Low Spatial Coherence Research Articles

Non-invasive imaging using a low-spatial-coherence multimode random polymer fiber laser.

Random lasers (RLs), with their low spatial coherence, are ideal illumination sources for speckle-free imaging. However, it is still challenging for RLs to maintain low spatial coherence with the need for integration and directionality. Here, a disordered multimode random polymer fiber laser (RPFL) is proposed and implemented as a low-spatial-coherence light source. Compared to typical multimode optical fibers, the number of accommodated modes is increased by about 11×, the speckle contrast is reduced to 0.013, and the spatial coherence factor is reduced to 0.08. The low-spatial-coherence property enables RPFL to produce significantly superior imaging quality in both speckle-free imaging and non-invasive imaging through opacity. This study provides a strategy for an integrated speckle-free imaging system and paves the way for non-invasive imaging.

Continuous-wave degenerate cavity laser for optical imaging in scattering media.

Lasers play a significant role in optical communication, medical, and scientific research, owing to their high brightness and high coherence. However, the high spatial coherence will lead to specific challenges, such as speckle noise in imaging and wavefront distortion during propagation through scattering media. Here, a continuous-wave (cw) degenerate cavity laser (DCL) with low spatial coherence is demonstrated with efficient suppression of the thermal lensing effect from the gain crystal. Experimentally, a cw degenerate laser output with about 2000 transverse modes corresponding to a speckle contrast of about 0.0224 is achieved. This laser can be used for speckle reduction and is robust against atmospheric turbulence, which may find applications in the field of laser imaging technology and illumination.

Imaging through scattering layers using a near-infrared low-spatial-coherence fiber random laser

Abstract Optical memory effect-based speckle-correlated technology has been developed for reconstructing hidden objects from disordered speckle patterns, achieving imaging through scattering layers. However, the lighting efficiency and field of view of existing speckle-correlated imaging systems are limited. Here, a near-infrared low spatial coherence fiber random laser illumination method is proposed to address the above limitations. Through the utilization of random Rayleigh scattering within dispersion-shifted fibers to provide feedback, coupled with stimulated Raman scattering for amplification, a near-infrared fiber random laser exhibiting a high spectral density and extremely low spatial coherence is generated. Based on the designed fiber random laser, speckle-correlated imaging through scattering layers is achieved, with high lighting efficiency and a large imaging field of view. This work improves the performance of speckle-correlated imaging and enriches the research on imaging through scattering medium.

Coherent random laser in Enteromorpha prolifera

This study presents a novel approach in coherent dye random laser (RL) generation by utilizing the special properties of Enteromorpha prolifera (EP) within a micro-glass tube configuration. The methodological simplicity of injecting a mixed solution of dye and EP into the tube at room temperature achieves a novel way of realizing the RL system, characterized by tunable random lasing thresholds contingent upon the concentration of EP. This tunability stems from the synergistic interplay between photon enhancement and fluorescent dye absorption facilitated by the unique nanoporous structure of EP, culminating in the attainment of a low lasing threshold of 21.7 μJ/mm2 at an optimized EP concentration of 2 mg/mL. With an excess EP concentration, the nanoporous structure could absorb large amounts of dye, resulting in dye fluorescence reabsorption, and causing the threshold to increase. A power Fourier transition was performed to calculate the effective optical cavity of the RL system, depicting the process of the coherent RL generation. Furthermore, a comprehensive investigation into the emission stability of the RL system revealed remarkable consistency in emission intensity over a prolonged excitation duration of 20 min, underscoring the potential for integration with optoelectronic devices. The L-shape mask imaging of the RL provides high-quality images with low spatial coherence, paving a useful application for speckle-free imaging. These results offer an affordable approach to achieving high-quality and low-threshold RLs using novel biomaterials.

Generation of 351 nm UV Q-switched laser beam with controllable spatial coherence

Ultraviolet laser with controllable spatial coherence is essential for a wide range of applications. In this study, we developed a pulsed, partially spatial coherence laser source operating in the ultraviolet (UV) range through extra-cavity frequency tripling of a solid-state Nd:glass degenerate laser. The oscillating modes of the laser can be flexibly controlled by size of the spatial filter inside the laser cavity, thus modulating the spatial coherence of the UV laser. The beam pattern and focusing, speckle suppression and imaging, coherence property of the partially coherent UV laser is investigated in detail. The third-harmonic generation (THG) supports many more transverse modes than the fundamental generation and the second-harmonic generation (SHG). Compared with high coherent UV laser, low spatial coherent UV laser has better speckle suppression effect and smoother focused spot intensity distribution. The high brightness, good monochromaticity and low spatial coherence make the UV degenerate laser an attractive illumination source for inertial confinement fusion, lithography, laser processing and others.

Manipulation of random laser by the bandgap in three-dimensional SiO2 photonic crystals

Photonic crystals (PCs) are artificial structures with different dielectric constants that are arranged periodically in space and can control the propagation of electromagnetic waves of specific frequencies. The photonic bandgap can tune the photon state density, which is beneficial for the generation of random lasers (RLs). In this paper, a thin film of PMMA-doped DCJTB is spun-coated on a silicon dioxide (SiO2)PC to produce an RL. The PC is fabricated by a convenient dip-coating method with variable PBGs by changing the diameter of SiO2 nanospheres. By changing the position of the PBG gap, the RL emission peak can be effectively manipulated with an emission wavelength of up to 16 nm. By adding Au nanoparticles, the lasing performance is improved due to the surface plasmon resonance effect and strong photon scattering, where the emission intensity is increased by 191.9 %, and the threshold is decreased by 37.8 %. The imaging results show that the RL has low spatial coherence, which has a good application in speckle-free imaging. This study provides an effective method to produce a low threshold, high-quality RL with convenient manipulation of lasing properties, which opens the door for expanding its application in optoelectronic devices and speckle-free imaging.

A micro random laser of dye solution-filled tube system based on electrospun fibers

Random lasers (RLs) have good application prospects in speckle-free imaging, micro sensing and micro-nano photonic devices due to their properties such as cavityless, miniaturization and low-spatial coherence. The minimization, emission mechanism and lasing property control of RLs still need to be explored. In this paper, we present a novel micro RL system in a dye solution-filled tube, where the coherent random lasing action is facilitated by the integration of electrospun fibers and gold nanoparticles (Au NPs). The electrospun fibers serve as the scattering medium, while the Au NPs produce the local plasmon resonance effect. The interaction effect improves the photon scattering rate, makes the photons form a ring resonator in the system, and realizes the coherent RL in the tube system. The microtube RL has a very small size, a convenient preparation method and stable emission properties. It can withstand more than 2000 times pumping while maintaining a stable emission intensity. The electrospun fibers are stable and uniform in morphology, which makes it easy to achieve mass production of microtube RL. The speckle-free imaging experiment shows the RL has low spatial coherence, which indicates that it’s a good choice for micro speckle-free imaging. This work provides a convenient way to realize micro coherent RLs by electrospun fibers and could pave a way for applications in photonic devices.

Random lasing in dye-doped electrospun PMMA fibers with different emission modes

Random lasers (RLs) have significant applications in micro-nano light emitters, micro-detection, and speckle-free imaging due to their novel properties in low spatial coherence, small size and multi-emission directions. A flexible, affordable approach for creating new scattering materials has recently been suggested for electrospun fibers, which are practical materials for RLs. In this article, we employ the electrospun PMMA fibers as scatters and the DCJTB as gain materials to generate an RL system. The sample shows obvious incoherent random lasing properties. A decrease of the lasing threshold by 32 % is realized when we increase the electrospun voltage from 12 kV to 15 kV due to the generation of a small mean diameter size and concentrated diameter distribution. The lasing threshold of the DCJTB/Au NPs-doped PMMA sample is further reduced to 19.8 μJ/mm2 by incorporating Au NPs into the electrospun dye-doped fibers. This leads to a lasing threshold reduction of 40 % from the electrospun sample without doping Au NPs and 52 % from the spin-coated film sample, respectively. It indicates that the electrospun method is an efficient and practical approach to make RLs. Furthermore, using single pulse laser stimulation, we investigated several different lasing modes, including spontaneous emission, short wavelength domain, mode competition, and single mode. The actions of photon scattering are analyzed to explain the dramatic lasing process. Due to the photon scattering optical path, there are mode competition and single mode emission regions under different pump energy. The light spot quality of RL shows low spatial coherence with the useful application of speckle-free imaging. This research provides a new method of electrospun to produce low-threshold and high-quality RLs and potentially opens the door for expanding their use in optics and optoelectronic equipment.

Robust speckle-free imaging using random lasers enhanced by TiN nanoparticles in complex scattering environments

Abstract A light source with narrowband, sufficient brightness, and low spatial coherence is required for certain applications such as optical imaging and free-space optical communication. In this study, our focus was to investigate a novel imaging laser source, specifically a low-threshold random laser enhanced by TiN nanoparticles. The results demonstrate that the random laser spectrum exhibits an impressive bandwidth of 0.23 nm, accompanied by an incredibly low spatial coherence factor of merely 0.15. Due to the low spatial coherence of random laser, the speck contrast is less than 0.02 when the light passes through a scattering system. Notably, when compared to traditional lasers, the use of a random laser yields significantly superior imaging quality in both scatterless and complex scattering environments. This finding highlights the immense potential of the random laser as a narrowband and low spatial coherence laser source for robust speckle-free imaging applications, particularly in environments with intricate scattering phenomena. Furthermore, this breakthrough can be extended to various other domains, including free-space optical communication.

Synchronized Structure and Teleconnection Patterns of Meteorological Drought Events over the Yangtze River Basin, China

Investigating the synchronized structure and teleconnection patterns of meteorological drought events (MDEs) contributes to elucidating drought’s evolution. In this study, the CN05.1 gridded meteorological dataset from 1961 to 2021 was utilized to calculate the 3-month standardized precipitation evapotranspiration index (SPEI-3) for each grid in the Yangtze River Basin (YRB). Based on these SPEI-3 series, the grid-based MDEs were then extracted. Subsequently, event synchronization and complex networks were employed to construct the MDE synchronized network over the YRB. This network was used to identify the MDEs’ topological structure, synchronized subregions, and representative grids. Finally, the MDE characteristics and MDE teleconnection patterns of individual subregions were investigated. The results of the MDE topological structure show that the northeastern portion of the YRB tends to experience widespread MDEs, while specific areas in the upper reaches are prone to localized MDEs. Synchronous MDEs mainly propagate along the central pathway and the eastern pathway, which display relatively low MDE spatial coherence. The YRB is partitioned into eight MDE synchronized subregions, each exhibiting distinct characteristics in terms of the frequency, duration, total severity, and peak of MDEs, as well as MDE temporal frequency distributions. Among all teleconnection factors, El Niño–Southern Oscillation (ENSO) exerts a strong influence on MDEs in all subregions, the Pacific Decadal Oscillation (PDO) shows a significant association with MDEs in all subregions except for Subregion 3 in the southeast, the North Atlantic Oscillation (NAO) displays a significant influence on MDEs in the southern subregions of the YRB, and the Arctic Oscillation (AO) has a more pronounced influence on MDEs in the northern subregions. This study provides valuable insights on drought’s evolution within the YRB and offers guidance to policymakers for advanced preventive measures.

Lensless microscopy by multiplane recordings: sub-micrometer, diffraction-limited, wide field-of-view imaging.

Lensless microscopy is attractive because lenses are often large, heavy and expensive. We report diffraction-limited, sub-micrometer resolution in a lensless imaging system that does not need a reference wave and imposes few restrictions on the density of the sample. We use measurements of the intensity of light scattered by the sample at multiple heights above the sample and a modified Gerchberg-Saxton algorithm to reconstruct the phase of the optical field. We introduce a pixel-splitting algorithm that increases resolution beyond the size of the sensor pixels, and implement high-dynamic-range measurements. The resolution depends on the numerical aperture of the first measurement height only, while the field of view is limited by the last measurement height only. As a result, resolution and field of view can be controlled independently. The pixel-splitting algorithm also allows imaging with light of low spatial coherence, and we show that such low coherence is beneficial for a larger field of view. Using illumination from three LEDs, we produce full-color images of biological samples. Finally, we provide a detailed analysis of the limiting factors of this lensless microscopy system. The good performance demonstrated here can allow lensless systems to replace conventional microscope objectives in some situations.

Propagation Characteristics of a Partially Coherent Gaussian Schell‐model Array Vortex Beam in the Joint Turbulence Effect of a Jet Engine and Atmosphere

AbstractThis work investigates the joint effects of jet engine exhaust‐induced turbulence and atmospheric turbulence on the propagation of a partially coherent Gaussian Schell‐model Array (GSMA) vortex beam. Using the two‐process propagation method, analytical formulae are derived for the cross‐spectral density, spectral density, degree of coherence, and beam width of the considered beam. The results show that the considered beam takes different shapes; when the spatial coherence is large, the spectral density of the GSMA vortex beam takes an elliptical shape, whereas when the spatial coherence is smaller, the spectral density remains a Gaussian shape. The evolution profile of the degree of coherence weakens gradually when the propagation distance, topological charge, and turbulence strength increase. Moreover, the profile of the degree of coherence takes the Gaussian profile when the propagation distance is longer or turbulence atmospheric is stronger. Furthermore, the results reveal that the corresponding beam spreads faster with a larger propagation distance, lower spatial coherence, and high‐strength turbulence. This study also concludes from the results that the beam is affected more when its propagation is near the jet engine exhaust, which means that this latter has a significant impact.

Coherent Backscattering Peak for Radiation with Low Spatial Coherence

Coherent Backscattering Peak for Radiation with Low Spatial Coherence

Abnormally autofocusing vortex Swallowtail Gaussian vector beam with low spatial coherence

The precondition for the application of light beams is the ability to devise light distribution with high precision. Controlling more dimensions for structured light fields is an effective method to improve the ability to devise light distribution. The Swallowtail beam, due to its rich regulatory parameters, provides the possibility to design a light field with a specific intensity distribution. Utilizing the Swallowtail beam as a foundation, we design its initial phase, polarization, and coherent structure, and propose a partially coherent azimuthally polarized circular vortex Swallowtail Gaussian beam (PCAPCVSGB) in our paper. This beam exhibits an abnormal self-focusing ability and forms an easily adjustable optical potential well at the focal plane, providing another effective tool for achieving optical manipulation. In addition, the PCAPCVSGB also shows an interesting vector property. It possesses a stable polarization singularity even with changes in coherence and topological charges, which exhibits a potential application value in optical communication.

Polarization Entanglement from Parametric Down-conversion with an LED Pump

Spontaneous parametric down-conversion (SPDC) is a reliable platform for entanglement generation. Routinely, a coherent laser beam is an essential prerequisite for pumping the nonlinear crystal. Here we break this barrier to generate polarization-entangled photon pairs by using a commercial light-emitting diode (LED) source to serve as the pump beam. This effect is counterintuitive, as the LED source is of extremely low spatial coherence, which is transferred during the down-conversion process to the biphoton wave function. However, the type-II phase matching condition naturally filters the specific frequency and wavelength of LED light exclusively to participate in SPDC such that localized polarization Bell states can be generated, regardless of the global incoherence over the full transverse plane. In our experiment, we characterize the degree of LED light-induced polarization entanglement in the standard framework of the violation of Bell's inequality. We achieve the Bell value $S=2.33\ifmmode\pm\else\textpm\fi{}0.097$, obviously surpassing the classical bound $S=2$ and thus witnessing the quantum entanglement. Our work can be extended to prepare polarization entanglement by using other natural light sources, such as sunlight and biolight, which holds promise for electricity-free quantum communications in outer space.

P-Band UAV-SAR 4D Imaging: A Multi-Master Differential SAR Tomography Approach

Due to its rapid deployment, high-flexibility, and high-accuracy advantages, the unmanned-aerial-vehicle (UAV)-based differential synthetic aperture radar (SAR) tomography (D-TomoSAR) technique presents an attractive approach for urban risk monitoring. With its sufficiently long spatial and temporal baselines, it offers elevation and velocity resolution beyond the dimensions of range and azimuth, enabling four-dimensional (4D) SAR imaging. In the case of P-band UAV-SAR, a long spatial-temporal baseline is necessary to achieve high enough elevation-velocity dimensional resolution. Although P-band UAV-SAR maintains temporal coherence, it still faces two issues due to the extended spatial baseline, i.e., low spatial coherence and high sidelobes. To tackle these problems, we introduce a multi-master (MM) D-TomoSAR approach, contributing three main points. Firstly, the traditional D-TomoSAR signal model is extended to a MM one, which improves the average coherence coefficient and the number of baselines (NOB) as well as suppresses sidelobes. Secondly, a baseline distribution optimization processing is proposed to equalize the spatial–temporal baseline distribution, achieve more uniform spectrum samplings, and reduce sidelobes. Thirdly, a clustering-based outlier elimination method is employed to ensure 4D imaging quality. The proposed method is effectively validated through computer simulation and P-band UAV-SAR experiment.

Angular Spectra Tunable Random Lasing for High Contrast Imaging

AbstractRandom lasers with low spatial coherence have important potential applications in speckle‐free imaging and displaying. Here, an angular spectra (AS) tunable random laser is proposed through directly coupling a microcavity with a commercial optical fiber. The local in‐plane AS distributions of the random lasing can be separately tuned by selectively using coupling filtering, free transmission, and intensity modulation. The obtained random lasing has the characteristics of non‐polarization and ring AS with distribution tunability. The unique properties endow the lasing as a promising illumination source for biological imaging. An excellent image with speckle‐free and high contrast is thus achieved with a maximal contrast improvement factor of 638% relative to that under the common white lighting. The results indicate that the designed AS‐tunable random laser has great application values in the fields of biological imaging, display, integration optics, and optical physics.

A Deep‐Learning Approach for Low‐Spatial‐Coherence Imaging in Computer‐Generated Holography

The low‐spatial‐coherence imaging capability of computer‐generated holography (CGH) is a key to high‐resolution displays, virtual reality, augmented reality, and holographic microscopy. The low spatial coherence caused by complex disturbances can damage the image quality irreversibly. The optical field with low spatial coherence has large fluctuations, making it difficult to be quantified and modeled directly. To tackle these challenges, a deep neural network‐based model U‐residual dense network (U‐RDN) is proposed, which obtains the optimal solution under the low‐spatial‐coherence condition. The large‐scale images are generated using optical experiments, with analysis and restoration by deep learning. Extensive experiments demonstrate the strong out‐of‐distribution robustness of U‐RDN, which is generalizable to unseen classes in unseen domains. The learning‐based approach and low‐spatial‐coherence dataset open a new path toward the next generation of CGH.

Controllable Self‐Focusing Circular Vortex Pearcey Gaussian Beam with Low Spatial Coherence

AbstractA new type of controllable self‐focusing circular vortex Pearcey Gaussian beam, which can create an optical potential well with adjustable depth and width at the center of the beam by adjusting the coherent width and the number of topological charges, is introduced. As the coherent width increases, the intensity becomes lower in the dark notch caused by the topological charges, which are consistent with the experimental results. In addition, the self‐focusing location and the strength of the circular vortex Pearcey Gaussian beam can be controlled by the spatial coherence in the simulation.

Single-shot off-axis full-field optical coherence tomography

Full field optical coherence tomography (FF-OCT) enables high-resolution in-depth imaging within turbid media. In this work, we present a simple approach which combines FF-OCT with off-axis interferometry for reconstruction of en-face images. With low spatial and temporal coherence illumination, this method is able to extract an FF-OCT image from only one interference acquisition. This method is described, and the proof-of-concept is demonstrated through the observation of scattering samples such as organic and ex vivo biomedical samples.