N. P. Barry

Sub-100- mu m depth-resolved holographic imaging through scattering media in the near infrared.

We discuss the compromise between depth and transverse spatial resolution for photorefractive holographic imaging through turbid media. Results from an optimized geometry for a 45°-cut rhodium-doped barium titanate photorefractive crystal are presented, demonstrating two-dimensional imaging through turbid media with both sub-100-μm depth and transverse spatial resolution.

DEPTH-RESOLVED HOLOGRAPHIC IMAGING THROUGH SCATTERING MEDIA BY PHOTOREFRACTION

A depth-resolved near-infrared imaging system has been demonstrated for recording three-dimensional images of objects embedded in diffuse media. Time-gated holographic imaging employing rhodium-doped barium titanate as the recording medium is used to acquire whole depth-resolved two-dimensional images in 1 s. Millimeter depth resolution has been achieved with a transverse resolution of ~ 30 microm.

Holographic storage and high background imaging using photorefractive multiple quantum wells

We report holographic, real time, depth‐resolved image acquisition, storage, and reconstruction in photorefractive GaAs/AlGaAs multiple quantum wells under high background radiation conditions. Reconstructed images of 50 μm transverse and depth resolution have been achieved using this device as a coherence gate to image through 9 mean free paths of turbid scattering medial

High resolution depth resolved imaging through scattering media using time resolved holography

Abstract We report on time-gated holographic imaging at 450 nm using photorefractive media to achieve high depth and transverse resolution through a turbid medium. Using 70 fs pulses from a frequency-doubled mode-locked Ti: sapphire, we have demonstrated that both depth and transverse spatial resolution of 50 μm are achievable through a depth of 10 scattering mean free paths. Whole-field depth-resolved 2-D image planes were acquired in a single integration of a few seconds.

Investigation of the temporal spread of an ultrashort light pulse on transmission through a highly scattering medium

An experimental and theoretical investigation of the temporal spread of an ultrashort light pulse on transmission through a highly scattering medium has been made. For the strongly diffuse light, the transmitted pulse may be described by a universal function whose duration can be directly related to the width of the sample. For sufficiently scattering samples, experimental data and the diffusion approximation indicate that the output pulse duration scales with the square root of the sample width.

Experimental study of a self-starting Kerr-lens mode-locked titanium-doped sapphire laser

Abstract We report an experimental investigation of self-starting operation of picosecond and femtosecond Kerr-lens mode-locked Ti:sapphire lasers. Pulses as short as 5 ps duration have been generated from self-starting lasers with no intracavity prisms to optimise group velocity dispersion. Self-starting dispersion-compensated lasers have generated pulses as short as 43 fs duration with mode-locking self-starting within 0.7 ms of the onset of laser action.

Imaging Biological Tissue Using Photorefractive Holography and Fluorescence Lifetime

This article reviews two approaches to biomedical imaging, namely photorefractive holography as a means of realising depth-resolved imaging through turbid media and fluorescence lifetime imaging as a spectroscopic imaging modality.

Evaluation of photorefractive holographic imaging through turbid media

We are pursuing high spatial resolution confocal imaging of reflected NIR light as a diagnostic tool. To determine the efficacy of confocal imaging for in vivo pathology, it is critical to know the maximum thickness of scattering tissue through which biological signals can be detected. This limit depends on three factors: the fraction of signal photons generated in the focal region, the attenuation of the light to and from the focal region, and the background signal generated outside the focal region. Previous measurements of this limit used signals from a mirror,l which are much stronger than those expected from biological sources. Our previous Monte Carlo simulations* found that the greatest sources of contrast in reflection confocal imaging are changes in the index of refraction. For the index mismatches found in cells (-0.05),3 using a system NA of 0.4 we estimated these changes could be detected through 2-3 optical depths (ODs) of scattering. In this work, we have measured the maximum depth in ODs at which physiologic index mismatches can be detected. A confocal reflectometer was constructed (NA = 0.8) as shown in Fig. l a with an axial resolution of 4 pm. Two types of samples were investigated: 1) background was measured from homogenous scattering samples and 2) signal plus background was measured from scattering samples containing a planar index mismatch located at variable depths. Scattering was generated by 1-pm diameter latex microspheres. Scattering coefficients were calculated using Mie theory. The background was assessed by translating optically thick samples of scattering gelatin through the focus of the objective while recording the detected signal as function of position. The measured background scans from samples with three different scattering coefficients are plotted in Fig. 2a as a function of optical depth, where OD = (pJ (Depth.) An exponential fit to the background data indicates that each is decaying at eCoD. The initial amplitude of the background signal (B,) is directly proportional to the scattering coefficient (Fig. 2b). These data suggest that the background signal is produced by photons initially scattered outside the focal volume. Confocal signals from inhomogeneous scattering samples were measured by scanning a layered phantom with known optical properties (Fig. Ib). An index mismatch of 0.05 or 0.1 between the scattering gelatin and the scattering index fluid (IF) was used as a signal source at variable OD. The appropriate background was subtracted from each measure(b)

Novel ultrafast tunable solid state lasers for real-world applications including medical imaging

This paper reviews ultrafast Kerr Lens Mode-locked solid- state lasers with particular emphasis on all-solid-state diode-pumped laser technology which has the potential to provide low-cost compact devices for ultrafast instrumentation, particularly for biomedical applications.We have demonstrated the use of ultrafast solid-state lasers for 3D imaging through turbid media using time-gated photorefractive holography, and for fluorescence lifetime imaging.

Time-gated holographic imaging using photorefractive multiple quantum well devices

A depth-resolved imaging system is described for recording three dimensional images of objects embedded in diffuse media. Time-gated holographic imaging, employing photorefractive multiple quantum well devices as the recording media, is used to obtain real-time whole-field depth-resolved two dimensional images. Infra-red radiation has been used which corresponds to the medical imaging window.