Chromatic confocal spectral interferometry combines the benefits of scanning free acquisition of the axial dimension with interferometrically increased depth accuracy. However, so far it has been difficult to separate the confocal signal from the interferometric signal. It is, of course, possible to apply the established CCM evaluation methods. In that case, the available phase information, that offers a decreased measurement uncertainty and to some degree the removal of disturbing artifacts at steep surface inclinations, is not taken into account. In fact, it is not straight forward to interpret the signal. In comparison to white light interference microscopy, the signal suffers from a chirp. This means that it cannot be associated with a single beating frequency, which corresponds to the interferometrically encoded z-value. However, a modified lock-in technique has in the past successfully been applied, demonstrating a significant advantage in comparison to the conventional CCM procedures. Here, we will introduce the concept of k-space phase equality, which enables the separation of the confocal and the interferometric signal and furthermore offers an extended measurement range. The principle is based on signal modification in the z-space, which corresponds to the Fourier domain of the recorded spectral signal. The evaluation is then performed in the spectral domain, where the phase signals for all z-positions with respect to the corresponding wavelength are evaluated. As a result, a phase signal with reduced aberration terms, similar to an interferometric signal, is obtained, which can hence be evaluated using established techniques.
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.
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.
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.
The EU-funded project VIAMOS1 proposes an optical coherence tomography system (OCT) for skin cancer detection, which combines full-field and full-range swept-source OCT in a multi-channel sensor for parallel detection. One of the project objectives is the development of new fabrication technologies for micro-optics, which makes it compatible to Micro-Opto-Electromechanical System technology (MOEMS). The basic system concept is a wafer-based Mirau interferometer array with an actuated reference mirror, which enables phase shifted interferogram detection and therefore reconstruction of the complex phase information, resulting in a higher measurement range with reduced image artifacts. This paper presents an experimental one-channel on-bench OCT system with bulk optics, which serves as a proof-of-concept setup for the final VIAMOS micro-system. It is based on a Linnik interferometer with a wavelength tuning light source and a camera for parallel A-Scan detection. Phase shifting interferometry techniques (PSI) are used for the suppression of the complex conjugate artifact, whose suppression reaches 36 dB. The sensitivity of the system is constant over the full-field with a mean value of 97 dB. OCT images are presented of a thin membrane microlens and a biological tissue (onion) as a preliminary demonstration.
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.
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.
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.