Markus Sticker

Measurement and imaging of birefringence and optic axis orientation by phase resolved polarization sensitive optical coherence tomography.

We present an improved method of polarization sensitive optical coherence tomography that enables measurement and imaging of backscattered intensity, birefringence, and fast optic axis orientation simultaneously with only one single A-scan per transverse measurement location. While intensity and birefringence data are obtained in a conventional way, the optic axis orientation is determined from the phase difference recorded in two orthogonal polarization channels. We report on accuracy and precision of the method by measuring birefringence and optic axis orientation of well defined polarization states in a technical object and present maps of birefringence and, what we believe for the first time, of optic axis orientation in biological tissue.

Spectral measurement of absorption by spectroscopic frequency-domain optical coherence tomography

A new method of measurement that essentially combines Fourier-domain optical coherence tomography with spectroscopy is introduced. By use of a windowed Fourier transform it is possible to obtain, in addition to the object structure, spectroscopic information such as the absorption properties of materials. The feasibility of this new method for performing depth-resolved spectroscopy is demonstrated with a glass filter plate. The results are compared with theoretically calculated spectra by use of the well-known spectral characteristics of the light source and the filter plate.

Numerical dispersion compensation for Partial Coherence Interferometry and Optical Coherence Tomography

Dispersive samples introduce a wavelength dependent phase distortion to the probe beam. This leads to a noticeable loss of depth resolution in high resolution OCT using broadband light sources. The standard technique to avoid this consequence is to balance the dispersion of the sample byarrangingadispersive materialinthereference arm. However, the impact of dispersion is depth dependent. A corresponding depth dependent dispersion balancing technique is diffcult to implement. Here we present a numerical dispersion compensation technique for Partial Coherence Interferometry (PCI) and Optical Coherence Tomography (OCT) based on numerical correlation of the depth scan signal with a depth variant kernel. It can be used a posteriori and provides depth dependent dispersion compensation. Examples of dispersion compensated depth scan signals obtained from microscope cover glasses are presented.

Dynamic coherent focus OCT with depth-independent transversal resolution

Abstract We present a new OCT technique which renders the transversal resolution depth independent. This is achieved by an optical setup which shifts the focus of the beam illuminating the object through the object depth without changing the path length in the corresponding interferometer arm. Therefore, the coherence gate remains at the beam focus without any readjustment of the reference arm. Depth resolution was tested with the help of microscopy cover-plates and transversal resolution was tested with the help of Ronchi rulings. Resolution was 100 lines mm−1 over an object depth of 430 μm. For a first demonstration of the properties of this dynamic coherent focus scheme in a biologic system a section of a human cornea was used. We expect that this technique can further be improved to obtain transversal resolution down to the 1-μm range

A thermal light source technique for optical coherence tomography

A new technique for optical coherence tomography imaging with spatially low-coherent light sources is presented. In this technique the low coherence interferometry (LCI) depth-scan is performed by the image of the light source, and, therefore, simultaneously by a multitude of mutually incoherent LCI channels, to increase the probe beam power. Thermal light sources have the advantage of extremely low time-coherence with coherence lengths in the 1 μm range. The performance of a tungsten halogen lamp with a thermal spectrum and a xenon arc lamp with broadened spectral lines superimposed on a thermal continuum are compared.

Quantitative differential phase measurement and imaging in transparent and turbid media by optical coherence tomography.

Differential phase-contrast optical coherence tomography allows one to measure the path-length differences of two transversally separated beams in the nanometer range. We calculate these path-length differences from the phase functions of the interferometric signals. Pure phase objects consisting of chromium layers containing steps of approximately 100-200-nm height were imaged. Phase differences can be measured with a precision of +/-2 degrees , corresponding to a path-difference resolution of 2-3 nm. To investigate the influence of scattering, we imaged the phase objects through scattering layers with increasing scattering coefficients. The limit of phase imaging through these layers was at approximately 8-9 mean free path lengths thick (single pass).

Halo size under distance and near conditions in refractive multifocal intraocular lenses

AIMS To calculate the diameter of halos perceived by patients with multifocal intraocular lenses (IOLs) and to stimulate halos in patients with refractive multifocal IOLs in a clinical experiment. METHODS Calculations were done to show the diameter of halos in the case of the bifocal intraocular lens. 24 patients with a refractive multifocal IOLs and five patients with a monofocal IOL were asked about their subjective observation of halos and were included in a clinical experiment using a computer program (Glare & Halo, FW Fitzke and C Lohmann, Tomey AG) which simulates a light source of 0.15 square degrees (sq deg) in order to stimulate and measure halos. Halo testing took place monoculary, under mesopic conditions through the distance and the near focus of the multifocal lens and through the focus of the monofocal lens. RESULTS The halo diameter depends on the pupil diameter, the refractive power of the cornea, and distance focus of the multifocal IOL as well as the additional lens power for the near focus. 23 out of 24 patients with a refractive multifocal IOL described halos at night when looking at a bright light source. Only one patient was disturbed by the appearance of halos. Under test conditions, halos were detected in all patients with a refractive multifocal IOL. The halo area testing through the distance focus was 1.05 sq deg ± 0.41, through the near focus 1.07 sq deg ± 0.49 and in the monofocal lens 0.26 sq deg ± 0.13. CONCLUSIONS Under high contrast conditions halos can be stimulated in all patients with multifocal intraocular lenses. The halo size using the distance or the near focus is identical.

Dispersion compensation for optical coherence tomography depth-scan signals by a numerical technique

A new numerical a posteriori dispersion compensation technique for partial coherence interferometry and optical coherence tomography depth-scan signals is presented. This technique is based on numerical correlation of the depth-scan interferometer signal with a depth-variant kernel. Examples of dispersion compensated depth-scan signals obtained from microscope cover glasses are presented.

En face imaging of single cell layers by differential phase-contrast optical coherence microscopy

Optical coherence microscopy (OCM) is capable of imaging the backscattering potential of a sample with high transversal and axial resolution. We report on a combination of OCM with a differential phase-contrast technique that permits imaging of the subwavelength optical path differences that occur between a narrow beam probing a sample and its surrounding. This technique allows small transversal refractive-index variations close to a selected interface to be seen. We report on the method and present first images of a test sample and a single cell layer. The cells act as phase objects; imaging the phase properties improves the contrast compared with that of intensity images.

Flow velocity measurements by frequency domain short coherence interferometry

A method to measure the longitudinal flow velocity component based on phase resolved frequency domain optical coherence tomography (FDOCT) is introduced. At a center wavelength of 800nm the accessible velocity components ranges from 2 micrometers /s up to 2 mm/s. The upper limit is set by half the maximum frame rate of the CCD detector array. The lower limit is determined by the minimum resolvable phase change in the system, which is set by the system phase noise of 1 deg. First tests of the method include the velocity measurement of a mirror mounted on an oscillating piezo translator, and the flow of 8 micrometers latex spheres dispersed in water through a glass capillary.