a different, limited region of the sample object's Fourier spectrum. They are "stitched together" to generate the synthetic aperture. A low-numerical-aperture (NA) objective lens provides the wide field of view, and the additional advantages of a long working distance, no immersion fluids, and an inexpensive, simple optical system. Following the first theoretical treatment of the technique, we present images of a microchip target derived from an annular synthetic aperture (NA = 0.61) whose area is 15 times that due to a single hologram (NA = 0.13); they exhibit a corresponding qualitative improvement. We demonstrate that a high-quality reconstruction may be obtained from a limited sub-region of Fourier space, if the object's structural information is concentrated there.

Detection of multiple scattering in optical coherence tomography using the spatial distribution of Stokes vectors.

Multiple scattering is one of the main degrading influences in optical coherence tomography, but to date its presence in an image can only be indirectly inferred. We present a polarization-sensitive method that shows the potential to detect it more directly, based on the degree to which the detected polarization state at any given image point is correlated with the mean state over the surrounding region. We report the validation of the method in microsphere suspensions, showing a strong dependence of the degree of correlation upon the extent to which multiply scattered light is coherently detected. We demonstrate the method's utility in various tissues, including chicken breast ex vivo and human skin and nailfold in vivo.

Modified discrete particle model of optical scattering in skin tissue accounting for multiparticle scattering.

We rigorously account for the effects of multiparticle light scattering from a fractal sphere aggregate in order to simulate the optical properties of a soft biological tissue, human skin. Using a computational method that extends Mie theory to the multisphere case, we show that multiparticle scattering significantly affects the computed optical properties, resulting in a reduction in both scattering coefficient and anisotropy for the wavelengths simulated, as well as a significantly enhanced forward peak in the simulated phase function. The model is extended to incorporate the contribution of Rayleigh scatterers, which we show is required to obtain reasonable agreement with experimentally measured optical properties of skin tissue.

Synthetic aperture fourier holographic optical microscopy.

We report a new synthetic aperture optical microscopy in which high-resolution, wide-field amplitude and phase images are synthesized from a set of Fourier holograms. Each hologram records a region of the complex two-dimensional spatial frequency spectrum of an object, determined by the illumination field's spatial and spectral properties and the collection angle and solid angle. We demonstrate synthetic microscopic imaging in which spatial frequencies that are well outside the modulation transfer function of the collection optical system are recorded while maintaining the long working distance and wide field of view.

Correlation of static speckle with sample properties in optical coherence tomography.

We present theoretical calculations, based on a random phasor sum model, which show that the optical coherence tomography speckle contrast ratio is dependent on the local density of scattering particles in a sample, provided that the effective number of scatterers in the probed volume is less than about five. We confirm these theoretical predictions experimentally, using suspensions of microspheres in water. The observed contrast ratios vary in value from the Rayleigh limit of 0.52 to in excess of 2, suggesting that the contrast ratio could be useful in optical coherence tomography, particularly when imaging in ultrahigh-resolution regimes.

Microscopic particle discrimination using spatially-resolved Fourier-holographic light scattering angular spectroscopy.

We utilize Fourier-holographic light scattering angular spectroscopy to record the spatially resolved complex angular scattering spectra of samples over wide fields of view in a single or few image captures. Without resolving individual scatterers, we are able to generate spatially-resolved particle size maps for samples composed of spherical scatterers, by comparing generated spectra with Mie-theory predictions. We present a theoretical discussion of the fundamental principles of our technique and, in addition to the sphere samples, apply it experimentally to a biological sample which comprises red blood cells. Our method could possibly represent an efficient alternative to the time-consuming and laborious conventional procedure in light microscopy of image tiling and inspection, for the characterization of microscopic morphology over wide fields of view.

Spatially resolved Fourier holographic light scattering angular spectroscopy.

We show for what is the first time to our knowledge that digital Fourier holography can be used to record spatially resolved angular light scattering spectra from microscopically structured samples. This is achieved in one or a few digital image captures over large millimeter-scale fields of view. Such spectra are a sensitive measure of microscopic morphology, with wide applications in biological and medical imaging. We demonstrate good agreement between results of experiment and Mie theory for the angular scattering spectra of microspheres in water extracted from local regions within reconstructed 2 x 1 millimeter image sets.