M. D. Duncan

Scanning coherent anti-Stokes Raman microscope.

We have constructed a spatially scanning coherent anti-Stokes Raman spectroscopic (CARS) apparatus that allows us to image the distribution of distinct chemical species in a microscopic sample region. Images of onion-skin cells have been obtained by using the CARS signal produced by the 2450-cm(-l) band of deuterated water. Future applications will be discussed.

Subsurface defect detection in ceramics by high-speed high-resolution optical coherent tomography

We use optical coherence tomography with a new configuration to determine the size and location of subsurface defects in solid ceramic and composite ceramic materials. Cross-sectional subsurface regions either parallel or perpendicular to the surface were examined. We present experimental results showing that the size and distribution of small subsurface defects can be determined with depth and lateral resolutions of 10 and 4 microm, respectively.

Resolution limits for imaging through turbid media with diffuse light.

We present analytic expressions for the achievable resolution for imaging through a turbid medium with multiply scattered light in the diffusion limit. We find that for detectable levels of light the spatial resolution ℛ (the half-width of the point-spread function) scales with thickness d of the sample as ℛ ≃ (0.2 ± 0.04)d over 10 orders of magnitude in input intensity and transport length. Experiments with a time-gated stimulated Raman amplifier are in good agreement with the calculations.

Subsurface defect detection in materials using optical coherence tomography

We have used optical coherence tomography to study the internal structure of a variety of non-biological materials. In particular, we have imaged internal regions from a commercial grade of lead zirconate titanate ceramic material, from a sample of single-crystal silicon carbide, and from a Teflon-coated wire. In each case the spatial positions of internal defects were determined.

Mine detection with a forward-looking ground-penetrating synthetic aperture radar

In order to detect buried land mines in clutter, Planning Systems Incorporated has adapted its Ground Penetrating Synthetic Aperture Radar (GPSAR) technology for forward-looking applications. The Forward Looking GPSAR (FLGPSAR), is a wide-band stepped-frequency radar operating over frequencies from 400 MHz to 4 GHz. The FLGPSAR system is based on a modified John Deere E-Gator turf vehicle that is capable of remote control. Custom Archimedean spiral antennas are used to populate the GPSAR array. These antennas are designed and built by PSI and have exceptional broad-band radiation characteristics. The FLGSPAR system has been used to detect plastic and metallic landmines at U.S. Army test facilities and at PSIs engineering center in Long Beach Mississippi. Multi-look SAR processing has been shown to significantly improve the quality of FLGPSAR imagery.

Parametric Raman gain suppression in D 2 and H 2

We report direct experimental evidence of the absence of exponential gain at the Stokes wavelength for Stokes/anti-Stokes phase-matched stimulated Raman scattering. The amplification of a Stokes seed pulse was measured at various propagation angles relative to the pump direction. Nonexponential growth was observed at the Stokes/ anti-Stokes phase-matching angle with an amplification that was smaller than at non-phase-matching angles by a factor of more than 10(7).

Characteristics of a Yb-doped superfluorescent fiber source for use in optical coherence tomography

We have used a newly developed Yb-doped high-power fiber source in an optical coherence tomography (OCT) apparatus. We have analyzed various properties of interest for OCT measurements such as spectral shape, related gate width, central wavelength, bandwidth, and power output.

Three-dimensional reflective image reconstruction through a scattering medium based on time-gated Raman amplification.

We present reconstructions of a three-dimensional object imaged in reflection mode through a turbid medium, using time-gated Raman amplification. We demonstrate a less than 1-cm depth resolution at a laser-power-limited viewing distance of 8.5 attenuation lengths from an object with a rough metallic surface. We show that, even at a viewing distance of 10.5 attenuation lengths, the imaging system is power limited rather than backscatter limited.

Imaging Biological Compounds Using The Coherent Anti-Stokes Raman Scattering Microscope

The microscopic spatial distribution of biological compounds can be imaged using a coherent anti-Stokes Raman scattering (CARS) microscope. Spatial resolution of less than one micrometer has been achieved, and excellent molecular discrimination using chemically similar molecular species has been obtained using digital image processing techniques.

Time-gated imaging through dense scatterers with a Raman amplifier

A time-gated Raman amplifier has been used to detect a bar chart hidden by a strongly scattering material. The time gating was provided by a frequency-doubled Nd:YAG pump laser having a pulse duration of 30 ps. We have amplified and detected images with resolved structures smaller than 125 µm through suspensions of polystyrene spheres and nondairy creamer for light extinction factors of up to e(33). The Raman amplifier system has been shown to produce images under conditions in which the scattering medium was sufficiently dense that an image could not be detected on either a streak camera or by integration on a sensitive, low-noise camera.