We propose and experimentally demonstrate a compact Mach–Zehnder interferometer (MZI)-based no-core fiber hollow-core fiber no-core fiber (NHN) structure. Two segments of the no-core fiber embedded on two ends of the hollow-core fiber are employed as beam splitter and combiner, which realize an MZI to measure temperature and strain. With the variation of the temperature and strain, the measurement is achieved by monitoring the transmission spectrum shift of the MZI. The results show that the proposed sensor has a temperature sensitivity of 30.92  pm/°C in the range from 30°C to 90°C, and strain sensitivity of 0.652  pm/μϵ in the range from 0 to 700  μϵ, respectively. The NHN structure exhibits the advantages of compactness, easy fabrication, and low cost, which shows the potential sensing applications of the proposed fiber structure.

This PDF file contains the editorial “Special Section Guest Editorial: Active Electro-Optical Sensing: Phenomenology, Technology, and Applications” for OE Vol. 56 Issue 03

The application of nonline-of-sight (NLoS) vision and seeing around a corner has been demonstrated in the recent past on a laboratory level with round trip path lengths on the scale of 1 m as well as 10 m. This method uses a computational imaging approach to analyze the scattered information of objects which are hidden from the sensor’s direct field of view. A detailed knowledge about the scattering surfaces is necessary for the analysis. The authors evaluate the realization of dual-mode concepts with the aim of collecting all necessary information to enable both the direct three-dimensional imaging of a scene as well as the indirect sensing on hidden objects. Two different sensing approaches, laser gated viewing (LGV) and time-correlated single-photon counting, are investigated operating at laser wavelengths of 532 and 1545 nm, respectively. While LGV sensors have high spatial resolution, their application for NLoS sensing suffers from a low temporal resolution, i.e., a minimal gate width of 2 ns. On the other hand, Geiger-mode single-photon counting devices have high temporal resolution (250 ps), but the array size is limited to some thousand sensor elements. The authors present detailed theoretical and experimental evaluations of both sensing approaches.

This paper reviews the work that has been done at Fraunhofer IOSB (and its predecessor institutes) in the past ten years in the area of laser gated-viewing (GV) in the short-wavelength infrared (SWIR) band. Experimental system demonstrators in various configurations have been built up to show the potential for different applications and to investigate specific topics. The wavelength of the pulsed illumination laser is 1.57  μm and lies in the invisible, retina-safe region allowing much higher pulse energies than for wavelengths in the visible or near-infrared band concerning eye safety. All systems built up, consist of gated Intevac LIVAR® cameras based on EBCCD/EBCMOS detectors sensitive in the SWIR band. This review comprises military and civilian applications in maritime and land domain—in particular vision enhancement in bad visibility, long-range applications, silhouette imaging, 3-D imaging by sliding gates and slope method, bistatic GV imaging, and looking through windows. In addition, theoretical studies that were conducted—e.g., estimating 3-D accuracy or modeling range performance—are presented. Finally, an outlook for future work in the area of SWIR laser GV at Fraunhofer IOSB is given.

Time-correlated single-photon-counting (TCSPC) lidar provides very high resolution range measurements. This makes the technology interesting for three-dimensional imaging of complex scenes with targets behind foliage or other obscurations. TCSPC is a statistical method that demands integration of multiple measurements toward the same area to resolve objects at different distances within the instantaneous field-of-view. Point-by-point scanning will demand significant overhead for the movement, increasing the measurement time. Here, the effect of continuously scanning the scene row-by-row is investigated and signal processing methods to transform this into low-noise point clouds are described. The methods are illustrated using measurements of a characterization target and an oak and hazel copse. Steps between different surfaces of less than 5 cm in range are resolved as two surfaces.

We developed a line scanning time-of-flight (TOF) laser sensor for an intelligent transport system (ITS), which combines wide field-of-view (FOV) receiving optics of 30 deg and a high-speed microelectro mechanical system scanner of 0.9  ms/line with a simple sensor configuration. The newly developed high-aspect ratio photodiode realizes the scanless and wide FOV receiver. The sinusoidal wave intensity modulation method is used for the TOF measurement. This enables the noise reduction of the trans-impedance amplifier by applying the LC-resonant method. The vehicle detection and axle counting, which are the important functions in ITS, are also demonstrated.

Long range identification (ID) or ID at closer range of small targets has its limitations in imaging due to the demand for very high-transverse sensor resolution. This is, therefore, a motivation to look for one-dimensional laser techniques for target ID. These include laser vibrometry and laser range profiling. Laser vibrometry can give good results, but is not always robust as it is sensitive to certain vibrating parts on the target being in the field of view. Laser range profiling is attractive because the maximum range can be substantial, especially for a small laser beam width. A range profiler can also be used in a scanning mode to detect targets within a certain sector. The same laser can also be used for active imaging when the target comes closer and is angularly resolved. Our laser range profiler is based on a laser with a pulse width of 6 ns (full width half maximum). This paper will show both experimental and simulated results for laser range profiling of small boats out to a 6 to 7-km range and a unmanned arrial vehicle (UAV) mockup at close range (1.3 km). The naval experiments took place in the Baltic Sea using many other active and passive electro-optical sensors in addition to the profiling system. The UAV experiments showed the need for a high-range resolution, thus we used a photon counting system in addition to the more conventional profiler used in the naval experiments. This paper shows the influence of target pose and range resolution on the capability of classification. The typical resolution (in our case 0.7 m) obtainable with a conventional range finder type of sensor can be used for large target classification with a depth structure over 5 to 10 m or more, but for smaller targets such as a UAV a high resolution (in our case 7.5 mm) is needed to reveal depth structures and surface shapes. This paper also shows the need for 3-D target information to build libraries for comparison of measured and simulated range profiles. At closer ranges, full 3-D images should be preferable.

In security operations or medical imaging, the main challenge is a need for fast identification, which is possible with a laser range profiler or 2-D–3-D laser imagery. This paper discusses scattering models for high temporal resolution ladar (laser detection and ranging) and 2-D–3-D imagery. We describe electromagnetic models of 1-D, 2-D, and 3-D laser signatures, whose surface reflectance of the objects is calculated with the second-order small-slope approximation for polarized incident laser wave and with a model of the effective permittivity. The physics-based model is designed to provide accurate results for the scattering from complex objects.

Measuring objects with high dynamic range (HDR) reflectivity by coded structured-light, captured stripes are usually seriously distorted by reflectivity, causing inaccurate measurement results. A stripe enhancement method is proposed to deal with the problem. The method is based on the correspondence between phase and intensity of the stripe. First, the phase map of the captured stripe pattern is retrieved by phase-shift algorithm and multiexposure method, where saturation and low contrast of the stripe are eliminated; then, the modulation of stripes is normalized to eliminate the influence of reflectivity; finally, the enhanced stripe is obtained by assembling the modulation and the phase map. Experimental results demonstrate that the method is efficient for objects with HDR reflectivity and achieves high accuracy.

The ability to create three-dimensional (3-D) image models, using registered texel images (fused ladar and digital imagery), is an important topic in remote sensing. These models are automatically generated by matching multiple texel images into a single common reference frame. However, rendering a sequence of independently registered texel images often provides challenges. Although accurately registered, the model textures are often incorrectly overlapped and interwoven when using standard rendering techniques. Consequently, corrections must be done after all the primitives have been rendered by determining the best texture for any viewable fragment in the model. This paper describes a technique to visualize a 3-D model image created from a set of registered texel images. The visualization is determined for each viewpoint. It is, therefore, necessary to determine which textures are overlapping and how to best combine them dynamically during the rendering process. The best texture for a particular pixel can be defined using 3-D geometric criteria, in conjunction with a real-time, view-dependent ranking algorithm. As a result, overlapping texture fragments can now be hidden, exposed, or blended according to their computed measure of reliability. The advantages of this technique are illustrated using artificial and real data examples.

A hyperentanglement-based atmospheric imaging/detection system involving only a signal and an ancilla photon will be considered for optical and infrared frequencies. Only the signal photon will propagate in the atmosphere and its loss will be classical. The ancilla photon will remain within the sensor experiencing low loss. Closed form expressions for the wave function, normalization, density operator, reduced density operator, symmetrized logarithmic derivative, quantum Fisher information, quantum Cramer–Rao lower bound, coincidence probabilities, probability of detection, probability of false alarm, probability of error after M measurements, signal-to-noise ratio, quantum Chernoff bound, time-on-target expressions related to probability of error, and resolution will be provided. The effect of noise in every mode will be included as well as loss. The system will provide the basic design for an imaging/detection system functioning at optical or infrared frequencies that offers better than classical angular and range resolution. Optimization for enhanced resolution will be included. The signal-to-noise ratio will be increased by a factor equal to the number of modes employed during the hyperentanglement process. Likewise, the measurement time can be reduced by the same factor. The hyperentanglement generator will typically make use of entanglement in polarization, energy-time, orbital angular momentum and so on. Mathematical results will be provided describing the system’s performance as a function of loss mechanisms and noise.

Liquid crystal polarization gratings (LCPGs) represent a relatively new technology capable of nonmechanically and efficiently steering light over a large field-of-regard in discrete steps. Due to their reliance on thin liquid crystal cells instead of mechanical moving parts, LCPG beam steering systems are attractive options for steering both active and passive optical sensors, especially in size, weight, and power (SWaP)-constrained platforms. This paper describes recent developments in large-aperture LCPG steering systems and summarizes the performance being achieved.

We developed an underwater three-dimensional (3-D) imaging sensor using a 532-nm laser. The sensor system combines a dome lens with coaxial optics to realize a wide-scanning angle of 120  deg (horizontal)×30   deg (vertical) while having a compact size of 25-cm diameter and 60-cm length. A detector sensitivity time control circuit and a time-to-digital converter are used to detect a small signal and suppress the unwanted backscattered signals due to marine snow. 3-D imaging of the seafloor with 20-m width and 60-m length was demonstrated in the sea around Ishigaki Island, Japan.

This paper develops wave-optics simulations which explore the estimation accuracy of digital-holographic detection for wavefront sensing in the presence of distributed-volume or “deep” turbulence and detection noise. Specifically, the analysis models spherical-wave propagation through varying deep-turbulence conditions along a horizontal propagation path and formulates the field-estimated Strehl ratio as a function of the diffraction-limited sampling quotient and signal-to-noise ratio. Such results will allow the reader to assess the number of pixels, pixel field of view, pixel-well depth, and read-noise standard deviation needed from a focal-plane array when using digital-holographic detection in the off-axis image plane recording geometry for deep-turbulence wavefront sensing.

We demonstrated the range imaging with high resolution of 256×256  pixels and high frame rate of 30 frames per second (fps) using a short wavelength infrared pulsed time-of-flight laser sensor, which is suitable for long range imaging. We additionally demonstrated the long range imaging of more than 1 km and wide field of view imaging of 12  deg× 4  deg, 768×256  pixels, and 10 fps. For these demonstrations, we developed the linear array devices of the aluminum indium arsenide avalanche photodiode array and silicon germanium bipolar complementary metal oxide semiconductor read-out integrated circuit array. We also deployed the flattop beam illumination optics with the beam division and recombination method and realized efficient line shape illumination.

Laser vibrometry based on coherent detection allows noncontact measurements of small-amplitude vibration characteristics of objects. This technique, commonly using the Doppler effect, offers high potential for short-range civil applications and for medium- or long-range applications in defense and security. Most commercially available laser Doppler vibrometers are for short ranges (up to a few tens of meters) and use a single beam from a low-power HeNe laser source (λ=633  nm). Medium- or long-range applications need higher laser output power, and thus, appropriate vibrometers typically operate at 1.5, 2, or 10.6  μm to meet the laser safety regulations. Spatially resolved vibrational information can be obtained from an object by using scanning laser vibrometers. To reduce measuring time and to measure transient object movements and vibrational mode structures of objects, several approaches to multibeam laser Doppler vibrometry have been developed, and some of them are already commercially available for short ranges. We focus on applications in the field of defense and security, such as target classification and identification, including camouflaged or partly concealed targets, and the detection of buried land mines. Examples of civil medium-range applications are also given.

We demonstrate a range imaging pulsed laser sensor with two-dimensional scanning of a transmitted beam and a scanless receiver using a high-aspect avalanche photodiode (APD) array for the eye-safe wavelength. The system achieves a high frame rate and long-range imaging with a relatively simple sensor configuration. We developed a high-aspect APD array for the wavelength of 1.5  μm, a receiver integrated circuit, and a range and intensity detector. By combining these devices, we realized 160×120  pixels range imaging with a frame rate of 8 Hz at a distance of about 50 m.

Two important enabling technologies for pulsed coherent detection wind lidar are the laser and real-time signal processing. In particular, fiber laser is limited in peak power by nonlinear effects, such as stimulated Brillouin scattering (SBS). We report on various technologies that have been developed to mitigate SBS and increase peak power in 1.5-μm fiber lasers, such as special large mode area fiber designs or strain management. Range-resolved wind profiles up to a record range of 16 km within 0.1-s averaging time have been obtained thanks to those high-peak power fiber lasers. At long range, the lidar signal gets much weaker than the noise and special care is required to extract the Doppler peak from the spectral noise. To optimize real-time processing for weak carrier-to-noise ratio signal, we have studied various Doppler mean frequency estimators (MFE) and the influence of data accumulation on outliers occurrence. Five real-time MFEs (maximum, centroid, matched filter, maximum likelihood, and polynomial fit) have been compared in terms of error and processing time using lidar experimental data. MFE errors and data accumulation limits are established using a spectral method.

Coherent image amplification is an important and difficult task that has a wide range of applications including surveillance, detection, and imaging of biological species. Earlier approaches to achieve a high-gain low-noise coherent amplification were met with little success in regards to gain and image quality. These methods included direct coherent beam propagation through the gain medium with inverted population levels as well as two- and four-beam coupling. This paper discusses a high-gain intracavity laser image amplifier. The approach enables time synchronization of the incoming and amplifying signals with an accuracy ≤1  ns. Matching the spectrum of the incoming signal to the cavity modes is no longer necessary with this method and is an essential advantage of the amplifier. Instead, the incoming signal is accepted within the spectral band of the amplifier’s gain material. We have gauged experimentally the performance of the amplifier with a 30-dB gain and an angle of view up to 30 mrad.

A regional elastic-scattering lidar network called Asian dust and aerosol lidar observation network (AD-Net) has operated for 15 years (since 2001) in East Asia. In this network, the extinction coefficient of aerosols below an altitude of 9 km is continuously obtained when conditions are clear; the coefficient is divided into two parts: dust extinction and spherical extinction coefficients. The dust extinction coefficient has been compared with several parameters measured by other instruments and utilized by various studies, including studies on the epidemiology of Asian dust. Recent expansion of the lidar system at some observatories allows more optical parameters to be retrieved at those observatories. All AD-Net products are used for monitoring global environmental change as an activity of global atmospheric watch lidar observation network.

This article discusses the history of laser radar development in America, Europe, and Asia. Direct detection laser radar is discussed for range finding, designation, and topographic mapping of Earth and of extraterrestrial objects. Coherent laser radar is discussed for environmental applications, such as wind sensing and for synthetic aperture laser radar development. Gated imaging is discussed through scattering layers for military, medical, and security applications. Laser microradars have found applications in intravascular studies and in ophthalmology for vision correction. Ghost laser radar has emerged as a new technology in theoretical and simulation applications. Laser radar is now emerging as an important technology for applications such as self-driving cars and unmanned aerial vehicles. It is also used by police to measure speed, and in gaming, such as the Microsoft Kinect.

The design of a compact, dual-polarization, nonscanning lidar system intended to fly in a small, single-engine aircraft for airborne study of freshwater marine ecosystems and mapping of fish schools in mountain lakes is discussed. Design trade-offs are presented with special attention paid to selecting the field of view and telescope aperture diameter. Example results and a comparison with a similar existing lidar system are presented.

The development of a three-beam aerosol backscatter correlation (ABC) light detection and ranging (lidar) to measure wind characteristics for wake vortex and plume tracking applications is discussed. This is a direct detection elastic lidar that uses three laser transceivers, operating at 1030-nm wavelength with ∼10-kHz pulse repetition frequency and nanosec class pulse widths, to directly obtain three components of wind velocities. By tracking the motion of aerosol structures along and between three near-parallel laser beams, three-component wind speed profiles along the field-of-view of laser beams are obtained. With three 8-in. transceiver modules, placed in a near-parallel configuration on a two-axis pan–tilt scanner, the lidar measures wind speeds up to 2 km away. Optical flow algorithms have been adapted to obtain the movement of aerosol structures between the beams. Aerosol density fluctuations are cross-correlated between successive scans to obtain the displacements of the aerosol features along the three axes. Using the range resolved elastic backscatter data from each laser beam, which is scanned over the volume of interest, a three-dimensional map of aerosol density can be generated in a short time span. The performance of the ABC wind lidar prototype, validated using sonic anemometer measurements, is discussed.

Three lidar receiver technologies using the total laser energy required to perform a set of imaging tasks are compared. The tasks are combinations of two collection types (3-D mapping from near and far), two scene types (foliated and unobscured), and three types of data products (geometry only, geometry plus 3-bit intensity, and geometry plus 6-bit intensity). The receiver technologies are based on Geiger mode avalanche photodiodes (GMAPD), linear mode avalanche photodiodes (LMAPD), and optical time-of-flight lidar, which combine rapid polarization rotation of the image and dual low-bandwidth cameras to generate a 3-D image. We choose scenarios to highlight the strengths and weaknesses of various lidars. We consider HgCdTe and InGaAs variations of LMAPD cameras. The InGaAs GMAPD and the HgCdTe LMAPD cameras required the least energy to 3-D map both scenarios for bare earth, with the GMAPD taking slightly less energy. We comment on the strengths and weaknesses of each receiver technology. Six bits of intensity gray levels requires substantial energy using all camera modalities.

Newly emerging accident-reducing, driver-assistance, and autonomous-navigation technology for automobiles is based on real-time three-dimensional mapping and object detection, tracking, and classification using lidar sensors. Yet, the lack of lidar sensors suitable for meeting application requirements appreciably limits practical widespread use of lidar in trucking, public livery, consumer cars, and fleet automobiles. To address this need, a system-engineering perspective to eyesafe lidar-system design for high-level advanced driver-assistance sensor systems and a design trade study including 1.5-μm spot-scanned, line-scanned, and flash-lidar systems are presented. A cost-effective lidar instrument design is then proposed based on high-repetition-rate diode-pumped solid-state lasers and high-gain, low-excess-noise InGaAs avalanche photodiode receivers and focal plane arrays. Using probabilistic receiver-operating-characteristic analysis, derived from measured component performance, a compact lidar system is proposed that is capable of 220 m ranging with 5-cm accuracy, which can be readily scaled to a 360-deg field of regard.

For digital image correlation to be firmly accepted as a validated displacement measurement system in the industrial arena, a measurement must be captured by the analysis system at time of test which confirms that the image correlation hardware and software system is performing as expected. To this end, a method for validating stereo digital image correlation optical test setups is presented, which is traceable to the length standard. The method employs a screen, on which is displayed a randomized speckle pattern of appropriate pitch for the test in question. This speckle pattern is then artificially translated by a known number of pixels on the screen, and image pairs captured of the original and translated speckles. Processing of these data image pairs with image correlation, and calibration of the pixel pitch of the display screen using a traceable measurement system, allows the image correlation test setup to be traceably calibrated in terms of in-plane displacement. The method is shown to be sufficiently sensitive and repeatable to provide a reasonably accurate, traceable validation in a practical environment.

The measurement accuracy of a phase-shifting measurement system is adversely affected by phase errors. This paper presents a theoretical analysis of phase errors caused by nonuniform surface reflectivity, such as varying reflectivity and a sharp change in reflectivity. Based on the analysis, a method to adaptively adjust the maximum input gray level of each pixel in projected fringe patterns to the local reflectivity was proposed to reduce phase errors. Experimental results for a planar checkerboard show that the measurement error can be reduced by 56.6% by using the proposed method.

The infrared signature has been used extensively to discriminate exoatmosphere objects. The performance of the discriminating system is heavily dependent on the choice of features. Although numerous features have been extracted, the shape and micromotion parameters still serve as important additional features. However, there is little research on extracting the shape and micromotion parameters from the infrared signature. An estimating method of the nutation angle and half cone angle for a precession conical object is investigated. The time variation of an IR signature is found to be complicated yet valuable for estimating the micromotion and shape parameters. Based on the fact that the temperature changes a small amount during a short interval, the band exitance of the object in a short time window is approximated to be a constant to reduce the number of the model parameters. A least square estimator is used to estimate the nutation angle and half cone angle from the IR signature. Simulation experiments and the resulting discussion are carried out to demonstrate the effectiveness of the proposed estimation method.

We present an approach for real-time camera tracking with depth stream. Existing methods are prone to drift in sceneries without sufficient geometric information. First, we propose a new weight method for an iterative closest point algorithm commonly used in real-time dense mapping and tracking systems. By detecting uncertainty in pose and increasing weight of points that constrain unstable transformations, our system achieves accurate and robust trajectory estimation results. Our pipeline can be fully parallelized with GPU and incorporated into the current real-time depth camera tracking system seamlessly. Second, we compare the state-of-the-art weight algorithms and propose a weight degradation algorithm according to the measurement characteristics of a consumer depth camera. Third, we use Nvidia Kepler Shuffle instructions during warp and block reduction to improve the efficiency of our system. Results on the public TUM RGB-D database benchmark demonstrate that our camera tracking system achieves state-of-the-art results both in accuracy and efficiency.

Electro-optical (EO) tracking systems, while exhibiting strong nonlinear characteristics, are difficult to accurately model. Nonlinear resistance torque is proposed to describe the system’s nonlinear phenomenon and the genetic algorithm is used to identify model parameters. The model’s root-mean-square error (RMSE) was reduced using nonlinear resistance torque by 2.5 times compared to the Stribeck friction model and by 12 times compared to the linear model. Under the identified model, the system’s nonlinearity was effectively compensated. The results demonstrate the feasibility of the proposed method for the identification of EO tracking systems.

An increasing number of heavy machinery and vehicles have come into service, giving rise to a significant concern over protecting these high-security systems from misuse. Conventionally, authentication performed merely at the initial login may not be sufficient for detecting intruders throughout the operating session. To address this critical security flaw, a line-scan continuous hand authentication system with the appearance of an operating rod is proposed. Given that the operating rod is occupied throughout the operating period, it can be a possible solution for unobtrusively recording the personal characteristics for continuous monitoring. The ergonomics in the physiological and psychological aspects are fully considered. Under the shape constraints, a highly integrated line-scan sensor, a controller unit, and a gear motor with encoder are utilized. This system is suitable for both the desktop and embedded platforms with a universal serial bus interface. The volume of the proposed system is smaller than 15% of current multispectral area-based camera systems. Based on experiments on a database with 4000 images from 200 volunteers, a competitive equal error rate of 0.1179% is achieved, which is far more accurate than the state-of-the-art continuous authentication systems using other modalities.

Per-pixel hand detection plays an important role in many human–computer interaction applications while accurate and robust hand detection remains a challenging task due to the large appearance variance of hands in images. We introduce a per-pixel hand detection system using one single depth image. We propose a circle sampling depth-context feature for hand regions representation, and a multilayered hand detection model is built for hand regions detection. Finally, a postprocessing step based on spatial constraints is applied to refine the detection results and further improve the detection accuracy. We evaluate the accuracy of our method on a public dataset and investigate the effect of key parameters in our system. The results of the qualitative and quantitative evaluation reveal that the proposed method performs well on per-pixel hand detection tasks. Furthermore, an additional experiment on hand parts segmentation proves that the depth-context feature has a generalization power for more complex multiclass classification tasks.

We present our work regarding the evaluation of protection measures against laser dazzling for imaging devices. Different approaches for the evaluation of dazzled sensor images are investigated to estimate the loss of information due to the dazzle spot: (1) counting the number of overexposed pixels, (2) based on triangle orientation discrimination, and (3) using the structural similarity index. The evaluation approaches are applied on experimental data obtained with two different sensors hardened against laser dazzling. The hardening concept of the first sensor is based on the combination of a spatial light modulator and wavelength multiplexing. This protection concept allows spatially and spectrally resolved suppression of laser radiation within the sensor’s field-of-view. The hardening concept of the second sensor utilizes the principle of “complementary bands.” The optical setup resembles a common three-chip camera, with the difference that dedicated filters with steep edges replace the regular spectral band filters. Although this concept does not really represent a “protection measure,” it allows the sensor to provide information even in laser dazzling situations. The data for the performance evaluation were acquired both in a laboratory setup using test charts comprising triangles of different size and orientation as well as in field trials.

Based on zero-padding and frequency-domain self-filtering, a robust and effective spectrum centroid carrier-removal method for carrier interferogram analysis is proposed. The interferogram is first spatial zero padded to increase the frequency resolution, and then the frequency domain self-filtering is carried out to suppress the noise and enhance the carrier component. Finally, the carrier frequencies are estimated by calculating the spectral centroid of the upside lobe. The simulations and experiments are carried out to testify the feasibility of this method. In addition, some factors, such as the carrier frequency values, the level of random noise, and the window size of the spectral filter, are analyzed and discussed. Compared with existing carrier-removal methods, the proposed method is effective and accurate for suppressing the carrier-removal error.

Internally scattered light in a Fizeau interferometer is generated from dust, defects, imperfect coating of the optical components, and multiple reflections inside the collimator lens. It produces additional noise fringes in the observed interference image and degrades the repeatability of the phase measurement. A method to reduce the phase measurement error is proposed, in which the test surface is mechanically translated between each phase measurement in addition to an ordinary phase shift of the reference surface. It is shown that a linear combination of several measured phases at different test surface positions can reduce the phase errors caused by the scattered light. The combination can also compensate for the nonuniformity of the phase shift that occurs in spherical tests. A symmetric sampling of the phase measurements can cancel the additional primary spherical aberrations that occur when the test surface is out of the null position of the confocal configuration.

An automatic focus determination method in digital holography that utilizes the cosine score (CS) of the inner angle between the vectors that result from adjacent axial reconstructed amplitude images is proposed. This method is based on the fact that the optical field near the focus plane contains more regular features of an object than the defocused region. Further, a modified CS (MCS) autofocusing method is proposed to extend the application range and improve the performance of the proposed method. First, a low-pass filter is designed for the object term holograms in the frequency domain. Second, the standardized Z-scores method is applied along the axis to the amplitude images reconstructed from the filtered hologram. Finally, the MCS of the standardized amplitude images is calculated by an improved cosine-like formula. Next, the focus plane is found at the minimum MCS for different types of objects. This method enables accurate focus determination for all types of objects, as well as for the image region with a slow change or small size, which offers time savings and the precise autofocusing of objects with large longitudinal volumes. The simulations and experimental results on a United States Air Force resolution chart and living cells are presented, illustrating the efficiency and potential of the methods.

A stationary phase method and ambiguity function (AF) method were used to conduct an analytical study of a modulation transfer function (MTF). From the perspective of mathematic models, there was a double integral relationship between the MTF and system wave aberrations. Based on the stationary phase and AF methods, the wave aberration term in the MTF expression was analytically discussed to achieve an aberration function derivation and extreme point analysis, thereby obtaining the analytical expressions of an every field-of-view (FOV) MTF and defocusing distance (i.e., the deviation of an MTF optimal imaging plane from an axial imaging plane) in the full FOV of an optical system. From the obtained image quality evaluation function, the defocusing distance and system image quality could be effectively controlled in a full FOV, which was verified using the examples.

The vertical optical measurement method with coaxial projection and imaging can measure the complex surface or step-like surface because it helps to avoid the shadow and occlusion. Instead of the phase calculation and phase unwrapping processing, only the modulation information is needed to reconstruct the surface of the tested object by this method. To improve the accuracy of the modulation calculation at each scanning position from only one fringe pattern, this paper introduces the two-dimensional (2-D) wavelet transform into modulation measurement profilometry. The relationship between the 2-D complex wavelet transform coefficients and the modulation distribution of the fringe is deduced. The computer simulation and experiment are carried out to verify that the method based on the 2-D complex wavelet analysis offers higher accuracy than that based on the Fourier transform analysis.

An effective correction model is proposed to eliminate the refraction error effect caused by an optical window of a furnace in digital image correlation (DIC) deformation measurement under high-temperature environment. First, a theoretical correction model with the corresponding error correction factor is established to eliminate the refraction error induced by double-deck optical glass in DIC deformation measurement. Second, a high-temperature DIC experiment using a chromium–nickel austenite stainless steel specimen is performed to verify the effectiveness of the correction model by the correlation calculation results under two different conditions (with and without the optical glass). Finally, both the full-field and the divisional displacement results with refraction influence are corrected by the theoretical model and then compared to the displacement results extracted from the images without refraction influence. The experimental results demonstrate that the proposed theoretical correction model can effectively improve the measurement accuracy of DIC method by decreasing the refraction errors from measured full-field displacements under high-temperature environment.

We proposed an optical sensing scheme, based on the mechanism of weak measurement, to detect the sonic pressure transversally without any disturbance. According to the theory of weak measurement in frequency domain, a tiny phase difference between the two eigenstates of the system could be detected, depending on the shift of central wavelength of output spectra. The optical path difference between two paths of a Mach–Zehnder interferometer is induced by the varied acoustic waves propagating in liquid. The propagating direction of the testing light beam is orthogonal to that of the acoustic wave. Thus, the phase shift corresponding to the changing acoustic pressure could be monitored transversally. The experiment with the hydrophone is implemented as a reference to demonstrate the feasibility and authenticity of this weak measurement scheme. With this method, an optical phase resolution of 1.12×10−5  rad was acquired in such a weak measurement system.

The continuous development of laser systems toward more compact and efficient devices constitutes an increasing threat to electro-optical imaging sensors, such as complementary metal–oxide–semiconductors (CMOS) and charge-coupled devices. These types of electronic sensors are used in day-to-day life but also in military or civil security applications. In camera systems dedicated to specific tasks, micro-optoelectromechanical systems, such as a digital micromirror device (DMD), are part of the optical setup. In such systems, the DMD can be located at an intermediate focal plane of the optics and it is also susceptible to laser damage. The goal of our work is to enhance the knowledge of damaging effects on such devices exposed to laser light. The experimental setup for the investigation of laser-induced damage is described in detail. As laser sources, both pulsed lasers and continuous-wave (CW)-lasers are used. The laser-induced damage threshold is determined by the single-shot method by increasing the pulse energy from pulse to pulse or in the case of CW-lasers, by increasing the laser power. Furthermore, we investigate the morphology of laser-induced damage patterns and the dependence of the number of destructive device elements on the laser pulse energy or laser power. In addition to the destruction of single pixels, we observe aftereffects, such as persistent dead columns or rows of pixels in the sensor image.

We propose a calibration method of a fringe projection system for surface profile measurement. The calibration method is divided into two parts: (1) phase to z calibration based on measuring the absolute phase distributions of a flat calibration board placed at several different known z positions. (2) Camera calibration based on Zhang’s method of imaging a checkerboard with different poses. Experiments of a flat plane, a sphere, and a Gorky sculpture demonstrate the validity and accuracy of the proposed technique.

Transforming polarization states of light from one to another in a controlled manner can be achieved with a minimal gadget consisting of two-quarter waveplates. Using a geometric approach, we demonstrate this result for transforming any completely polarized states of light (CPSL) to any other CPSL, including the orthogonal states. Furthermore, we describe a working algorithm for identifying the fast axis setting of the two-quarter waveplates essential for the state transformation.

In-situ intelligent manufacturing for large-volume equipment requires industrial robots with absolute high-accuracy positioning and orientation steering control. Conventional robots mainly employ an offline calibration technology to identify and compensate key robotic parameters. However, the dynamic and static parameters of a robot change nonlinearly. It is not possible to acquire a robot’s actual parameters and control the absolute pose of the robot with a high accuracy within a large workspace by offline calibration in real-time. This study proposes a real-time online absolute pose steering control method for an industrial robot based on six degrees of freedom laser tracking measurement, which adopts comprehensive compensation and correction of differential movement variables. First, the pose steering control system and robot kinematics error model are constructed, and then the pose error compensation mechanism and algorithm are introduced in detail. By accurately achieving the position and orientation of the robot end-tool, mapping the computed Jacobian matrix of the joint variable and correcting the joint variable, the real-time online absolute pose compensation for an industrial robot is accurately implemented in simulations and experimental tests. The average positioning error is 0.048 mm and orientation accuracy is better than 0.01 deg. The results demonstrate that the proposed method is feasible, and the online absolute accuracy of a robot is sufficiently enhanced.

For single slabs of uniform material, such as bulk semiconductors, we derive closed-form expressions for absorption and reflection coefficients, α and R , respectively, in terms of measured reflectance and transmittance, R m and T m . The formula for α can replace the several commonly used approximations for α as a function of T m and in particular does not require α d ≫ 1 , where d is the thickness. Thus, it can be applied to weak impurity absorptions, such as Fe absorption in Fe-doped GaN. Finally, the real ( η ) and imaginary ( κ ) parts of the index of refraction ( n = η + i κ ) can be obtained from α and R and agree well with η and