Beam Illumination Research Articles

Spatially modulated scene illumination for intensity-compensated two-dimensional array photon-counting LiDAR imaging

Photon-counting LiDAR using a two-dimensional (2D) array detector has the advantages of high lateral resolution and fast acquisition speed. The non-uniform intensity profile of the illumination beam and non-uniform quantum efficiency of the detectors in the 2D array deteriorate the imaging quality. Herein, we propose a photon-counting LiDAR system that uses a spatial light modulator to control the spatial intensity to compensate for both the non-uniform intensity profile of the illumination beam, and the variation in the quantum efficiency of the detectors in the 2D array. By using a 635 nm peak wavelength and 4 mW average power semiconductor laser, lab-based experiments at a 4.27 m stand-off distance are performed to verify the effectiveness of the proposed method. Compared with the unmodulated method, the standard deviation of the intensity image of the proposed method is reduced from 0.109 to 0.089 for a whiteboard target, with an average signal photon number of 0.006 per pixel.

Digital holographic camera with extended stochastic illumination for non-destructive inspection of silicon optics

In the present work, we propose a compact digital holographic camera (DHC) with extended stochastic illumination for full-field non-destructive inspection of silicon optics fabricated in a computerized numerical control machine. The developed technique overcomes the limitation of digital holography imparted by the finite size of the active area of the recording sensor that can image a specular surface. The original aspect of this research work is to develop a system that enables reconstruction and testing of a specular surface. For this a dual diffuser configuration is incorporated in a compact DHC developed for non-destructive testing (NDT) applications. The generation of a stochastic illumination beam using the diffusers is explained by simulating the propagation of a light beam through a random phase function of the scattering medium. The stochastic optical field produced by the combination of diffusers in the DHC makes the camera suitable for NDT of the specular surface of silicon optics. The effects of the number of diffusers and their relative positions on the imaging area of the specular object are studied for development of an optimized configuration of DHC. Applicability of the proposed scheme is demonstrated through detection of defects in silicon optics using digital holographic interferometry.

Elongated Focus Optoacoustic Microscopy with Matched Bessel Beam Illumination and Ultrabroadband Axicon Detection

Current optoacoustic (OA) microscopy configurations often have narrow focal ranges that limit their use for fast volumetric imaging applications. Herein, the focal range of OA microscopes is extended by matching the elongated optical illumination profile of a Bessel beam with the pencil beam acoustic sensitivity profile of a broadband axicon detector. An inverted OA microscope is developed with interchangeable optical illumination and acoustic detection units to assess the working distance and resolutions retained with several combinations of illumination and detection profiles. Matching Bessel illumination with axicon detection extends the depth of focus 17‐fold over traditional configurations. Imaging a tilted mouse ear with the matched Bessel−axicon configuration reveals vasculature over a working distance exceeding 4.2 mm with optical resolution, while affording a sixfold increase in imaging volume over the same scanning duration compared with configurations using standard Gaussian illumination, demonstrating this approach's promise for increasing applications for OA microscopy in preclinical research.

Illumination angle correction during image acquisition in light-sheet fluorescence microscopy using deep learning.

Light-sheet fluorescence microscopy (LSFM) is a high-speed imaging technique that provides optical sectioning with reduced photodamage. LSFM is routinely used in life sciences for live cell imaging and for capturing large volumes of cleared tissues. LSFM has a unique configuration, in which the illumination and detection paths are separated and perpendicular to each other. As such, the image quality, especially at high resolution, largely depends on the degree of overlap between the detection focal plane and the illuminating beam. However, spatial heterogeneity within the sample, curved specimen boundaries, and mismatch of refractive index between tissues and immersion media can refract the well-aligned illumination beam. This refraction can cause extensive blur and non-uniform image quality over the imaged field-of-view. To address these issues, we tested a deep learning-based approach to estimate the angular error of the illumination beam relative to the detection focal plane. The illumination beam was then corrected using a pair of galvo scanners, and the correction significantly improved the image quality across the entire field-of-view. The angular estimation was based on calculating the defocus level on a pixel level within the image using two defocused images. Overall, our study provides a framework that can correct the angle of the light-sheet and improve the overall image quality in high-resolution LSFM 3D image acquisition.

Lateral Resolution of a Commercial Optical Coherence Tomography Instrument.

PurposeThe lateral resolution of an optical coherence tomography (OCT) instrument was considered to be equal to the illumination spot size on the retina. To evaluate the potential lateral resolution of the Spectralis OCT, an instrument calculated to have a 14 µm resolution.MethodsThe lateral point spread function (PSF) was evaluated using diamond abrasive powder 0 to 1 µm in diameter in silicone elastomer and a validated target with 800 nm FeO particles in urethane. The amplitude transfer function was calculated from human OCT images. Finally, resolution was measured using the 1951 USAF target.ResultsMeasurement of the lateral PSF from 1215 diamond particle images yielded a full-width half maximum (FWHM) to be 5.11 µm and for 732 FeO particles, 4.9 µm. From the amplitude transfer function, the FWHM of the diffraction limited PSF was calculated to be 5.0 µm. The USAF target imaging showed a lateral resolution of 4.6 µm.ConclusionsAlthough a calculation of the spot size of the illumination beam was reported in the past as the lateral resolution of the OCT instrument, the actual lateral resolution is better by a factor of at least 2.5 times. The clinically used A-scan spacing was derived from the calculated, and not the true resolution, and results in under sampling. This set of findings likely apply to all commercial clinical instruments.Translational RelevanceThe scan density parameters of past and present commercial OCT instruments were based on earlier translational concepts, which now appear to have been incorrect.

Optical encryption using phase modulation generated by thermal lens effect

We present the implementation of an experimental optical encryption configuration in the Fresnel domain using the thermal lens (TL) effect to modulate the phase of the illumination beam. We corroborate the basic performance of our cryptosystem by performing the encryption-decryption procedure of a single object. Then, we test the tolerance of the proposed scheme to decryption with keys registered for different modulations of the illumination beam induced by the phase generated due to TL effect. These different phase modulations are obtained by introducing axial displacements of a sample, where the TL effect is induced by an excitation beam (EB). Considering the phase modulation changes in the illumination due to sample displacements, we propose an undercover procedure to further protect the target information using decoy data. The key associated with the target information is obtained when the probe beam (PB) and EB are focused on the same point of the sample, while the decoy object key is registered when the beams are not focused on the same region of the sample. Finally, we analyzed the resistance of the undercover ciphertext to some of the attacks reported in the literature. All experimental results presented in this work demonstrate the basic capabilities, viability, and versatility of our proposals.

Light Sheet Microscopy Using Line Scan Confocal with Bidirectional Bessel Beam Illumination

Light Sheet Microscopy Using Line Scan Confocal with Bidirectional Bessel Beam Illumination

Adjustable Resolution for Localized FSS-Based Sensing by Synthetic Beamforming

Recently, frequency-selective surface (FSS)-based sensors have shown potential for structural health monitoring due to their sensitivity to changes in element geometry, interelement spacing, substrate properties, and local environment. In addition, these sensors are remotely interrogated and are planar in design, thereby providing a wireless sensing solution that can cover large areas. Traditionally, FSS sensors are analyzed assuming a uniform (plane wave) illumination. However, practically speaking, the sensor will be illuminated with a nonuniform excitation. In this way, the resolution of the sensor is limited to the illumination pattern (footprint) on the sensor. As such, an adjustable illumination pattern is advantageous as it relates to the ability to interrogate the sensor on a localized or comprehensive basis. To this end, this article considers a synthetic beamforming approach to adjust the beamwidth of the focused illumination beam on the sensor as a solution for localized sensing applications. This approach is proposed to generate an arbitrary beam shape with a desired footprint. Moreover, the illumination and spillover efficiencies of the synthetic beam (SB) are defined, simulated, and discussed. The results show that a minimum focal area of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1\lambda _{0} \times 1\lambda _{0}$ </tex-math></inline-formula> ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3\times3$ </tex-math></inline-formula> unit cells for the FSS sensors considered here) is essential to achieve a spillover efficiency of greater than 80%, thereby reducing the contribution of other neighboring unit cells on the sensor response. In addition, the illumination efficiency when an SB is utilized for illumination is greater than 80% due to the uniformity of the SB. Representative measurements were also performed on a sensor with a simulated strain profile.

Patterned photocrosslinking to establish stiffness anisotropies in fibrous 3D hydrogels

Cells are known to constantly interact with their local extracellular matrix (ECM) and respond to a variety of biochemical and mechanical cues received from the ECM. Nonetheless, comprehensive understanding of cell-ECM interactions has been elusive. Many studies rely on analysis of cell behavior on 2D substrates, which do not reflect a natural cell environment. Further, lack of dynamic control over local stiffness anisotropies and fiber alignment hinders progress in studies in naturally derived fibrous 3D cultures. Here, we present a cell-safe method of patterned photocrosslinking, which can aid in studying biological hypotheses related to mechanotransduction in 3D hydrogels. As previously described by our group, ruthenium-catalyzed photocrosslinking (RCP) of selected ECM regions promotes localized increase in stiffness mediated by focused blue laser light in a confocal microscope. In this study, we further demonstrate that RCP can induce localized strain stiffening and fiber alignment outside of the selected crosslinked region and induce stiffness anisotropy biased towards the direction of fiber alignment. MDA-MB-231 cells are shown to respond to RCP-induced changes in local ECM architecture and display directional bias towards the direction of fiber alignment, as compared to control cells. Further, the effect of patterned crosslinking on a stiffness landscape is measured using multi-axes optical tweezers active microrheology (AMR) with backscattered laser beam illumination. AMR validates RCP as a suitable tool for creating distinct stiffness anisotropies which promote directed migration of cells, further underscoring the usefulness of RCP in cell-ECM studies. Statement of significanceStudies on cell-ECM interactions in 3D cultures have often been hindered by the lack of available tools to dynamically alter local ECM stiffness and fiber alignment. Here, we present a non-invasive, cell-safe and easily applicable method of patterned photocrosslinking, which can aid in studying biological hypotheses in fibrous 3D hydrogels. Ruthenium-catalyzed crosslinking (RCP) of selected fibrin ECM regions promotes localized increase in stiffness and creates distinct stiffness anisotropies in the presence of the focused blue laser light. Outside of the crosslinked region, RCP causes fiber alignment and strain stiffening in the ECM, verified using multi-axes optical tweezers active microrheology (AMR). Following RCP, human breast cancer MDA-MB-231 exhibit directed cell migration, validating usefulness of this method in cell-ECM studies.

Investigation of Dual Color Angular Beam Scanning (DCABS) PIV system

In a scanning-based PIV system, Continuous Wave (CW) laser beam(s) sweep through, and illuminate the region of interest. Use of CW lasers instead of high energy pulsed lasers (as used in conventional PIV) makes the system cheaper and robust. Here, we present mathematical model and analysis of a Dual Color Angular Beam Scanning (DCABS) PIV system. We consider two CW laser beams (red and green) angularly scanning through the test section. The color image of tracer particles is processed to get the velocity field. The model considers spatial variation of ∆t (i.e. the time delay between two laser beam illuminations), unlike most of the previous works. We test the model on different flow cases and compare the results with conventional PIV.

Superresolution Linear Optical Imaging in the Far Field.

The resolution of optical imaging devices is ultimately limited by the diffraction of light. To circumvent this limit, modern superresolution microscopy techniques employ active interaction with the object by exploiting its optical nonlinearities, nonclassical properties of the illumination beam, or near field probing. Thus, they are not applicable whenever such interaction is not possible, for example, in astronomy or noninvasive biological imaging. Far field, linear optical superresolution techniques based on passive analysis of light coming from the object would cover these gaps. In this Letter, we present the first proof-of-principle demonstration of such a technique for 2D imaging. It works by accessing information about spatial correlations of the image optical field and, hence, about the object itself via measuring projections onto Hermite-Gaussian transverse spatial modes. With a basis of 21 spatial modes in both transverse dimensions, we perform two-dimensional imaging with twofold resolution enhancement beyond the diffraction limit.

Sharper photonic nanojets generated by microspheres under higher-order radially polarized beam illumination.

Photonic nanojets (PNJs) generated from a single microsphere illuminated by higher-order radially polarized (RP) beams are investigated. The effects of the size parameters of higher-order RP beams, the refractive index, and radius of the dielectric microsphere on the full width at half-maximum and peak intensity of the PNJ are numerically discussed and qualitatively interpreted. The results show that the minimal width of the PNJ can be obtained by optimally adjusting the size parameter. The PNJ beam waist becomes gradually narrower with increasing the radial mode number. As compared to the case of plane wave illumination, sharper PNJs are more easily generated when irradiated by a higher-order RP beam, even for microspheres with lower refractive indices or larger radii. Our findings can promote potential applications of PNJs in a variety of fields including super-resolution microscopy, nanolithography, and optical data storage.

Gain-switched short pulse generation from InAs-InP (1 1 3)B quantum dot laser excited state

The gain switching characteristics and the effect of laser parameters on the output pulses of an InAs-InP (113)B quantum dot laser are investigated for the first time using theoretical models to obtain shorter pulses by applying an external optical beam to the laser excited state. The model is based on the rate equations, which are solved by the Runge –Kutta method. The results showed that without an optical beam, long pulse widths are obtained because of the ground state transition, and the change in the parameters strongly affects the laser output. However, with an optical beam illumination, short pulses with a width of 25–40 ps with a high peak power are generated owing to the excited state transition at low currents, and the change in the laser parameters does not affect the output pulses strongly. These results are important for long-distance optical transmissions as well as medical biotechnology.

Shaping the illumination beams for STED imaging through highly scattering media

Stimulated emission depletion (STED) microscopy has been widely applied in single-cell and single-molecular studies benefiting from its ability of super-resolution imaging. However, it is challenging to perform STED imaging in highly scattering media, such as biological tissues, due to high scattering, which destroys the structured illumination beams. Here, we present a technique to produce the desired illumination beams beyond high scattering for STED microscopy. A dual-wavelength transmission matrix method is employed for shaping the excitation beam and the STED beam simultaneously, achieving the temporal and spatial overlap of the two beams behind a scattering medium. Experimentally, the overlapped foci were constructed by using a digital micromirror device (DMD). The extinction ratio of the STED focus reaches −23.8 dB, which compares favorably to the oft-quoted desirable value for STED applications. In particular, raster scanning of overlapped foci and the switching of the STED focus were demonstrated by adopting the fast switching ability of the DMD. Construction of the illumination beams beyond scattering will benefit STED microscopy in deep tissues.

Speckle in polarization structured light

We show that the far-field speckle patterns observed on the transmission of polarization structured light beams through a random diffuser screen have distinct spatial textures, which significantly differ from the texture of the fully developed scalar speckle. In particular, we report on speckle observed using the V-point and C-point polarization singular beams for illumination in this work. The V-point and C-point polarization singularities are generated by a combination of different orbital angular momentum (OAM) states in orthogonal polarizations. When passed through a random diffuser screen, the on-axis far-field diffraction intensity is an incoherent summation of speckle intensities due to the individual OAM states. Unlike the typical grainy structure in the scalar speckle, the speckle patterns observed with V-point and C-point illuminating beams are seen to have a noodle-like and sponge-like textures respectively. The speckle from V-point and C-point illuminations is difficult to distinguish using the usual intensity statistics which does not account for the distinct spatial structure of the speckle patterns. The Gray Level Co-occurrence Matrix (GLCM) methodology was therefore used to extract a number of spatial textural features from the speckle patterns observed using scalar and polarization structured illuminations. The principal component analysis (PCA) technique when applied to the texture parameters shows that it is possible to identify the nature of the illuminating beam by simply observing the texture properties of the resultant speckle pattern. When coherent light beams pass through a random medium resulting in a fully developed speckle pattern, the information regarding the profile of the input beam is generally considered to be lost. Our new observations on distinct textural features in speckle from structured light therefore suggest the possibility of information transmission through random media by means of switching the polarization structure of the incident beam.

Data inversion algorithms for droplet characterization based on simulated rainbows

In characterizing droplets, the rainbow reveals information about the size of individual drops, their refractive index, temperature, and in some instances composition. Previous experience in exploiting rainbow scattering for this purpose has led to classical rainbow refractometry, but also to the global rainbow technique. To improve on these existing techniques, a highly focused Gaussian illumination beam offers some advantages for achieving higher spatial resolution in dense spray situations. This is explored in the present study, using simulations of rainbow scattering from a spherical droplet in the primary and secondary rainbow regions. These simulations are performed using the generalized Lorenz-Mie theory (GLMT), allowing for a focused Gaussian beam illumination. For droplet characterization, three inversion algorithms are investigated, designated as the peak-trough method, inflection-inflection method and trough-trough method. The effect of the incident position of the beam on the droplet characterization is considered, as are effects of white noise superimposed on the rainbow. For a spherical water droplet in the size range 50–200 μm, an accuracy of the measured relative refractive index up to 4 decimal places and a relative error in size smaller than 5% can be achieved in most cases.

Enhanced Sub‐Terahertz Microscopy based on Broadband Airy Beam

AbstractNon‐diffraction electromagnetic beams such as Airy beams attract wide exploration in various disciplines owing to its fascinating features and great potential of applications. Although generation and applications of Airy beams are actively studied in the optical regime, applications of Airy beams in the THz or sub‐THz domain are rarely explored. This work proposes a new microscopic technique leveraging a sub‐THz broadband non‐diffraction Airy beam as the illumination beam to improve the image quality, which is named as enhanced sub‐THz microscopy (ESTM). The effectiveness of the ESTM is validated by a metallic sample, which shows that applying broadband measured information can render a better image quality than only using single‐frequency information. By blocking the sample using a scattering obstacle, it is demonstrated that the self‐reconstruction property of the Airy beam can efficiently suppress the distortion induced by the obstacle and good images can still be obtained. In addition, it is shown that the image quality applying a broadband Airy beam is superior to that of non‐diffraction quasi‐Bessel beams and other conventionally used illumination beams for microscopic technique.

Optical method for simultaneous thickness measurements of two layers with a significant thickness difference.

In this study, an optical method that allows simultaneous thickness measurements of two different layers distributed over a broad thickness range from several tens of nanometers to a few millimeters based on the integration of a spectroscopic reflectometer and a spectral-domain interferometer is proposed. Regarding the optical configuration of the integrated system, various factors, such as the operating spectral band, the measurement beam paths, and the illumination beam type, were considered to match the measurement positions and effectively separate two measurement signals acquired using both measurement techniques. Furthermore, for the thickness measurement algorithm, a model-based analysis method for high-precision substrate thickness measurements in thin-film specimens was designed to minimize the measurement error caused by thin films, and it was confirmed that the error is decreased significantly to less than 8 nm as compared to that when using a Fourier-transform analysis. The ability to undertake simultaneous thickness measurements of both layers using the proposed system was successfully verified on a specimen consisting of silicon dioxide thin film with nominal thicknesses of 100 nm and 150 nm and a 450 µm-thick silicon substrate, resulting in the exact separation between the two layers. From measurement uncertainty evaluation of a thin-film, a substrate in a thin-film specimen, and a single substrate, the uncertainties were estimated to be 0.12 nm for the thin-film, 0.094 µm for the substrate in a thin-film specimen, and 0.076 µm for the substrate. The measurement performance of thicknesses distributed on multi-scale was verified through comparative measurements using standard measurement equipment for several reference samples.

Multiview tiling light sheet microscopy for 3D high-resolution live imaging.

Light-sheet or selective plane illumination microscopy (SPIM) is ideally suited for in toto imaging of living specimens at high temporal-spatial resolution. In SPIM, the light scattering that occurs during imaging of opaque specimens brings about limitations in terms of resolution and the imaging field of view. To ameliorate this shortcoming, the illumination beam can be engineered into a highly confined light sheet over a large field of view and multi-view imaging can be performed by applying multiple lenses combined with mechanical rotation of the sample. Here, we present a Multiview tiling SPIM (MT-SPIM) that combines the Multi-view SPIM (M-SPIM) with a confined, multi-tiled light sheet. The MT-SPIM provides high-resolution, robust and rotation-free imaging of living specimens. We applied the MT-SPIM to image nuclei and Myosin II from the cellular to subcellular spatial scale in early Drosophila embryogenesis. We show that the MT-SPIM improves the axial-resolution relative to the conventional M-SPIM by a factor of two. We further demonstrate that this axial resolution enhancement improves the automated segmentation of Myosin II distribution and of nuclear volumes and shapes.

Temporal and spatial domain detection model and method of infrared photoelectric detection target.

To improve the detection ability of photoelectric detection targets under low-illumination conditions, based on the traditional infrared photoelectric detection targets, in this paper, we propose a design method for an infrared detection screen by using a double high-power line laser interactive layout auxiliary mode, and we form an active infrared photoelectric detection target. We then establish the calculations of the laser beam's illumination and the emission power of the pulse laser as a projectile passes through the infrared detection screen. According to the relation of the thickness of the infrared detection screen, the time period over which the projectile passes through the infrared detection screen, and the projectile's spatial position, we use the grid division method to establish the temporal- and spatial-domain echo power model and the output signal voltage function of the active infrared photoelectric detection target. Through calculations and experimental analysis, it is found that the echo power is reduced as the detection distance is increased, and the intensity of the echo power is not linear with the thickness of the detection screen or the length of the projectile in the temporal and spatial domains. The detection performance of the proposed active infrared photoelectric detection target is better than the traditional infrared photoelectric detection target, and it can be improved to a certain extent by increasing the laser emission power appropriately.