Low-coherence Interference Microscopy Research Articles

Effects of refractive index mismatch between sample and immersion medium in line-field confocal optical coherence tomography

Line-field confocal optical coherence tomography (LC-OCT) is an optical technique based on low-coherence interference microscopy with line illumination, designed for tomographic imaging of semi-transparent samples with micrometer-scale spatial resolution. A theoretical model of the signal acquired in LC-OCT is presented. The model shows that a refractive index mismatch between the sample and the immersion medium causes a dissociation of the coherence plane and the focal plane, leading to a decrease in the signal amplitude and a degradation of the image’s lateral resolution. Measurements are performed to validate and illustrate the theoretical predictions. A mathematical condition linking various experimental parameters is established to ensure that the degradation of image quality is negligible. This condition is tested experimentally by imaging a phantom. It is verified theoretically in the case of skin imaging, using experimental parameters corresponding to those of the commercially available LC-OCT device.

Simultaneous local spectral, colorimetric, and topographic characterization of laser-induced colored stainless steel with low coherence interference microscopy

Laser marking is a well-established advanced technology for changing the color properties of an object. The spectral response or color information induced by the laser is generally measured with a spectrometer or an ellipsometer. These instruments only provide an average response over an area of a few thousand µm² at best. However, color laser marking is a local process that modifies the structure of the metal at a micrometer scale. In this work, we propose a technique for extracting simultaneously the 3D topography and the 2D spectral response from 460 nm to 900 nm, providing color information over this spectral range, in a single measurement. The proposed technique uses a low-coherence interference microscope to provide a laterally diffraction-limited measurement (∼1 µm) over a large field of view (up to 200 × 200 µm²) with the advantages of being rapid (<3 min) and contactless. Through the Fourier Transform analysis of the measured interferogram, a multimodal characterization (quantitative spectra, color (xy) and topography) of reflective samples can be achieved. Results obtained on laser-induced colored stainless steel are reported.

Unbalanced low coherence interference microscopy

Low coherence interference microscopy (LCIM) provides high spatial phase sensitivity, i.e., speckle free and coherent noise free quantitative phase images of the test specimens. Due to low temporal coherence (TC) length of the light source, LCIM requires precise adjustment of the optical path difference (OPD) between the object and the reference arm, which is only a few micrometers. Consequently, previously demonstrated LCIM systems are implemented with the use of identical objective lenses in both the arms and also known as balanced interferometric configuration. The use of identical objective lens hinders both the use of high numerical aperture objective lens and also the swift change of the objective lens during imaging. In the present work, LCIM is implemented with non-identical objective lenses in the object and the reference arm also called unbalanced optical configuration. A range of objective lenses 10 × /0.25NA, 20 × /0.45NA and 60 × /1.2NA are employed in the object arm of the system while keeping single objective lens 10 × /0.25NA in the reference arm. To resolve the challenges associated with unbalanced configuration, advanced iterative algorithm (AIA) and principal component analysis (PCA) algorithms are integrated to recover quadratic phase error free phase images of the test specimens. The capabilities of the proposed method are exhibited on various specimens like USAF resolution, step-like test object and for the biological cells, HeLa cells. The proposed approach enables scalable magnification and resolution by simply rotating the imaging objective turret without the need of changing objective lens in the reference arm.

Focus defect and dispersion mismatch in full-field optical coherence microscopy.

Full-field optical coherence microscopy (FFOCM) is an optical technique, based on low-coherence interference microscopy, for tomographic imaging of semi-transparent samples with micrometer-scale spatial resolution. The differences in refractive index between the sample and the immersion medium of the microscope objectives may degrade the FFOCM image quality because of focus defect and optical dispersion mismatch. These phenomena and their consequences are discussed in this theoretical paper. Experimental methods that have been implemented in FFOCM to minimize the adverse effects of these phenomena are summarized and compared.

Signal modeling in low coherence interference microscopy on example of rectangular grating.

Besides the illumination wavelength also the numerical aperture (NA) of a microscope objective affects the fringe spacing in interference microscopy. Therefore, at high NA values an effective wavelength should be obtained by calibration. At step height structures both, the effective wavelength and the batwing effect strongly depend on the height-to-wavelength-ratio (HWR). Therefore, changes of the effective wavelength considering temporal and spatial coherence enable us to estimate the batwing effect in measurement results. For high NA systems and broadband illumination two different theoretical approaches for signal modeling are introduced to study the influence of the center wavelength, the temporal, and the spatial coherence of the illuminating light on measurement results of a rectangular grating. In both models diffraction is considered. While the first simulation model (Kirchhoff) is mostly analytical the second one (Richards-Wolf) is primarily numerical. Simulation results of both models show a good agreement with experimental measurement results.

Three-band, 19-μm axial resolution full-field optical coherence microscopy over a 530–1700 nm wavelength range using a single camera

Full-field optical coherence microscopy is an established optical technology based on low-coherence interference microscopy for high-resolution imaging of semitransparent samples. In this Letter, we demonstrate an extension of the technique using a visible to short-wavelength infrared camera and a halogen lamp to image in three distinct bands centered at 635, 870, and 1170nm. Reflective microscope objectives are employed to minimize chromatic aberrations of the imaging system operating over a spectral range extending from 530 to 1700nm. Constant 1.9-μm axial resolution (measured in air) is achieved in each of the three bands. A dynamic dispersion compensation system is set up to preserve the axial resolution when the imaging depth is varied. The images can be analyzed in the conventional RGB color channels representation to generate three-dimensional images with enhanced contrast. The capability of the system is illustrated by imaging different samples.

Numerical correction of coherence gate in full-field swept-source interference microscopy

A big problem in low-coherence interference microscopy is the degradation of the coherence signal caused by shift of the angular and temporal spectrum gates. It limits the depth of field in confocal optical coherence microscopy and degrades images of sample inner structure in most interference microscopy techniques. To overcome this problem we propose numerical correction of the coherence gate in application to full-field swept-source interference microscopy. The proposed technique allows three-dimensional sample imaging without mechanical movement of the microscope components and is also capable of determining separately the geometrical thickness and the refractive index of the sample layers, when the sample contains a transversal pattern. The applicability of the proposed technique is verified with numerical simulation.

Spectroscopic polarization-sensitive full-field optical coherence tomography

Full-field optical coherence tomography (FF-OCT) is a recent optical imaging technology based on low-coherence interference microscopy for imaging of semi-transparent samples with ~1 µm spatial resolution. FF-OCT produces en-face tomographic images obtained by arithmetic combination of interferometric images acquired by an array camera. In this paper, we demonstrate a unique multimodal FF-OCT system, capable of measuring simultaneously the intensity, the power spectrum and the phase-retardation of light backscattered by the sample being imaged. Compared to conventional FF-OCT, this multimodal system provides enhanced imaging contrasts at the price of a moderate increase in experimental complexity and cost.

A five-point stencil based algorithm used for phase shifting low-coherence interference microscopy

The phase shifting technique is the most widely used approach for detecting the envelope in low coherence interferometry. However, if the phase shifts calibration contains errors, some parasitic fringe structure will propagate into the calculated envelopes and cause imprecision in the envelope peak detection. To tackle these problems, a five-point stencil algorithm is introduced into the phase shifting interference microscopy. Considering the amount of parasitic fringes, envelope peak detection and computational efficiency, the presented approach leads to satisfactory results in performance. In combination with a simple polynomial curve fitting method the proposed algorithm exhibits good performance on envelope peak detection in surface profiling. Both of the simulated results and the experimental results indicated that the presented approach can be taken as an alternative to the currently existing methods used for phase shifting low-coherence interference microscopy.

Achromatic phase shifter with eight times magnification of rotation angle in low coherence interference microscopy

An achromatic phase shifter with a rotating half-wave plate (HWP) used at the input and output of the low coherence interference microscopy is presented. This novel achromatic phase-shifter configuration is first investigated, and then its performances are compared with those of traditional phase shifters theoretically by means of Jones matrices. It is evident that the achromatism and the variation of the amplitude ratio of the proposed achromatic phase-shifter configuration is much better than other traditional phase-shifter configurations, and it can provide a phase shift of eight times the rotation angle of the HWP, which is the largest magnification achieved by far. A low coherence interference microscopy system based on the proposed achromatic phase-shifter configuration is also established to confirm the eight times relation between phase-shift and rotation angle of the HWP experimentally. At last, the three-dimensional profile and the groove depth of a step height calibration standard are obtained by using the traditional four-step algorithm to illustrate the capability and the accuracy of the low coherence interference microscopy system based on the proposed achromatic phase-shifter configuration.

Flexible contrast for low-coherence interference microscopy by Fourier-plane filtering with a spatial light modulator

We propose a full-field low-coherence interference (LCI) microscope that can provide different contrast modes using Fourier-plane filtering by means of a spatial light modulator. By altering the phase and spatial frequencies of the backreflected wavefront from the sample arm of the interferometer, we are able to change the contrast in the depth-resolved LCI images. We demonstrate that different types of contrast modes, such as, e.g., spiral phase contrast, can successfully be emulated to provide specific enhancement of internal structures and edges and to reveal complementary details within the samples under investigation.

Low-Coherence interference microscopy of the internal structure of crystallized blood plasma

The possibility of using full-field low-coherence (white light) interferometry to investigate the surface profile and the internal spatial structure of crystallized drops of blood plasma and to study plasma structuring in real time is discussed. The experimental microtomograms of the central and peripheral regions of a plasma drop are presented. A method is proposed to estimate the contact angle using two-dimensional cross sections of the drop optical structure in the peripheral region.

Low-coherence interference microscopy with an improved switchable achromatic phase-shifter

We present results obtained with a computer-controlled low-coherence interference microscope featuring a modified optical layout that uses a pair of fast switchable achromatic phase-shifters. This layout makes it possible to use ferro-electric liquid-crystal devices made with readily available FLC materials having a switching angle of 45 degrees , and obtain phase shifts of 0 degrees and +/-90 degrees . The use of phase shifts of 0 degrees and +/-90 degrees simplifies calculations of the fringe visibility and the fractional fringe-order and yields maximum accuracy.