Tobias Boettcher

Optical Design of a Vertically Integrated Array-type Mirau-based OCT system

The presented paper shows the concept and optical design of an array-type Mirau-based OCT system for early diagnosis of skin cancer. The basic concept of the sensor is a full-field, full-range optical coherence tomography (OCT) sensor. The micro-optical interferometer array in Mirau configuration is a key element of the system allowing parallel imaging of multiple field of views (FOV). The optical design focuses on the imaging performance of a single channel of the interferometer array and the illumination design of the array. In addition a straylight analysis of this array sensor is given.

Multi-layer topography measurement using a new hybrid single-shot technique: Chromatic Confocal Coherence Tomography (CCCT)

Often measurement tasks occur, where specimens consist of multiple layers or topography shall be examined through contaminations. Especially for unknown layer materials, it is important to measure the layers refractive index to compensate for the errors induced on the measurement of underlying surfaces. Chromatic Confocal Coherence Tomography is proposed as a new hybrid single-shot scheme for a simultaneous measurement of thickness and refractive index of semitransparent layers, combining chromatic confocal and interferometric information. As a proof of concept, first measurements are presented along with a short discussion about their uncertainties, where minimal layer thickness and resolution are dominated by the confocal part of the signal, that is mainly influenced by the chosen microscope objective.

Robust signal evaluation for Chromatic Confocal Spectral Interferometry

The hybrid measurement principle Chromatic Confocal Spectral Interferometry combines Spectral Interferometry with Chromatic Confocal Microscopy and therefore benefits from their respective advantages. Our actual demonstrator setup enables an axial measurement range up to 100 μm with resolution up to 5 nm depending on the employed evaluation method and the characteristics of the object’s surface. On structured surfaces, lateral features down to 1 μm can be measured. As the sensor raw signal consists of a Spectral Interferometry type wavelet modulated by a confocal envelope, two classes of evaluation methods working on the phasing or the position of the envelope are employed. Even though both of these information channels are subject to their respective problems, we show that a proper combination of the individual methods leads to a robust signal evaluation. In particular, we show that typical artifacts on curved surfaces, that are known from Chromatic Confocal Microscopy, are minimized or completely removed by taking the phasing of the Spectral Interferometry wavelet into consideration. At the same time the problem of determining the right fringe order of the Spectral Interferometry signal at surface discontinuities can be solved by evaluation of the confocal envelope. We present here a first approach using a contrast threshold on the signal and a median referencing for trusted sections of the analysed topography, which yields a reduction of artifacts in a submicron range on steep gradients, discontinuous specimen or curved mirror-like surfaces.

Development of a realistic wave propagation-based chromatic confocal microscopy model

A model describing the signal generation in chromatic confocal imaging is presented here. It can be used to understand the signal development process accounting for wave-optical phenomena using scalar wave theory. The influence of the optics in terms of aberrations, the specimen in terms of roughness and further parameters on the signal generation process will be investigated. Moreover, the possibility to adapt the model to investigate other spectral imaging systems, such as chromatic confocal spectral interferometry will also be shown.

Von Topographie bis Brechungsindex: Flexibles multi-modales Messen mittels Fusion chromatisch-konfokaler und spektralinterferometrischer Technik

Zusammenfassung Bei der Messung technischer Oberflächen treten insbesondere im Umfeld flexibler Fertigung unterschiedlichste Anforderungen auf. Häufig ist eine schnelle hochgenaue Topographie-Messung erwünscht. Durch die Kombination des chromatisch-konfokalen Verfahrens mit der spektralen Interferometrie (Chromatisch-Konfokale Spektral-Interferometrie) ist es möglich, single-shot Messungen mit einem weiten Messbereich und gleichzeitig geringer Unsicherheit zu erzielen. Dabei können ebenso die den beiden einzelnen Verfahren inhärenten Probleme umgangen und so der Einsatzbereich erweitert werden. Mit demselben optischen Aufbau ist es auch möglich, gleichzeitig Brechungsindex und Dicke von Mehrschichtproben mit hoher Auflösung zu vermessen (Chromatisch-Konfokale Kohärenz-Tomographie). Dies ist etwa von Vorteil, wenn das Messobjekt mit funktionalen Schichten versehen ist oder beispielsweise durch eine Ölschicht gemessen werden muss.

Single-shot multilayer measurement by chromatic confocal coherence tomography

We propose Chromatic Confocal Coherence Tomography as a new system able to achieve corrected topography measurements of multi-layered specimens by measuring position, thickness and refractive index of each layer simultaneously at each measurement point. This feature is achieved by a combination of a chromatic confocal scheme and an interferometric one. The numerical aperture of the used microscope objective has a significant effect on the measurement uncertainty. Hence, its contribution to uncertainty is discussed in more detail.

Performance analysis of a full-field and full-range swept-source OCT system

In recent years, optical coherence tomography (OCT) became gained importance in medical disciplines like ophthalmology, due to its noninvasive optical imaging technique with micrometer resolution and short measurement time. It enables e. g. the measurement and visualization of the depth structure of the retina. In other medical disciplines like dermatology, histopathological analysis is still the gold standard for skin cancer diagnosis. The EU-funded project VIAMOS (Vertically Integrated Array-type Mirau-based OCT System) proposes a new type of OCT system combined with micro-technologies to provide a hand-held, low-cost and miniaturized OCT system. The concept is a combination of full-field and full-range swept-source OCT (SS-OCT) detection in a multi-channel sensor based on a micro-optical Mirau-interferometer array, which is fabricated by means of wafer fabrication. This paper presents the study of an experimental proof-of-concept OCT system as a one-channel sensor with bulk optics. This sensor is a Linnik-interferometer type with similar optical parameters as the Mirau-interferometer array. A commercial wavelength tunable light source with a center wavelength at 845nm and 50nm spectral bandwidth is used with a camera for parallel OCT A-Scan detection. In addition, the reference microscope objective lens of the Linnik-interferometer is mounted on a piezo-actuated phase-shifter. Phase-shifting interferometry (PSI) techniques are applied for resolving the conjugate complex artifact and consequently contribute to an increase of image quality and depth range. A suppression ratio of the complex conjugate term of 36 dB is shown and a system sensitivity greater than 96 dB could be measured.

A more robust and flexible approach to laterally chromatically dispersed, spectrally encoded interferometry (LCSI)

Derived from Spectral Interferometry, a line sensor named Laterally Chromatically Dispersed, Spectrally Encoded Interferometer has been developed lately. The basic setup features a single SLD in the near infra-red range, whose light is laterally spread over a measurement line of about 1mm by a diffraction grating. The signal encodes the lateral position as well as the respective optical path difference for every pixel on the spectrometer. Thus, an elaborated evaluation strategy is needed for precise measurement, including the need for a priori knowledge of the surface or multiple related measurements. To overcome this limitation and provide a real single-shot measurement, the setup can be extended by a second light source. However, the sources have to meet some strong requirements, such as sufficient spectral separation. Sensor simulations for different classes of objects show, that an accurate reconstruction of many surfaces can be achieved with the extended setup in a real single-shot line measurement without the need for a priori information.