Background: Non-invasive optical measurements such as diffuse correlation spectroscopy and photoplethysmography provide critical physiological information, including cardiovascular parameters. Compact and wearable optical devices enable point-of-care and daily monitoring of cardiovascular signals. Objective: In this study, we propose a comprehensive methodology for informed design of optical transceivers to optimize signal acquisition. Specifically, we investigated the dependence of depth sensitivity on scattering as a function of source-detector distance (SDD). Methods: Speckle variance optical coherence tomography was performed on healthy adult volunteers (3 female, 3 male) to obtain three-dimensional angiograms of the skin microvascular network. Using machine vision algorithms, we quantified microvascular parameters including average depth, width, and volumetric density. These parameters were incorporated into a multi-layer skin digital twin model, and Monte Carlo simulations of light transport at 660 nm were performed across a range of SDD values. By analyzing scattering events in each skin layer, we quantified the SDD-dependent depth sensitivity. Results: Our results indicate that at short SDDs (i.e., 0.15 mm), scattering predominantly occurs in the upper dermis (i.e., 49%), whereas at longer SDDs (i.e., 4 mm), the hypodermis becomes dominant (i.e., 41%). With an average microvascular depth of 130±30μm (within the upper dermis), we identified an optimal SDD of 0.9 mm, yielding a maximum scattering contribution of 72% for the studied population. Conclusion: Our methodology establishes a foundation for patient-specific optimization of optical signal acquisition, with potential applications in diverse populations, including hypertensive elderly patients. Significance: Our study enables patient-specific device design addressing physiological variations across individuals (e.g. differences in microvascular networks and skin tone).

We study the evolution of the kinetic energy (or gradient norm) of an incident linearly polarized monochromatic wave propagating in correlated random media. We explore the optical flux transverse to the mean Poynting flux at the paraxial-nonparaxial (vectorial) transition along with vortex counting. Here, by paraxial-nonparaxial transition we mean a gradual loss of validity of the paraxial approximation such that it is necessary to solve Maxwell-consistently employing the dyadic Green’s function. The vortex number appears to increase approximately with a cubic root of the propagation distance for sufficiently small correlation length. Furthermore, a kink appears in nucleation rate at the position of maximum scintillation upon increasing correlation length. A driven steady state is reached due to the filtering of evanescent waves upon propagation. Finally, we present the spectrum of the incompressible kinetic energy and how it evolves from the paraxial case to that of a (nonparaxial) random field.

Optical wearable sensors provide crucial information on cardiovascular biomarkers used to estimate heart rate variability, blood oxygen saturation, and arterial blood pressure. However, environmental factors, including external contact pressure, can significantly affect the quality of the acquired signal but have been poorly studied. Objective: In this work, we investigate the influence of external contact pressure on the photoplethysmography (PPG) signal in reflection mode. By systematic application of external contact pressure to the fingertips of volunteers, we aim to examine how such pressure affects the morphological features of a representative cardiac cycle. Methods: We designed, and 3D printed a mounting system to apply controlled pressure to the fingertips of volunteers. This system generated a controllable force using a spring mechanism coupled with a rotating screw. First, we quantified the spring constant and the pressure it applies per revolution. Then, using a PPG sensor operating at a wavelength of 660 ± 20nm, we recorded raw photocurrent signals from three healthy adults. Using our proposed signal processing algorithm, we created ensemble-averaged representative cardiac cycles. Results: We calculated the spring's constant within the mounting system as 339.3N/m. Using this system, we applied external contact pressure values from 20 to 180mmHg. Our results show that the amplitude of systolic peak, dicrotic notch, and diastolic peak for this external contact pressure range continuously rise with a factor of 2.5, 5, and 2, respectively. Conclusion: Our 3D printed mounting system provided a reliable means of applying controlled and reproducible external contact pressure to the fingertips of adult volunteers. We conclude that such contact pressure substantially influences the amplitude of the obtained PPG photocurrent, being a crucial factor in optical wearable sensor design. Significance: Our findings pave the way for determining the optimal level of external contact pressure, as an environmental factor. Such optimal pressure should balance user comfort with the quality of the measured PPG signal, thereby supporting the reliable estimation of cardiovascular parameters.

We propose to use a frequency-modulated light source and a high-speed camera to simultaneously measure the optical and depth-dependent dynamic properties of turbid media. This approach mitigates bandwidth limitations encountered by previously demonstrated interferometric near-infrared spectroscopy techniques.

Interferometric diffuse optics (iDO) enables non-invasive measurement of deep tissue blood flow without requiring photon-counting detectors. Due to hardware constraints, achieving both optical properties and depth-dependent dynamics within a single modality remains a challenge for iDO. We present a simple method based on frequency-modulated light scattering that overcomes this limitation.

We study straylight of metalenses both by systematically adding controlled manufacturing errors as well as numerically. For the experimental realisation, we nanofabricate amorphous silicon (a-Si) nanopillars on a silicon nitride (SiN) membrane via electron beam lithography. For the numerical comparison employ a Finite-Difference in Time-Domain solver.

The need of atmospheric information with a higher spatial and temporal resolution drives the development of small satellites and satellite constellations to complement satellite flagship missions. Since optical systems are a main contributor to the satellite size, these are the prime candidate for their miniaturization. We present here a novel optical system where the complete spectrometer part of the optical system is compressed in one flat optical element. The element consists of an array of photonic crystals which is directly placed on a detector. The photonic crystals act as optical filters with a tunable spectral transmission response. From the integrated optical signals per filter and the atmosphere model, greenhouse gas concentrations are obtained using computational inversion. We present in this article the instrument concept, the manufacturing and measurement of the photonic crystals, methods for the filter array optimization, and discuss the predicted retrieval performance for the detection of methane and carbon dioxide.

In measuring cerebral blood flow (CBF) noninvasively using optical techniques, diffusing-wave spectroscopy is often combined with near-infrared spectroscopy to obtain a reliable blood flow index. Measuring the blood flow index at a determined depth remains the ultimate goal. In this study, we present a simple approach using dual-comb lasers where we simultaneously measure the absorption coefficient (μ_a), the reduced scattering coefficient (μ_s^′), and dynamic properties. This system can also effectively differentiate dynamics from various depths, which is crucial for analyzing multilayer dynamics. For CBF measurements, this capability is particularly valuable as it helps mitigate the influence of the scalp and skull, thereby enhancing the specificity of deep tissue.

The disease atherosclerosis causes stenosis inside the patient's arteries, which often eventually turns lethal. Our goal is to detect a stenosis in a non-invasive manner, preferably in an early stage. To that end, we study whether and how laser speckle contrast imaging (LSCI) can be deployed. We start out by using computational fluid dynamics on a patient-specific stenosed carotid artery to reveal the flow profile in the region surrounding the stenosis, which compares well with particle image velocimetry experiments. We then use our own fully interferometric dynamic light scattering routines to simulate the process of LSCI of the carotid artery. Our approach offers an advantage over the established Monte Carlo techniques because they cannot incorporate dynamics. From the simulated speckle images, we extract a speckle contrast time series at different sites inside the artery, of which we then compute the frequency spectrum. We observe an increase in speckle boiling in sites where the flow profile is more complex, e.g., containing regions of backflow. In the region surrounding the stenosis, the measured speckle contrast is considerably lower due to the higher local velocity, and the frequency signature becomes notably different with prominent higher-order frequency modes that were absent in the other sites. Although future work is still required to make our new approach more quantitative and more applicable in practice, we have provided a first insight into how a stenosis might be detected in vivo using LSCI.

We study how the speckle contrast depends on scatterer velocity, with the goal of further developing laser speckle imaging as a quantitative measurement technique. To that end, we perform interferometric computer simulations on a dilute plug flow. The results of our numerical experiment, that we compare with known analytical expressions to confirm their veracity, match well at low velocities with the Gaussian expression. Finally, we address the issue of how velocity depends on speckle decorrelation time, and show that the speckle size is most likely the relevant connecting length scale.

Distance measurement using frequency sweeping interferometry is an absolute distance measurement technique that allows for high accuracy over long distances. Notwithstanding, the measurement accuracy is affected by laser sweeping nonlinearity and limited sweeping range. In this work, an optimized post-processing linearization method is demonstrated to realize high-accuracy arbitrary distance measurement using a laser with small modulation range. The interference signal is sparsely resampled to eliminate the influence of the sweeping nonlinearity, and the absolute distance is obtained by analyzing the phase of the resampled signal. In the measurement system, a high-finesse Fabry-Pérot cavity placed in vacuum is used as the measurement reference, so the effect of dispersion mismatch is negligible. Moreover, the distance measurement result is determined by the linear fit of the phase of each resampled point. Therefore, the influence of target vibration and other external random noise can be partially eliminated, and the reliability of the result is high. In the experiment, the sweeping range of the laser source is only 88 GHz. Comparing with a fringe-counting interferometer, the standard deviation of the residual errors is 34 µm within a distance of 6.7 m.

The measurement of distance plays an integral part in many aspects of modern societies. In this paper an integrated mode-locked laser on a chip is used for distance measurement based on mode-resolved interferometry. The emission from the on-chip source with a repetition rate of 2.5 GHz and a spectral bandwidth of 3 nm is coupled into a Michelson interferometer. The interferometer output is recorded as a spectral interferogram, which is captured in a single camera image. The images are analyzed using Hilbert transform to extract the distance. The distance derived shows a deviation of 6 \mum from the reference, for a distance up to 25 mm. We also demonstrate interferometry with repetition frequency sweep which can also be used with the source. Performance is expected to be better in the near future with the rapid developments in the field of on-chip laser sources which are demonstrating larger spectral widths and coherence lengths.

Laser speckle imaging (LSI) can be used to study dynamic processes in turbid media, such as blood flow. However, it is presently still challenging to obtain meaningful quantitative information from speckle, mainly because speckle is the interferometric summation of multiply scattered light. Consequently, speckle represents a convolution of the local dynamics of the medium. In this paper, we present a computational model for simulating the LSI process, which we aim to use for improving our understanding of the underlying physics. Thereby reliable methods for extracting meaningful information from speckle can be developed. To validate our code, we apply it to a case study resembling blood flow: a cylindrical fluid flow geometry seeded with small spherical particles and modulated with a heartbeat signal. From the simulated speckle pattern, we successfully retrieve the main frequency modes of the original heartbeat signal. By comparing Poiseuille flow to plug flow, we show that speckle boiling causes a small amount of uniform spectral noise. Our results indicate that our computational model is capable of simulating LSI and will therefore be useful in future studies for further developing LSI as a quantitative imaging tool.

In this paper a generic monolithic photonic integration technology platform and tunable laser devices for gas sensing applications at 2 μm will be presented. The basic set of long wavelength optical functions which is fundamental for a generic photonic integration approach is realized using planar, but-joint, active-passive integration on indium phosphide substrate with active components based on strained InGaAs quantum wells. Using this limited set of basic building blocks a novel geometry, widely tunable laser source was designed and fabricated within the first long wavelength multiproject wafer run. The fabricated laser operates around 2027 nm, covers a record tuning range of 31 nm and is successfully employed in absorption measurements of carbon dioxide. These results demonstrate a fully functional long wavelength photonic integrated circuit that operates at these wavelengths. Moreover, the process steps and material system used for the long wavelength technology are almost identical to the ones which are used in the technology process at 1.5μm which makes it straightforward and hassle-free to transfer to the photonic foundries with existing fabrication lines. The changes from the 1550 nm technology and the trade-offs made in the building block design and layer stack will be discussed.

In this work we present a design of an external optical cavity based on Fabry-Perot etalons applied to a 100 MHz Er-doped fiber optical frequency comb working at 1560 nm to increase its repetition frequency. A Fabry-Perot cavity is constructed based on a transportable cage system with two silver mirrors in plano-concave geometry including the mode-matching lenses, fiber coupled collimation package and detection unit. The system enables full 3D angle mirror tilting and x-y off axis movement as well as distance between the mirrors. We demonstrate the increase of repetition frequency by direct measurement of the beat frequency and spectrally by using the virtually imaged phased array images.

The scattering of coherent light from a system with underlying flow can be used to yield essential information about dynamics of the process. In the case of pulsatile flow, there is a rapid change in the properties of the speckle images. This can be studied using the standard laser speckle contrast and also the fractality of images. In this paper, we report the results of experiments performed to study pulsatile flow with speckle images, under different experimental configurations to verify the robustness of the techniques for applications. In order to study flow under various levels of complexity, the measurements were done for three in-vitro phantoms and two in-vivo situations. The pumping mechanisms were varied ranging from mechanical pumps to the human heart for the in vivo case. The speckle images were analyzed using the techniques of fractal dimension and speckle contrast analysis. The results of these techniques for the various experimental scenarios were compared. The fractal dimension is a more sensitive measure to capture the complexity of the signal though it was observed that it is also extremely sensitive to the properties of the scattering medium and cannot recover the signal for thicker diffusers in comparison to speckle contrast.