The 50 most-cited articles in Applied Optics, 2023

We present chemical kinetics and environmental monitoring applications in the long-wavelength mid-infrared (LW-MIR) region using a new diagnostic that exploits a widely tunable light source emitting in the LW-MIR. The custom-designed laser source is based on a difference-frequency generation (DFG) process in a nonlinear orientation-patterned GaAs crystal. The pump laser, an external-cavity quantum cascade laser, is tuned in a continuous-wave (cw) mode, while the signal laser, a C O 2 gas laser, is operated in a pulsed mode with a kilohertz repetition rate. The idler wavelength can be tuned between 11.58 ( 863.56 c m − 1 ) and 15.00 µm ( 666.67 c m − 1 ) in a quasi-cw manner. We discuss the unique prospective applications offered by probing the LW-MIR region for chemical kinetics and environment-monitoring applications. We showcase the potential of the DFG laser source by some representative applications.

) laser diagnostics for hypersonic flows is reviewed. Owing to the ultrahigh flow speed, a laser frequency of 100 kHz or higher is required for hypersonic diagnostics. Here, two main laser diagnostic techniques are discussed: focused laser differential interferometry (FLDI) and pulse-burst laser-based diagnostics. Single- and multiple-point FLDI measurements have been widely applied to hypersonic flows for flow velocity and density fluctuation measurements. The progress of pulse-burst laser-based hypersonic diagnostics, including flow velocity measurements and 2D flow visualization, is also discussed.

Free-space optics (FSO) and visible light communication (VLC) are interesting solutions for last mile access due to their benefits over conventional wired and wireless communication systems, including easy installation, low latency, high bandwidth, and low cost. This study aims to present last mile access using hybrid FSO/VLC, addressing the shortcomings of each technology. A hybrid system might, for instance, use VLC to give last meter connectivity to specific buildings and FSO to provide high-bandwidth outdoor connectivity across vast distances. In this paper, three proposed scenarios are designed using Optisystem and Matlab software, employing pulse amplitude modulation with four levels (PAM-4). The first scenario involves designing the last mile of the FSO-VLC system. In contrast, the second uses wavelength division multiplexing (WDM) technology to increase the data transmission capacity by enabling multiple optical signals at various wavelengths. The last scenario entails using VLC systems and the non-orthogonal multiple access (NOMA) approach to accommodate several users…

We present current results of a novel, to the best of our knowledge, type of setup for quantum ghost imaging based on asynchronous single photon timing using single photon avalanche diode (SPAD) detectors, first presented in [Appl. Opt.60, F66 (2021)APOPAI0003-693510.1364/AO.423634]. The scheme enables photon pairing without fixed delays and, thus, overcomes some limitations of the widely used heralded setups for quantum ghost imaging [Nat. Commun.6, 5913 (2015)NCAOBW2041-172310.1038/ncomms6913]. It especially allows three-dimensional (3D) imaging by direct time of flight methods, the first demonstration of which will be shown here. To our knowledge, it is also the first demonstration of 3D quantum ghost imaging at all.

In recent years, with the development of nano-photonics, Fano resonance has gained increasing attention. Due to its high sensitivity, real-time detection, and label-free properties, the Fano resonance sensor has been widely applied in the fields of biochemistry and environmental detection. To improve the sensing characteristics of Fano resonance, an Au-TiO 2 -Ag grating structure is proposed in this paper, and the sensing performance is enhanced by a bi-metallic grating and deposited TiO 2 . The characteristics of both sensing and field distribution of the model are accordingly analyzed using the finite-difference time-domain method. By varying the structural parameters such as grating period, grating height, silver film thickness, and TiO 2 layer thickness, the tuning of sensing characteristics can be realized, and afterwards, the sensing performance is improved; consequently, the Fano resonance reflection spectrum with high sensitivity and a high figure of merit (FOM) value is obtained. When the grating period P = 200 nm, grating height T1 = 90 nm, silver film thickness T2 = 20…

This paper elaborates on the design and simulation of a multifunctional optical sensor that features simultaneous detection of pressure and temperature, which is based on the metal-insulator-metal waveguide structure with two T-shaped resonant cavities. Depending on the simulation findings, pressure and temperature can be measured separately by two T-shaped cavities at different Fano resonance wavelengths. As the pressure applied to the upper T-shaped cavity increases, the resonance wavelength first shifts linearly due to the slight deformation of the cavity, and the maximum pressure sensitivity reaches 12.48 nm/MPa. After the pressure exceeds a threshold, the relationship between pressure and resonance wavelength transforms into a quadratic polynomial. In the lower T-shaped cavity, solid polydimethylsiloxane is sealed as a thermal-sensitive material, effectively preventing material overflow brought on by structural micro-vibration under pressure, and its high thermo-optical coefficient prompts a temperature sensitivity of 0.36 nm/°C. Furthermore, by optimizing the choice of Fano…

The detection of oil in water is of great importance for maintaining subsurface infrastructures such as oil pipelines. As a potential technology for oceanic application, an oceanic lidar has proved its advantages for remote sensing of optical properties and subsea materials. However, current oceanic lidar systems are highly power-consuming and bulky, making them difficult to deploy underwater to monitor oil in water. To address this issue, we have developed a compact single-photon Raman lidar by using a single-photon detector with high quantum efficiency and low dark noise. Due to the single-photon sensitivity, the detection of the relatively weak Raman backscattered signal from underwater oil was realized with a laser with a pulse energy of 1 µJ and a telescope with a diameter of 22.4 mm. An experimental demonstration was conducted to obtain the distance-resolved Raman backscatter of underwater oil of different thicknesses up to a distance of 12 m. The results indicate the single-photon Raman lidar's potential for inspecting underwater oil pipelines.

Baseline correction is necessary for the qualitative and quantitative analysis of samples because of the existence of background fluorescence interference in Raman spectra. The asymmetric least squares (ALS) method is an adaptive and automated algorithm that avoids peak detection operations along with other user interactions. However, current ALS-based improved algorithms only consider the smoothness configuration of regions where the signals are greater than the fitted baseline, which results in smoothing distortion. In this paper, an asymmetrically reweighted penalized least squares method based on spectral estimation (SEALS) is proposed. SEALS considers not only the uniform distribution of additive noise along the baseline but also the energy distribution of the signal above and below the fitted baseline. The energy distribution is estimated using inverse Fourier and autoregressive models to create a spectral estimation kernel. This kernel effectively optimizes and balances the asymmetric weight assigned to each data point. By doing so, it resolves the issue of local…

Analytical spectroscopy methods have shown many possible uses for nuclear material diagnostics and measurements in recent studies. In particular, the application potential for various atomic spectroscopy techniques is uniquely diverse and generates interest across a wide range of nuclear science areas. Over the last decade, techniques such as laser-induced breakdown spectroscopy, Raman spectroscopy, and x-ray fluorescence spectroscopy have yielded considerable improvements in the diagnostic analysis of nuclear materials, especially with machine learning implementations. These techniques have been applied for analytical solutions to problems concerning nuclear forensics, nuclear fuel manufacturing, nuclear fuel quality control, and general diagnostic analysis of nuclear materials. The data yielded from atomic spectroscopy methods provide innovative solutions to problems surrounding the characterization of nuclear materials, particularly for compounds with complex chemistry. Implementing these optical spectroscopy techniques can provide comprehensive new insights into the chemical…

Distributed acoustic sensors (DAS) are effective apparatuses that are widely used in many application areas for recording signals of various events with very high spatial resolution along optical fibers. To properly detect and recognize the recorded events, advanced signal processing algorithms with high computational demands are crucial. Convolutional neural networks (CNNs) are highly capable tools to extract spatial information and are suitable for event recognition applications in DAS. Long short-term memory (LSTM) is an effective instrument to process sequential data. In this study, a two-stage feature extraction methodology that combines the capabilities of these neural network architectures with transfer learning is proposed to classify vibrations applied to an optical fiber by a piezoelectric transducer. First, the differential amplitude and phase information is extracted from the phase-sensitive optical time domain reflectometer (Φ-OTDR) recordings and stored in a spatiotemporal data matrix. Then, a state-of-the-art pre-trained CNN without dense layers is used as a feature…

Microplastic (MP) pollution is seriously threatening the environmental health of the world, which has accelerated the development of new identification and characterization methods. Digital holography (DH) is one of the emerging tools to detect MPs in a high-throughput flow. Here, we review advances in MP screening by DH. We examine the problem from both the hardware and software viewpoints. Automatic analysis based on smart DH processing is reported by highlighting the role played by artificial intelligence for classification and regression tasks. In this framework, the continuous development and availability in recent years of field-portable holographic flow cytometers for water monitoring also is discussed.

We have demonstrated a stable and low-noise all-polarization-maintaining (PM) ultrafast erbium-doped fiber laser mode-locked via nonlinear polarization evolution (NPE) in a linear cavity with dual outputs. A detailed design strategy is presented. The all-PM configuration enhances the capability of resistance to environmental fluctuations. Self-starting mode-locking is realized by using a non-reciprocal phase shifter. The dual-output structure offers the intracavity filtering effect, where the reflective port serves as a bandpass spectral filter, significantly improving the transmissive-port optical properties. The laser directly generates ultrashort pulses with a pulse duration of 129 fs operating at a fundamental repetition rate of 105.8 MHz. The integrated root-mean-square (RMS) relative intensity noise from 10 Hz to 10 MHz is ∼0.008%, and the integrated RMS timing jitter from 5 kHz to 10 MHz is ∼36fs. Long-term stability is confirmed in 25 h with a RMS power fluctuation of ∼0.10%. Our high-performance fiber laser is a prospective candidate for low-noise applications.

In this paper, an approach for detecting branch points using a Shack–Hartmann wavefront sensor (SHWFS) is introduced. Simulated data are created using Monte Carlo wave-optics simulations of varying turbulence strengths. It is assumed that the presence of a branch point in the SHWFS subaperture lenslet pupils causes appreciable beam spreading in the image plane. Therefore, second-moment statistics are used to quantify beam spread for each subaperture image-plane irradiance pattern. Thresholding is then employed to dictate what degree of beam spreading is sufficient to determine the presence of a branch point. Three different thresholds are imposed: liberal, moderate, and conservative. Furthermore, the collected SHWFS signal is treated as analog, digitized, and digitized with three levels of additive noise: low, moderate, and high. Monte Carlo simulations are conducted for 20 different spherical-wave Rytov numbers (RSW) ranging from 0.1 to 2.0. It was found that when conservative thresholds were employed, for the analog signal, digitized signal with no noise, and digitized signal with…

We report an experimental proof of concept for speckle-based one-to-three non-line-of-sight (NLOS) free space optical (FSO) communication channels employing structured light shift-keying. A 3-bit gray image of resolution 100×100 pixels is encoded in Laguerre–Gaussian or Hermite–Gaussian beams and decoded using their respective intensity speckle patterns via trained 1D convolutional neural network. We have achieved an average classification accuracy of 96% and 93% using LG ml and HG pq beams, respectively, among all three channels. It demonstrates the directional independence and broadcasting capability of speckle-based decoding (SBD) in FSO communication using structured light. Further, we have extended the study from 2D to 1D SBD in one-to-three NLOS FSO communication channels to decrease the computational cost and to emphasize the importance of the 1D SBD approach.

This paper proposes a technique combining an adaptive fringe projection algorithm and a curve fitting algorithm for the three-dimensional (3D) reconstruction of highly reflective objects. First, an adaptive projection algorithm is proposed to avoid image saturation. The phase information obtained by projecting vertical and horizontal fringes is used to establish the pixel coordinate mapping between the camera image and the projected image, and the highlight area in the camera image is found and linearly interpolated. By modifying the mapping coordinates of the highlight area, the optimal light intensity coefficient template of the projection image is calculated, the coefficient template is mapped to the projector image, and it is multiplied by the standard projection fringes to obtain the adaptive projection fringes we need. Secondly, after obtaining the absolute phase map, the phase at the hole is calculated by fitting the correct phase values at both ends of the data hole, and the phase value closest to the actual surface of the object is obtained by performing a fitting in the…

We propose a machine-learning-based method for grating waveguides and augmented reality, significantly reducing the computation time compared with existing finite-element-based numerical simulation methods. Among the slanted, coated, interlayer, twin-pillar, U-shaped, and hybrid structure gratings, we exploit structural parameters such as grating slanted angle, grating depth, duty cycle, coating ratio, and interlayer thickness to construct the gratings. The multi-layer perceptron algorithm based on the Keras framework was used with a dataset comprised of 3000-14,000 samples. The training accuracy reached a coefficient of determination of more than 99.9% and an average absolute percentage error of 0.5%-2%. At the same time, the hybrid structure grating we built achieved a diffraction efficiency of 94.21% and a uniformity of 93.99%. This hybrid structure grating also achieved the best results in tolerance analysis. The high-efficiency artificial intelligence waveguide method proposed in this paper realizes the optimal design of a high-efficiency grating waveguide structure. It can…

To evaluate corn quality quickly, the feasibility of near-infrared spectroscopy (NIRS) coupled with chemometrics was analyzed to detect the moisture, oil, protein, and starch content in corn. A backward interval partial least squares (BiPLS)-principal component analysis (PCA)-extreme learning machine (ELM) quantitative analysis model was constructed based on BiPLS in conjunction with PCA and the ELM. The selection of characteristic spectral intervals was accomplished by BiPLS. The best principal components were determined by the prediction residual error sum of squares of Monte Carlo cross validation. In addition, a genetic simulated annealing algorithm was utilized to optimize the parameters of the ELM regression model. The established regression models for moisture, oil, protein, and starch can meet the demand for corn component detection with the prediction determination coefficients of 0.996, 0.990, 0.974, and 0.976; the prediction root means square errors of 0.018, 0.016, 0.067, and 0.109; and the residual prediction deviations of 15.704, 9.741, 6.330, and 6.236, respectively.…

In this study, we have undertaken a comprehensive numerical investigation of a refractive index sensor designed around a metal-insulator-metal (MIM) plasmonic waveguide. Our approach utilizes the finite element method to thoroughly analyze the sensor's performance. The sensor's configuration utilizes a ring resonator design, which has been slightly modified at the coupling segment. This modification enhances the efficiency of light coupling between a bus waveguide and the ring resonator, particularly at the resonance wavelength. This strategic adjustment significantly improves the device's extinction ratio, a critical factor in its functionality. Remarkably, the sensitivity of this sensor is determined to be approximately 1155.71 nm/RIU, while it possesses a figure of merit of 25.9. Furthermore, our study delves into the intricate mechanism governing the injection of light into the nanoscale MIM waveguide. We achieve this through the incorporation of silicon-tapered waveguides, which play a pivotal role in facilitating the transformation of a dielectric mode into a plasmonic mode,…

The optical-fiber-based dosimeter of the LUMINA project was deployed in August 2021 in the International Space Station in the framework of the Alpha mission. The sensing elements of the dosimeter are P-doped optical fibers, which were proven to be excellent candidates for dosimetry applications. The twofold objective of this paper is to provide a theoretical model for the radiation response of the dosimeter and to report on the experimental work carried out at CERN for the qualification and calibration of the engineering model of the LUMINA dosimeter. Combining the theoretical response and experimental data, the calibration curve of the flight model is obtained. Finally, this study broadens the investigation of the room temperature radiation response of P-doped optical fibers in a range of dose rates 10 4 times lower than previously reported, from 21µGy(SiO 2 )/h to 145mGy(SiO 2 )/h.

An optical vortex beam carrying fractional topological charge (TC) has become an immerging field of interest due to its unique intensity distribution and fractional phase front in a transverse plane. Potential applications include micro-particle manipulation, optical communication, quantum information processing, optical encryption, and optical imaging. In these applications, it is necessary to know the correct information of the orbital angular momentum, which is related to the fractional TC of the beam. Therefore, the accurate measurement of fractional TC is an important issue. In this study, we demonstrate a simple technique to measure the fractional TC of an optical vortex with a resolution of 0.05 using a spiral interferometer and fork-shaped interference patterns. We further show that the proposed technique provides satisfactory results in cases of low to moderate atmospheric turbulences, which has relevance in free-space optical communications.

The Laser Communications Relay Demonstration is NASA's multi-year demonstration of laser communication from the Earth to a geosynchronous satellite. The mission currently has two optical ground stations (OGSs), with one in California (OGS1) and one in Hawaii (OGS2). Each ground terminal optical system consists of a high-order adaptive optics (AO) system, a laser transmit system, and a camera for target acquisition. The OGS1 AO system is responsible for compensating for the downlink beam for atmospheric turbulence and coupling it into the modem's single mode fiber. The mission requires a coupling efficiency of 50%, which necessitates a high-order AO system. To achieve this performance, the AO system uses two deformable mirrors with one mirror correcting for low-spatial-frequency aberrations with large amplitude and a second deformable mirror correcting for high-spatial-frequency aberrations with small amplitude. Turbulence is sensed with a Shack-Hartmann wavefront sensor. To meet its performance requirements in the most stressing conditions, the system can operate at frame rates of…

As a significant and cost-effective method of obtaining shallow seabed topography, satellite derived bathymetry (SDB) can acquire a wide range of shallow sea depth by integrating a small quantity of in-situ water depth data. This method is a beneficial addition to traditional bathymetric topography. The seafloor's spatial heterogeneity leads to inaccuracies in bathymetric inversion, which reduces bathymetric accuracy. By utilizing multispectral data with multidimensional features, an SDB approach incorporating spectral and spatial information of multispectral images is proposed in this study. In order to effectively increase the accuracy of bathymetry inversion throughout the entire area, first the random forest with spatial coordinates is established to control bathymetry spatial variation on a large scale. Next, the Kriging algorithm is used to interpolate bathymetry residuals, and the interpolation results are used to adjust bathymetry spatial variation on a small scale. The data from three shallow water sites are experimentally processed to validate the method. Compared with…

A laser absorption spectroscopy diagnostic integrated within a hardened optical probe was used to measure temperature and water mole fraction at 500 kHz in post-detonation fireballs of explosives. In the experiments, an exploding-bridgewire detonator initiated a 25 g hemisphere of explosive (N5 or PETN). This produced a hemispherical fireball that traveled radially towards a hardened measurement probe. The probe contained a pressure transducer and optical equipment to pitch fiber-coupled laser light across a 12.6 cm gap onto a detector. Tunable diode lasers emitting near 7185.6 and 6806 c m − 1 were used to measure the absorbance spectrum of H 2 O utilizing peak-picking scanned-wavelength-modulation spectroscopy with a scan frequency of 500 kHz and modulation frequencies of 35 and 45.5 MHz, respectively. This enabled measurements of temperature and X H 2 O in the shock-heated air and trailing fireball at 500 kHz. Time histories of pressure, temperature, and H 2 O mole fraction were acquired at different standoff distances to quantify how the fireball evolved in space and time as…

The main objective of the present research is to develop an optical fiber relative humidity (RH) sensor with a high sensitivity, linear response over a wide dynamic range, and optimum response/recovery times by employing the simplest optical fiber sensing configuration. An optical fiber RH sensor that exploits the intensity modulation scheme through evanescent wave absorption spectroscopy and fulfills the objective is reported here. The fiber sensor employs graphene oxide (GO) diffused silica nanostructured thin sensing film as the cladding on the centrally decladded straight and uniform multimode optical fiber. Detailed experimental investigations are carried out to analyze the response characteristics of the proposed sensor. In comparison to other GO based optical fiber RH sensors, a linear response over the widest dynamic range (15.0%-95.3%RH) and shortest (fastest) response/recovery times (0.1436 s/0.1547 s) are observed for the proposed sensor. The linear sensitivity for the developed sensor is observed to be 0.1036 dB/%RH. In addition, the proposed sensor exhibits a very good…

The absorption, scattering, and turbulence effects have a significant impact on the performance of underwater wireless optical communication (UWOC). Therefore, it is crucial to consider seawater's optical parameters comprehensively when designing UWOC systems. So far, most studies on the UWOC channel have separately modeled the absorption and scattering, and turbulence of seawater, and furthermore, the continuous phase perturbations caused by turbulence are neglected to simplify the model when modeling turbulence channels. Hence, this paper simultaneously considers the absorption, scattering, and turbulence effects of seawater and proposes a UWOC channel modeling method that combines Monte Carlo simulation with multiple phase screen approaches. Subsequently, the impacts of different systems and channel conditions on system performance are explored, and simulation results indicate that as the turbidities and turbulence intensities of the seawater increase, the probability density function of received light signal intensity becomes more dispersed. The turbulence introduces an increase…

A method of optical fiber composite overhead ground wire (OPGW) positioning based on a Brillouin distributed optical fiber sensor and machine learning is proposed. A distributed Brillouin optical time-domain reflectometry (BOTDR) and Brillouin optical time-domain analyzer (BOTDA) are designed, where the ranges of BOTDR and the BOTDA are 110 km and 125 km, respectively. An unsupervised machine learning method density-based spatial clustering of applications with noise (DBSCAN) is proposed to automatically identify the splicing point based on the Brillouin frequency shift (BFS) difference of adjacent sections. An adaptive parameter selection method based on k-distance is adapted to overcome the parameter sensitivity. The validity of the proposed DBSCAN algorithm is greater than 96%, which is evaluated by three commonly external validation indices with five typical BFS curves. According to the clustering results of different fiber cores and the tower schedule of the OPGW, the connecting towers are distinguished, which is proved as a 100% recognition rate. According to the…

The quick and accurate retrieval of an object's depth from a single-shot fringe pattern in fringe projection profilometry has been a topic of ongoing research. In recent years, with the development of deep learning, a deep learning technique to FPP for single-shot 3D measurement is being used. To improve the accuracy of depth estimation from a single-shot fringe pattern, we propose the depthwise separable Dilation Inceptionv2-UNet (DD-Inceptionv2-UNet) by adjusting the depth and width of the network model simultaneously. And we evaluate the model on both simulated and experimental datasets. The experimental results show that the error between the depth map predicted by the proposed method and the label is smaller, and the depth curve map is closer to the ground truth. And on the simulated dataset, the MAE of the proposed method decreased by 35.22%, compared to UNet. On the experimental dataset, the MAE of the proposed method decreased by 34.62%, compared to UNet. The proposed method is relatively outstanding in both quantitative and qualitative evaluations, effectively improving the…

In this paper, a polarization-insensitive sensor based on graphene electromagnetically induced transparency (EIT) is proposed. The device consists of two graphene orthogonal T-shaped structures. This T-shaped resonator produces transparent windows that largely overlap under x and y polarizations, and the results demonstrate its good polarization insensitivity. The device can accomplish detection performance with sensitivity higher than 4960 nm/RIU and figure of merit (FOM) greater than 11.4. Meanwhile, when the Fermi energy level of graphene changes from 0.5 to 0.8 eV, it enables arbitrary modulation of the operating frequency over a wide frequency range of about 4.5 terahertz in the mid-infrared band. Our work has the potential to significantly advance the area of biological molecular detection.

Quantum ghost imaging (QGI) is an intriguing imaging protocol that exploits photon-pair correlations stemming from spontaneous parametric down-conversion (SPDC). QGI retrieves images from two-path joint measurements, where single-path detection does not allow us to reconstruct the target image. Here we report on a QGI implementation exploiting a two-dimensional (2D) single-photon avalanche diode (SPAD) array detector for the spatially resolving path. Moreover, the employment of non-degenerate SPDC allows us to investigate samples at infrared wavelengths without the need for short-wave infrared (SWIR) cameras, while the spatial detection can be still performed in the visible region, where the more advanced silicon-based technology can be exploited. Our findings advance QGI schemes towards practical applications.

In this work, we investigate an approach to scale up the output pulse energy in an all-polarization-maintaining 17.3 MHz Yb-doped fiber oscillator via implementation of a 25 µm core-diameter large-mode-area fiber. The artificial saturable absorber is based on a Kerr-type linear self-stabilized fiber interferometer, enabling non-linear polarization rotation in polarization-maintaining fibers. Highly stable mode-locked steady states in the soliton-like operation regime are demonstrated with 170 mW average output power and a total output pulse energy of ∼10nJ distributed between two output ports. An experimental parameter comparison with a reference oscillator constructed with 5.5 µm core-sized standard fiber components reveals an increase of pulse energy by a factor of 36 with simultaneously reduced intensity noise in the high-frequency range >100kHz.

Metasurfaces are a new class of diffractive optical elements with subwavelength elements whose behavior can be lithographically tailored. By leveraging form birefringence, metasurfaces can serve as multifunctional freespace polarization optics. Metasurface gratings are novel, to the best of our knowledge, polarimetric components that integrate multiple polarization analyzers into a single optical element enabling the realization of compact imaging polarimeters. The promise of metasurfaces as a new polarization building block is contingent on the calibration of metagrating-based optical systems. A prototype metasurface full Stokes imaging polarimeter is compared to a benchtop reference instrument using an established linear Stokes test for 670, 532, and 460 nm gratings. We propose a complementary full Stokes accuracy test and demonstrate it using the 532 nm grating. This work presents methods and practical considerations involved in producing accurate polarization data from a metasurface-based Stokes imaging polarimeter and informs their use in polarimetric systems more generally.

A broad range of imaging and sensing technologies in the infrared require large field-of-view (FoV) operation. To achieve this, traditional refractive systems often employ multiple elements to compensate for aberrations, which leads to excess size, weight, and cost. For many applications, including night vision eye-wear, air-borne surveillance, and autonomous navigation for unmanned aerial vehicles, size and weight are highly constrained. Sub-wavelength diffractive optics, also known as meta-optics, can dramatically reduce the size, weight, and cost of these imaging systems, as meta-optics are significantly thinner and lighter than traditional refractive lenses. Here, we demonstrate 80° FoV thermal imaging in the long-wavelength infrared regime (8-12 µm) using an all-silicon meta-optic with an entrance aperture and lens focal length of 1 cm.

We design a four-band narrow-band near-perfect absorber based on bulk Dirac semimetal (BDS) metamaterial in the terahertz region. The absorber has a top-to-bottom three-layer structure of a BDS layer, an insulating dielectric slab, and a gold layer. The BDS is flexible and tunable, allowing the Fermi energy level to be adjusted by changing the applied bias voltage, thus changing the absorption characteristics of the absorber. We use the time-domain finite-difference method to simulate the absorption characteristics of the absorber, which could achieve four discrete near-perfect absorption peaks at 0.98 THz, 1.70 THz, 2.02 THz, and 2.36 THz. The absorber is polarization sensitive, and the conversion between four-band absorption and three-band absorption is achieved by changing the incident polarization angle. We also change the structure of the absorber to study the absorption characteristics and break the structural symmetry to achieve a larger number of absorption peaks. Besides, the sensing performance of four-band narrow-band absorption is analyzed, and the maximum sensitivity of…

The extraction of the center of a laser stripe is a key step in line-structure measurement, where noise interference and changes in the surface color of an object are the main factors affecting extraction accuracy. To obtain sub-pixel level center coordinates under such non-ideal conditions, we propose LaserNet, a novel deep learning-based algorithm, to the best of our knowledge, which consists of a laser region detection sub-network and a laser position optimization sub-network. The laser region detection sub-network is used to determine potential stripe regions, and the laser position optimization sub-network uses the local image of these regions to obtain the accurate center position of the laser stripe. The experimental results show that LaserNet can eliminate noise interference, handle color changes, and give accurate results under non-ideal conditions. The three-dimensional reconstruction experiments further demonstrate the effectiveness of the proposed method.

Photothermoelectric conversion in chiral metasurfaces with thermoelectric material provides an effective way to achieve circular polarization recognition. In this paper, we propose a circular-polarization-sensitive photodetector in a mid-infrared region, which is mainly composed of an asymmetric silicon grating, a film of gold (Au), and the thermoelectric Bi 2 Te 3 layer. The asymmetric silicon grating with the Au layer achieves high circular dichroism absorption due to a lack of mirror symmetry, which results in a different temperature increasing on the surface of the Bi 2 Te 3 layer under right-handed circularly polarized (RCP) and left-handed circularly polarized (LCP) excitation. Then the chiral Seebeck voltage and output power density are obtained, thanks to the thermoelectric effect of Bi 2 Te 3 . All the works are based on the finite element method, and the simulation results are conducted by the Wave Optics module of COMSOL, which is coupled with the Heat Transfer module and Thermoelectric module of COMSOL. When the incident flux is 1.0W/cm 2 , the output power density under…

This research reports an advancement in splicing silica glass fiber to sapphire single-crystal optical fiber (SCF) using a specialized glass processing device, including data that demonstrate the thermal stability of the splice to 1000°C. A filament heating process was used to produce a robust splice between the dissimilar fibers. A femtosecond laser is used to inscribe a fiber Bragg gratings sensor into the SCF to measure the high-temperature capabilities and signal attenuation characteristics of the splice joint. The experimental results demonstrate that the proposed splicing method produces a splice joint that is robust, stable, repeatable, and withstands temperatures up to 1000°C with a low attenuation of 0.5 dB. The proposed method allows placement of SCF-based sensors in the extreme environments encountered in various engineering fields, such as nuclear, chemical, aviation, and metals manufacturing, to enable improvements in process monitoring, product quality, and production efficiency.

In this work, the fixed right shift (FRS) code is utilized for the optical code division multiple access (OCDMA) technique in an underwater optical wireless communication (UOWC) system. Additionally, in this system, a 532 nm laser diode (LD) source is employed to generate optical signals. The investigation encompasses an analysis of five distinct Jerlov water types, each exhibiting diverse chlorophyll concentrations. The performance of the proposed system is evaluated when each channel that is assigned a unique FRS code sequence carries different data rates (2.5, 5, and 10 Gbps). Underwater (UW) ranges, bit error rate (BER), eye diagrams, and quality factor (Q-factor) are the performance metrics used to evaluate the system performance. The proposed UOWC-FRS/OCDMA system is simulated, and the obtained results show that the eye diagram openings close, the BER increases, and the Q-factor decreases as the data rate per each channel increases from 2.5 to 10 Gbps, and the attenuation of water becomes higher. Moreover, the lower attenuation values caused by the Jerlov type I (JI) waterbody…

I propose methods for reducing the number of exposures in incoherent digital holography with two polarization-sensitive phase-only spatial light modulators (IDH with TPP-SLMs). In IDH with TPP-SLMs, no polarization filters are required, and not only three-dimensional (3D), but polarization information is also obtained. However, seven exposures are required to conduct filter-free polarimetric incoherent holography. In this article, the optical designs and modified phase-shifting interferometry to reduce the number of recordings are described. IDH with TPP-SLMs has the potential for filter-free single-shot multidimensional incoherent holographic imaging.

Fiber imaging bundles allow the transfer of optical images from place-to-place along narrow and flexible conduits. Traditionally used extensively in medical endoscopy, bundles are now finding new applications in endoscopic microscopy and other emerging techniques. PyFibreBundle is an open-source Python package for fast processing of images acquired through imaging bundles. This includes detection and removal of the fiber core pattern by filtering or interpolation, and application of background and flat-field corrections. It also allows images to be stitched together to create mosaics and resolution to be improved by combining multiple shifted images. This paper describes the technical implementation of PyFibreBundle and provides example results from three endomicroscopy imaging systems: color transmission, monochrome transmission, and confocal fluorescence. This allows various processing options to be compared quantitatively and qualitatively, and benchmarking demonstrates that PyFibreBundle can achieve state-of-the-art performance in an open-source package. The paper demonstrates…

In this paper, we propose a new, to the best of our knowledge, saturable absorber (SA) based on sodium carbonate (Na 2 CO 3 ) for producing an ultrafast mode-locked stretched pulse in a passively erbium-doped fiber laser at near-zero dispersion. The solid film of a Na 2 CO 3 -SA was fabricated by the drop-casting method using polyvinyl alcohol as a host polymer. The modulation depth of the proposed SA, which was measured by a balanced twin detector technique, was 2.3% with saturation intensity of 181MW/cm 2 . The mode-locking operation of the EDFL-based Na 2 CO 3 -SA was observed at a pump power of 117 mW. A stable stretched pulse was generated by using the proposed Na 2 CO 3 -SA. The laser can generate pulses with a repetition rate and duration of 1.87 MHz and 820 fs, respectively, within a bandwidth of 6.6 nm. The single pulse energy reaches up to 5 nJ, which is equivalent to the average output power of 9.3 mW. Finally, to the best of our knowledge, this is the first report on using the Na 2 CO 3 -SA for generating a stretched-pulse mode-locked fiber laser.

Retrieval algorithms for aerosol elastic backscatter lidars are most commonly based on a signal inversion known as the Klett solution. While often used, a sign error in the original publication has barely been mentioned or recognized in the scientific community. In this study, we present a corrected Klett inversion and a sensitivity analysis of its implementation under different atmospheric conditions. We show that the error’s dimension depends on multiple factors, thus preventing trivial a posteriori corrections on the results calculated with the original, uncorrected Klett solution. Comparing the uncorrected with the corrected Klett solution, long integration pathways in combination with low aerosol concentrations lead to substantial relative deviations of more than 100%, whereas short integration pathways with high aerosol concentrations considerably reduce the relative deviations to magnitudes around 15%. The higher the altitude of layers with increased aerosol concentrations, the higher the deviations due to the incorrect usage of Klett’s inversion, however on a slight level ().

With the development of computational imaging, the integration of optical system design and digital algorithms has made more imaging tasks easier to perform. Wavefront coding (WFC) is a typical computational imaging technique that is used to address the constraints of optical aperture and depth of field. In this paper, we demonstrated a low-cost and simple optical system based on WFC and deep learning. We constructed an optimized encoding method for the phase plate under the framework of deep learning, which reduces the requirement for aberration correction in the full field of view. Optical coding was achieved with just a double-bonded lens and a simple cubic phase mask, and digital decoding used the deep residual UNet++ network framework. The final image obtained has good resolution, whereas the depth of field of the system expanded by a factor of 13, which is of great significance for the high-precision inspection and attaching of small parts of machine vision.

Multispectral quantitative phase imaging (MS-QPI) is a high-contrast label-free technique for morphological imaging of the specimens. The aim of the present study is to extract spectral dependent quantitative information in single-shot using a highly spatially sensitive digital holographic microscope assisted by a deep neural network. There are three different wavelengths used in our method: λ =532, 633, and 808 nm. The first step is to get the interferometric data for each wavelength. The acquired datasets are used to train a generative adversarial network to generate multispectral (MS) quantitative phase maps from a single input interferogram. The network was trained and validated on two different samples: the optical waveguide and MG63 osteosarcoma cells. Validation of the present approach is performed by comparing the predicted MS phase maps with numerically reconstructed (FT+TIE) phase maps and quantifying with different image quality assessment metrices.

Satellite free-space optical (FSO) communication is very promising in improving the bandwidth and capacity of space information networks in the future. However, the inter-satellite transmission distance of over 1000 km leads to unstable optical beam pointing, acquisition, and tracking and then generates optical power jitter by a large margin before detection-demodulation. Therefore, it is difficult to realize high-stability and long-time FSO communication between satellites due to the generated bit error rate (BER) by jitter. In this paper, we report an autonomously self-designed and high-integration laser communication payload (LCP) and on-orbit-demonstrated inter-satellite 145 min, zero-BER FSO stable communication with a line rate of 2.8 Gbps. Moreover, based on the inter-satellite laser communication link, a video phone was clearly implemented for more than 10 min, and authentic user data transmitted 459,149 packets, achieving results of zero-packet loss. Summarily, this on-orbit experiment demonstrated an excellent performance of the LCP owing to the distinctive design of…

Focused laser differential interferometry (FLDI) measures the phase shift corresponding to localized fluctuations in the refractive index of a medium. The sensitivity, bandwidth, and spatial filtering properties of FLDI make it particularly suited to applications in high-speed gas flows. Such applications often require the quantitative measurement of density fluctuations, which are related to changes in the refractive index. In a two-part paper, a method is presented for the recovery of a spectral representation of density disturbances from the measured time-dependent phase shift for a particular class of flows able to be modeled using sinusoidal plane waves. The approach is based on the ray-tracing model of FLDI due to Schmidt and Shepherd [Appl. Opt.54, 8459 (2015)APOPAI0003-693510.1364/AO.54.008459]. In this first part, the analytical results for FLDI response to single- and multiple-frequency plane waves are derived and validated against a numerical implementation of the instrument. A spectral inversion method is then developed and validated, including consideration for the…

The distributed acoustic sensor (DAS) is a promising technology for real-time monitoring of wellbores and other infrastructures. However, the desired signals are often overwhelmed by background and environmental noise inherent in field applications. We present a suite of computationally inexpensive techniques for the real-time extraction of the gas signatures from noisy DAS data acquired in a 5163 ft. deep wellbore. The techniques are implemented on three well-scale DAS datasets, each representing multiphase flow conditions with different gas injection volumes, fluid circulation rates, and injection methods. The proposed denoising techniques not only helped in optimizing the gas slug signature despite the high background noise, but also reduced the DAS data size without compromising the signal quality.

Breast cancer (BC) molecular subtypes diagnosis involves improving clinical uptake by Fourier transform infrared (FTIR) spectroscopic imaging, which is a non-destructive and powerful technique, enabling label free extraction of biochemical information towards prognostic stratification and evaluation of cell functionality. However, methods of measurements of samples demand a long time to achieve high quality images, making its clinical use impractical because of the data acquisition speed, poor signal to noise ratio, and deficiency of optimized computational framework procedures. To address those challenges, machine learning (ML) tools can facilitate obtaining an accurate classification of BC subtypes with high actionability and accuracy. Here, we propose a ML-algorithm-based method to distinguish computationally BC cell lines. The method is developed by coupling the K-neighbors classifier (KNN) with neighborhood components analysis (NCA), and hence, the NCA-KNN method enables to identify BC subtypes without increasing model size as well as adding additional computational parameters.…

The development of atmospheric hypersonic flight and re-entry capabilities requires the characterization of the thermo-chemical state of representative test environments. This study demonstrates the usage of multiplex nanosecond N 2 coherent anti-Stokes Raman scattering (CARS) to measure temperatures in an atmospheric, high-temperature (>6000K), air plasma plume, generated by an inductively coupled plasma torch. These are some of the highest temperatures ever accessed via gas-phase CARS, to our knowledge. Temperatures of N 2 in the equilibrium plasma plume are determined via theoretical fits to measured CARS spectra. We discuss the practical implementation of CARS at very high temperatures, including the scaling of the N 2 CARS signal strength from 300 to 6700 K, where the expected peak signal from the high-temperature plasma torch gases is two orders of magnitude less than commonly encountered in combustion environments. An intensified CCD camera enables single-laser-shot detection at temperatures as high as 6200 K, by increasing sensitivity and providing a time gate against…

Free space optics communication (FSO) offers several advantages over its counterpart radio frequency (RF) systems in terms of bandwidth, data rates, and cost efficiency. However, FSO, being a line-of-sight (LoS) communication system, is hindered by various environmental factors. In this paper, we propose a multihop decode-and-forward (DF) relaying-based intelligent reflecting surface (IRS)-assisted FSO system with an aim to deal with the impediments in FSO communication, and it is assumed that each IRS consists of multiple reflecting elements. Specifically, we derive end-to-end expressions for cumulative distribution function (CDF) of output instantaneous signal-to-noise ratio (SNR). Furthermore, with the aid of the CDF statistics, we study the performance of the proposed system by obtaining the expressions for outage probability (OP) and average symbol error rate (ASER) in terms of multivariate Fox's H-function. Moreover, we have quantified the system performance by numerical results. The numerical results show that the multihop IRS-assisted FSO system provides better performance…

A highly sensitive and reliable tunable diode laser absorption spectroscopy gas detection system with a temperature–pressure compensation algorithm is demonstrated for detecting CH 4 concentrations in near space. Near space generally refers to the airspace 20–100 km away from the ground, where temperature and pressure changes are complex. Since the gas absorption spectrum is easily affected by temperature and pressure, a temperature–pressure compensation algorithm is proposed and used in the CH 4 sensor to improve the detection accuracy of the sensor. First, we measured the basic characteristics of the sensor in the laboratory, such as linearity and long-term stability. Experimental results showed that the linear correlation coefficient R-square can reach 0.999, and the concentration fluctuation of CH 4 is less than 0.17 ppm within 3.5 h. Then the sensor was applied to a research activity in Qinghai Province, China, in September, and the results show that the sensor can effectively monitor the CH 4 concentration in near space.