Conventional methods used to inspect the molten pool surface induced by arc welding processes are based on post-welding evaluation using destructive or semi-destructive testing methods. Many attempts were performed to develop alternative and nondestructive tests. In this context, a polarimetric vision methodology is evaluated. The polarimetric parameters are measured from the thermal radiations emitted by the hot liquid metal at a wavelength within an arc plasma blind spectral window. The Stokes parameters and their segmentation within a Gaussian mixture model allow to discriminate artifacts at the surface of the liquid metal. After weld pool solidification, the use of scanning electron microscopy and energy dispersive spectroscopy allow to identify the artifacts, i.e., floating alumina particles.

A scheme of parabolic pulse generation is proposed and verified by simulations and experiments. Based on a multiply operation, a parabolic pulse can be obtained by carving a triangular envelope with a triangular time window. Theoretical analysis shows that an ideal dark parabolic pulse and frequency-doubled parabolic pulse can be generated under the triangular profile with the second-order approximation. The corresponding fitting degrees of the two types of parabolic pulses are beyond 99% and 97%, respectively. In the experiment, the flexibility is demonstrated, which perfectly agrees with the theoretical expectation.

This guest editorial introduces the Special Section on Holography.

The resolution optimization and sampling strategies in practical digital in-line holography (DIH) with spherical wave illumination are analytically studied by optimizing system parameters. Optimal parameters of holographic recording, including illumination wavelength, numerical aperture of the spherical illumination wave, source-sensor distance, object–sensor distance, and sampling parameters of the sensor, for achievable resolution are worked out. A formula is derived to guide the DIH system design in general cases. Different sampling strategies associated with corresponding reconstruction waves (plane wave and spherical waves with various curvatures) are analyzed. The reason why recording with spherical wave and reconstructing with plane wave works well in practice is explained in detail. We also present how to determine the reconstruction distance and magnification to reconcile the curvature difference. The analysis is carried out mathematically and verified by simulated holograms. Optical experiments with U.S. Air Force resolution target are carried out based on the analysis for further verification.

The label-free analysis of leukocytes by holographic microscopy has the potential to overcome the extensive labeling procedures of conventional analysis by automated hematology analyzers and manual blood smear analysis. For the enrichment of leukocytes, selective lysis of erythrocytes is applied in flow cytometry to minimize the background. In the case of quantitative phase measurements, lysis procedures may affect the morphology of leukocytes inducing refractive index changes, which can deteriorate the specificity and sensitivity of the method. To study the effects of selective erythrocyte lysis on phase measurements of leukocytes, we compared two widespread methods: lysis by hypotonic shock followed by magnetic depletion of erythrocytes and lysis with a commercial lysis buffer containing ammonium chloride and potassium bicarbonate. We observed that lymphocytes were less affected than granulocytes and monocytes by both methods and that the intracellular changes increased with increasing incubation times. These findings indicate a significant effect of erythrocyte lysis on leukocytes and need to be studied for future hematology applications.

A quick and accurate way for testing collimation of laser beams is desired to achieve correct results in many optical instruments. Laser beam collimation is generally performed using glass wedge plates. For better accuracy, a set of wedge plates of different thicknesses and wedge angles are required to acquire first-hand visual information about collimation of laser beams having a wide range of diameters. Use of a compact holographic lateral shearing interferometer (HLSI) is demonstrated for collimation testing of laser beams having different beam diameters. The interferometer uses two holographic optical elements (HOEs) with provision of variable tilt in one of the HOEs to maintain a constant number of fringes in the interferograms for different diameters of input test beams. An angle of rotation of interference fringes from a reference position is measured to be 0.9 deg for a test beam of diameter 2 mm and 16 deg for the test beam of diameter 35 mm. We experimentally demonstrated range of optimum number of interference fringes in the interferogram for visual inspection of the collimation state of the test beam. Thus the proposed interferometer could replace a set of wedge plates generally required for collimation testing of beams having different diameters. The functioning of the interferometer for collimation testing of beams having different diameters is discussed and experimentally demonstrated.

We introduce a technique for recording volume holographic optical elements (vHOEs) designed for operation at a wavelength different from the recording wavelength. This technique relies on our holographic wave front printer, which employs two spatial light modulators for recording wave front generation. We present the procedure for computing the required recording wave fronts accurately and noniteratively from the desired vHOE optical transformation function. We exploit this technique for the fabrication of a converging lens vHOE with an operation wavelength of 915 nm at a recording wavelength of 515 nm. Finally, we demonstrate the holographic lens functionality directly in the infrared spectral region.

To simulate the effects of multiple-longitudinal modes and rapid fluctuations in center frequency, we use sinusoidal phase modulation and linewidth broadening, respectively. These effects allow us to degrade the temporal coherence of our master-oscillator laser, which we then use to conduct digital holography experiments. In turn, our results show that the coherence efficiency decreases quadratically with fringe visibility and that our measurements agree with our models to within 1.8% for sinusoidal phase modulation and 6.9% for linewidth broadening.

In this work, we propose a combination of the Teager–Kaiser energy operator (TKEO) and the spiral phase transform (SPT) for robust instant energy estimation of amplitude-modulated and frequency-modulated (AM–FM) signals, where the energy extraction is followed by a high-frequency component, generally considered as noise. We demonstrate that this noise component can be subtracted mathematically using the SPT transformation applied to the AM–FM signal. The improvement in demodulation is tested using a simulated AM–FM image and evaluated by the image quality index. An experimental speckle fringe pattern obtained by digital speckle pattern interferometry on a hard disk is denoised using a multiband approach and demodulated using the proposed method.

The extreme computational complexity in generating a hologram makes the interactive holographic display very difficult. An optimized layered rendering method for real-time interactive holographic display based on ray-tracing technique is presented in order to overcome this challenge and realize real-time interaction of three-dimensional scenes. Ray tracing is able to render more complex and photorealistic images for different illuminations and circumstances. Rays are launched in parallel and traced to obtain ray–object intersections, and then the complex wave field of the effective pixels in different layers is determined. The occlusions between different layers are also considered to effectively reduce the overall computation. A computer-generated hologram is fused with the multilayer-hologram-diffraction results using the layered fast Fourier transform algorithm. The experimental results demonstrate the effectiveness of the proposed method. The reconstructed holographic image with real depth cues is demonstrated by the experiments, and the optical reconstruction images can be interacted in real-time.

Faithful reconstruction, which is a key phenomenon in optical data storage, is significant in polarization holography and has attracted much attention. It is defined as the reconstructed wave being identical to the signal wave. We design an experiment to observe the faithful reconstruction of elliptical polarization holography in which the two orthogonal elliptical polarization waves are applied in the recording stage. In the experiment, phenanthrenequinone-doped poly methyl methacrylate is used as the recording material, and the angle between the signal and reference waves is ∼56  deg. We control the exposure time of recording material to observe the faithful reconstruction of orthogonal elliptical polarization holography. How to obtain the faithful reconstruction is interesting work; therefore, we propose a prerequisite for faithful reconstruction. Although the prerequisite is derived from orthogonal elliptical polarization holography, it could also be applied in orthogonal linear and circular polarization holography. The result may be helpful for understanding the faithful reconstruction of orthogonal polarization holography.

Floating three-dimensional (3-D) display can provide a natural and realistic 3-D scene. Nowadays, because of stray light and aberration, most floating 3-D displays cannot realize a large viewing angle and high resolution simultaneously, and it directly deteriorates the viewing effect of the 3-D scene. A large viewing angle floating and high-resolution 3-D display based on multichannel and multivariable (MCMV) correction algorithm are presented. The optical system consists of a time-sequential autostereoscopic 3-D display with the directional backlight, a floating lens, and an eye tracker. The 3-D display with the directional backlight provides loss-free resolution for the system. To realize a large viewing angle floating 3-D display, MCMV correction algorithm based on eye tracking is proposed. The 3-D images can be reconstructed by the optical system within the viewing angle of 60 deg. The feasibility of the proposed display method is verified with the experimental results.

A method is proposed for multiple-image encryption based on optical scanning holography (OSH) using a random phase mask (RPM) and orthogonal compressive sensing (CS). It can destroy the linearity of the traditional OSH system and possess a superior security. On the one hand, images are preferentially preprocessed by utilizing the orthogonal CS to provide a single-layer section for OSH. Therefore, the defocus noise as well as information leakage can be controlled effectively in decryption. On the other hand, each image can be extracted separately without the others’ contents. In addition, the use of RPM can bring about a more ulterior distribution to the cyphertext, which may be better than the case without the RPM. The use of orthogonal modulation matrices and RPM can provide the additional key spaces to guarantee the security of this holography cryptosystem. Simulations and discussions are also made on the cyphertext characteristics as well as the ability of resisting occlusive attack.

A modified single-pass multiview rendering method is proposed to achieve real-time large viewing angle volume data three-dimensional (3-D) display. By modifying the volume rendering pipeline, the mapping of the synthetic image pixel and multiperspective information is directly accomplished in once rendering pass. And then shear-transformed ray is cast to integrate multiperspective information into the synthetic image used for 3-D display. Proof-of-concept experimental setups are constructed with the autostereoscopic 3-D display and the 3-D light-field display to demonstrate the proposed method without loss of generality. The experimental results show that the real-time frame rates of the autostereoscopic 3-D display and the 3-D light-field display are more than 30 and 60 fps at 4K (3840  ×  2160) resolution, respectively. The rendering efficiency of the proposed method is irrespective of the viewpoint number and viewing angle size. In addition, it is not only compatible with the volume drawing pipeline but also robust to different types of 3-D displays based on pixel coding. A feasible way of applying 3-D display to the biological and medical fields is presented.

Computer-generated holography (CGH) is a technique to generate holographic interference patterns. One of the major issues related to computer hologram generation is the massive computational power required. Hardware accelerators are used to accelerate this process. Previous publications targeting hardware platforms lack performance comparisons between different architectures and do not provide enough information for the evaluation of the suitability of recent hardware platforms for CGH algorithms. We aim to address these limitations and present a comprehensive review of CGH-related hardware implementations.

We present validation of a tilt angle illumination technique for height maps’ reconstruction using digital holography and propose a few solutions for improvement of the inclined probe wave front approach. The general algorithm of tilt angle digital holography approach is described and validated in experimental monitoring of a coin relief. Several approaches of height map quality improvement concerning tilt angle estimation, relation between measurement accuracy and dynamic range, and selective averaging algorithm are proposed and discussed.

Non-interferometric phase retrieval is a fundamental technique for phase-modulated holographic data storage due to its advantages of easy implementation, simple system setup, and robust noise tolerance. Usually, the iterative algorithm of non-interferometry needs hundreds of iteration numbers to retrieve phase accurately, which decreased the data transfer rate severely. Strong constraint conditions, such as embedded data, can be used on the phase data page to reduce the iteration numbers. However, introducing embedded data will reduce the code rate of the system. We proposed a method that combined the single-shot interferometric method with the non-interferometric iterative Fourier transform algorithm method. We used the phase decoding result by single-shot interferometry as the embedded data in the process of non-interferometry. Therefore, no extra embedded data are needed in the signal code. We realized the code rate improvement as well as keeping fast data transfer rate. In the demonstration, we recorded a four-level phase pattern and retrieved the phase correctly. The bit error rate of phase retrieval is less than 1% within 20 iterations, which proves our approach is practical. In our case, the code rate is increased by two times.

For many years, the use of advanced composite materials in aeronautics, automotive, and sports applications has been well established. The characteristics of composite materials in terms of weight reduction, fatigue, and corrosion resistance make them competitive in many cases with respect to conventional ones (i.e., metal alloys). On the other hand, the fabrication process of the most employed composites reinforced by carbon or glass fibers requires complex steps that are not always environmentally friendly. In fact, such composite materials are not themselves “green.” For these reasons, in the last decades, the use of natural reinforcing fibers has gained increasing attention, allowing the development of new materials sharing the same advantages with conventional composite systems while respecting the environment. Due to their structural complexity, these materials are not always compatible with the use of standard nondestructive evaluation methods, e.g., ultrasounds testing. The effective use of speckle holographic techniques as nondestructive evaluation methods in full field and noncontact modality is proved on “green” composite materials. In particular, electronic speckle pattern interferometry is tested on different kinds of specimens preliminarily subjected to flexural tests. Results show that, in most cases, the damage appears more severe on the back side of specimens, opposite to the loaded one, with the sole exception of basalt laminates.

We demonstrate an application of digital holography for detection of a new type of defect, i.e., delamination of polyurethane pads on a polishing tool used on a high-speed computerized numerical control machine for production of precision optics. The method enabled us to acquire both qualitative and quantitative information about the delaminated region in the polishing pad. A delaminated region of 13.6  mm  ×  10.8  mm is detected and measured on the surface of the polishing pad. An increase in the delaminated region by 30% is noticed after completion of the polishing process with the delaminated polishing pad. The effect of delamination of the polishing pad on an optical surface is experimentally demonstrated using it for polishing a BK-7 glass substrate. These investigations may enable production of improved quality precision optics at the commercial level.

A self-contained platform for realistic modeling of digital holographic microscopy (DHM) is presented. Amplitude and phase samples, imaged in different architectures, can be modeled to produce numerical DHM holograms that include all the parameters that are present in real experiments, providing an accessible way for newcomers to have a first approach to this research field. The platform is based on considering the imaging arm of the DHM that produces the object wave as the result of the convolution process between the geometrical-optics image prediction of the sample with the point spread function introduced by diffraction. The DHM hologram is produced by the amplitude superposition of complex-valued object wave with a reference wave of arbitrary description. The sampling of the analytically produced DHM holograms is set from the input discretized image according to the specifications of the digital camera aimed to be used. The feasibility of the realistic platform is exemplified by contrasting the wrong results of a nonrealistic simulation with experimental results to show the need for using a complete realistic simulation like the one presented; further applications of the platform to numerical modeling speckle noise reduction over samples with controlled levels of roughness, and phase-shifting DHM techniques, are included.

In computer-generated holograms (CGH), a random phase (RP) is usually introduced to spread the object information in the hologram and avoid the frequency-filtering effect in the reconstructed image. However, this action introduces speckle noise in the reconstructed image but also permits one to reconstruct objects with bigger size than the hologram. In recent years, alternative RPs have been proposed to reduce the speckle noise in comparison with the uniform distributed RP that is usually applied. However, its effects in different types of holograms, its capacity to reconstruct big objects, and its capability to eliminate the frequency-filtering effect have not been reviewed in detail. We present an analysis of the normal distributed, pink, brown, and Perlin phases in Fourier and Fresnel holograms coded as amplitude and phase (Kinoform and complex amplitude encoded). Only in the Kinoform holograms, the RP is required to eliminate the frequency-filtering effect. In other cases, the phase is useful to record big objects in the hologram. The Perlin phase had a better performance than the other phases, achieving the reconstruction of objects with bigger size than the hologram and reducing the speckle noise. We present a quantitative and qualitative analysis of each distribution, including simulated and experimental results that confirm the numerical analysis. We believe these results could be helpful in the selection of an adequate RP in CGH, according to the hologram configuration.

We dealt with the problem of shading of dense polydispersed aerosol particles during registration of holographic images. It is a suggested technique that is based on the three-wave holographic recording of a cloud of micron and submicron particles. We present the calculated and experimental results of effective reconstruction of cloud particles with concentration of up to 100  ×  10⁶  /  cm³. Simultaneous use of three-wave scheme provides complete analysis of the resulting holograms and more effectively restore the volume distribution of dense aerosol formatting.

A digital domain dynamic path accumulation method for a time-delay-integration (TDI) image sensor that can realize on-chip compensation for image distortion caused by sensor vibration is proposed. Through the proposed method, the exposure signals are controlled to enter the corresponding accumulator according to the vibration vector, ensuring the synchronous accumulation of different stages of pixels on the same object. The behavior level model of the method is established. The structural similarity (SSIM) between images without distortion, distorted images, and images compensated for by the proposed method is simulated and analyzed to verify the effectiveness of the method. Compared with conventional TDI, the SSIM between images with and without vibration is improved from 0.2322 to 0.9858, as the vibration level is 1 pixel per stage. With fixed TDI stages and the vibration level varying from 0 to 2 pixels per stage, the SSIM of conventional TDI drops rapidly to about 0.2 while that of the dynamic path accumulation method maintains at about 0.95. The proposed method is suitable for an antivibration CMOS-TDI image sensor of remote imaging systems.

Waveform light detection and ranging detection (LiDAR) systems capture the entire backscattered signal from the interaction of the laser beam with objects located within the laser footprint. The target response (TR) is a time-dependent curve implying the geophysical attribute of the detected objects. TR restoration by removing the effect of the system waveform (SW) from the received waveform is crucial but suffers from ill-posedness. We recast the deconvolution problem to a nonlinear least squares problem and use the proposed method to deal with the LiDAR waveforms with negative tails. The proposed method is an iterative algorithm starting with a practical initial TR seen as the combination of parts of the deconvolution results obtained by the L1- and L2-regularization methods, then ending by a stopping criteria set empirically. A set of hybrid LiDAR waveforms constructed by the SW of our LiDAR and the synthetic TRs are employed to evaluate the performance of the TR retrieval. The results show the superior performance of the proposed method in both reconstructions of the flattened and sharp curves of the TRs as compared to the L1- and L2-regularization methods. This demonstrates the potential of the nonlinear least squares method for retrieving the range and geometric physical information from the LiDAR waveforms.

Object tracking technology is being continually improved to ensure robust tracking performance under real conditions, such as cluttered background, fast motion, and lengthy occlusions. The performance of these video trackers depends significantly on evaluation methodologies adopted during the assessment of tracking algorithms. Most of these video trackers are evaluated using video sequences annotated with the simulated visual attributes of objects. Generating such sequences with the requisite attributes is laborious and time consuming. These annotated video sequences may fail to address real challenges; hence, the trackers must be tested using available opportunity targets in lengthy field trials. We present an innovative scheme for the hardware-in-loop testing of such video trackers for efficient evaluation under complex background scenarios. The designed simulator can provide the real-time visual characteristics of object parameters with a live background without halting the evaluation process. The proposed method facilitates the efficient evaluation of a closed-loop video tracker by providing a virtually endless video sequence containing realistic visual attributes of potential objects. In addition, the validation of the tracker’s performance in realistic scenarios, the comparative performance of tracking algorithms under varying operational conditions, and an in-depth analysis of the tracker system parameters are presented.

A real-time viewpoint generation-based 3D display with low crosstalk and high resolution is proposed. The proposed 3D display consists of a pattern image display (PID), a lenticular lens, and a transparent liquid crystal display (LCD) panel. For each eye of the observer, the PID can provide a pattern image with stripes that have a calculated pitch and displacement according to the eye position. The light from the stripes can be concentrated 3D display eye tracking by the lenticular lens and generate a viewpoint at the eye position. The transparent LCD panel is used to provide parallax images. This transparent LCD panel will not change the direction of light propagation and the location of the generated viewpoints. Then the whole parallax image on the transparent LCD panel can be projected by the PID and the lenticular lens to the viewpoint, so the resolution will not suffer like that in the convention 3D displays. Because the viewpoints are real-time generated according to the position of the observer’s eyes, the observer’s eyes will completely coincide with the position of the generated viewpoint. So crosstalk can be limited at a low level, and the viewing range is almost unlimited.

As insulators play an important role in power transmission lines, we propose an intelligent method based on the convolution neural network (CNN) to evaluate the corona discharge of an insulator. In this method, the imaging device is a dual-spectra camera with a visible channel and an ultraviolet (UV) channel. The CNN is adopted to identify the detection distance of the insulator with the visible channel. To train the network, the dataset of the insulator is obtained by the experimental setup and deep convolutional generative adversarial networks. Through adjusting the training parameters and optimizing the network structure, an optimal trained model is achieved. Then the image pixel ratio method is adopted to measure the UV signal strength of the images captured by the UV channel. Meanwhile, the relationship between the detection distance and the UV signal strength is discussed. The critical value for the corona discharge of the insulator is obtained via experiments at the standard detection distance. Finally, the corona discharge of the insulator is evaluated by combining the detection distance with the UV signal strength. The experimental results show the method has the advantages of high accuracy and robustness and can effectively evaluate the corona discharge of the insulator.

Antireflective nanostructured surfaces (ARSS) enhance optical transmission through suppression of Fresnel reflection at boundaries between layered media. Previous studies show that random ARSS (rARSS) exhibit broadband enhancement and polarization insensitivity in transmission when applied to flat optical windows. Zinc selenide windows with rARSS treatment were fully characterized (transmittance, reflectance, and angular scatter) in the midwave and long-wave infrared range (2 to 12  μm). Four morphologically different, random nanoroughness, antireflective surfaces were tested at: normal incidence transmission, at 15 deg angle of incidence, and 15 deg to 45 deg angle of reflection. The angular reflectance distribution resembles a diffuse dipole radiator due to the finite elongated beam cross section at the incidence surface. Scattering diagrams with main and side lobes are presented. Partially integrated scatter values were obtained, allowing the comparison of random antireflective boundary performance to optically flat surfaces. Comparing axial transmission and specular reflection with the scattered performance, an accurate determination of the redistribution of the incident energy is obtained. Measurements of the rARSS feature topology were determined from autocorrelation of the scanning electron microscope images of the nanoroughened substrates, to assess the structured surfaces’ feature scales. The results show differences in scattered intensity over the wavelength bands of interest, correlating with surface random feature populations.

Laser active imaging is widely used in many fields. The intensity image quality of laser active imaging is affected by various degradations, such as speckle effect and noise. Most existing objective image quality assessment (IQA) methods that consider only a single distorted image are not suitable for an intensity image. A multiscale full-reference intensity IQA method is presented. The proposed method is based on an improved gradient magnitude similarity deviation (GMSD) in the nonsubsampled contourlet transform (NSCT) domain. The reference and distorted images are decomposed by NSCT to emulate the multichannel structure of the human visual system. Then, the GMSD of each sub-band is computed to capture the intensity image quality. At last, the contrast sensitivity function implementation is employed in the sub-bands of the NSCT domain. All sub-bands’ GMSD is evaluated and pooled together to yield the objective quality index of a distorted intensity image. Experimental results show that the proposed method can effectively and accurately evaluate the quality of intensity images, and it is highly consistent with subjective perception.

The intensity of objects in the infrared is an important quantity for a number of applications. Intensity in watts per steradian is the parameter that is used to describe either small targets or targets that are far away. Intensity is used because these cases are usually presented to a detection sensor where the object is smaller than the sensor detector angular subtense, a situation known as an “unresolved target.” In the military, unresolved targets can be rocket-propelled grenades, man-portable air defense threats, enemy aircraft at long range, or even ground vehicles that are being engaged by ground-to-ground or air-to-ground missiles. Typical “resolved target” metrics such as root-sum-squared differential temperature do not work well for unresolved targets. In addition, a given target intensity coupled with range, atmospheric transmission, and sensor noise equivalent irradiance can provide a quick signal-to-noise estimate of a particular sensor against a particular target. Target intensity can even be a measure of how visible ones platform is to other sensor and can be used to reduce platform signatures. Measurement of intensity is always a difficult procedure, where there is typically a sensor that does not encompass all aspects of measurement parameters. For example, there are very few radiometers that include high-resolution spatial measurements with high-resolution spectral measurements with high-resolution temporal measurements, not to mention polarization. For the few systems that exist that can provide a simultaneous measurement with most of these parameters, the cost is prohibitive. Usually, a spectral radiometer will provide high spectral resolution with no spatial information and a slow temporal rate. These measurements are common. In the case we describe here, the intensity measurement is taken broadband in the midwave or longwave infrared regions with good spatial resolution. This measurement provides a band integrated intensity measurement. We describe an approach for sensor calibration and object intensity measurement that can be used for broadband sensors applications.

Three-dimensional light field imaging in laparoscopic surgery is an emerging technology, which has the potential to enable three-dimensional imaging. Calibration of the three-dimensional light field endoscope (3D LFE) is essential but challenging as the disparity is much smaller than that of the conventional light field camera. The geometrical model for 3D LFE is established, and a calibration method based on virtual objective lens and virtual feature points is proposed. First, the virtual objective lens is introduced and the parameters about it are calibrated using corner features in center subaperture images. Second, two types of virtual feature points are proposed to calibrate the parameters about the microlens array, one is on the black-and-white board line and the other is selectively determined but can be anywhere on the checkerboard. Moreover, the relationship between the virtual feature points mapping in the microlens image and the virtual feature points mapping in the central subaperture image is deduced to overcome tiny light field disparity. Experimental results verify the performance of our calibration method.

We present an accurate laser radar receiver with a wide dynamic range intended for ranging applications based on an event-based approach, in which a receiver-time-to-digital converter is used to extract the timing information from the reflected echo. The receiver is based on LC resonance pulse shaping at the input so that the unipolar pulse detected by the avalanche photodiode is converted to a bipolar signal, and the first zero-crossing of this converted signal is marked as the only timing point. One important aspect of the proposed scheme is that it does not need any postcompensation or gain control for achieving a wide dynamic range. The receiver chip was fabricated in a 0.35-μm standard CMOS technology, and a laser radar platform was developed to verify the functionality of the proposed receiver channel. The measured accuracy of the receiver is ±3.5  cm within a dynamic range of more than 1:250,000 using 3-ns FWHM pulses when target materials with different reflectivities are used in the measurements. The single-shot precision of the receiver (σ value) is ∼5  cm for a minimum SNR of ∼10.

We present the results of research of autocollimating null-indicator in the angle measurement system intended for measuring angles between normals to faces of optical polygons or mirrors. The autocollimating null-indicator is considered with analog signal processing based on its digitization and center of mass estimation. We present the results of modeling and analyze the dependence of the sampling point number and signal-to-noise ratio on the accuracy of the signal center of mass determination. The results of experimental research of the system are presented.

High speed time-resolved wavefront and imaging measurements were taken synchronously in-flight through both boundary layer and shear layer environments around the Airborne Aero-Optical Laboratory for Beam Control. Instantaneous modulation transfer functions and point spread functions (PSFs), which characterize image degradation, were generated using wavefront data. Instantaneous power-in-bucket ratios were extracted from both the image data and computed from the wavefront data, and the ratios were found to correlate well with each other. The lower power-in-bucket values and related increased blurring that occurred predominantly in the streamwise direction were associated with large-scale, large-amplitude wavefront spatial variations due to large organized vortical structures present in the shear layer. The boundary layer did not create any significant image blurring due to the low level of aero-optical distortions. Finally, spatial autocorrelation functions were extracted from the wavefront data using the stitching method and were used to compute time-averaged PSFs for different aperture diameters.

Lensless microscopes are simple, portable, and cost effective compared with the sophisticated microscopes of today that require high-end objectives, lenses, and filters. We demonstrate a lensless on-chip phase microscope based on the holographic principle to image 3D nanometric depth information from transparent and weakly scattering biological samples. We characterize the microscope using standard quantitative phase resolution target (PRT) charts with feature depths from 50 to 350 nm and report a signal-to-noise ratio value of 23 dB. Further, we apply a gradient descent-based constrained optimization approach for phase retrieval to eliminate the twin image and noise from the hologram. The device, operating as an inline holographic microscope, can perform quantitative phase imaging with nanometric depth sensitivity for a field of view of 29.4  mm² using an LED-based light source butt-coupled to an optical fiber cable.

Due to the unique mechanical and optical characteristics, it is difficult to carry out experimental research and online measurement for large-aperture ICF frequency converters. To analyze and optimize the performance of the frequency converters under complex process loads, we present an integrated optomechanical method that correlates actual process loads with laser critical characteristics. Based on the established optomechanical model, the key factor that induces the loss of harmonic generation efficiency is identified. In addition, the proposed method is conductive to rapid evaluation, prediction, and optimization of comprehensive performance of frequency converters. Thereby, we propose an adaptive frequency conversion system (AFCS). The results indicate that AFCS can not only minimize the phase mismatching of second harmonic generation doubler but also significantly improve the quality of far-field focal spot.

Multimode interference (MMI)-based optical splitter is designed and experimentally demonstrated on silicon on insulator for on-chip optical interconnect. We have proposed MMI-based 1  ×  2 optical 3-dB splitter having a compact footprint (2.8  ×  27  μm²). The device shows low excess loss of 0.05 dB for TE mode at operating wavelength 1550 nm and <0.45  dB over broad wavelength 1430 to 1675 nm (S–C–L–U band) using eigenmode expansion (EME) method. The proposed MMI-based Mach–Zehnder interferometer structure is implemented for arbitrary power splitting applications having excess loss <1  dB for wavelength range 1520 to 1580 nm. Moreover, the fabrication tolerance of the device is analyzed using the EME method. The proposed device can be used for on-chip optical interconnects in photonic integrated circuits.

Photonic firewalls that can directly conduct the data discrimination and intrusion detection at the optical layer are an important security technique for optical networks. In the photonic firewall, binary sequence matching module is the core part and is implemented by multiple all-optical logic gates such as all-optical AND gate, all-optical XOR gate, etc. However, the semiconductor optical amplifier (SOA)-based all-optical logic gates have the disadvantages of low operation rate which leads to low matching rate. We design a binary sequence matching module that is implemented by the cross-phase modulation effect (XPM) and the four-wave mixing effect (FWM) of the highly nonlinear fiber (HNLF). The nonlinear effect of HNLF has a very short response time, thus it can realize the logic operation at a very high optical signal rate. The developed binary sequence matching module is verified through VPI Transmission Maker 8.5. The simulation results show that it can achieve the matching of 8-bit data in 64-bit binary data at 160-Gbps signal rate.

We describe the development of a lightweight, high-resolution surveillance camera for deployment on high altitude platform systems. The instrument is designed to operate at an altitude of ∼20  km and has an expected ground resolution of better than 120 mm with an appropriate sensor. While designed specifically for imaging at visible wavelengths, it is shown that the design is capable of diffraction-limited imaging at NIR and SWIR wavelengths up to 2.5  μm. We have combined a range of materials from aluminum and titanium alloys through to carbon fiber-reinforced plastic to produce an instrument with structural components that match the thermal expansion of the optical glasses used. The use of these materials has resulted in an instrument that weighs <2  kg, including a sensor package, and is designed to weigh <3  kg once integrated with an enclosure and actuated gimbal. The successful testing of two prototype systems is described, including several design outcomes from the program intended for implementation in advance of flight trials.

We present a MoS2–graphene bilayer-based high-performance refractive index surface plasmon resonance (SPR) sensor applying an angular interrogation technique. We used it for inspecting the reflected optical signal from the sensing medium. For enhancing the sensor angular sensitivity (S), signal-to-noise ratio (SNR), and quality factor (QF), we undertook some steps sequentially. First, the impact of gold layer thickness was investigated and optimized to 40 nm. Second, the MoS2 and graphene coating layers were optimized to four and three, respectively. Finally, the minimum reflectance and SPR angle were identified with this optimum structure. It is seen that the angular sensitivity of this optimum structure improved to an excellent value of 130  deg-RIU^−  1 along with an improved angular SNR of 1.37 and QF of 17.02  RIU^−  1. Then we compared the proposed sensor with the existing works in terms of angular sensitivity, SNR, and QF to prove its effective application. Finally, an analysis of DNA hybridization is presented as an application of the proposed structure. Our offered configuration is capable of recognizing absorption of DNAs by means of the attenuated total reflection method. The effective refractive index of the sensing medium alters because of the absorption of various concentrated DNAs. Computational investigation demonstrates that the change of SPR angle and minimum reflectance for mismatched DNA strands is inconsiderable, though it is significant for complementary DNA strands, which is crucial for the sensing of accurate DNA hybridization.

It is critical to accurately obtain a phase position in a digital fringe projection system. Due to stray light in a complex environment, when projecting an image of a code fringe, the projected light intensity can be influenced. As a result, the intensity of fringe light captured with the camera may be defective, resulting in the decline of measurement precision. Hence, we propose a strong suppression technology for stray light based on polarization. Through processing fringe images in the frequency domain, this technology can effectively suppress interference of stray light on the three-dimensional (3D) imaging system. This method is verified in the experiment. The results show that the method can accurately capture the 3D profile of real-world targets under the interference of stray light.

We propose a self-powered signal processing system that can transfer a single byte of data without needing any external power supply. The proposed system uses a local extreme point reconstruction algorithm for reconstructing the waveform. In addition, we use Manchester coding in the proposed method for reducing the byte error rate. The experimental results show that the self-powered processor needs 3 mW generated from the solar panel to maintain 1  MSa  /  s and 8-bit sample depth when the visible light communication link distance is fixed as 30 cm. The maximum bit rate is 500  kbit  /  s and the corresponding byte error rate is 6  ×  ^{10  −  2}.

Laser-based displays have attracted much attention because they are the only displays that can express the full color gamut of Ultra-High Definition Television (U-HDTV), International Telecommunication Union Radio Communication Sector Broadcasting Service (ITU-R BT).2020. We have developed 638-nm broad-area laser diode (LD) with 75-μm-wide dual emitters and achieved wall plug efficiency of over 40.5% at 25°C. But slow degradation is still a factor limiting the lifetime due to the requirement for a high temperature and high-power operation. We performed the long-term aging test of 638-nm LDs under the different conditions, including a high-power operation such as 1.3 to 2.5 W, and investigated the behavior of output power characteristics, especially the dependence of its output characteristics on temperature. After long-term aging, the threshold current of the LD increased and its slope efficiency (SE) decreased. The measured dependence of the output characteristics of the LD on temperature was converted into that on the junction temperature using the thermal resistance between the junction and package. The latter dependence showed an increase of threshold current and no change of SE. This result indicates that the slow degradation of the red broad-area LD was caused by the increase of the nonradiative recombination rate in the active layer during aging.

With the development of technology, the total extent of global pipeline transportation is also increased each year. Nevertheless, owing to the wide distribution of gas and oil pipelines and the complex laying environment, the traditional long-distance optical fiber pre-warning system (OFPS) has a high false alarm rate when recognizing events threatening the pipeline safety, and it is difficult to define the type of intrusion events. Therefore, an improved high-intelligence long-distance OFPS was proposed, and the generalized adaptability of the system in various environments was studied. Φ-optical time-domain reflectometry technology was used in the distributed sensing part of the system, and a neural network (NN) was used in the signal recognition part to identify and classify intrusion events. Three methods were used in this system including an improved NN, a wavelet packet decomposition-based artificial NN, and a five-layer deep NN. Finally, through experiments in these three methods, the adaptability of the system was explored. The results show that the system possesses an excellent classification effect in the recognition of intrusion events with an average recognition rate reaching over 95%. Thus, it has good adaptability under various real environmental circumstances.

Because well-considered optical buffers have not been developed to date, the optical circuit switching (OCS) mechanism has been used widely for data transmission applications in existing optical network-on-chip (ONoC) devices. Unfortunately, with increasing network loads and increasing numbers of intellectual property (IP) cores, ONoCs based on the OCS mechanism are facing serious node congestion problems that greatly limit their network communication performances. A chain-building scheme is proposed to achieve high chain-building efficiency with low overheads. The proposed scheme uses a network state partitioning method based on time prediction to optimize the chain-building process and performs congestion control using a dynamic back-off approach. The simulation results obtained using OPNET software show that the network delay and throughput performances of the ONoC device using the proposed chain-building scheme represent significant improvements when compared with common existing schemes.

A probabilistically shaped 9-ary carrierless amplitude and phase (CAP-9) optical modulation format based on a reverse distribution model is proposed. We experimentally demonstrate the transmission of 33.3-Gb  /  s PS-CAP-9 signals over 25-km standard single-mode fiber in an intensity-modulation and direct-detection system. The results indicate that the proposed scheme outperforms the conventional uniform CAP-16 and the conventional PS-CAP-16 in receiver sensitivity by 1.2 and 0.4 dB at the threshold of 3.8  ×  10^−  3   BER, respectively.

We have successfully compressed the pulse duration of an all-normal-dispersion (ANDi) Yb-fiber laser to sub-100 fs by a single-mode nonlinear fiber amplifier. Through optimizing the input power, prechirp, and the pump power of the amplifier, the pulse duration of the ANDi Yb-fiber laser is compressed to 68 fs when a 2.5-m active fiber is used, although for a 3.5-m active fiber, the pulse duration of the laser can be compressed to 58 fs. We have also shown that a low relative intensity noise is maintained within the nonlinear amplification process. The experimental results prove this ultrashort-pulse-duration and low-noise laser source with moderate output power can be a good candidate for frequency comb, multiphoton imaging, coherent Raman spectroscopy, and other applications.

We present a state-of-the-art ultraviolet (UV)-based multiuser indoor communication over power-constrained discrete-time Poisson channels. To adhere to the UV exposure limit and safety, we consider a low-power transmission regime where strict peak and average transmit power constraints are imposed. These constraints are different from conventional electrical power constraints. Therefore, performance metrics, such as bit error rate (BER) and channel capacity, are newly characterized in relation to the discrete-time Poisson channel. We develop a minimum mean-square error receiver to reject the interference from multiple users with a detection scheme robust against channel state information (CSI) uncertainty. Furthermore, a downlink beamforming optimization problem is formulated using a second-order cone program to maximize the received signal-to-interference-plus-noise ratio. We bound the behavior of the mutual information in the low-power regime, when dark current increases linearly with average transmit power. In addition, the power penalty of multiuser link is analyzed. The proposed system is also realized in the experiment. The influence of the control parameter, i.e., average-to-peak power ratio, on the BER relative to the peak transmit power is experimentally investigated. The experimental results are found consistent with the theoretical simulation results. Further, from the simulation results, it is found that the proposed system approaches the CSI lower bound performance with a minimum sequence length and can provide ubiquitous link connectivity in indoor applications while keeping network structure and management simple.

We present the development of a multi-user cross-layer security network implemented on the optical code-division multiple access (OCDMA) with newly designed hybrid 2D-modified Walsh code (MWC) along with an algorithmic approach in cryptography domain. Minimum BER value is evaluated on the order of 10^−  50 using our designed MWC, which is comparatively lower than other existing codes. The security performance of the network was analyzed using the probability detection parameter, which was found to be superior apart from few other available existing codes. The enhancement of the quality and grade of security in this network eavesdropper can intercept the designed optical code of OCDMA in the physical layer. It compels one to break the security in the data-layer to access the original code. We also calculated the probability of successful data interception as a function of time complexity. Furthermore, we presented the simulation of the hybrid system using the OptiSystem simulator.

We propose to introduce a phase shift in each ring of the multi-stage interleavers based on add-drop resonator Mach–Zehnder interferometer to quantitatively measure the wavelength offset of each interleaver, thus a demultiplexer with uniformly segmented multi-channel output can be obtained. It is found that the additional phase of a certain interleaver is equal to the offset value of this stage plus the accumulated offset value of all previous stages. Following this rule, the quantitative cascade rule can be summarized, and theoretically an infinite-channel demultiplexer will be obtained. Combining fine-tuning and thermal tuning machining method, this cascade rule can guide the fabrication of stable and tunable demultiplexers in the integrated silicon photonics technology. Our method will provide a potential application for the fabrication of dense wavelength-division multiplexing systems with ultra-large capacity.

Our work studies the spatial distribution of ocean optical turbulence through numerical simulation. We used the ocean optical turbulence phase screen created by the ocean power spectrum to study the effect of varying the refractive index of water via a laser beam propagating through ocean optical turbulence. Because the intensity of ocean optical turbulence is much higher than that of atmospheric turbulence, when simulating the ocean phase screen, traditional methods based on the fast Fourier transform (FFT) algorithm introduce enormous errors in the low-frequency band. Some interpolation algorithms commonly used in atmospheric turbulence simulation can reduce these errors. However, their calculation complexity is always high, so the computational speed is slow. We combine a nonuniform sampling method based on variations in the ocean turbulent power spectrum and the nonuniform FFT algorithm to generate phase screens of ocean optical turbulence. The proposed algorithm can solve the deficiency of the long runtime of the traditional algorithm with even better accuracy. In addition to the traditional phase structure function, we measured the far-field distribution of the laser transmitting through water turbulence caused by temperature contrasts, and the experimental results have outstanding agreement with the simulated far-field image from our ocean phase screen. To the best of our knowledge, this is the first work to verify the accuracy of the phase screen by conducting far-field image experiments.

We experimentally demonstrate the performance of a complex deep neural network (CDNN) equalizer in multi-band super-Nyquist carrier-less amplitude and phase (m-SCAP) modulation for underwater visible-light communications (UVLC). The in-phase and quadrature of the complex data after the match filter and down-sampling are combined as real number pairs and input to the CDNN, which outputs the real part and the imaginary part of the equalized complex data. We compare the different performances of three pulse shapings [better-than-Nyquist pulse shaping (BTN), square-root raised cosine (SRRC), and Xia] utilized in the m-SCAP UVLC system based on the CDNN. We demonstrate that the CDNN equalizer can outperform the traditional equalizer based on the Volterra series and least-mean-square algorithm. The experiments show that the low-roll-off BTN performs best, and the high-roll-off SRRC performs best in the m-SCAP system. In our experiment, the bit-error rate (BER) of the BTN system is 2.6  ×  ^{10  −  4}, the BER of the SRRC is 3.6  ×  10^−  4, and the BER of the Xia system is 7.3  ×  10^−  4 when the spectral efficiency is 3.76  bit  /  s  /  Hz and the signal peak-to-peak voltage (Vpp) is 0.9 V.

Quasicontinuous wave operation of midinfrared quantum cascade lasers are shown to have increased average output power with good beam quality. The ability to enhance average power by a significant fraction of CW power motivates the development of a model to estimate and project performance at varying duty cycles based on a previously developed continuous wave power projection model. The model takes into account pulse to pulse changes in temperature profile to project a transient steady-state temperature distribution. This temperature distribution is used to project both peak and average power in agreement with measurements. Preliminary model projections suggest that high average brightness may be achieved using a reduced number of stages and a greater scaling of core width than would be permissible for CW lasing.

Efficient dielectric–plasmonic interconnect is significant in the design of future electronic–photonic integrated circuits that deal with large data transfer. We propose three designs and analysis of small-footprint couplers that are located at the interface between dielectric and plasmonic waveguides. To the best of our knowledge, our proposed couplers outperform all the works reported in the literature in several aspects including size and coupling efficiency (CE). Our results indicate that the optimum dimensions of the proposed couplers are determined based on whether the coupler is located in the metal side or dielectric side, or the coupler extends equally in both types of materials. The proposed couplers work over a broad frequency range achieving a CE above 88% at the optical communications wavelength 1550 nm. In addition, our results indicate that the CE can be further increased to above 93% by increasing the width of the dielectric waveguide before it is connected to the coupler. Moreover, our proposed designs provide a considerable alignment tolerance, which is needed when aligning the dielectric waveguide to the metal–dielectric–metal waveguide. Our proposed couplers have an impact on the design and miniaturization of nanoscale all-optical devices.

We propose a mathematical model of multiple interference phenomena occurring inside the dielectric cavity of the metasurface absorber. Subsequently, it derived the expressions for electric and magnetic fields within the structure. The model enables one to mathematically interpret wave propagation, standing wave formation, and absorption in terms of electric and magnetic fields. We also discussed the effect of change in the polarization angle of the incident electromagnetic wave on the standing wave pattern and surface-current orientation. The study reveals that the theory is equally applicable for transverse electric and transverse magnetic polarized incident waves. All the mathematical aspects of the theory have been analytically established through the validation of simulation data. It has been found that the theory is in good agreement with the analytically obtained results.

High explosives (HE) represent a high risk to the safety and health of the general population. Therefore, there is an ongoing demand for methods of analysis with limits of detection at trace levels for these hazardous chemicals. Since human hair is one of the main physical attributes of our bodies, the interaction of hair surfaces with HE can be of critical importance in forensic and security applications. Noninvasive in situ approaches such as spectroscopic methods are preferred for elucidating these interactions. Among the spectroscopy-based methodologies, those based on Raman scattering are often favored because the sharpness of the vibrational signals facilitates the precise location of the peak maxima and enables more precise determination of the band shifts due to intermolecular interactions. This preliminary study is based on the detection of the HEs 2,4,6-trinitrotoluene (TNT), 1,3,5-trinitroperhydro-1,3,5-triazine (RDX), and pentaerythritol tetranitrate (PETN) on human hair strands by RS. Raman spectral libraries obtained with a 660-nm laser excitation line were generated for black, bleached, and natural gray hair using direct deposition of PETN, TNT, and RDX. Results obtained were confirmed using portable systems operating at 785 and 1064 nm. Spectral data were preprocessed to correct a high fluorescence background exhibited by the samples due to the indole groups and melanin present in hair. Despite the high fluorescence levels that characterized all the samples, the vibrational signatures that identify the presence of the HEs studied could be detected once the optimum acquisition parameters were established. Among the three hair types studied, gray hair was the best substrate to observe the vibrations of HE crystals on hair. A method limit of detection of 2  ±  1  ng was estimated from image analysis of the HE crystals observed and confirmed as ∼3  ±  1  ng using spectroscopic analysis.

Studies on low-cost sol–gel spin-coated TiO₂ thin film with high refractive index (HRI), which may be used as an intermediate layer to enhance the light extraction efficiency (LEE) of a typical bottom-emitting organic light emitting diode (OLED), are reported. The TiO₂ solution is prepared using titanium tetra iso-propoxide, acetic acid, and ethanol. Spin coating method is used to deposit TiO₂ thin films on glass substrate. Different optical characterizations of as-deposited and annealed (150°C, 300°C, and 450°C) TiO₂ thin films on glass substrate are done, from which different properties of the film are derived. Ellipsometric measurement shows shrinkage in the thickness of the as-deposited TiO₂ films after annealing at different temperatures. X-ray diffraction reveals amorphous TiO₂ formation for all the samples. The RI of the coated film increases with the increase in annealing temperatures. Its value at 633 nm wavelength for the as-deposited film is found to be 2.1, which is quite high and it is seen that this RI can be further increased to 2.78 by annealing the samples at 450°C. This value is comparatively high compared to several other reported values of other researchers. The as-deposited sample reveals highest porosity, which further decreases with rise in annealing temperature. Our calculation of LEE for a typical OLED with the intermediate layer of this HRI as-deposited TiO₂ film shows improvement of the LEE. However for annealed films, the experimentally obtained thicknesses are not adequate for this improvement, but it is shown that by increasing the film thickness, further improvement of LEE is possible.