The Top 10 2017 reviewers are acknowledged in this editorial.

Optical Engineering thanks these individuals for reviewing manuscripts in 2017.

Dense wavelength division multiplexed (DWDM) transmission equal to 1.2 Tbps over 25 to 50 km across standard single-mode fiber (SSMF) in the C band is performed based on an optical frequency comb generator. Sixty-one flattened optical frequency tones were realized with 30-GHz frequency spacing, high side-mode suppression ratio over 35 dB, and minimum amplitude difference was realized using amplitude modulator for first time in cascade mode with two Mach–Zehnder modulators (MZMs) where all the modulators were tailored by RF signals. 20×61  Gbps DWDM-based differential quadrature phase shift keying modulated signals were successfully transmitted over SSMF and analyze its transmission capability for range of 25 to 50 km with acceptable power penalties and bit error rates.

A virtually optical system with a hierarchical structure is designed for optical verification. At each hierarchical level, two phase-only masks are alternately generated using an iterative approach and then are sparsified. All sparse phase-only masks generated at the lower hierarchical levels are fixed and applied as constraints at the higher hierarchical level. Since sparse phase-only masks are applied for the decoding, the recovered images are invisible and instead can be further verified by a nonlinear correlation algorithm. The results are presented to show validity of the proposed method, and the proposed method provides a promising strategy for optical verification.

This guest editorial introduces the Special Section on Quantum and Interband Cascade Lasers with Applications.

We report on the experimental demonstration of a high-power (>100  mW) quantum cascade superluminescent (SL) emitter employing a spiral cavity with a passive loop back facet. By replacing the rounded, wet-etched, antireflection coated back facet of a spiral cavity with a passive loop facet, the lasing action was suppressed (i.e., threshold current density increased) due to a decrease in the back facet reflectivity. This type of facet allowed us to achieve a twofold increase in the peak SL power by employing a 16-mm-long spiral cavity.

We report interband cascade light-emitting devices (ICLEDs) emitting at peak wavelengths of 3.1 to 3.2  μm that display higher maximum output powers, radiances, and efficiencies than any earlier midwave-infrared LEDs when operated at 10 to 105°C. To enhance the output power, we split the ICLED’s 22 active stages into four groups positioned at antinodes of the optical field so that the emission interferes constructively when reflected at near-normal incidence from the metal contact of the epitaxial-side-down mounted device. At an applied bias of 9.6 V and injection current of 0.6 A, an ICLED with mesa diameter of 400  μm produces 3.1 mW of continuous-wave output power at T=10°C, which corresponds to a radiance of 0.79  W/cm2/sr. The same device generates 1.7 mW at T=105°C.

We demonstrate standoff detection of turbulent mixed-chemical plumes using a broadly tunable external cavity quantum cascade laser (ECQCL). The ECQCL was directed through plumes of mixed methanol/ethanol vapor to a partially reflective surface located 10 m away. The reflected power was measured as the ECQCL was swept over its tuning range of 930 to 1065  cm−1 (9.4 to 10.8  μm) at rates up to 200 Hz. Analysis of the transmission spectra though the plume was performed to determine chemical concentrations with a time resolution of 0.005 s. Comparison of multiple spectral sweep rates of 2, 20, and 200 Hz shows that higher sweep rates reduce effects of atmospheric and source turbulence, resulting in lower detection noise and more accurate measurement of the rapidly changing chemical concentrations. Detection sensitivities of 0.13 ppm*m for MeOH and 1.2 ppm*m for EtOH are demonstrated for a 200-Hz spectral sweep rate and normalized to a 1-s detection time.

We report single-mode midinfrared laser beam delivery through a 50-cm-long tapered hollow-core waveguide (HCW) having bore diameter linearly increasing from 200 to 260  μm. We performed theoretical calculations to identify the best HCW-laser coupling conditions in terms of optical losses and single-mode fiber output. To validate our modeling, we coupled the HCW with an interband cascade laser and four quantum cascade lasers with their emission wavelengths spanning 3.5 to 7.8  μm, using focusing lenses with different focal lengths. With the best coupling conditions, we achieved single-mode output in the investigated 3.5 to 7.8  μm spectral range, with minimum transmission losses of 1.27 dB at 6.2  μm.

We review recent advances in chemical sensing applications based on surface emitting ring quantum cascade lasers (QCLs). Such lasers can be implemented in monolithically integrated on-chip laser/detector devices forming compact gas sensors, which are based on direct absorption spectroscopy according to the Beer–Lambert law. Furthermore, we present experimental results on radio frequency modulation up to 150 MHz of surface emitting ring QCLs. This technique provides detailed insight into the modulation characteristics of such lasers. The gained knowledge facilitates the utilization of ring QCLs in combination with spectroscopic techniques, such as heterodyne phase-sensitive dispersion spectroscopy for gas detection and analysis.

We report on a comparison study of the electrical and optical properties of a set of device structures with different numbers of cascade stages, type-II superlattice (T2SL) absorber thickness, and doping variations, as well as a noncurrent-matched interband cascade infrared photodetectors (ICIP) structure with equal absorbers. Multistage ICIPs were demonstrated to be capable of operating at high temperatures at zero-bias with superior carrier transport over comparable conventional one-stage detectors. Based on the temperature dependence and bias sensitivity of their responsivities with various absorber thicknesses, the diffusion length is estimated to be between 0.6 and 1.0  μm for T2SL materials at high temperatures (>250  K). A comparison of responsivities between current matched ICIPs with varied absorber thicknesses and noncurrent-matched ICIPs with equal absorbers shows that the current-matching among cascade stages is necessary to maximize responsivity. Additionally, electrical gain exceeding unity is demonstrated in these detectors in the reverse-illumination configuration.

We present terahertz (THz) surface emission by difference frequency generation in nonlinear quantum cascade lasers operating at room temperature. The device comprises two separate, transversely superimposed gratings, one for selective feedback for the midinfrared (MIR) modes and one for normal surface emission of the generated THz radiation. This allows for narrow far-field THz emission with an enhanced extraction over the entire device length with improved electrical and thermal properties compared to previous devices relying on Cherenkov phase matching. The MIR grating is realized as a higher order loss-coupled distributed feedback grating for single-mode emission at two distinct wavelengths. Its position determines the position of the standing wave pattern of the MIR modes and of the nonlinear polarization wave. This allows for a precise placement of the second-order top grating, which leads to perpendicular surface emission. The device emits in a single-lobed far-field with a full width half maximum of 3.5 deg in single-mode operation at room temperature.

An interband cascade laser multiemitter with single-mode distributed feedback (DFB) emission at two wavelengths is presented. Continuous-wave laser operation is measured from 0°C to 40°C with threshold currents of around 25 mA and output powers of around 9 mW at 20°C. The ridge waveguide DFB structures are monolithically integrated with a spacing of 70  μm and each is provided with an individual metal DFB grating to select specific single-mode wavelengths of interest for absorption spectroscopy. The emission windows at 3.92 and 4.01  μm are targeting hydrogen sulfide and sulfur dioxide, which are of importance for industrial applications since both gases are reagents of the Claus process in sulfur recovery units, recovering elemental sulfur from gaseous hydrogen sulfide.

Recent research has shown that free-running quantum cascade lasers are capable of producing frequency combs in midinfrared and THz regions of the spectrum. Unlike familiar frequency combs originating from mode-locked lasers, these do not require any additional optical elements inside the cavity and have temporal characteristics that are dramatically different from the periodic pulse train of conventional combs. Frequency combs from quantum cascade lasers are characterized by the absence of sharp pulses and strong frequency modulation, periodic with the cavity round trip time but lacking any periodicity within that period. To explicate for this seemingly perplexing behavior, we develop a model of the gain medium using optical Bloch equations that account for hole burning in spectral, spatial, and temporal domains. With this model, we confirm that the most efficient mode of operation of a free-running quantum cascade laser is indeed a pseudorandom frequency-modulated field with nearly constant intensity. We show that the optimum modulation period is commensurate with the gain recovery time of the laser medium and the optimum modulation amplitude is comparable to the gain bandwidth, behavior that has been observed in the experiments.

We report on mid-IR spectroscopic measurements performed with rapidly tunable external cavity quantum cascade lasers (EC-QCLs). Fast wavelength tuning in the external cavity is realized by a microoptoelectromechanical systems (MOEMS) grating oscillating at a resonance frequency of about 1 kHz with a deflection amplitude of up to 10 deg. The entire spectral range of the broadband QCL can therefore be covered in just 500  μs, paving the way for real-time spectroscopy in the mid-IR region. In addition to its use in spectroscopic measurements conducted in backscattering and transmission geometry, the MOEMS-based laser source is characterized regarding pulse intensity noise, wavelength reproducibility, and spectral resolution.

Experimental and model results for 15-stage broad area quantum cascade lasers (QCLs) are presented. Continuous wave (CW) power scaling from 1.62 to 2.34 W has been experimentally demonstrated for 3.15-mm long, high reflection-coated QCLs for an active region width increased from 10 to 20  μm. A semiempirical model for broad area devices operating in CW mode is presented. The model uses measured pulsed transparency current, injection efficiency, waveguide losses, and differential gain as input parameters. It also takes into account active region self-heating and sublinearity of pulsed power versus current laser characteristic. The model predicts that an 11% improvement in maximum CW power and increased wall-plug efficiency can be achieved from 3.15  mm×25  μm devices with 21 stages of the same design, but half doping in the active region. For a 16-stage design with a reduced stage thickness of 300 Å, pulsed rollover current density of 6  kA/cm2, and InGaAs waveguide layers, an optical power increase of 41% is projected. Finally, the model projects that power level can be increased to ∼4.5  W from 3.15  mm×31  μm devices with the baseline configuration with T0 increased from 140 K for the present design to 250 K.

The external cavity tunable mid-infrared emitters based on Littrow configuration and utilizing three stages type-I quantum well cascade diode laser gain elements were designed and fabricated. The free-standing coated 7.5-μm-wide ridge waveguide lasers generated more than 30 mW of continuous wave power near 3.25  μm at 20°C when mounted epi-side-up on copper blocks. The external cavity lasers (ECLs) utilized 2-mm-long gain chips with straight ridge design and anti-/neutral-reflection coated facets. The ECLs demonstrated narrow spectrum tunable operation with several milliwatts of output power in spectral region from 3.05 to 3.25  μm corresponding to ∼25  meV of tuning range.

The modal characteristics of nonresonant five-element phase-locked arrays of 4.7-μm emitting quantum cascade lasers (QCLs) have been studied using spectrally resolved near- and far-field measurements and correlated with results of device simulation. Devices are fabricated by a two-step metal-organic chemical vapor deposition process and operate predominantly in an in-phase array mode near threshold, although become multimode at higher drive levels. The wide spectral bandwidth of the QCL’s core region is found to be a factor in promoting multispatial-mode operation at high drive levels above threshold. An optimized resonant-array design is identified to allow sole in-phase array-mode operation to high drive levels above threshold, and indicates that for phase-locked laser arrays full spatial coherence to high output powers does not require full temporal coherence.

While midinfrared radiation can be used to identify and quantify numerous chemical species, contemporary broadband midinfrared spectroscopic systems are often hindered by large footprints, moving parts, and high power consumption. In this work, we demonstrate multiheterodyne spectroscopy (MHS) using interband cascade lasers, which combines broadband spectral coverage with high spectral resolution and energy-efficient operation. The lasers generate up to 30 mW of continuous-wave optical power while consuming <0.5  W of electrical power. A computational phase and timing correction algorithm is used to obtain kHz linewidths of the multiheterodyne beat notes and up to 30 dB improvement in signal-to-noise ratio. The versatility of the multiheterodyne technique is demonstrated by performing both rapidly swept absorption and dispersion spectroscopic assessments of low-pressure ethylene (C2H4) acquired by extracting a single beat note from the multiheterodyne signal, as well as broadband MHS of methane (CH4) acquired with all available beat notes with microsecond temporal resolution and an instantaneous optical bandwidth of ∼240  GHz. The technology shows excellent potential for portable and high-resolution solid-state spectroscopic chemical sensors operating in the midinfrared.

For many applications, optical emission power of many watts in the midinfrared part of the optical spectrum is required. Quantum cascade lasers for these applications typically contain 30 to 40 cascades and have laser ridge widths of about 10  μm. These lasers cannot, however, be readily scaled up in power. This paper demonstrates a path for further power scaling that is based on broad stripes with fewer than 15 cascades. Although the confinement factor Γ for this design approach is lower than for designs with 30 to 40 cascades, the ability to efficiently extract heat from the top and bottom of the lasers more than offsets the Γ disadvantage. We expect this approach to deliver tens of watts of power with high beam quality leading to high brilliance. The paper discusses the design approach, thermal conduction considerations, and confinement factor. Room temperature cw operation of broad-area lasers based on this approach is demonstrated and described.

The effects of gamma radiation on Fabry–Perot interband cascade lasers (ICLs) were investigated. Two ICLs were exposed to cobalt-60 gamma rays for a total dose of 500 krad(Si) each. The ICLs do not show any evidence of changes in performance, including output power, threshold current, slope efficiency, or spectral frequency. These results demonstrate that ICLs are insensitive to gamma irradiation up to exposure rates above those normally encountered within a shielded spacecraft.

Quantum cascade lasers (QCLs) that employ metamorphic buffer layers as substrates of variable lattice constant have been designed for emission in the 3.0- to 3.5-μm wavelength range. Theoretical analysis of the active-region (AR) energy band structure, while using an 8-band k•p model, reveals that one can achieve both effective carrier-leakage suppression as well as fast carrier extraction in QCL structures of relatively low strain. Significantly lower indium-content quantum wells (QWs) can be employed for the AR compared to QWs employed for conventional short-wavelength QCL structures grown on InP, which, in turn, is expected to eliminate carrier leakage to indirect-gap valleys (X, L). An analysis of thermo-optical characteristics for the complete device design indicates that high-Al-content AlInAs cladding layers are more effective for both optical confinement and thermal dissipation than InGaP cladding layers. An electroluminescence-spectrum full-width half-maximum linewidth of 54.6 meV is estimated from interface roughness scattering and, by considering both inelastic and elastic scattering, the threshold-current density for 3.39-μm-emitting, 3-mm-long back-facet-coated QCLs is projected to be 1.40  kA/cm2.

We present recent progress on the development of monolithic, broadband, widely tunable midinfrared quantum cascade lasers. First, we show a broadband midinfrared laser gain realized by a heterogeneous quantum cascade laser based on a strain balanced composite well design of Al0.63In0.37As/Ga0.35In0.65As/Ga0.47In0.53As. Single mode emission between 5.9 and 10.9  μm under pulsed mode operation was realized from a distributed feedback laser array, which exhibited a flat current threshold across the spectral range. Using the broadband wafer, a monolithic tuning between 6.2 and 9.1  μm was demonstrated from a beam combined sampled grating distributed feedback laser array. The tunable laser was utilized for a fast sensing of methane under pulsed operation. Transmission spectra were obtained without any moving parts, which showed excellent agreement to a standard measurement made by a Fourier transform infrared spectrometer.

Due to its high toxicity, monitoring of hydrogen sulfide (H2S) concentration is essential in many industrial sites (such as natural gas extraction sites, petroleum refineries, geothermal power plants, or waste water treatment facilities), which require sub-parts-per-million sensitivities. We report on a quantum cascade laser-based spectroscopic system for detection of H2S in the midinfrared at ∼7.2  μm. We present a sensor design utilizing Herriott multipass cell and a wavelength modulation spectroscopy to achieve a detection limit of 140 parts per billion for 1-s integration time.

We report broadly tunable terahertz (THz) sources based on intracavity Cherenkov difference-frequency generation in quantum cascade lasers transfer-printed on high-resistivity silicon substrates. Spectral tuning from 1.3 to 4.3 THz was obtained from a 2-mm long laser chip using a modified Littrow external cavity setup. The THz power output and the midinfrared-to-THz conversion efficiency of the devices transferred on silicon are dramatically enhanced, compared with the devices on a native semi-insulating InP substrate. Enhancement is particularly significant at higher THz frequencies, where the tail of the Reststrahlen band results in a strong absorption of THz light in the InP substrate.

InAs-based interband cascade (IC) lasers with improved optical confinement have achieved high-temperature operation with a threshold current density as low as 333  A/cm2 at 300 K for emission at 6003 nm. The threshold current density is the lowest ever reported among semiconductor midinfrared lasers at similar wavelengths. These InAs-based IC devices lased in pulsed mode at temperatures up to 357 K near 6.28  μm. A narrow-ridge device was able to operate in continuous-wave mode at temperatures up to 293 K at 6.01  μm.

Tracking clustering objects with similar appearances simultaneously in collective scenes is a challenging task in the field of collective motion analysis. Recent work on clustering-object tracking often suffers from poor tracking accuracy and terrible real-time performance due to the neglect or the misjudgment of the motion differences among objects. To address this problem, we propose a subgroup motion pattern segmentation framework based on a multilayer clustering structure and establish spatial constraints only among objects in the same subgroup, which entails having consistent motion direction and close spatial position. In addition, the subgroup segmentation results are updated dynamically because crowd motion patterns are changeable and affected by objects’ destinations and scene structures. The spatial structure information combined with the appearance similarity information is used in the structure preserving object tracking framework to track objects. Extensive experiments conducted on several datasets containing multiple real-world crowd scenes validate the accuracy and the robustness of the presented algorithm for tracking objects in collective scenes.

In ground-based large aperture solar telescopes, the speckle image reconstruction technique combined with adaptive optics (AO) correction is generally used to get near diffraction-limited solar images. In order to select the high quality images from the AO-corrected high resolution solar image sequence, an automated no-reference image quality assessment (IQA) is needed. According to the noise characteristics of solar AO images, an IQA metric based on an image power spectrum and human visual system is developed. By the incorporation of noise masking and shifting the spatial frequency range of summation, our IQA metric could select sharper images with less noise than previous works based on the image power spectrum, even if there is image scaling. Compared with existing general-purpose IQA metrics and previous metrics specifically designed for solar IQA, experimental results verify that the proposed metric gains better performance whether on robustness to blur and noise, or on selecting high quality frames containing solar granulations, sunspots, or both of them.

This paper explores the problem of signal processing in optical current transformers (OCTs). Based on the noise characteristics of OCTs, such as overlapping signals, noise frequency bands, low signal-to-noise ratios, and difficulties in acquiring statistical features of noise power, an improved standard Kalman filtering algorithm was proposed for direct current (DC) signal processing. The state-space model of the OCT DC measurement system is first established, and then mixed noise can be processed by adding mixed noise into measurement and state parameters. According to the minimum mean squared error criterion, state predictions and update equations of the improved Kalman algorithm could be deduced based on the established model. An improved central difference Kalman filter was proposed for alternating current (AC) signal processing, which improved the sampling strategy and noise processing of colored noise. Real-time estimation and correction of noise were achieved by designing AC and DC noise recursive filters. Experimental results show that the improved signal processing algorithms had a good filtering effect on the AC and DC signals with mixed noise of OCT. Furthermore, the proposed algorithm was able to achieve real-time correction of noise during the OCT filtering process.

The High-Energy Replicated Optics to Explore the Sun (HEROES) program is a balloon-borne x-ray telescope mission to observe hard x-rays (∼20 to 70 keV) from the sun and multiple astrophysical targets. The payload consists of eight mirror modules with a total of 114 optics that are mounted on a 6-m-long optical bench. Each mirror module is complemented by a high-pressure xenon gas scintillation proportional counter. Attached to the payload is a camera that acquires star fields and then matches the acquired field to star maps to determine the pointing of the optical bench. Slight misalignments between the star camera, the optical bench, and the telescope elements attached to the optical bench may occur during flight due to mechanical shifts, thermal gradients, and gravitational effects. These misalignments can result in diminished imaging and reduced photon collection efficiency. To monitor these misalignments during flight, a supplementary Bench Alignment Monitoring System (BAMS) was added to the payload. BAMS hardware comprises two cameras mounted directly to the optical bench and rings of light-emitting diodes (LEDs) mounted onto the telescope components. The LEDs in these rings are mounted in a predefined, asymmetric pattern, and their positions are tracked using an optical/digital correlator. The BAMS analysis software is a digital adaption of an optical joint transform correlator. The aim is to enhance the observational proficiency of HEROES while providing insight into the magnitude of mechanically and thermally induced misalignments during flight. Results from a preflight test of the system are reported.

An adaptive target binarization method based on a dual-camera system that contains two dynamic vision sensors was proposed. First, a preprocessing procedure of denoising is introduced to remove the noise events generated by the sensors. Then, the complete edge of the target is retrieved and represented by events based on an event mosaicking method. Third, the region of the target is confirmed by an event-to-event method. Finally, a postprocessing procedure of image open and close operations of morphology methods is adopted to remove the artifacts caused by event-to-event mismatching. The proposed binarization method has been extensively tested on numerous degraded images with nonuniform illumination, low contrast, noise, or light spots and successfully compared with other well-known binarization methods. The experimental results, which are based on visual and misclassification error criteria, show that the proposed method performs well and has better robustness on the binarization of degraded images.

A method for moving target detection and segmentation using Markov random field (MRF)-based evaluation metric in infrared videos has been proposed. Starting with the most useful seeds of a moving object, which are extracted based on the “holes” effect of temporal difference; the proposed method employs a region growing method using local gray information and a spatial and temporal MRF model-based evaluation metric without ground truth for moving target segmentation in infrared videos. The segmented mask of a moving target is grown from the most useful seeds using the region growing method with thresholds. The proposed evaluation metric is utilized to determine the best growing threshold, where the performance of moving target segmentation is measured by that of segmented mask’s boundary. Thus, an MRF modeling for each boundary point of the segmented mask in spatial and temporal directions was considered by us. This problem is formulated using maximum a posteriori (MAP) estimation principle. At last, the global optimum of MRF-MAP framework is achieved using simulated annealing algorithm. The best segmented mask of a moving target is grown from the most useful seeds with the best growing threshold. Experimental results are reported to demonstrate the accuracy and robustness of our algorithm.

Monocular image-based three-dimensional (3-D) human pose recovery aims to retrieve 3-D poses using the corresponding two-dimensional image features. Therefore, the pose recovery performance highly depends on the image representations. We propose a multispectral embedding-based deep neural network (MSEDNN) to automatically obtain the most discriminative features from multiple deep convolutional neural networks and then embed their penultimate fully connected layers into a low-dimensional manifold. This compact manifold can explore not only the optimum output from multiple deep networks but also the complementary properties of them. Furthermore, the distribution of each hierarchy discriminative manifold is sufficiently smooth so that the training process of our MSEDNN can be effectively implemented only using few labeled data. Our proposed network contains a body joint detector and a human pose regressor that are jointly trained. Extensive experiments conducted on four databases show that our proposed MSEDNN can achieve the best recovery performance compared with the state-of-the-art methods.

Object recognition and delineation is an important task in many environments, such as in crime scenes and operating rooms. Marking evidence or surgical tools and attracting the attention of the surrounding staff to the marked objects can affect people’s lives. We present an optical system comprising a camera, computer, and small laser projector that can detect and delineate objects in the environment. To prove the optical system’s concept, we show that it can operate in a hypothetical crime scene in which a pistol is present and automatically recognize and segment it by various computer-vision algorithms. Based on such segmentation, the laser projector illuminates the actual boundaries of the pistol and thus allows the persons in the scene to comfortably locate and measure the pistol without holding any intermediator device, such as an augmented reality handheld device, glasses, or screens. Using additional optical devices, such as diffraction grating and a cylinder lens, the pistol size can be estimated. The exact location of the pistol in space remains static, even after its removal. Our optical system can be fixed or dynamically moved, making it suitable for various applications that require marking of objects in space.

We proposed an optonumerical method that supports the optimization of the fabrication process of polymer microtips manufactured at the ends of optical fibers. The optimization was aimed at obtaining the functional parameter of microtips—the output beam distribution in the far-field diffraction region. This parameter depends on refractive index distribution within the microtip and its geometrical properties, which are determined by the optical power distribution of the actinic light and the exposition time during the photopolymerization process. The proposed method constitutes a convenient feedback loop for modification of the fabrication parameters. A single cycle of the proposed scheme includes numerical simulations and measurements of the functional parameter, tomographic measurements, and modifications of the fabrication process. We proposed utilization of the measured values of three-dimensional refractive index distribution of microtips as input data for the finite-difference time-domain simulations. It was proven that the iterative process leads to controlled modification of the technology parameters and finally to obtaining the desired functional parameter of fabricated microtips.

This paper presents a kind of miniature handheld laser fluorescence spectrometer, which integrates a laser emission system, a spectroscopic system, and a detection system into a volume of 100×50×20  mm3. A universal serial bus interface is connected to PC for data processing and spectrum display. The emitted laser wavelength is 405 nm. A spectral range is 400 to 760 nm and 2-nm optical resolution has been achieved. This spectrometer has the advantages of compact structure, small volume, high sensitivity, and low cost.

ISO 12233 slanted-edge method experiences errors using fast Fourier transform (FFT) in the camera modulation transfer function (MTF) measurement due to tilt angle errors in the knife-edge resulting in nonuniform sampling of the edge spread function (ESF). In order to resolve this problem, a modified slanted-edge method using nonuniform fast Fourier transform (NUFFT) for camera MTF measurement is proposed. Theoretical simulations for images with noise at a different nonuniform sampling rate of ESF are performed using the proposed modified slanted-edge method. It is shown that the proposed method successfully eliminates the error due to the nonuniform sampling of the ESF. An experimental setup for camera MTF measurement is established to verify the accuracy of the proposed method. The experiment results show that under different nonuniform sampling rates of ESF, the proposed modified slanted-edge method has improved accuracy for the camera MTF measurement compared to the ISO 12233 slanted-edge method.

The optical whispering-gallery mode (WGM) resonators are axially symmetrical resonators with smooth edges, supporting the existence of the WGMs by the total internal reflection on the surface of the resonator. As of today, various types of such resonators have been developed, namely the ball shaped, tor shaped, bottle shaped, disk shaped, etc. The movement of WGM resonators in inertial space causes the changes in their shape. The result is a spectral shift of the WGMs. Optical methods allow to register this shift with high precision. It can be used in particular for the measurement of angular velocities in inertial orientation and navigation systems. However, different types of resonators react to the movement in different manners. In addition, their sensitivity to movement can be changed when changing the geometric parameters of these resonators. The work is devoted to investigation of these aspects.

A method for improving accuracy in Wigner–Ville distribution (WVD)-based particle size measurements from inline holograms using flip and replication technique (FRT) is proposed. The FRT extends the length of hologram signals being analyzed, yielding better spatial-frequency resolution of the WVD output. Experimental results verify reduction in measurement error as the length of the hologram signals increases. The proposed method is suitable for particle sizing from holograms recorded using small-sized image sensors.

We study a simulation method that uses the Wigner distribution function to incorporate wave optical effects in an established framework based on geometrical optics, i.e., a ray tracing engine. We use the method to calculate point spread functions and show that it is accurate for paraxial systems but produces unphysical results in the presence of aberrations. The cause of these anomalies is explained using an analytical model.

Calibration of CCD arrays for identifying bad pixels and achieving nonuniformity correction is commonly accomplished using dark frames. This kind of calibration technique does not achieve radiometric calibration of the array since only the relative response of the detectors is computed. For this, a second calibration is sometimes utilized by looking at sources with known radiances. This process can be used to calibrate photodetectors as long as a calibration source is available and is well-characterized. A previous attempt at creating a procedure for calibrating a photodetector using the underlying Poisson nature of the photodetection required calculations of the skewness of the photodetector measurements. Reliance on the third moment of measurement meant that thousands of samples would be required in some cases to compute that moment. A photocalibration procedure is defined that requires only first and second moments of the measurements. The technique is applied to image data containing a known light source so that the accuracy of the technique can be surmised. It is shown that the algorithm can achieve accuracy of nearly 2.7% of the predicted number of photons using only 100 frames of image data.

A polar-coordinate optical encoder that can realize synchronous measurement of angular and radial displacements is introduced. It has potential application in concurrent monitoring of radial and angular motions for rotating spindles and in real-time detection and compensation of eccentricity error for rotary stages. We explain its operating principle and develop a prototype. Experimental results demonstrate the feasibility and effectiveness of the proposed optical encoder in polar-coordinate displacement measurement.

A dual-wavelength phase-shifting speckle interferometry approach has been proposed to diagnose the topography of plasma-facing materials (PFMs) in tokamak. The conventional speckle interferometric surface measurement, which uses single-wavelength, can offer excellent vertical resolution, but limitation in measuring large height step and phase ambiguity would occur during their application. To solve the problem, a dual-wavelength method was developed. Experiments were conducted on molybdenum (Mo) sample, which is related to PFMs of experimental advanced superconducting tokamak (EAST), and a laser ablation method was adopted to simulate the erosion happed on PFMs. The laser ablation craters were measured by both single-wavelength and dual-wavelength phase-shifting speckle interferometry, and a multistep phase-shifting method has been investigated for their effectiveness on reducing noise in calculating the phase map. This work demonstrates the superiority of the dual-wavelength speckle interferometry and the feasibility of applying the measurement system in topographic measurement of PFMs in EAST.

A structured light system with spinning fringe projection has been an excitement for three-dimensional (3-D) shape measurement since high-speed phase-shifting fringe patterns can be produced by mechanical spinning slide projectors. However, two cameras are typically required for 3-D reconstruction since the calibration method of spinning fringe projection is not well documented yet. This article introduces a computational framework for the calibration of spinning fringe projection profilometry using polar coordinate representation. In this preliminary study, we emulated the ideal spinning fringe projection with a video projector that can precisely control phase shifts with minimized harmonic errors. Results show that under the ideal scenario, our calibration method can reach an accuracy of 0.10 mm with a standard deviation of 0.73 mm by measuring a spherical object with 150 mm diameter.

Pencil beam deflectometric profilers are common instruments for high-accuracy surface slope metrology of x-ray mirrors in synchrotron facilities. An f-theta optical system is a key optical component of the deflectometric profilers and is used to perform the linear angle-to-position conversion. Traditional optimization procedures of the f-theta systems are not directly related to the angle-to-position conversion relation and are performed with stops of large size and a fixed working distance, which means they may not be suitable for the design of f-theta systems working with a small-sized pencil beam within a working distance range for ultra-high-accuracy metrology. If an f-theta system is not well-designed, aberrations of the f-theta system will introduce many systematic errors into the measurement. A least-squares’ fitting procedure was used to optimize the configuration parameters of an f-theta system. Simulations using ZEMAX software showed that the optimized f-theta system significantly suppressed the angle-to-position conversion errors caused by aberrations. Any pencil-beam f-theta optical system can be optimized with the help of this optimization method.

Previous studies have shown that molecular contamination outgassed from nonmetallic materials tends toward deposition on optical surfaces as droplets instead of nearly uniform thin films. Failure to consider the sources and effects of these droplets in an optical instrument omits large throughput losses due to scattering. This paper demonstrates that a simple treatment of optical system surfaces using vacuum ultraviolet (VUV) radiation reduces the formation of molecular contaminant droplets. VUV radiation exposure of a nominally clean silicon surface using a deuterium lamp suffices to remove hydrocarbon and carbonyl species that allow wetting of the surface by the contaminant. The throughput losses of the contamination due to droplet scattering can be reduced significantly.

Rotary symmetric error (RSE) is a typical type of surface machining errors for precision components, related to the relative tool-workpiece movements adopted. A quick removal method of the workpiece surface RSE using a fluid jet polishing (FJP) process, in which the tool nozzle and workpiece move relatively as in an end-face turning process, is presented. Efficient improvement in the workpiece surface form accuracy was achieved using a relatively simple device. Two methods to extract the RSE from the measured surface error curves, i.e., the average and minimum methods, are introduced and compared. The influence of the local jet rotation radius on the removal function of FJP was investigated experimentally. The algorithm for solving the dwell time curve in two-dimensional FJP process was derived. Deterministic FJP experiments were conducted to remove the RSE by means of the end-face turning movement. Experimental results show that the proportion of the RSE to the overall surface error on most workpieces was reduced from >60% to <20%, whereas the volume of the nonrotary symmetric error (non-RSE) fluctuates slightly before and after the FJP process. Using the average RSE extraction method and combining it with the zero-phase filtering method to filter the dwell time can effectively reduce the frequency of residual circular error in the polishing surface and avoid increasing of power spectral density in the mid-high frequency after polishing.

The chemical mechanical polishing (CMP) is a key process during the machining route of plane optics. To improve the polishing efficiency and accuracy, a CMP model and machine tool were developed. Based on the Preston equation and the axial run-out error measurement results of the m circles on the tin plate, a CMP model that could simulate the material removal at any point on the workpiece was presented. An analysis of the model indicated that lower axial run-out error led to lower material removal but better polishing efficiency and accuracy. Based on this conclusion, the CMP machine was designed, and the ultraprecision gas hydrostatic guideway and rotary table as well as the Siemens 840Dsl numerical control system were incorporated in the CMP machine. To verify the design principles of machine, a series of detection and machining experiments were conducted. The LK-G5000 laser sensor was employed for detecting the straightness error of the gas hydrostatic guideway and the axial run-out error of the gas hydrostatic rotary table. A 300-mm-diameter optic was chosen for the surface profile machining experiments performed to determine the CMP efficiency and accuracy.

Momentum exchange theory (MET) provides an alternative picture for optical diffraction based on a distribution of photon paths through momentum transfer probabilities determined at the scattering aperture. This is contrasted with classical optical wave theory that uses the Huygens–Fresnel principle and sums the phased contributions of wavelets at the point of detection. Single-slit, multiple-slit (Talbot effect), and straight-edge diffraction provide significant clues to the geometric parameters controlling momentum transfer probabilities and the relation to Fresnel zone numbers. Momentum transfer is primarily dependent on preferred momentum states at the aperture and the specific location and distance for momentum exchange. Diffraction by an opaque disc provides insight to negative (attractive) dispersions. MET should simplify the analysis of a broadened set of aperture configurations and experimental conditions.

Influence of radiation force of a high-energy laser beam on the second harmonic generation (SHG) efficiency through stress within a mounted potassium dihydrogen phosphate (KDP) crystal is studied, as well as an active method of improving the SHG efficiency by controlling the stress is proposed. At first, the model for studying the influence of the radiation force on the SHG efficiency is established, where the radiation force is theoretically analyzed, the stress caused by the radiation force is theoretically analyzed and numerically calculated using the finite-element method, and the influence of the stress on the SHG efficiency is theoretically analyzed. Then, a method of improving the SHG efficiency by controlling the stress through adjusting the structural parameters of the mounting set of the KDP crystal is examined. It demonstrates that the radiation force causes stress within the KDP crystal and further militates against the SHG efficiency; however, the SHG efficiency could be improved by controlling the stress through adjusting the structural parameters of the mounting set of the KDP crystal.

We present the performance benefits of differential phase-shift keying (DPSK) modulation in eliminating influence from atmospheric turbulence, especially for coherent free space optical (FSO) communication with a high communication rate. Analytic expression of detected signal is derived, based on which, homodyne detection efficiency is calculated to indicate the performance of wavefront compensation. Considered laser pulses always suffer from atmospheric scattering effect by clouds, intersymbol interference (ISI) in high-speed FSO communication link is analyzed. Correspondingly, the channel equalization method of a binormalized modified constant modulus algorithm based on set-membership filtering (SM-BNMCMA) is proposed to solve the ISI problem. Finally, through the comparison with existing channel equalization methods, its performance benefits of both ISI elimination and convergence speed are verified. The research findings have theoretical significance in a high-speed FSO communication system.

The modeling of the scattering phenomena for the multielement telescope for imaging and spectroscopy (METIS) coronagraph on board the European Space Agency Solar Orbiter is reported. METIS is an inverted occultation coronagraph including two optical paths: the broadband imaging of the full corona in linearly polarized visible-light (580 to 640 nm) and the narrow-band imaging of the full corona in the ultraviolet Lyman-α (121.6 nm). METIS will have the unique opportunity of observing the solar outer atmosphere as close to the Sun as 0.28 AU and from up to 35 deg out-of-ecliptic. The stray-light simulations performed on the UV and VL channels of the METIS analyzing the contributors of surface microroughness, particulate contamination, cosmetic defects, and diffraction are reported. The results obtained with the nonsequential modality of Zemax OpticStudio are compared with two different approaches: the Monte Carlo ray trace with Advanced Systems Analysis Program (ASAP®) and a semianalytical model. The results obtained with the three independently developed approaches are in considerable agreement and show compliance to the requirement of stray-light level for both the UV and VL channels.

In high-power laser system, the surface wavefront of large optics has a close link with its structure design and mounting method. The back-support transport mirror design is presently being investigated as a means in China’s high-power laser system to hold the optical component firmly while minimizing the distortion of its reflecting surface. We have proposed a comprehensive analytical framework integrated numerical modeling and precise metrology for the mirror’s mounting performance evaluation while treating the surface distortion as a key decision variable. The combination of numerical simulation and field tests demonstrates that the comprehensive analytical framework provides a detailed and accurate approach to evaluate the performance of the transport mirror. It is also verified that the back-support transport mirror is effectively compatible with state-of-the-art optical quality specifications. This study will pave the way for future research to solidify the design of back-support large laser optics in China’s next generation inertial confinement fusion facility.

This paper proposes a three-dimensional (3-D) high-precision indoor positioning strategy using Tabu search based on visible light communication. Tabu search is a powerful global optimization algorithm, and the 3-D indoor positioning can be transformed into an optimal solution problem. Therefore, in the 3-D indoor positioning, the optimal receiver coordinate can be obtained by the Tabu search algorithm. For all we know, this is the first time the Tabu search algorithm is applied to visible light positioning. Each light-emitting diode (LED) in the system broadcasts a unique identity (ID) and transmits the ID information. When the receiver detects optical signals with ID information from different LEDs, using the global optimization of the Tabu search algorithm, the 3-D high-precision indoor positioning can be realized when the fitness value meets certain conditions. Simulation results show that the average positioning error is 0.79 cm, and the maximum error is 5.88 cm. The extended experiment of trajectory tracking also shows that 95.05% positioning errors are below 1.428 cm. It can be concluded from the data that the 3-D indoor positioning based on the Tabu search algorithm achieves the requirements of centimeter level indoor positioning. The algorithm used in indoor positioning is very effective and practical and is superior to other existing methods for visible light indoor positioning.

We present results of experimental and numerical investigation of supercontinuum (SC) generation in polarization-maintaining photonic crystal fiber (PCF) using chirped femtosecond pulses. The initial unchirped pump pulse source was a mode-locked Yb:KGW laser generating 52-nJ energy, 110-fs duration pulses at 1030 nm with a 76-MHz repetition rate. The nonlinear medium was a 32-cm-long polarization-maintaining PCF manufactured by NKT Photonics A/S. We demonstrated the influence of pump pulse chirp on spectral characteristics of a SC. We showed that by chirping pump pulses positively or negatively one can obtain a broader SC spectrum than in the case of unchirped pump pulses at the same peak power. Moreover, the extension can be controlled by changing the amount of pump pulse chirp. Numerical simulation results also indicated that pump pulse chirp yields an extension of SC spectrum.

Based on vibration signals detected by a phase-sensitive optical time-domain reflectometer distributed optical fiber sensing system, this paper presents an implement of time-frequency analysis and convolutional neural network (CNN), used to classify different types of vibrational events. First, spectral subtraction and the short-time Fourier transform are used to enhance time-frequency features of vibration signals and transform different types of vibration signals into spectrograms, which are input to the CNN for automatic feature extraction and classification. Finally, by replacing the soft-max layer in the CNN with a multiclass support vector machine, the performance of the classifier is enhanced. Experiments show that after using this method to process 4000 vibration signal samples generated by four different vibration events, namely, digging, walking, vehicles passing, and damaging, the recognition rates of vibration events are over 90%. The experimental results prove that this method can automatically make an effective feature selection and greatly improve the classification accuracy of vibrational events in distributed optical fiber sensing systems.

We propose a W-band wavelength-division multiplexing (WDM)-over-optical code-division multiple access radio-over-fiber system. This system offers capacity expansion by increasing the working frequency to millimeter wave region and by introducing optical encoding and multiwavelength multiplexing. The system’s functionality is investigated by software modeling, and the results are presented. The generated signals are data modulated at 10  Gb/s and optically encoded for two wavelength channels and transmitted with a 20-km length of fiber. The received signals are optically decoded and detected. Also, encoding has improved the bit error rate (BER) versus the received optical power margin for the WDM setting by about 4 dB. In addition, the eye-diagram shows that the difference between received optical power levels at the BER of 10−12 to 10−3 is about 1.3% between two encoded channels. This method of capacity improvement is significantly important for the next generation of mobile communication, where millimeter wave signals will be widely used to deliver data to small cells.

We propose an accurate and nondata-aided chromatic dispersion (CD) estimation method involving the use of the cross-correlation function of two heterodyne detection signals for coherent optical communication systems. Simulations are implemented to verify the feasibility of the proposed method for 28-GBaud coherent systems with different modulation formats. The results show that the proposed method has high accuracy for measuring CD and has good robustness against laser phase noise, amplified spontaneous emission noise, and nonlinear impairments.

A microwave phase-control scheme is proposed and experimentally demonstrated. Two lasers are combined in an optical fiber coupler to generate a beat signal. The beat frequency is tuned by controlling the frequency of one laser. Using the phase shift of the beat waves with different frequencies during the propagation in an optical fiber, the phase of the radio-frequency (RF) signal generated by a photodetector (PD) can be controlled. Using the phase shift during the propagation of beat waves in an optical fiber with different beat frequencies, the phase of the RF signal generated by a PD connected to the fiber can be controlled. A tunable phase shift ranging from 0 deg to 1400 deg is obtained for frequencies from 6 to 10 GHz. This scheme offers the advantages of fast tuning and precise phase control of an RF signal.

Visible light communication (VLC) based on light-emitting diodes (LEDs) technology not only provides higher data rate for indoor wireless communications and offering room illumination but also has the potential for indoor localization. VLC-based indoor positioning using the received optical power levels from emitting LEDs is investigated. We consider both scenarios of line-of-sight (LOS) and LOS with non-LOS (LOSNLOS) positioning. The performance of the proposed system is evaluated under both noisy and noiseless channel as is the impact of different location codes on positioning error. The analytical model of the system with noise and the corresponding numerical evaluation for a range of signal-to-noise ratio (SNR) are presented. The results show that an accuracy of <10  cm on average is achievable at an SNR>12  dB.

We propose an efficient partial transmit sequence technique based on genetic algorithm and peak-value optimization algorithm (GAPOA) to reduce high peak-to-average power ratio (PAPR) in visible light communication systems based on orthogonal frequency division multiplexing (VLC-OFDM). By analysis of hill-climbing algorithm’s pros and cons, we propose the POA with excellent local search ability to further process the signals whose PAPR is still over the threshold after processed by genetic algorithm (GA). To verify the effectiveness of the proposed technique and algorithm, we evaluate the PAPR performance and the bit error rate (BER) performance and compare them with partial transmit sequence (PTS) technique based on GA (GA-PTS), PTS technique based on genetic and hill-climbing algorithm (GH-PTS), and PTS based on shuffled frog leaping algorithm and hill-climbing algorithm (SFLAHC-PTS). The results show that our technique and algorithm have not only better PAPR performance but also lower computational complexity and BER than GA-PTS, GH-PTS, and SFLAHC-PTS technique.

A high-birefringence photonic crystal fiber polarization filter is proposed. The coupling theory is used to explain full and incomplete couplings. The resonance point can be adjusted to the communication band by optimizing the fiber structure parameters. Numerical simulation results indicate that the resonance strength can reach 924.96 and 710.28  dB.cm−1 at the communication wavelength of 1.31 and 1.55  μm in x- and y-polarized directions, respectively. By filling liquid analyte, the confinement loss can reach 804.52  dB.cm−1 at the wavelength of 1.55  μm. Furthermore, when the fiber length of L equals 500  μm, the peak value of the cross talk (CT) can reach 389.15 and −280.52  dB, respectively. When the length of the fiber L equals 200  μm, the bandwidth of the CT better than 20 dB is up to 120 nm at the wavelength of 1.31  μm, and the bandwidth of the CT<−20  dB is up to 140 nm at the wavelength of 1.55  μm. These properties make it a good candidate for designing types of polarization filter devices.

The image scene is limited by the small size of the spot, which is linked with the short wavelength of the laser in synthetic aperture ladar (SAL). The common wide-scene imaging mode in synthetic aperture radar, i.e., the terrain observation by progressive scans imaging mode, is extended and applied in SAL. A suitable imaging algorithm is also proposed according to the characteristics of the signal. First, the azimuth prefiltering based on the spectrum analysis is adopted to eliminate the azimuth spectrum aliasing caused by the steering of the antenna beam. Then, the improved frequency scaling algorithm is applied to complete the range compression and range cell migration correction. Meanwhile, the Doppler frequency shift induced by the continuous moving of the platform is compensated. Finally, the azimuth compression is completed in the Doppler domain, and the SAL image without ambiguity is obtained with the application of the dechirp technique. Simulation results provide the evidence on the effectiveness of the proposed method.

A scheme is proposed to realize the all-optical phase regeneration of four-channel quadrature phase shift keying (QPSK) signal based on phase-sensitive amplification. By utilizing conjugate pump and common pump in a highly nonlinear optical fiber, degenerate four-wave mixing process is observed, and QPSK signals are regenerated. The number of waves is reduced to decrease the cross talk caused by undesired nonlinear interaction during the coherent superposition process. In addition, to avoid the effect of overlapping frequency, frequency spans between pumps and signals are set to be nonintegral multiples. Optical signal-to-noise ratio improvement is validated by bit error rate measurements. Compared with single-channel regeneration, multichannel regeneration brings 0.4-dB OSNR penalty when the value of BER is 10−3, which shows the cross talk in regeneration process is negligible.