Low Spatial Coherence Research Articles

Diffraction enhanced imaging utilizing a laser produced x-ray source.

Image formation by Fresnel diffraction utilizes both absorption and phase-contrast to measure electron density profiles. The low spatial and spectral coherence requirements allow the technique to be performed with a laser-produced x-ray source coupled with a narrow slit. This makes it an excellent candidate for probing interfaces between materials at extreme conditions, which can only be generated at large-scale laser or pulsed power facilities. Here, we present the results from a proof-of-principle experiment demonstrating an effective ∼2 μm laser-generated source at the OMEGA Laser Facility. This was achieved using slits of 1 ×30μm2 and 2 ×40μm2 geometry, which were milled into 30 μm thick Ta plates. Combining these slits with a vanadium He-like 5.2keV source created a 1D imaging system capable of micrometer-scale resolution. The principal obstacles to achieving an effective 1 μm source are the slit tilt and taper-where the use of a tapered slit is necessary to increase the alignment tolerance. We demonstrate an effective source size by imaging a 2 ± 0.2 μm radius tungsten wire.

Low-spatial coherence vortex beam generation by random distributed feedback fibre laser

Vortex beams with low spatial coherence have great potential in applications such as imaging, medical diagnosis, optical communication, and manipulation. In this study, we experimentally demonstrated a radially polarised vortex beam with extremely low spatial coherence using a random distributed feedback fibre laser (RDFB-FL). The results show that the spatial coherence of vortex beams can be reduced in an RDFB-FL and further reduced by introducing radial polarisation. In addition, the evolution of the spatial coherence of a RDFB-FL-generated vortex beam with varying pump power was clearly revealed for the first time. The spatial coherence first decreases sharply up to a point as the pump power increases and then remains constant, and this behaviour is quite different from that of vortex beams generated by conventional fibre lasers. The present study provides an effective method for achieving a low-spatial coherence vortex beam source for application in telecommunications and imaging systems.

Difference-Frequency Generation of Random Fiber Lasers for Broadly Tunable Mid-Infrared Continuous-Wave Random Lasing Generation

Random fiber lasers (RFLs) can generate flexible near-infrared lasing wavelengths with high power/high efficiency in simple configurations, which could be excellent candidates to realize low spatial and/or temporal coherence mid-infrared (MIR) random lasing via the difference-frequency generation (DFG) process. Here, we explore the feasibility, and demonstrate the proof of concept of MIR continuous-wave (CW) random lasing through DFG of RFLs in a new promising nonlinear optical crystal BaGa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> Se <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sub> for the first time, to the best of our knowledge. The watt-level ytterbium-doped RFL with the tuning range covering 1038 to 1091 nm and the cascaded random Raman fiber laser at 1213 nm or 1281 nm serve as pump and signal sources, respectively for DFG in a 15 mm-long BaGa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> Se <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sub> crystal. As a result, we experimentally demonstrate the MIR random lasing with the tuning wavelengths ranging from 5.5 μm to 9.5 μm. The output power of the MIR DFG at 5.6 μm is measured as 26.8 μW with a conversion efficiency of 8.4 μW/W <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . This work proves that RFLs could provide a new compact and cost-effective platform to realize broadly tunable modeless CW MIR lasers, which can potentially extend the applications of RFLs such as temporal ghost imaging and speckle-free imaging to MIR region.

Electrically driven random lasing from a modified Fabry–Pérot laser diode

Random lasers (RLs) are intriguing devices with promising applications as light sources for imaging, sensing, super resolution spectral analysis or complex networks engineering. RLs can be obtained from optically pumped dyes, optical fibers and crystals, or electrically pumped semiconductor heterostructures. Semicon-ductor RLs are usually fabricated by introducing scattering defects into the active layer, adding a degree of complexity to the fabrication process and losing the ease of realization potentially offered by disordered structures. Ready availability of electrically pumped RLs, avoiding costly fabrication approach, would boost the use of these devices in research and applications. Here, we realize an incoherent semiconductor RL by simply processing the output mirror of an off-the-shelf Fabry-Perot laser diode via controlled laser ablation. Optical feedback provided by the intact back mirror and the ablated front mirror results in multi-mode ran-dom lasing with low spatial coherence and speckled output emission profile.

Spatial coherence of electrically pumped random terahertz lasers

Light sources with high radiance and tailored coherence properties are highly desirable for imaging applications in the mid-infrared and terahertz (THz) spectral regions, which host a large variety of molecular absorptions and distinctive fingerprints to be exploited for sensing and tomography. Here, we characterize the spatial coherence of random multimode THz quantum cascade lasers (QCLs) emitting &gt; mW optical power per mode and showing low divergence (10°–30°), performing a modified Young’s double-slit experiment. Partial spatial coherence values ranging between 0.16 and 0.34 are retrieved, depending on the specific degree of disorder. These values are significantly lower than those (0.82) of conventional Fabry–Perot THz QCLs exploiting an identical active region quantum design. We then incorporate the devised low spatial coherence random lasers into a confocal imaging system with micrometer spatial resolution and demonstrate notable imaging performances, at THz frequencies, against spatial cross talk and speckles.

High‐Power Multimode Random Fiber Laser for Speckle‐Free Imaging

AbstractNew light sources with low spatial coherence and high brightness are crucial for high‐quality imaging. In this paper, such a light source is reported, which is formed by combining random lasing used as seed light with a multimode fiber (MMF) mediated master oscillator power amplifier (MOPA) for power scaling and spatial decoherence. Experimental results show that the proposed multimode random fiber laser (MM‐RFL), with low noise, high spectral density, and high power efficiency, can achieve a maximum output power of 100.6 W and speckle contrast of as low as 0.037. Furthermore, MM‐RFL‐based speckle‐free imaging is demonstrated. The imaging quality can be improved by increasing the output power of the MM‐RFL as its spatial coherence decreases with an increasing number of effective modes excited in the MMF. Such a MM‐RFL provides a powerful light source for speckle‐free imaging applications where high power and low spatial coherence are essential for high‐quality imaging.

Multispot optics for beam shaping of high-power single-mode and multimode lasers

The performance of various laser technologies, such as welding, laser powder bed fusion (LPBF), brazing, cladding, and sheet metal cutting, based on the use of high-power multimode fiber lasers, fiber-coupled solid-state, and diode lasers, can be improved using the patent pending beam-shaping optics providing optimal energy distributions by splitting the laser beam into several separate spots in the working plane and variable energy sharing between these spots. Various patterns, such as square, line, and rhombus, consisting of four or nine separate spots, are expected to eliminate or reduce spatter and to realize optimum temperature distributions in the melt pool and stabilizing the processes in the welding of tailored blanks, copper and aluminum parts in the production of batteries, zinc-coated steel, cladding, and LPBF. Because multimode lasers have a comparably low spatial coherence characterized by large beam parameter products or beam quality (M²) values, it is difficult to control the intensity distribution by methods other than imaging the fiber end with a collimator and a focusing objective. The proposed solution is a combination of fiber end imaging and geometrical separation of focused spots perpendicular to the optical axis using special optical components and creating a working spot as a combination of several spots. Varying the energy portions in separate spots and the distances between them make it possible to optimize common spot intensity distributions for particular applications. To ensure reliable operation with multi-kW lasers, the refractive optical components of the multispot devices are implemented from athermal optical materials characterized by insignificant thermal lensing and, hence, negligible thermal focus shift and spherical aberration. The article presents descriptions of multispot optics and examples of intensity profile measurements and application results, while the reduction in spattering was observed using multispot laser welding. It is concluded that the melt pool flows homogenize when applying several laser spots compared to a single spot. The possibility of tailoring melt pool dimensions in LPBF was shown.

BiconNet: An edge-preserved connectivity-based approach for salient object detection

Salient object detection (SOD) is viewed as a pixel-wise saliency modeling task by traditional deep learning-based methods. A limitation of current SOD models is insufficient utilization of inter-pixel information, which usually results in imperfect segmentation near edge regions and low spatial coherence. As we demonstrate, using a saliency mask as the only label is suboptimal. To address this limitation, we propose a connectivity-based approach called bilateral connectivity network (BiconNet), which uses connectivity masks together with saliency masks as labels for effective modeling of inter-pixel relationships and object saliency. Moreover, we propose a bilateral voting module to enhance the output connectivity map, and a novel edge feature enhancement method that efficiently utilizes edge-specific features. Through comprehensive experiments on five benchmark datasets, we demonstrate that our proposed method can be plugged into any existing state-of-the-art saliency-based SOD framework to improve its performance with negligible parameter increase.

Light Disorder as a Degree of Randomness to Improve the Performance of Random Lasers

The operation of random lasers (RLs) is possible due to multiple light scattering, which provides optical feedback inside the gain medium. No conventional optical cavity is required for the operation of RLs and, therefore, RLs provide the best evidence to defy the conventional view that disorder is an unwanted phenomenon when present in physical systems. Mirrorless lasers demonstrate that the degree of disorder, associated with the spatial distribution of scattering particles, is the main factor responsible for generating a light source with low spatial coherence; this is reason why RLs are so attractive for modern imaging systems. Here, we demonstrate how to optimize the performance of typical diffusive RLs by exploiting a different degree of randomness in the system, which is loaded onto an induced disorder in the transverse intensity and wave-vector distributions of the light pattern that pumps the random-lasing medium. Quantitative analyses show that the RL's intensity emission and threshold fluence can be optimized by more than 20 times, when excited by an optical field presenting a random spatial-intensity distribution, with high speckle contrast, becoming an efficient methodology to induce high photon degeneracy in RLs required for high sensitivity applications.

Spatiotemporal Coherence Weighting for In Vivo Cardiac Photoacoustic Image Beamformation.

Photoacoustic (PA) image reconstruction generally utilizes delay-and-sum (DAS) beamforming of received acoustic waves from tissue irradiated with optical illumination. However, nonadaptive DAS reconstructed cardiac PA images exhibit temporally varying noise which causes reduced myocardial PA signal specificity, making image interpretation difficult. Adaptive beamforming algorithms such as minimum variance (MV) with coherence factor (CF) weighting have been previously reported to improve the DAS image quality. In this article, we report on an adaptive beamforming algorithm by extending CF weighting to the temporal domain for preclinical cardiac PA imaging (PAI). The proposed spatiotemporal coherence factor (STCF) considers multiple temporally adjacent image acquisition events during beamforming and cancels out signals with low spatial coherence and temporal coherence, resulting in higher background noise cancellation while preserving the main features of interest (myocardial wall) in the resultant PA images. STCF has been validated using the numerical simulations and in vivo ECG and respiratory-signal-gated cardiac PAI in healthy murine hearts. The numerical simulation results demonstrate that STCF weighting outperforms DAS and MV beamforming with and without CF weighting under different levels of inherent contrast, acoustic attenuation, optical scattering, and signal-to-noise (SNR) of channel data. Performance improvement is attributed to higher sidelobe reduction (at least 5 dB) and SNR improvement (at least 10 dB). Improved myocardial signal specificity and higher signal rejection in the left ventricular chamber and acoustic gel region are observed with STCF in cardiac PAI.

Landscape resistance affects individual habitat selection but not genetic relatedness in a reintroduced desert ungulate

The long-term success of species reintroductions is strongly dependent on the availability of large areas of suitable habitat and the genetic make-up of the population. If available habitat is poorly connected this can hinder gene flow and lead to genetic fragmentation of the population, potentially increasing its extinction risk. We employed a conservation genomics approach in which we combined analyses of genetic structure with testing for potential landscape effects on habitat selection and gene flow in reintroduced Asiatic wild ass Equus hemionus ssp. in the Israeli Negev desert. Genetic structure and pairwise relatedness were first investigated followed by examination of landscape effects on individual habitat selection using records of GPS collared individuals. We then built habitat resistance surfaces and used electrical circuit theory to test for landscape effects on genetic relatedness. We detected weak genetic structuring, yet low spatial coherence among individuals from the same genetic cluster. Landscape variables had a significant impact on individual habitat selection, with wild ass avoiding steep slopes and habitats of low suitability as predicted by a species distribution model. However, the landscape genetic analysis revealed no effect of habitat resistance on genetic relatedness. These results suggest that gene flow in the reintroduced population is not impacted by landscape resistance. Indeed, the high mobility of the species may increase its resistance to the genetic effects of habitat fragmentation, at least over a small number of generations. We discuss other potential causes for the observed genetic structure including a behavioural effect. Our study highlights the importance of understanding species-habitat interactions for the long-term success of reintroductions.

Plasmonic random laser from dye-doped cholesteric liquid crystals incorporating silver nanoprisms.

Plasmonic random lasers have been demonstrated in combining dye-doped cholesteric liquid crystals (DD-CLCs) and silver nanoparticles (AgNPs). The DD-CLC laser reveals the lowest threshold and highest slope efficiency through the localized surface plasmon resonance of AgNPs with the best coupling of the emission spectrum of lasing dye and resonance of electron oscillation on the metal surface. Thermal control of the DD-CLC lasers has been achieved to simultaneously shift the long- and short-edge lasing peaks. By the α-stable analysis, the DD-CLC random laser (RL) reveals heavy tail distribution with relatively low α∼1.06 to show the Lévy behavior. Owing to its low spatial coherence, the DD-CLC RL has been demonstrated to produce a speckle-reduced image with a lower contrast of about 0.04.

Imaging through opacity using a near-infrared low-spatial-coherence fiber light source.

Memory-effect-based speckle correlation is one of the most practical techniques for imaging through scattering opaque media, where a light source with low spatial coherence and moderate bandwidth plays a pivotal role. Usually, a rapidly rotating diffuser is applied to make the light source spatially decoherent. Here, an all-fiber-based low-spatial-coherence light source is proposed and demonstrated for such speckle-correlated imaging. The illumination structure is greatly simplified, the lightening efficiency is enhanced, and the wavelength is extended to the near-infrared band, which is favorable for a larger memory effect range and deeper penetrating depth through opacity. Moreover, the proposed local illumination method can identify the orientation of the object, which has not been revealed by former methods. This work would facilitate the research in optical biomedical imaging and broaden the applications of multimode random fiber lasers.

Effects of source spatial partial coherence on intensity statistics of optical beams in mono-static turbulent channels.

We investigate, via both experimental measurements and wave-optics computer simulations, the statistical characteristics of the fluctuating intensity, such as the average intensity, the beam wander and the scintillation index, of the Gaussian Schell-model (GSM) beams on passing through double-pass, monostatic turbulence channels with either a retro-reflector (RR) or a flat mirror (FM). Our experimental results reveal that the enhanced backscatter (EBS) gradually weakens as the spatial coherence of the GSM source decreases, and eventually disappears for the sufficiently low source spatial coherence states. The r.m.s beam wander remains practically invariant with the variation of the source coherence width in the range from 0.2 to 6.0 mm both in the case of the RR and the FM, which formed the RR case being much smaller. In addition, it is found that the long-term scintillation index of the untracked beam with the RR is smaller than that with the FM, while the situation is reversed for the short-term scintillation index of the tracked beam. In both cases, the scintillation index decreases as the spatial coherence of the GSM source decreases. The obtained computer simulation results agree reasonably well with the experimental results. In addition, the effects of spatial coherence on statistical characteristics of the GSM beams along a 1 km propagation distance through the double-pass monostatic turbulence are also investigated using wave-optics simulation. We also carry out evaluation and comparison of the intensity probability density functions for the RR and FM cases and for various source coherence states that are of utmost importance for free-space optical communications in retro-reflection modulation regime. In addition, our findings will be beneficial for the development of remote sensing and directed energy laser applications in the presence of air turbulence.

Effect of Passivation Layer on the Thin Film Perovskite Random Lasers.

Novel functionalities of disorder-induced scattering effect in random lasers, attributed to low spatial coherence, draw remarkable attention in high-contrast to superior quality speckle-free imaging applications. This paper demonstrates perovskite-polystyrene (PS)-based random lasing action with robust optical performance at room temperature. Optical characterizations are carried out upon perovskite thin films addition with polystyrene of different mixing concentrations (wt.%). A low threshold lasing operation is achieved with an increasing concentration of polystyrene, accompanying a wavy surface texture with high surface roughness. The rough surface dominating multiple scattering effects leads to enhanced feedback efficiency. Moreover, this study also elucidates efficient fabrication process steps for the development of high quality and durable PS-based random lasers. With the advantages of reduced coherent artifacts and low spatial coherence, speckle free projection images of the USAF (U. S. Air Force MIL-STD-150A standard of 1951) resolution test chart are shown for different PS-based random lasers.

One-dimensional, surface emitting, disordered Terahertz lasers

Quantum cascade lasers are, by far, the most compact, powerful, and spectrally pure sources of radiation at terahertz frequencies, and, as such, they are of crucial importance for applications in metrology, spectroscopy, imaging, and astronomy, among many others. However, for many of those applications, particularly imaging, tomography, and near-field microscopy, undesired artifacts, resulting from the use of a coherent radiation source, can be detrimental. Random lasers can offer a concrete technological solution to the above issue. They, indeed, maintain a high degree of temporal coherence, as traditional lasers, while only exhibiting low spatial coherence, which can allow for the prevention of coherent artifacts, such as speckles. In this study, we report on the development of one-dimensional THz-frequency random wire lasers, patterned on the top surface of a double-metal quantum cascade laser with fully randomly arranged apertures, not arising from the perturbation of a regular photonic structure. By performing finite element method simulations, we engineer photonic patterns supporting strongly localized random modes in the 3.05–3.5 THz range. Multimode laser emission over a tunable-by-design band of about 400 GHz and with ∼2 mW of peak power has been achieved, associated with 10° divergent optical beam patterns. The achieved performances were then compared with those of perturbed Fabry–Perot disordered lasers, showing continuous-wave operation in the 3.5–3.8 THz range with an order of magnitude larger average power output than their random counterpart, and an irregular far field emission profile.

Investigation on Intracavity SHG With Controllable Coherence in a Degenerate Laser

We investigate intracavity second harmonic generation (SHG) in a degenerate laser for generating green laser with controllable degree of coherence. It is shown that spatial coherence of the SHG emission can be controlled by varying the diameter of the limiting aperture in the degenerate laser resonator. The pumping power also affects the number of the oscillating transverse modes, so that influences the spatial coherence of the output laser. The laser of low coherence for speckle reduction has been demonstrated. It is shown that the laser of low spatial coherence may find applications in imaging, and laser display etc.

Impact of coherent light on interaction of fungi and bacteria cells cultivated in vitro

This article considers impact of coherent red quasi-monochromatic light on interaction of colonies of the Pseudomonas syringae bacteria and the Fusarium macroceras fungus in an in vitro culture. A helium-neon laser and a heat source with a system of light filters and aperture diaphragms were used for irradiation. Two light fluxes were obtained with energy parameters close in magnitude, but significantly different in spatio-temporal coherence. Light with a high statistical ordering stimulated growth of both colonies. Irradiation from the same spectral range and intensity, but with low spatial coherence, increased the functional activity of only small bacteria cells. As a result, there was a suppression of larger fungal cells development that were interacting with them. Therefore, it was the statistical (coherent) properties of light that affected the change in the equilibrium of microorganisms in an artificial biocenosis. This approach can be used in practice for increasing the activity of bacteria antagonists of pathogenic fungi and the non-chemical disease protection of plants.

Control of LED Emission with Functional Dielectric Metasurfaces

AbstractThe improvement of light‐emitting diodes (LEDs) is one of the major goals of optoelectronics and photonics research. LED integration to complex photonic devices requires precise control of the wavefront of the emitted light. Metasurfaces are spatial arrangements of engineered scatters that may enable this light manipulation capability with unprecedented resolution. Most of these devices, however, are only able to function properly under irradiation of laser light with a large spatial coherence. LEDs, on the other hand, have angularly broad, Lambertian‐like emission patterns characterized by a low spatial coherence, which makes the integration of metasurface devices on LEDs challenging. A novel concept for metasurface integration on LED is proposed, using a cavity to increase the LED spatial coherence through an angular collimation. The experimental demonstration of the proposed concept is implemented on a GaP LED architecture including a hybrid metallic‐Bragg cavity. By integrating a silicon metasurface on top, two different functionalities of these compact devices are demonstrated: directional LED emission at a desired angle and LED emission of a vortex beam with an orbital angular momentum. The presented concept is general, being applicable to other incoherent light sources and enabling metasurfaces designed for plane waves to work with incoherent light emitters.

Surface profile measurement of doped silicon using near-infrared low-coherence light.

A scanning interferometry system based on near-infrared low-coherence interferometry by using a superluminescent diode (SLD) as the light source is presented. The system cannot only measure the surface profile of doped double-sided polished silicon but also measure its optical thickness and refractive index. The measurement system uses near-infrared CCD to detect interferometric light in the near infrared, based on the Michelson interference principle. SLD has low temporal and high spatial coherence and high penetration to doped silicon wafers; thus, higher visibility can be obtained when measuring the rear side. Meanwhile, the optical thickness measured by low coherent scanning interference is compared with the optical thickness obtained by spectrally resolved interferometry to verify the accuracy of the system. Due to the periodic characteristics of interference fringes, the coherence length of the narrowband light source is usually greater than the path length difference of the interferometer. It gives the measurement a phase ambiguity of 2π, which may severely limit the application of the measurement. However, the short coherent length SLD light source used in the project can avoid 2π phase ambiguity problems. Besides, this system can perform full scanning in a larger step and achieve rapid on-line measurement of the target surface.