Norman Lippok

Dispersion compensation in Fourier domain optical coherence tomography using the fractional Fourier transform.

We address numerical dispersion compensation based on the use of the fractional Fourier transform (FrFT). The FrFT provides a new fundamental perspective on the nature and role of group-velocity dispersion in Fourier domain OCT. The dispersion induced by a 26 mm long water cell was compensated for a spectral bandwidth of 110 nm, allowing the theoretical axial resolution in air of 3.6 μm to be recovered from the dispersion degraded point spread function. Additionally, we present a new approach for depth dependent dispersion compensation based on numerical simulations. Finally, we show how the optimized fractional Fourier transform order parameter can be used to extract the group velocity dispersion coefficient of a material.

Degree of polarization (uniformity) and depolarization index: unambiguous depolarization contrast for optical coherence tomography.

The degree of polarization (uniformity) has attracted increased interest as a functional contrast in optical coherence tomography (OCT). However, its computation from a single polarization state suggests an ambiguity that is strongly dependent on a samples orientation. We here propose an improved metric to present depolarization with respect to the optical system rather than the propagating field. Using numerical simulations and optical frequency domain imaging, we evaluate the conventional DOP(U) for different polarization states and compare its performance with the unambiguous depolarization index.

Single input state, single-mode fiber-based polarization-sensitive optical frequency domain imaging by eigenpolarization referencing

Fiber-based polarization-sensitive optical frequency domain imaging is more challenging than free-space implementations. Using multiple input states, fiber-based systems provide sample birefringence information with the benefit of a flexible sample arm but come at the cost of increased system and acquisition complexity, and either reduce acquisition speed or require increased acquisition bandwidth. Here we show that with the calibration of a single polarization state, fiber-based configurations can approach the conceptual simplicity of traditional free-space configurations. We remotely control the polarization state of the light incident at the sample using the eigenpolarization states of a wave plate as a reference, and determine the Jones matrix of the output fiber. We demonstrate this method for polarization-sensitive imaging of biological samples.

Dispersion mapping at the micrometer scale using tri-band optical frequency domain imaging

Techniques to differentiate between materials are a powerful addition to the structural information traditionally available from optical coherence tomography images. We present label-free detection of water and lipid at a micrometer scale by evaluating their unique dispersion properties. Using a tri-band swept source configuration, we measure both β(2) and β(3) and show how to identify the two materials at sample thicknesses of 40 and 90 μm, respectively.

Simple and versatile long range swept source for optical coherence tomography applications

We present a versatile long coherence length swept-source laser design for optical coherence tomography applications. This design consists of a polygonal spinning mirror and an optical gain chip in a modified Littman–Metcalf cavity. A narrowband intra-cavity filter is implemented through multiple passes off a diffraction grating set at grazing incidence. The key advantage of this design is that it can be readily adapted to any wavelength regions for which broadband gain chips are available. We demonstrate this by implementing sources at 1650 nm, 1550 nm, 1310 nm and 1050 nm. In particular, we present a 1310 nm swept source laser with 24 mm coherence length, 95 nm optical bandwidth, 2 kHz maximum sweep frequency and 7.5 mW average output power. These parameters make it a suitable source for the imaging of biological samples.

Instantaneous quadrature components or Jones vector retrieval using the Pancharatnam-Berry phase in frequency domain low-coherence interferometry.

We use the Pancharatnam-Berry phase as a multifunctional tool for low-coherence interferometry. This geometric phase shift enables instantaneous retrieval of the quadrature components of the complex interferometric signal. The phase shift is independent of wavelength and allows for a complex conjugate suppression of 40 dB for an optical bandwidth of 115 nm. Furthermore, this paper investigates the versatility of the geometric phase to perform polarization sensitive measurements. The Jones vector of the sample was obtained numerically, allowing sample birefringence and optical axis calculation.

Single-shot speckle reduction and dispersion compensation in optical coherence tomography by compounding fractional Fourier domains.

We present speckle suppression and dispersion compensation for Fourier-domain optical coherence tomography based on fractional Fourier transforms of a single A scan. A 1.54-fold reduction in speckle contrast was achieved with group velocity dispersion compensation. The method is demonstrated on biological samples using a swept source configuration at 1310 nm and a spectral-domain system at 840 nm.

High-speed optical coherence tomography by circular interferometric ranging

Existing three-dimensional optical imaging methods excel in controlled environments, but are difficult to deploy over large, irregular and dynamic fields. This means that they can be ill-suited for use in areas such as material inspection and medicine. To better address these applications, we developed methods in optical coherence tomography to efficiently interrogate sparse scattering fields, that is, those in which most locations (voxels) do not generate meaningful signal. Frequency comb sources are used to superimpose reflected signals from equispaced locations through optical subsampling. This results in circular ranging, and reduces the number of measurements required to interrogate large volumetric fields. As a result, signal acquisition barriers that have limited speed and field in optical coherence tomography are avoided. With a new ultrafast, time-stretched frequency comb laser design operating with 7.6 MHz to 18.9 MHz repetition rates, we achieved imaging of multi-cm3 fields at up to 7.5 volumes per second.Using an ultrafast, time-stretched frequency comb laser operating with repetition rates from 7.6 MHz to 18.9 MHz, a rapid and large-volumetric-field optical coherence tomography at an imaging rate of up to 7.5 volumes per second is demonstrated.

Coronary Plaque Microstructure and Composition Modify Optical Polarization : A New Endogenous Contrast Mechanism for Optical Frequency Domain Imaging

OBJECTIVES This study aimed to evaluate whether polarimetry, performed using a modified optical frequency domain imaging (OFDI) system, can improve the assessment of histological features relevant to characterizing human coronary atherosclerosis. BACKGROUND The microscopic structure and organization of the arterial wall influence the polarization of the infrared light used by OFDI. Modification of the OFDI apparatus, along with recently developed image reconstruction methods, permits polarimetric measurements simultaneously with conventional OFDI cross-sectional imaging through standard intravascular imaging catheters. METHODS The main coronary arteries of 5 cadaveric human hearts were imaged with an OFDI system capable of providing polarimetric assessment. Cross-sectional views of tissue birefringence, measured in refractive index units, and depolarization, expressed as the ratio of depolarized signal to total intensity, were reconstructed, together with conventional OFDI images. Following imaging, the vessels underwent histological evaluation to enable interpretation of the observed polarization features of individual tissue components. RESULTS Birefringence in fibrous tissue was significantly higher than in intimal tissue with minimal abnormality (0.44 × 10−3 vs. 0.33 × 10−3; p < 0.0001). Birefringence was highest in the tunica media (p < 0.0001), consistent with its high smooth muscle cell content, cells known to associate with birefringence. In fibrous areas, birefringence showed fine spatial features and close correspondence with the histological appearance of collagen. In contrast, necrotic cores and regions rich in lipid elicited significant depolarization (p < 0.0001). Depolarization was also evident in locations of cholesterol crystals and macrophages. CONCLUSIONS Intravascular measurements of birefringence and depolarization can be obtained using conventional OFDI catheters in conjunction with a modified console and signal processing algorithms. Polarimetric measurements enhance conventional OFDI by providing additional information related to the tissue composition and offer quantitative metrics enabling characterization of plaque features.

Intravascular polarization sensitive optical coherence tomography in human patients

Polarization sensitive optical coherence tomography (PS-OCT) provides measures of tissue birefringence and depolarization. Here, we demonstrate catheter-based PS-OCT in the coronary arteries of human patients to obtain valuable insight into the morphology of atherosclerotic plaques.