J. Reintjes

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.

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.

Time-gated imaging through scattering media using stimulated Raman amplification.

We propose the use of stimulated Raman scattering for time-gated image amplification and demonstrate its use for the detection of images through a strongly scattering material. Using 30-ps pulses from a frequency-doubled Nd:YAG laser, we have amplified and detected images through a suspension of nondairy creamer with a spatial resolution of less than 300 microm and at concentrations such that the nonscattered extinction ratio was e(-33). Our time-gated image amplifier can produce images under conditions in which the scattering medium is sufficiently dense that an image cannot be seen by using multiple exposures on a streak camera or time-integrated exposures on a sensitive, low-noise CCD camera.

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.

Generation of coherent radiation in XUV by fifth- and seventh-order frequency conversion in rare gases

Results of experimental studies of the generation of coherent radiation in the XUV by high order optical nonlinearities in the rare gases are described. Fifth- and seventh-harmonic conversion and six wave mixing of harmonic pulses from an Nd:YAG laser were used to produce radiation at several discrete wavelengths between 38 and 76 nm. Experimental measurements of fifth-haxmonic conversion of pump pulses at 266.1 nm in helium are compared with theoretical predictions. Discrepancies between theory and experiment are observed at high laser powers, and the role of competing processes is discussed.

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.

Efficient harmonic generation from 532 to 266 nm in ADP and KD*P

Second harmonic generation of mode‐locked pulses in ADP and KD*P from 532 to 266 nm with 85% conversion is reported. Two‐photon absorption coefficients are measured for the two materials and found to be relatively unimportant in the limitation of harmonic conversion compared to the effects of phase mismatch and dispersion.

Beam cleanup with stimulated Raman scattering in the intensity-averaging regime

We describe experimental and theoretical investigations of beam cleanup with highly aberrated pump beams in the intensity-averaging regime. Distortion-free amplification of a diffraction-limited Stokes beam is demonstrated in a crossed-beam geometry with a pump beam aberrated to 120 times its diffraction limit, resulting in a brightness increase of 5000 times. Moderately aberrated pump beams produce off-axis Stokes components, while collinear interactions introduce distortion on the Stokes beam. Phase conjugation is combined with stimulated Raman scattering to remove both the aberrations of the pump beam and the aberrations on the Stokes beam itself.