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.

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 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.

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.

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.

Generation of high-contrast narrow-band subpicosecond correlations from broadband stochastic fields.

We demonstrate that two broadband stochastic fields generated by use of either parametric frequency conversion or stimulated Raman scattering can be mixed in a nonlinear crystal to produce an output spectrum with a pronounced narrow-band feature. This narrow-band feature, which we show is due to the phase-correlated nature of the beams, is present when the two broadband beams are delayed less than a coherence length relative to each other. We apply this technique to a novel measurement of the field cross-correlation function between two broadband stochastic fields with different center frequencies.

Coherently amplified Raman polarization gate for imaging through scattering media.

We demonstrate a new nonlinear optical field cross-correlation technique for two-dimensional imaging through scattering media. The technique, the coherently amplified Raman polarization gate, relies on both the polarization and coherence sensitivity of broadband stimulated Raman amplification to produce a high-contrast gate with temporal resolution of the order of the inverse bandwidth of the pump.

Image resolution by use of multiply scattered light.

We present calculations based on the Markov-chain technique of the intensity and direction of light traversing a multiply scattering medium in the isotropic scattering case for pulsed point-source illumination. From these calculations we derive expressions for the trade-off between spatial image resolution and detector integration time when the diffusion approximation is not valid. We show that image resolution better than the diffusion limit is theoretically possible use of multiply scattered light for samples thinner than ~35 scattering lengths. Monte Carlo simulations are used to extend these results to the anisotropic scattering case.

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.

Nonlinear-optical field cross-correlation techniques for medical imaging with lasers.

We show that certain resonant nonlinear-optical processes are capable of producing a cross-correlation function of the incident fields. This property can be used for time-gating purposes. Using a coherent anti-Stokes Raman scattering process as a time gate, we reproduce an image of an object obscured by a scattering medium.

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.

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.

Chromatic time lag in picosecond streak camera measurements.

Ihlemann et al. [Rev. Sci. Instrum. 59, 2502 (1988)] have found unexpectedly large group velocity dispersion produced by commercial picosecond streak camera objective lenses. Our work supports and extends their lens data.

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).

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.