A recent JEOS-RP publication proposed Comments about Dispersion of Light Waves, and we present here complementary comments for birefringence dispersion in polarization-maintaining (PM) fibers, and for its measurement techniques based on channeled spectrum analysis. We start by a study of early seminal papers, and we propose additional explanations to get a simpler understanding of the subject. A geometrical construction is described to relate phase birefringence to group birefringence, and it is applied to the measurement of several kinds of PM fibers using stress-induced photo-elasticity, or shape birefringence. These measurements confirm clearly that the difference between group birefringence and phase birefringence is limited to 15–20% in stress-induced PM fibers (bow-tie, panda, or tiger-eye), but that it can get up to a 3-fold factor with an elliptical-core (E-core) fiber. There are also surprising results with solid-core micro-structured PM fibers, that are based on shape birefringence, as E-core fibers.

A wavelength-switchable L-band erbium-doped fiber laser (EDFL) assisted by an artificially controlled backscattering (ACB) fiber reflector is here presented. This random reflector was inscribed by femtosecond (fs) laser direct writing on the axial axis of a multimode fiber with 50 μm core and 125 μm cladding with a length of 17 mm. This microstructure was placed inside a surgical syringe to be positioned in the center of a high-precision rotation mount to accurately control its angle of rotation. Only by rotating this mount, three different output spectra were obtained: a single wavelength lasing centered at 1574.75 nm, a dual wavelength lasing centered at 1574.75 nm and 1575.75 nm, and a single wavelength lasing centered at 1575.5 nm. All of them showed an optical signal-to-noise ratio (OSNR) of around 60 dB when pumped at 300 mW.

Bismuth-doped fiber amplifiers offer an attractive solution for meeting continuously growing enormous demand on the bandwidth of modern communication systems. However, practical deployment of such amplifiers require massive development and optimization efforts with the numerical modeling being the core design tool. The numerical optimization of bismuth-doped fiber amplifiers is challenging due to a large number of unknown parameters in the conventional rate equations models. We propose here a new approach to develop a bismuth-doped fiber amplifier model based on a neural network purely trained with experimental data sets in E- and S-bands. This method allows a robust prediction of the amplifier operation that incorporates variations of fiber properties due to manufacturing process and any fluctuations of the amplifier characteristics. Using the proposed approach the spectral dependencies of gain and noise figure for given bi-directional pump currents and input signal powers have been obtained. The low mean (less than 0.19 dB) and standard deviation (less than 0.09 dB) of the maximum error are achieved for gain and noise figure predictions in the 1410–1490 nm spectral band.

We propose an ultra-broadband near- to mid-infrared (NMIR) tunable absorber based on VO2 hybrid multi-layer nanostructure by hybrid integration of the upper and the lower parts. The upper part is composed of VO2 nanocylinder arrays prepared on the front illuminated surface of quartz substrate, and VO2 square films and VO2/SiO2/VO2 square nanopillar arrays prepared on the back surface. The lower part is an array of SiO2/Ti/VO2 nanopillars on Ti substrate. The effects of different structural parameters and temperature on the absorption spectra were analyzed by the finite-difference time-domain method. An average absorption rate of up to 94.7% and an ultra-wide bandwidth of 6.5 μm were achieved in NMIR 1.5–8 μm. Neither vertical incident light with different polarization angles nor large inclination incident light has a significant effect on the absorption performance of the absorber. The ultra-broadband high absorption performance of this absorber will be widely used in NMIR photodetectors and other new optoelectronic devices.

Extreme Learning Machines (ELMs) are a versatile Machine Learning (ML) algorithm that features as the main advantage the possibility of a seamless implementation with physical systems. Yet, despite the success of the physical implementations of ELMs, there is still a lack of fundamental understanding in regard to their optical implementations. In this context, this work makes use of an optical complex media and wavefront shaping techniques to implement a versatile optical ELM playground to gain a deeper insight into these machines. In particular, we present experimental evidences on the correlation between the effective dimensionality of the hidden space and its generalization capability, thus bringing the inner workings of optical ELMs under a new light and opening paths toward future technological implementations of similar principles.

Space Debris Laser Ranging (DLR) is a technique to measure range to defunct satellites, rocket bodies or other space targets in orbits around Earth. The analysis shows that one of the reasons for the low success probability of DLR is the inaccurate orbital prediction of targets. Then it is proposed to use the Superconducting Nanowire Single-Photon Detector (SNSPD) running in automatic-recoverable range-gate-free mode, in which case, the effect of the accuracy of the target’s orbital prediction on the success probability of DLR is greatly reduced. In this way, 249 space debris were successfully detected and 532 passes of data were obtained. The smallest target detected was the space-debris (902) with an orbital altitude of about 1000 km and a Radar Cross Section (RCS) of 0.0446 m2. The farthest target detected was the space-debris (12,445) with a large elliptical orbit and an RCS of 18.2505 m2, of which the range of the normal point (NPT) of the measured arc-segment on January 27, 2019 was 6260.805 km.

The focus of this study was to develop a method to demonstrate the feasibility of obtaining useful and high-value resources from Phoenix dactylifera residues and, to determine the physical and chemical properties of the ash of dates-palm-tree remains. Date-palm leaves and fiber samples were combusted for 50 s, using an Nd: YAG laser with 40 W output power. It was found, that combustion of one gram of agricultural waste could be completed in 50 s and 40 W by laser while 10 g required 1.5–10 min and 300–800 W power by microwave and at least 2 h with 1500 W power for conventional heating for 10 g. The subjects of this treatment, the leaves and fiber samples, before and after combustion were investigated by X-Ray Diffraction (XRD) and Fourier Transform Infrared (FTIR). The XRD results of the palm-fiber after combustion reveal that the samples were crystallized with a rhombohedral phase of acetamide and hatrurite, orthorhombic finite, and Ca4Si2O6(CO3)(OH)2, and a monoclinic phase of ikaite properties. The XRD patterns of palm-leaf after combustion reveal that the samples were crystallized with orthorhombic hillebrandite, rhombohedral acetamide, and the monoclinic phase of each karpatite, morganite, and howlite. Finally, the FTIR exhibited several absorbance peaks, assigned to silica.

We report on subwavelength reflective gratings for hyperspectral applications operating in a very large spectral band (340–1040 nm). Our study concerns a blazed-binary grating having a period of 30 μm and composed of 2D subwavelength structures with size from 120 nm to 350 nm. We demonstrate the manufacturing of the gratings on 3″ wafers by two lithography technologies (e-beam and nanoimprint) followed by classical dry etching process. Optical measurements show that the subwavelength grating approach enables a broadband efficiency, polarization behaviour and wavefront quality improvement with respect to the requirements for the next generation of spectro-imagers for Earth observation missions. An outlook towards spherical substrate based on nanoimprint lithography is also reported with the results of mixed features replication (holes and pillars in the range of 160–330 nm) on a 540 mm concave substrate which demonstrate uniformity and accuracy capabilities over 3″ surface.

This work presents a new technique based on modulating the IR absorbance of each substance in a mixture in a chirped manner to reduce the effect of their partial spectral absorption overlap on the accuracy of determining their concentrations. This chirped spectral modulation CSM algorithm can deal with mixtures containing unknown substances rather than the substances whose concentrations are aimed. This novel algorithm, when compared to existing pattern recognition techniques, makes it easy to analyze the constituents of a mixture with high accuracy in the presence of traces of unknown components. It is found that the new algorithm can detect the presence of gas pollutants such as sulfur dioxide, carbon monoxide, carbon dioxide, nitrogen dioxide in a sample containing many other unknown polluting substances. This new algorithm is tested on air samples composed of predetermined percentages of air constituents and the results of calculations are compared with those of classical least squares CLS pattern recognition algorithm. The comparison showed that the new algorithm can detect down to very small traces of harmful gases such as NO2, and SO2, at least one order of magnitude less than those detected by the CLS approach. Finally, the new algorithm is used to examine collected air samples from an industrial zone, and in the middle and at the exit of a road tunnel in Riyadh area which showed that the percentages of sulfur dioxide, nitrogen dioxide, and carbon monoxide are well below the safe levels.

Transient dynamical–thermoelastic–optical system simulation is an important expansion of classical ray tracing through rigid, resting lenses because the operating performance of high-precision optical systems can be influenced by dynamical excitations or thermal gradients. In this paper an approach for an integrated optical system simulation using the coupling of elastic multibody system simulations, thermoelastic finite element analysis and gradient-index ray tracing is presented. Transient mechanical rigid body motions and elastic deformations, thermally induced refraction index changes, and thermal elastic deformations can be considered simultaneously in the ray tracing using the presented method. The calculation of the dynamical and thermal disturbances, the data transfer and coupling, and the gradient index ray tracing method are introduced. Finally, the approach is applied on a transient triplet lens optical system and some investigation results are shown.

Robot polishing is increasingly used in the production of high-end glass work pieces such as astronomy mirrors, lithography lenses, laser gyroscopes or high-precision coordinate measuring machines. The quality of optical components such as lenses or mirrors can be described by shape errors and surface roughness. Whilst the trend towards sub nanometre level surfaces finishes and features progresses, matching both form and finish coherently in complex parts remains a major challenge. With larger or more precise optics, the influence of process instabilities on the quality of the optics to be polished has a greater impact. Vibrations at a polishing head have a negative influence on the polishing result. These vibrations are caused by bearing damage, motors and other excitations (e.g. gears, belts). The aim of this work is the determination of vibrations at a polishing head and their avoidance strategies. Different bearing conditions are considered: new and perfect bearing, a bearing that has been in contact with polish (rust) and a bearing with repeatable damage (groove milled on the running surface). It can be shown that the frequencies of bearings affect the polishing tool. Furthermore, reasons for and against vibrations in the process are discussed. For the case of vibrationless machining, avoidance strategies were presented.

Vibrational dephasing times for benzene and carbon disulfide are measured using a custom single-beam Coherent Anti-Stokes Raman Spectroscopy (CARS) setup. A femtosecond oscillator is used to pump a polarization maintaining all normal dispersion photonic crystal fibre (PM-ANDi-PCF) to generate a broad band supercontinuum, covering a spectral region from 680 to 900 nm. The dispersion properties of the PM-ANDi-PCF ensures the supercontinuum is stable and there exists a fixed phase relationship between the spectral components of the supercontinuum. This enables its temporal compression using i2PIE, implemented using a liquid crystal spatial light modulator (SLM) in a 4f geometry. This SLM is also used to shape the pulse spectrally and temporally. With this setup we could demonstrate time-resolved CARS, measuring the vibrational relaxation times of a carbon disulfide (CS2)/benzene mixture, and eliminate the non-resonant background completely. The main advantage of this setup is the fact that it is a single beam technique, eliminating the requirement for aligning the overlap of the pump and probe, both spatially and temporally, in the focal plane of the microscope. The strengths and limitations of the technique are highlighted and the route to time-resolved/background free vibrational microscopy is proposed.

In this study, CdO/Cu/CdO multilayers thin films were organized on glass substrates with different Cu intermetallic layer thickness engaging DC plasma magnetron sputtering. The optoelectronic properties and structural characteristics of the multilayers at various Cu intermetallic layer thicknesses which were varied from 4 to 16 nm were explored. The calculated band gap was reduced from 2.66 eV to 2.48 eV as the Cu intermetallic layer thickness increased from 4 to 16 nm. The refractive index and coefficient of extinction of CdO/Cu/CdO multilayers increased with increasing the Cu intermetallic layer thickness. The resistivity is reduced from 1.8 × 10−2 Ω cm for CdO single layer to reach a value of 2.7 × 10−4 Ω cm for CdO/Cu (16 nm)/CdO multilayer. Further, the sheet resistance is decreased from 1000 to 13.8 Ω/sq. with the variation in Cu intermetallic layer thickness from 0 to 16 nm. CdO/Cu (4 nm)/CdO multilayer film recorded the best figure of merit (2.3 × 10−4 Ω−1). After sunlight illumination for the multilayers, the surface wettability was improved and the contact angle recorded lowest value of nearly 24° for CdO/Cu (8 nm)/CdO and CdO/Cu (12 nm)/CdO.

In this study a commercial particle analyzer was used to image and help sorting microplastic particles (MPs) dispersed in filtrated and de-aerated tap water. The device provides a relatively easy and fast procedure for obtaining ultra-high-definition imaging, allowing the determination of shape, size, and number of 2D-projections of solid particles. The image analysis revealed clear differences among the studied different MPs originating from the grinding of five common grades of plastic sheets as they affect the image rendering differently, principally due to the light scattering either at the surface or in the volume of the microplastics. The high-quality imaging of the device also allows the discrimination of the microplastics from air bubbles with well-defined spherical shapes as well as to obtain an estimate of the size of MPs in a snapshot. We associate the differences among the shapes of the identified MPs in this study depending on the plastic type with known physical properties, such as brittleness, crystallinity, or softness. Furthermore, as a novel method we exploit a parameter based on the light intensity map from moving particles in cuvette flow to sort MPs from other particles, such as, wood fiber, human hair, and air bubbles. Using the light intensity map, which is related to the plastic-water refractive index ratio, the presence of microplastics in water can be revealed among other particles, but not their specific plastic type.

Advances in the generation of the shortest optical laser pulses down to the sub-cycle regime promise to break new ground in ultrafast science. In this work, we theoretically demonstrate the potential scaling capabilities of soliton self-compression in hollow capillary fibers with a decreasing pressure gradient to generate near-infrared sub-cycle pulses in very different dispersion and nonlinearity landscapes. Independently of input pulse, gas and fiber choices, we present a simple and general route to find the optimal self-compression parameters which result in high-quality pulses. The use of a decreasing pressure gradient naturally favors the self-compression process, resulting in shorter and cleaner sub-cycle pulses, and an improvement in the robustness of the setup when compared to the traditional constant pressure approach.

Digital speckle photography is a displacement field measurement method that employs laser speckles as surface markers. Since the approach requires only one reference image without a preparation of the sample and provides a fast, single-shot measurement with interferometric precision, the method is applied for in-process measurements in manufacturing engineering. Due to highly localized loads, higher-order displacement gradients occur in manufacturing processes and it is an open research question how these gradients affect the measurement errors of digital speckle photography. We simulate isotropic Gaussian surface topographies, apply a displacement field and then generate laser speckle patterns, which are evaluated with digital image correlation and subsequently the resulting random and systematic errors of the displacement field are analyzed. We found that the random error is proportional to the first-order displacement gradient and results from decorrelation of the laser speckles. The systematic error is mainly caused by the evaluation algorithm and is linearly dependent on the second-order gradient and the subset size. We evaluated in-process displacement measurements of laser hardening, grinding and single-tooth milling where we determined the relative error caused by displacement gradients to be below 2.5% based on the findings from the simulative study.

Purpose: Despite theoretical models for achieving laser-based ablation smoothness, methods do not yet exist for assessing the impact of residual roughness after corneal ablation, on retinal polychromatic vision. We developed a method and performed an exploratory study to qualitatively and quantitatively analyze the impact of varying degree of corneal roughness simulated through white and filtered noise, on the retinal image. Methods: A preliminary version of the Indiana Retinal Image Simulator (IRIS) [Jaskulski M., Thibos L., Bradley A., Kollbaum P., et al. (2019) IRIS – Indiana Retinal Image Simulator. https://blogs.iu.edu/corl/iris] was used to simulate the polychromatic retinal image. Using patient-specific Zernike coefficients and pupil diameter, the impact of different levels of chromatic aberrations was calculated. Corneal roughness was modeled via both random and filtered noise [(2013) Biomed. Opt. Express4, 220–229], using distinct pre-calculated higher order Zernike coefficient terms. The outcome measures for the simulation were simulated retinal image, Strehl Ratio and Visual Strehl Ratio computed in frequency domain (VSOTF). The impact of varying degree of roughness (with and without refractive error), spatial frequency of the roughness, and pupil dilation was analyzed on these outcome measures. Standard simulation settings were pupil size = 6 mm, Defocus Z[2, 0] = 2 μm (−1.54D), and Spherical Aberrations Z[4, 0] = 0.15 μm. The signal included the 2–4th Zernike orders, while noise used 7–8th Zernike orders. Noise was scaled to predetermined RMS values. All the terms in 5th and 6th Zernike order were set to 0, to avoid overlapping of signal and noise. Results: In case of a constant roughness term, reducing the pupil size resulted in improved outcome measures and simulated retinal image (Strehl = 0.005 for pupil size = 6 mm to Strehl = 0.06 for pupil size = 3 mm). The calculated image quality metrics deteriorated dramatically with increasing roughness (Strehl = 0. 3 for no noise; Strehl = 0.03 for random noise of 0.25 μm at 6 mm diameter; Strehl = 0.005 for random noise of 0.65 μm at 6 mm diameter). Clear distinction was observed in outcome measures for corneal roughness simulated as random noise compared to filtered noise, further influenced by the spatial frequency of filtered noise. Conclusion: The proposed method enables quantifying the impact of residual roughness in corneal ablation processes at relatively low cost. Since normally laser ablation is an integral process divided on a defined grid, the impact of spatially characterized noise represents a more realistic simulation condition. This method can help comparing different refractive laser platforms in terms of their associated roughness in ablation, indirectly improving the quality of results after Laser vision correction surgery.

We present a novel approach of modelling surface light scattering in the context of two-dimensional reflector design, relying on energy conservation and optimal transport theory. For isotropic scattering in cylindrically or rotationally symmetric systems with in-plane scattering, the scattered light distribution can be expressed as a convolution between a scattering function, which characterises the optical properties of the surface, and a specular light distribution. Deconvolving this expression allows for traditional specular reflector design procedures to be used, whilst accounting for scattering. This approach thus constitutes solving the inverse problem of light scattering, allowing for direct computation of the reflector surface, without the need for design iterations.

The use of convolutional neuronal networks (CNN) for the treatment of interferometric fringes has been introduced in recent years. In this paper, we optimize and build a CNN model, based U-NET architecture, to maximize its performance processing electronic speckle interferometry fringes (ESPI). The proposed approach is based on quick and light trainings to select the architecture parameters (network depth and kernel sizes) to maximize the performance of the neural network improving the visibility of ESPI images. To measure the performance, the structural similarity index (SSMI) will be the lead indicator, and the need for large datasets to train neural networks, unavailable for ESPI images, forces the use of a simulated ESPI image dataset along the process. This dataset is computed using Zernike polynomials to simulate local surface deformations in the specimen under test and simulated true speckle fields for the reference and object field involved in ESPI techniques.

One of the most profound and philosophically captivating foci of modern astronomy is the study of Earth-like exoplanets in the search for life in the Universe. The paradigm-shifting investigation described here calls for a new type of scalable space telescope that redefines the available light-collecting area in space. The Nautilus Space Observatory, enabled by multiple-order diffractive optics (the MODE lens), is ushering in the advent of large space telescope lenses designed to search for biosignatures on a thousand exo-earths. The Kinematically Engaged Yoke System (KEYS) was developed to align a segmented version of the MODE lens. A technology demonstration prototype of KEYS was built and tested using scanning white light interferometry and deflectometry. A deflectometry system was also developed to monitor the closed-loop alignment of the segmented MODE lens during its UV (i.e., Ultraviolet) curing.

A camera-based single-image sensor is presented, that is able to measure the distance of one or multiple object points (light emitters). The sensor consists of a camera, whose lens is upgraded with a diffractive optical element (DOE). It fulfils two tasks: adding a vortex point spread function (PSF) and replication of the vortex PSFs to a predefined pattern of K spots. Both, shape and rotation of the vortex PSF is sensitive to defocus. The sensor concept is presented and its capabilities evaluated both on axis and off-axis. The achieved standard deviation of the error ranges between 8.5 μm (on-axis) and 3.5 μm (off-axis) within a measurement range of 20 mm. However, as soon as calibration and measurement position no longer match, the accuracy is limited. An analysis of the effects responsible for this are also part of the publication.

Critical defects, also known as device killers, in wide bandgap semiconductors significantly affect the performance of power electronic devices. We used the methods imaging ellipsometry (IE) and white light interference microscopy (WLIM) in a hybrid optical metrology study for fast and non-destructive detection, classification, and characterisation of defects in 4H–SiC homoepitaxial layers on 4H–SiC substrates. Ellipsometry measurement results are confirmed by WLIM. They can be successfully applied for wafer characterisation already during production of SiC epilayers and for subsequent industrial quality control.

Vanadium dioxide (VO2) has promising applications in smart windows and active micro-optical devices due to its thermochromic properties. However, the successful fabrication and patterning of VO2 thin films with the correct stoichiometry and phase are challenging. In this study, we investigated lithographically patterned and non-patterned VO2 thin films fabricated by reactive ion beam deposition, using variable angle spectroscopic ellipsometry, Raman spectroscopy, and transmission and reflection measurements. The results show that the refractive index and extinction coefficient exhibit significant changes for near-infrared wavelengths when heated above 68 °C, confirming its thermochromic properties. The Raman spectroscopy results indicate the formation of the monoclinic phase VO2(M) after annealing, which was not changed by reactive ion etching. Lithographically structured VO2-layers were successfully realized demonstrating the potential of VO2 as a material for active micro-optical devices, such as guided mode resonance filters with switchable reflectance. The results suggest that VO2 has great potential as a promising material for actively switched optical elements and micro-optical devices.

Space division multiplexing (SDM) is promising to enhance capacity limits of optical networks. Among implementation options, few-mode fibres (FMFs) offer high efficiency gains in terms of integratability and throughput per volume. However, to achieve low insertion loss and low crosstalk, the beam launching should match the fiber modes precisely. We propose an all-optical data-driven technique based on multiplane light conversion (MPLC) and neural networks (NNs). By using a phase-only spatial light modulator (SLM), spatially separated input beams are transformed independently to coaxial output modes. Compared to conventional offline calculation of SLM phase masks, we employ an intelligent two-stage approach that considers knowledge of the experimental environment significantly reducing misalignment. First, a single-layer NN called Model-NN learns the beam propagation through the setup and provides a digital twin of the apparatus. Second, another single-layer NN called Actor-NN controls the model. As a result, SLM phase masks are predicted and employed in the experiment to shape an input beam to a target output. We show results on a single-passage configuration with intensity-only shaping. We achieve a correlation between experiment and network prediction of 0.65. Using programmable optical elements, our method allows the implementation of aberration correction and distortion compensation techniques, which enables secure high-capacity long-reach FMF-based communication systems by adaptive mode multiplexing devices.

This work presents a promising method for automatic, non-contact, detection and counting of salmon lice infested on salmon in an aquacultural farm setting. The method uses fluorescence from chitin in the visual part of spectrum to enhance the contrast between fish skin and salmon lice, and show that the fluorescence is even strong enough to give a real-time view of the digestive and reproduction system in live lice without use of staining dyes. The wavelengths used are compatible with an underwater measurement system.

Light sheet fluorescence microscope with single light sheet illumination enables rapid 3D imaging of living cells. In this paper we show the design, fabrication and characterization of a diffractive optical element producing several light sheets along a 45° inclined tube. The element, which is based on a multi-focal diffractive lens and a linear grating, generates five thin light sheets with equal intensities when combined with a refractive cylindrical lens. The generated uniform light sheets can be applied for the scanning of samples in tubes enabling flow-driven 3-dimensional imaging.

We propose a time-gated-single-pixel-camera as a promising sensor for image-free object detection for automotive application in adverse weather conditions. By combining the well-known principles of time-gating and single-pixel detection with neural networks, we aim to ultimately detect objects within the scene rapidly and robustly with a low-cost sensor. Here, we evaluate the possible data reduction such a system can provide compared to a conventional time-gated camera.

Interferometric detection enables the acquisition of the amplitude and phase of the optical field. By making use of the synthetic wavelength as a computational construct arising from digital processing of two off-axis digital holograms, it is possible to identify the shape of an object obscured by fog and further increase the imaging range due to the increased sensitivity in coherent detection. Experiments have been conducted inside a 27 m long fog tube filled with ultrasonically generated fog. We show the improved capabilities of synthetic phase imaging through fog and compare this technique with conventional active laser illumination imaging.

The objective of the present study is to examine the behaviors of chirped optical solitons in fiber Bragg gratings (BGs) with dispersive reflectivity. The form of nonlinear refractive index represents polynomial law nonlinearity. By virtue of phase-matching condition, the discussed model of coupled nonlinear Schrödinger equation is reduced to an integrable form. Consequently, chirped optical solitons having various profiles such as W-shaped, bright, dark, kink and anti-kink solitons are derived. Further to this, the chirp associated with these soliton structures are extracted. The impact of dispersive reflectivity, self-phase modulation and cross-phase modulation on the pulse propagation is investigated and it is induced that the changes of self-phase modulation and cross-phase modulation cause a marked rise in soliton amplitude which is subject to minor variations by dispersive reflectivity. The physical evolutions of chirped optical solitons are described along with the corresponding chirp to pave the way for possible applications in the field of fiber BGs.

Mechanical forces play an important role in the behaviour of cells, from differentiation to migration and the development of diseases. Optical tweezers provide a quantitative tool to study these forces and must be combined with other tools, such as phase contrast and fluorescence microscopy. Detecting the retro-reflected trap beam is a convenient way to monitor the force applied by optical tweezers, while freeing top access to the sample. Accurate in situ calibration is required especially for single cells close to a surface where viscosity varies rapidly with height. Here, we take advantage of the well contrasted interference rings in the back focal plane of the objective to find the height of a trapped bead above a cover slip. We thus map the viscous drag dependence close to the surface and find agreement between four different measurement techniques for the trap stiffness down to 2 μm above the surface. Combining this detection scheme with phase contrast microscopy, we show that the phase ring in the back focal plane of the objective must be deported in a conjugate plane on the imaging path. This simplifies implementation of optical tweezers in combination with other techniques for biomechanical studies.

Brillouin light scattering (BLS) spectroscopy is a label-free method of measuring the GHz-frequency viscoelastic properties. The measured longitudinal modulus is acutely sensitive to the degree of hydration, crosslinking, and temperature, which can be indicative of tissue health. As such, performing in situ measurements on humans is particularly desirable for exploring potential clinical translation, however, is not possible with existing designs which are coupled to bench-top microscopes. Here we introduce a robust fiber coupled hand-held BLS probe and demonstrate its reliability for measuring excised human tissue. We verify its accuracy using confocal BLS microscopy and further show that it is possible to distinguish veins, arteries, nerves and muscles based on their BLS-measured viscoelasticity. This provides a necessary first step towards in situ clinical BLS viscoelasticity studies of human tissue.

Enhancing the lateral resolution in optical microscopy and interferometry is of great interest in recent research. In order to laterally resolve structures including feature dimensions below the Abbe resolution limit, microspheres in the optical near-field of the specimen are shown to locally improve the resolution of the imaging system. Experimental and simulated results following this approach are obtained by a high NA Linnik interferometer and analyzed in this contribution. They show the reconstructed surface of a 1D phase grating below the resolution limit. For further understanding of the transfer characteristics, measured interference data are compared with FEM (finite element method) based simulations with respect to the polarization dependency of the relevant image information for 1D phase gratings. Therefore, the implemented Koehler illumination as well as the experimental setup utilize polarized light.

Infrared spectroscopy is often used to spot differences between benign and malignant tissue. Due to the proliferation of tumorous cells, the composition of tissue changes drastically. In the consequence shifts occur in its optical properties that are indicated by spectral biomarkers in the so-called fingerprint region. In this work, we propose a new concept for a sparsified multi-spectral measurement of the most important and informative biomarker signals. The results of a data-driven feature selection approach show that a reliable discrimination of the tissue is still possible, even though utilizing only a small fraction of the measured data. A selected arrangement of only a few narrow-band quantum cascade lasers could provide proficient signal-to-noise ratios and can noticeably reduce the data acquisition time. Consequentially, real-time applications will be possible in short-term and in-vivo diagnostics in the long-term. First measurements of silicone phantoms validate the imaging capability of the sensor concept.