Mark Bashkansky

Statistics and reduction of speckle in optical coherence tomography

Studies have shown that optical coherence tomography (OCT) is useful in imaging microscopic structures through highly scattering media. Because spatially coherent light is used in OCT, speckle in the reconstructed image is unavoidable, resulting in degradation of the quality of the OCT images and impaired ability to differentiate subsurface structures. Therefore speckle reduction is an important issue in OCT imaging. We develop speckle statistics that are appropriate to the OCT measurements and demonstrate a simple and practical speckle-reduction technique.

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.

Fiber Bragg reflectors prepared by a single excimer pulse

Narrow-line, permanent Bragg reflection gratings have been created in Ge-doped silica-core optical fibers by interfering beams of a single 20-ns pulse of KrF excimer laser light. Of the fibers studied, the highest reflectance value of ~2% was observed with a linewidth (FWHM) of 0.1 nm, which corresponds to a 2-mm grating length with an index modulation of ~3 x 10(-5).

Two-dimensional synthetic aperture imaging in the optical domain

In scan-mode synthetic aperture imaging radar, spatial resolution in a range is given by a frequency-swept waveform, whereas resolution in the orthogonal direction is derived from the record of phase as the beam footprint executes linear motion over the object. We demonstrate here what is to our knowledge the first two-dimensional imaging that uses exactly this process in the optical domain for a 1 cm x 1 cm object with 90 mumx170 mum resolution.

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.

Signal Processing for Improving Field Cross-correlation Function in Optical Coherence Tomography

Signal processing is used to enhance the field cross-correlation function in optical coherence tomography (OCT). This paper shows how the spurious signal resulting from an imperfect spectrum of broadband emitters can be cleaned up. It also demonstrates that, using a fairly simple deconvolution procedure, a factor of two enhancement in depth resolution can be achieved. An example of the enhanced resolution is shown using the signal from two microscope cover slides.

Role of the ponderomotive potential in above-threshold ionization

Ponderomotive scattering strongly affects photoelectron spectra in above-threshold ionization. We review experimental studies of intensity-dependent angular distributions and inelastic electron scattering that demonstrate how electrons may be scattered, accelerated, or decelerated by the laser beam after photoionization.

Stimulated Brillouin scattering in single-mode As 2 S 3 and As 2 Se 3 chalcogenide fibers

Stimulated Brillouin scattering was investigated for the first time in As(2)S(3) single-mode fibers, and also in As(2)Se(3). The propagation loss and numerical aperture of the fibers at 1.56 mum, along with the threshold intensity for the stimulated Brillouin scattering process were measured. From the threshold values we estimate the Brillouin gain coefficient and demonstrate record figure of merit for slow-light based applications in chalcogenide fibers.

Azimuthally and radially polarized light with a nematic SLM

We demonstrate a technique for generating azimuthally and radially polarized beams using a nematic liquid crystal spatial light modulator and a pi phase step. The technique is similar in concept to prior techniques that interfere TEM(01) and TEM(10) laser modes, but the presented technique removes the requirement of interferometric stability. We calculate an overlap integral of >0.96 with >70% efficiency from an input Gaussian mode. The technique can easily switch between beams with azimuthal and radial polarization.

Generation of hollow beams by using a binary spatial light modulator

We demonstrate the generation of hollow laser beams by using a binary spatial light modulator and compare the results with those for a continuous modulator. The binary phase modulator produces beams that have continuous, azimuthally varying phase profiles and can be dynamically changed with kilohertz refresh rates. The intensity and phase profiles are recorded through the focus of an imaging lens and are compared with scalar diffraction theory. We highlight properties of the beams relevant to optical dipole traps.