Markus Laubscher

Parallel optical coherence tomography in scattering samples using a two-dimensional smart-pixel detector array

Parallel optical coherence tomography in scattering samples is demonstrated using a 58 x 58 smart-pixel detector array. A femtosecond mode-locked Ti:Sapphire laser in combination with a free space Michelson interferometer was employed to achieve 4 mum longitudinal resolution and 9 mum transverse resolution on a 260 x 260 mum field of view. We imaged a resolution target covered by an intralipid solution with different scattering coefficients as well as onion cells

Speckle statistics in optical coherence tomography

The two previously reported calculations of the amplitude distribution of speckles in optical coherence tomography, each based on a different mathematical formulation, yield different results. We show that a modification of an initial assumption in one of the formulations leads to equivalent results.

Multiple scattering in optical coherence tomography. I. Investigation and modeling

We present a new model of optical coherence tomography (OCT) taking into account multiple scattering. A theoretical analysis and experimental investigation reveals that in OCT, despite multiple scattering, the field backscattered from the sample is generally spatially coherent and that the resulting interference signal with the reference field is stationary relative to measurement time. On the basis of this result, we model an OCT signal as a sum of spatially coherent fields with random-phase arguments--constant during measurement time--caused by multiple scattering. We calculate the mean of such a random signal from classical results of statistical optics and a Monte Carlo simulation. OCT signals predicted by our model are in very good agreement with a depth scan measurement of a sample consisting of a mirror covered with an aqueous suspension of microspheres. We discuss other comprehensive OCT models based on the extended Huygens-Fresnel principle, which rest on the assumption of partially coherent interfering fields.

Video-rate three-dimensional optical coherence tomography

Most current optical coherence tomography systems provide two-dimensional cross-sectional or en face images. Successive adjacent images have to be acquired to reconstruct three-dimensional objects, which can be time consuming. Here we demonstrate three-dimensional optical coherence tomography (3D OCT) at video rate. A 58 by 58 smart-pixel detector array was employed. A sample volume of 210x210x80 m3 (corresponding to 58x58x58 voxels) was imaged at 25 Hz. The longitudinal and transverse resolutions are 3 m and 9 m respectively. The sensitivity of the system was 76 dB. Video rate 3D OCT is illustrated by movies of a strand of hair undergoing fast thermal damage.

Spatially incoherent illumination as a mechanism for cross-talk suppression in wide-field optical coherence tomography

Comparison of two illumination modes for wide-field optical coherence tomography has revealed that spatially coherent illumination generates coherent cross talk, causing significant image degradation, and that spatially incoherent illumination, with an adequate interferometer design, provides an efficient mechanism for suppression of coherent cross talk. This is shown by comparison of a pulsed laser with a thermal light source for a U.S. Air Force resolution target covered with a scattering solution made from microbeads as well as for an ex vivo tooth.

Multiple scattering in optical coherence tomography. II. Experimental and theoretical investigation of cross talk in wide-field optical coherence tomography

We present a comprehensive study of multiple-scattering effects in wide-field optical coherence tomography (OCT) realized with spatially coherent illumination. Imaging a sample made of a cleaved mirror embedded in an aqueous suspension of microspheres revealed that, despite temporal coherence gating, multiple scattering can induce significant coherent optical cross talk. The latter is a serious limitation to the method, since it prevents shot-noise-limited detection and diffraction-limited imaging in scattering samples. We investigate the dependence of cross talk on important system design parameters, as well as on some relevant sample properties. The agreement between theoretical and experimental results for the wide range of parameters investigated was very good, in both the lateral and the axial dimensions. This further confirms the validity of the model developed in our companion paper [J. Opt. Soc. Am. A 22, 1369-1379 (2005)].

Self-referenced method for optical path difference calibration in low-coherence interferometry.

A simple method for the calibration of optical path difference modulation in low-coherence interferometry is presented. Spectrally filtering a part of the detected interference signal results in a high-coherence signal that encodes the scan imperfections and permits their correction. The method is self-referenced in the sense that no secondary high-coherence light source is necessary. Using a spectrometer setup for spectral filtering allows for flexibility in both the choice of calibration wavelength and the maximum scan range. To demonstrate the methods usefulness, it is combined with a recently published digital spectral shaping technique to measure the thickness of a pellicle beam splitter with a white-light source.

A semi-analytical model accounting for multiple scattering in optical coherence tomography

We present a semi-analytical model of optical coherence tomography (OCT) taking into account multiple scattering. The model rests on the assumptions that the measured portion of the backscattered sample field is spatially coherent and that the sample is motionless relative to measurement time. This allows modeling an OCT signal as a sum of spatially coherent fields with random phase arguments-constant during measurement time-caused by multiple scattering. We calculate the mean OCT signal from classical results of statistical optics and a Monte Carlo simulation. Our model is shown to be in very good agreement with a whole range of experimental data gathered in a comprehensive study of cross-talk in wide-field OCT realized with spatially coherent illumination. The study consists of depth scan measurements of a mirror covered with an aqueous suspension of microspheres. We investigate the dependence of cross-talk on important optical system parameters, as well as on some relevant sample properties. We discuss the more complex OCT models based on the extended Huygens-Fresnel principle, which rest on different assumptions since they assume partially coherent interfering fields.

Video-rate high-resolution parallel optical coherence tomography

Parallel optical coherence tomography is demonstrated at video rate using a 58 by 58 smart-pixel detector array. A sample volume of 210x210x80 micrometers3 (corresponding to 58x58x58 voxels) was imaged at 25 Hz. A femtosecond mode-locked Ti:Sapphire laser in combination with a free space Michelson interferometer was employed to achieve a 3 micrometer longitudinal resolution. We used 20x microscope objectives in both sample arm and reference arm and measured a 8 micrometer transverse resolution. The sensitivity of the system was 76 dB.

High-resolution parallel optical coherence tomography in scattering samples

Parallel optical coherence tomography in scattering samples is demonstrated using a 58 by 58 smart-pixel detector array. A femtosecond mode-locked Ti:Sapphire laser in combination with a free space Michelson interferometer was employed to achieve 4micrometers longitudinal resolution and 9mm transverse resolution on a 260x260 micrometers 2 field of view. We imaged a resolution target covered by an intralipid solution with different scattering coefficients as well as onion cells.