Visible light positioning (VLP) has attracted much attention in both academic and industrial areas due to the extensive deployment of light-emitting diodes (LEDs) as next-generation green lighting. Generally, the coverage of a single LED lamp is limited, so LED arrays are always utilized to achieve uniform illumination within the large-scale indoor environment. However, in such dense LED deployment scenario, the superposition of the light signals becomes an important challenge for accurate VLP. To solve this problem, we propose a forward and correctional orthogonal frequency division multiplexing (OFDM)-based VLP (FCO-VLP) scheme with low complexity in generating and processing of signals. In the first forward procedure of FCO-VLP, an initial position is obtained by the trilateration method based on OFDM-subcarriers. The positioning accuracy will be further improved in the second correctional procedure based on the database of reference points. As demonstrated in our experiments, our approach yields an improved average positioning error of 4.65 cm and an enhanced positioning accuracy by 24.2% compared with trilateration method.

Fiber Bragg grating (FBG)-based high-temperature sensor with enhanced-temperature range and stability has been developed and tested. The sensor consists of an FBG and a mechanical transducer, which furnishes a linear temperature-dependent tensile strain on FBG by means of differential linear thermal expansion of two different ceramic materials. The designed sensor is tested over a range: 20°C to 1160°C and is expected to measure up to 1500°C.

GaN provides the highest electron saturation velocity, breakdown voltage, operation temperature, and thus the highest combined frequency-power performance among commonly used semiconductors. The industrial need for compact, economical, high-resolution, and high-power terahertz (THz) imaging and spectroscopy systems are promoting the utilization of GaN for implementing the next generation of THz systems. As it is reviewed, the mentioned characteristics of GaN together with its capabilities of providing high two-dimensional election densities and large longitudinal optical phonon of ∼90  meV make it one of the most promising semiconductor materials for the future of the THz emitters, detectors, mixers, and frequency multiplicators. GaN-based devices have shown capabilities of operation in the upper THz frequency band of 5 to 12 THz with relatively high photon densities in room temperature. As a result, THz imaging and spectroscopy systems with high resolution and deep depth of penetration can be realized through utilizing GaN-based devices. A comprehensive review of the history and the state of the art of GaN-based electronic devices, including plasma heterostructure field-effect transistors, negative differential resistances, hetero-dimensional Schottky diodes, impact avalanche transit times, quantum-cascade lasers, high electron mobility transistors, Gunn diodes, and tera field-effect transistors together with their impact on the future of THz imaging and spectroscopy systems is provided.

This PDF file contains the editorial “Special Section Guest Editorial: Infrared Sensors” for OE Vol. 56 Issue 09

We present progress in metal organic chemical vapor deposition (MOCVD) growth of (100) HgCdTe epilayers achieved recently at the Institute of Applied Physics, Military University of Technology and Vigo System S.A. It is shown that MOCVD technology is an excellent tool for the fabrication of different HgCdTe detector structures with a wide range of composition, donor/acceptor doping, and without post grown ex-situ annealing. Surface morphology, residual background concentration, and acceptor doping efficiency are compared in (111) and (100) oriented HgCdTe epilayers. At elevated temperatures, the carrier lifetime in measured p-type photoresistors is determined by Auger 7 process with about one order of magnitude difference between theoretical and experimental values. Particular progress has been achieved in the growth of (100) HgCdTe epilayers for medium wavelength infrared photoconductors operated in high-operating temperature conditions.

The theory of excitonic quasimolecules (biexcitons) (formed of spatially separated electrons and holes) in a nanosystem that consists of semiconductor quantum dots synthesized in a borosilicate glass matrix is presented. It is shown that exciton quasimolecule formation is of a threshold character and is possible in nanosystem, if the spacing between the quantum dots surfaces is larger than a certain critical spacing. It was found that the binding energy of the singlet ground state of an exciton quasimolecule, consisting of two semiconductor quantum dots is a significant large values, larger than the binding energy of the biexciton in a semiconductor single crystal by almost two orders of magnitude.

Significantly improved carrier lifetimes in very long-wave infrared (VLWIR) InAs/GaInSb superlattice (SL) absorbers are demonstrated using time-resolved microwave reflectance (TMR) measurements. A nominal 47.0  Å InAs/21.5  ÅGa0.75In0.25Sb SL structure that produces an ∼25  μm response at 10 K has a minority carrier lifetime of 140±20  ns at 18 K, which is an order-of-magnitude improvement compared with previously reported lifetime values for other VLWIR detector absorbers. This improvement is attributed to the strain-engineered ternary SL design, which offers a variety of epitaxial advantages and ultimately leads to the improvements in the minority carrier lifetime by mitigating defect-mediated Shockley–Read–Hall (SRH) recombination centers. By analyzing the temperature dependence of TMR decay data, the recombination mechanisms and trap states that currently limit the performance of this SL absorber were identified. The results show a general decrease in the long-decay lifetime component, which is dominated by SRH recombination at temperatures below ∼30  K and by Auger recombination at temperatures above ∼45  K. Since the strain-balanced ternary SL design offers a reasonably good absorption coefficient and many epitaxial advantages during growth, this VLWIR SL material system should be considered as a competitive candidate for VLWIR photodetector technology.

Extension of the wavelength threshold of an infrared detector beyond λt=hc/Δ is demonstrated, without reducing the minimum energy gap (Δ) of the material. Specifically, a photodetector designed with Δ=0.40  eV, and a corresponding λt=3.1  μm, was shown to have an extended threshold of ∼45  μm at 5.3 K, at zero bias. Under negative and positive applied bias, this range was further extended to ∼60 and ∼68  μm, respectively, with the photoresponse becoming stronger at increased biases, but the spectral threshold remained relatively constant. The observed wavelength extension arises from an offset between the two potential barriers in the device. Without the offset, another detector with Δ=0.30  eV showed a photoresponse with the expected wavelength threshold of ∼4  μm.

The depletion and surface leakage dark current suppression properties of unipolar barrier device architectures such as the nBn have been highly beneficial for III–V semiconductor-based infrared detectors. Using a one-dimensional drift-diffusion model, we theoretically examine the effects of contact doping, minority carrier lifetime, and absorber doping on the dark current characteristics of nBn detectors to explore some basic aspects of their operation. We found that in a properly designed nBn detector with highly doped excluding contacts the minority carriers are extracted to nonequilibrium levels under reverse bias in the same manner as the high operating temperature (HOT) detector structure. Longer absorber Shockley–Read–Hall (SRH) lifetimes result in lower diffusion and depletion dark currents. Higher absorber doping can also lead to lower diffusion and depletion dark currents, but the benefit should be weighted against the possibility of reduced diffusion length due to shortened SRH lifetime. We also briefly examined nBn structures with unintended minority carrier blocking barriers due to excessive n-doping in the unipolar electron barrier, or due to a positive valence band offset between the barrier and the absorber. Both types of hole blocking structures lead to higher turn-on bias, although barrier n-doping could help suppress depletion dark current.

Effect of iodine-doping in the deposition solution and iodine vapor pressure during the sensitization process on the morphological, microstructural, electrical, and optical properties of PbSe films was studied. Undoped and iodine-doped PbSe films of polycrystalline particles were coated on thermally oxidized silicon substrates by chemical bath deposition. The PbSe films were oxidized at 380°C for 30 min and then iodinated at different iodine vapor pressures at 380°C for 5 min. When the iodine vapor pressure was below 20 Pa, PbSeO3 was the main phase formed on the surface of PbSe microcrystals for both undoped and iodine-doped films. As the iodine vapor pressure was increased above 20 Pa, Pb3I2O2 and PbI2 phases were formed in both types of films and PbSeO3 disappeared in the undoped film. Only the iodine-doped films showed photo response. The sheet resistance and IR signal-to-noise ratio had maximum values at the iodine vapor pressure of 17.5 Pa in the iodine-doped film. The x-ray diffraction spectra, scanning electron microscopy morphologies, and EDS analyses of the sensitized PbSe films show that the main role of iodine in the sensitization is helping solid-state sintering of PbSe microcrystals which may lead to redistribution of oxygen atoms in the effective atomic sites.

Highly sensitive photon detectors are regarded as the key enabling elements in many applications. Due to the low photon energy at the short-wave infrared (SWIR), photon detection and imaging at this band are very challenging. As such, many efforts in photon detector research are directed toward improving the performance of the photon detectors operating in this wavelength range. To solve these problems, we have developed an electron-injection (EI) technique. The significance of this detection mechanism is that it can provide both high efficiency and high sensitivity at room temperature, a condition that is very difficult to achieve in conventional SWIR detectors. An EI detector offers an overall system-level sensitivity enhancement due to a feedback stabilized internal avalanche-free gain. Devices exhibit an excess noise of unity, operate in linear mode, require bias voltage of a few volts, and have a cutoff wavelength of 1700 nm. We review the material system, operating principle, and development of EI detectors. The shortcomings of the first-generation devices were addressed in the second-generation detectors. Measurement on second-generation devices showed a high-speed response of ∼6  ns rise time, low jitter of less than 20 ps, high amplification of more than 2000 (at optical power levels larger than a few nW), unity excess noise factor, and low leakage current (amplified dark current ∼10  nA at a bias voltage of −3  V and at room temperature. These characteristics make EI detectors a good candidate for high-resolution flash light detection and ranging (LiDAR) applications with millimeter scale depth resolution at longer ranges compared with conventional p-i-n diodes. Based on our experimentally measured device characteristics, we compare the performance of the EI detector with commercially available linear mode InGaAs avalanche photodiode (APD) as well as a p-i-n diode using a theoretical model. Flash LiDAR images obtained by our model show that the EI detector array achieves better resolution with higher signal-to-noise compared with both the InGaAs APD and the p-i-n array (of 100×100 elements). We have designed a laboratory setup with a receiver optics aperture diameter of 3 mm that allows an EI detector (with 30-μm absorber diameter) to be used for long-range LiDAR imaging with subcentimeter resolution.

Type-II strained-layer superlattices (T2SL) based on InAs1−xSbx are a promising photovoltaic detector material technology for thermal imaging; however, Shockley–Read–Hall recombination and generation rates are still too high for thermal imagers based on InAs1−xSbx T2SL to reach their ideal performance. Molecular dynamics simulations using the Stillinger–Weber (SW) empirical potentials are a useful tool to study the growth of tetrahedral coordinated crystals and the nonequilibrium formation of defects within them, including the long-range effects of strain. SW potentials for the possible atomic interactions among {Ga, In, As, Sb} were developed by fitting to ab initio calculations of elastically distorted zinc blende and diamond unit cells. The SW potentials were tested against experimental observations of molecular beam epitaxial (MBE) growth and then used to simulate the MBE growth of InAs/InAs0.5Sb0.5 T2SL on GaSb substrates over a range of processes parameters. The simulations showed and helped to explain Sb cross-incorporation into the InAs T2SL layers, Sb segregation within the InAsSb layers, and identified medium-range defect clusters involving interstitials and their induction of interstitial-vacancy pairs. Defect formation was also found to be affected by growth temperature and flux stoichiometry.

The Schroedinger eigenmaps (SE) algorithm using spatial and spectral information has been applied to supervised classification of hyperspectral imagery (HSI). We have previously introduced the use of SE in spectral target detection problems. The original SE-based target detector was built on the spectral information encoded in the Laplacian and Schroedinger operators. The original SE-based detector is extended such that spatial connectivity of target-like pixels is explored and encoded into the Schroedinger operator using a “knowledge propagation” scheme. The modified SE-based detector is applied to two HSI data sets that share similar target materials. Receiver operating characteristic curves and rates of detection and false alarm at object level are used as quantitative metrics to assess the detector. In addition, the Schroedinger embedding performance in target detection is compared against the performances of principal component embedding and the Laplacian embedding. Results show that the SE-based detector with spatial–spectral features outperforms the other considered approaches.

The random selection of phase mask parameters will cause the degradation of imaging quality, which can be fixed through the optimization process. We introduce an evaluation function based on the use of multitarget optimization to obtain optimal phase mask parameters. The proposed method gives the optimal phase mask parameters, which produce together an imaging quality at all defocus positions as best as possible. The cubic phase mask and the general cubic phase mask are both used to perform optimization with a proposed evaluation function. The simulation result gives quite interesting values for the two types of these phase masks.

A constrained optimization approach with faster convergence is proposed to recover the complex object field from a near on-axis digital holography (DH). We subtract the DC from the hologram after recording the object beam and reference beam intensities separately. The DC-subtracted hologram is used to recover the complex object information using a constrained optimization approach with faster convergence. The recovered complex object field is back propagated to the image plane using the Fresnel back-propagation method. The results reported in this approach provide high-resolution images compared with the conventional Fourier filtering approach and is 25% faster than the previously reported constrained optimization approach due to the subtraction of two DC terms in the cost function. We report this approach in DH and digital holographic microscopy using the U.S. Air Force resolution target as the object to retrieve the high-resolution image without DC and twin image interference. We also demonstrate the high potential of this technique in transparent microelectrode patterned on indium tin oxide-coated glass, by reconstructing a high-resolution quantitative phase microscope image. We also demonstrate this technique by imaging yeast cells.

Considering that manual inspection of the yarn-dyed fabric can be time consuming and inefficient, we propose a yarn-dyed fabric defect classification method by using a convolutional neural network (CNN) based on a modified AlexNet. CNN shows powerful ability in performing feature extraction and fusion by simulating the learning mechanism of human brain. The local response normalization layers in AlexNet are replaced by the batch normalization layers, which can enhance both the computational efficiency and classification accuracy. In the training process of the network, the characteristics of the defect are extracted step by step and the essential features of the image can be obtained from the fusion of the edge details with several convolution operations. Then the max-pooling layers, the dropout layers, and the fully connected layers are employed in the classification model to reduce the computation cost and extract more precise features of the defective fabric. Finally, the results of the defect classification are predicted by the softmax function. The experimental results show promising performance with an acceptable average classification rate and strong robustness on yarn-dyed fabric defect classification.

Pansharpening is an effective way to enhance the spatial resolution of a multispectral (MS) image by fusing it with a provided panchromatic image. Instead of restricting the coding coefficients of low-resolution (LR) and high-resolution (HR) images to be equal, we propose a pansharpening approach via sparse regression in which the relationship between sparse coefficients of HR and LR MS images is modeled by ridge regression and elastic-net regression simultaneously learning the corresponding dictionaries. The compact dictionaries are learned based on the sampled patch pairs from the high- and low-resolution images, which can greatly characterize the structural information of the LR MS and HR MS images. Later, taking the complex relationship between the coding coefficients of LR MS and HR MS images into account, the ridge regression is used to characterize the relationship of intrapatches. The elastic-net regression is employed to describe the relationship of interpatches. Thus, the HR MS image can be almost identically reconstructed by multiplying the HR dictionary and the calculated sparse coefficient vector with the learned regression relationship. The simulated and real experimental results illustrate that the proposed method outperforms several well-known methods, both quantitatively and perceptually.

This paper presents a robust iterative algorithm, known as hybrid Wirtinger flow (HWF), for phase retrieval (PR) of complex objects from noisy diffraction intensities. Numerical simulations indicate that the HWF method consistently outperforms conventional PR methods in terms of both accuracy and convergence rate in multiple phase modulations. The proposed algorithm is also more robust to low oversampling ratios, loose constraints, and noisy environments. Furthermore, compared with traditional Wirtinger flow, sample complexity is largely reduced. It is expected that the proposed HWF method will find applications in the rapidly growing coherent diffractive imaging field for high-quality image reconstruction with multiple modulations, as well as other disciplines where PR is needed.

In a compact digital lensless inline holographic microscope (LIHM), where the sample-to-sensor distance is short, the imaging resolution is often limited by sensor pixel size instead of the system numerical aperture. We propose to solve this problem by applying data interpolation with an iterative holographic reconstruction method while using grating illumination in the LIHM system. In the system setup, the Talbot self-image of a Ronchi grating was used to illuminate the sample, and the inline hologram was recorded by a CMOS imaging sensor located behind the sample. The hologram was then upsampled by data interpolation before the reconstruction process. In the iterative holographic reconstruction, the sample support was defined by the bright areas of the grating illumination pattern and was used as constraint. A wide-field image can also be obtained by shifting the grating illumination pattern. Furthermore, we assumed that the sample was amplitude object, i.e., no obvious phase change was caused by the sample, which provided additional constraint to refine the interpolated data values. Besides improved resolution, the iterative holographic reconstruction also helped to reduce the twin-image background. We demonstrated the effectiveness of our method with simulation and imaging experiment by using the USAF target and polystyrene microspheres with 1  μm diameter as the sample.

The workshop Measurement Positioning System (wMPS) is a large-scale measurement system that better copes with the current challenges of dimensional metrology. However, as a distributed measuring system with multiple transmitters forming a spatial measurement network, the network topology of transmitters relative to the receiver exerts a significant influence on the measurement accuracy albeit one that is difficult to quantify. An evaluation metric, termed the geometric dilution of precision (GDOP), is introduced to quantify the quality of the network topology of the wMPS. The GDOP is derived from the measurement error model of wMPS and its mathematical derivation is expounded. Two significant factors (density and layout of the transmitter) affecting the network topology are analyzed by simulations and experiments. The experimental results show that GDOP is approximately proportional to the measurement error. More transmitters, and a relatively good layout thereof, can decrease the value of GDOP and the measurement error.

For the improvement of monitoring accuracy, a vibration monitoring for aircraft wing model using a fiber Bragg grating (FBG) array packaged by vacuum-assisted resin transfer molding (VARTM) is proposed. The working principle of the vibration monitoring using FBG array has been explained, which can theoretically support the idea of this paper. VARTM has been explained in detail, which is suitable for not only the single FBG sensor but also the FBG array within a relatively large area. The calibration experiment has been performed using the FBG sensor packaged by VARTM. The strain sensitivity of the VARTM package is 1.35  pm/μϵ and the linearity is 0.9999. The vibration monitoring experiment has been carried out using FBG array packaged by VARTM. The measured rate of strain changes across the aluminum test board used to simulate the aircraft wing is 0.69  μϵ/mm and the linearity is 0.9931. The damping ratio is 0.16, which could be further used for system performance evaluation. Experimental results demonstrate that the vibration monitoring using FBG sensors packaged by VARTM can be efficiently used for the structural health monitoring. Given the validation and great performance, this method is quite promising for in-flight monitoring and holds great reference value in other similar engineering structures.

We consider a computational superresolution inverse diffraction problem for phase retrieval from phase-coded intensity observations. The optical setup includes a thin lens and a spatial light modulator for phase coding. The designed algorithm is targeted on an optimal solution for Poissonian noisy observations. One of the essential instruments of this design is a complex-domain sparsity applied for complex-valued object (phase and amplitude) to be reconstructed. Simulation experiments demonstrate that good quality imaging can be achieved for high-level of the superresolution with a factor of 32, which means that the pixel of the reconstructed object is 32 times smaller than the sensor’s pixel. This superresolution corresponds to the object pixel as small as a quarter of the wavelength.

A look-up table (LUT) method for solving the problem of phase unwrapping is presented. Considering the effect of noise on the unwrapping process, a concept called “tolerance” is advanced, and an associated algorithm called the “equipartition of tolerance” algorithm is proposed. The proposed algorithm eliminates the need for a high signal-to-noise ratio while retaining the LUT method’s advantages of extended measurement range and high precision. Further, it improves the tolerance of the LUT method and enables reconstruction of discontinuous objects. In simulations and experiments conducted, the proposed algorithm successfully unwrapped the absolute phase of a slope model and a three-step model. The proposed algorithm is significantly more accurate and has better stability and sensitivity than the heterodyne algorithm.

High-speed cameras provide full field measurement of structure motions and have been applied in nondestructive testing and noncontact structure monitoring. Recently, a phase-based method has been proposed to extract sound-induced vibrations from phase variations in videos, and this method provides insights into the study of remote sound surveillance and material analysis. An efficient singular value decomposition (SVD)-based approach is introduced to detect sound-induced subtle motions from pixel intensities in silent high-speed videos. A high-speed camera is initially applied to capture a video of the vibrating objects stimulated by sound fluctuations. Then, subimages collected from a small region on the captured video are reshaped into vectors and reconstructed to form a matrix. Orthonormal image bases (OIBs) are obtained from the SVD of the matrix; available vibration signal can then be obtained by projecting subsequent subimages onto specific OIBs. A simulation test is initiated to validate the effectiveness and efficiency of the proposed method. Two experiments are conducted to demonstrate the potential applications in sound recovery and material analysis. Results show that the proposed method efficiently detects subtle motions from the video.

Machining multiple mirror surfaces on one common substrate during the fabrication of off-axis three-mirror or four-mirror optical systems can take less time and drastically improve the alignment efficiency. However, the difficulty of the surface test remains the same. We theoretically propose a subaperture test method to carry out the null test of two mirrors on the synthetic reflective mirror. Specifically, we design a special zoom null lens and selectively use its subaperture wavefront aberrations of different configurations to nullify the surface normal wavefront aberrations of the according mirrors on the synthetic reflective mirror. The proposed method is verified by simulating the null test process of a synthetic reflective mirror integrating an off-axis high-order primary mirror (PM) and a coaxial high-order tertiary mirror (TM) of one off-axis three-mirror system, with the consideration of the fabrication and alignment errors. Simulation results show that, within a limited range of feasible tolerances, the residual wavefront aberration is 0.033λ root mean square (RMS) for the PM and 0.025λ RMS for the TM, at a wavelength of 1064 nm.

We report an optical implementation of a parallel phase-shifting quasi-common path interferometer using two modified Michelson interferometers to generate two interferograms. By using a displaceable polarizer’s array, placed on the image plane, we can obtain four phase-shifted interferograms in two captures. The system operates as a quasi-common path interferometer generating four beams, which are to interfere with alignment procedures on the mirrors of the Michelson configurations. The optical phase data are retrieved using the well-known four-step algorithms. To present the capabilities of the system, experimental results obtained from transparent structures are presented.

An approach employing ultrafast laser hybrid subtractive-additive microfabrication, which combines ablation, three-dimensional nanolithography, and welding, is proposed for the realization of a lab-on-chip (LOC) device. A single amplified Yb:KGW femtosecond (fs)-pulsed laser source is shown to be suitable for fabricating microgrooves in glass slabs, polymerization of fine-meshes microfilter out of hybrid organic–inorganic photopolymer SZ2080 inside them, and, finally, sealing the whole chip with cover glass into a single monolithic piece. The created microfluidic device proved its particle sorting function by separating 1- and 10-μm polystyrene spheres in an aqueous mixture. All together, this proves that laser microfabrication based on a single amplified fs laser source is a flexible and versatile approach for the hybrid subtractive-additive manufacturing of functional mesoscale multimaterial LOC devices.

Eigenfrequency is a key parameter for the fiber optic gyroscope (FOG). An eigenfrequency detecting method for FOGs, especially for high-grade FOGs, such as the navigation grade FOGs, is proposed. The eigenfrequency is detected with the sawtooth wave modulation theory. Adjusting the frequency of the sawtooth wave to an even integer of the eigenfrequency, the error signal caused by the sawtooth wave modulation will be zero, then the eigenfrequency can be calculated by the value of the sawtooth wave frequency exactly and the bias modulation frequency is at the eigenfrequency accurately. It is demonstrated experimentally with an FOG, the length of whose sensing coil is about 1200 m, that the accuracy of the eigenfrequency measurement is better than 1.2 ppm (0.1 Hz). With its high accuracy, not only can the frequency of the bias modulation be adjusted to the eigenfrequency precisely, but also this method can be used as an eigenfrequency detector for studying the characteristics of the sensing coil according to the eigenfrequency to study the mechanism of the errors generated in the FOGs.

X-ray phase-contrast imaging has experienced rapid development over the last few decades, and, in this technology, the phase modulation strategy of phase stepping (PS) is used most widely to measure the sample’s phase signal. However, because of its discontinuous nature, PS has the defects of worse mechanical stability and high exposure dose, which greatly hinder its wide use in dynamic phase measurement and potential clinical applications. We demonstrate preliminary research on the use of integrating-bucket (IB) phase modulation method to retrieve the phase information in grating-based x-ray phase-contrast imaging. Experimental results show that our proposed method can be well employed to extract the differential phase-contrast image, compared with the commonly used PS strategy, the advantage of the IB phase modulation technique is that fast measurement and low dose are promising.

Fringe projection profilometry is a popular optical method for three-dimensional (3-D) shape measurement because of its high accuracy, fast measurement speed, and full-field inspection nature. However, due to the limited dynamic range of the digital camera, saturated pixels in the captured images will lead to serious phase errors and measurement errors when the measured object has a drastic texture variation. To deal with such a problem, an adaptive digital fringe projection technique for high dynamic 3-D shape measurement is proposed. In this method, phase-shifting fringes are adaptively generated with the aid of a coordinates mapping process and binary-search technique to eliminate saturation. Compared with previous adaptive fringe projection techniques, the camera response function and homographic mapping between the camera and projector are not needed, making the whole measurement easier to carry out and less laborious. Experiments validate the effectiveness and superiority of the proposed method for high-dynamic range 3-D shape measurement.

The intensity and polarization dynamics of a laser subjected to anisotropic high-order optical feedback have been investigated. The feedback system is realized by a high-reflectivity feedback mirror and a wave plate in the feedback cavity. The high-resolution optical fringes with nanometer order are obtained owing to the high-reflectivity feedback mirror, and the modulation depth of these optical fringes is relatively uniform, which is different from that of instable fluctuation or intensity noise that has been previously observed. In particular, the polarization flipping is found in each fringe, and the flipping position can be easily changed by rotating the wave plate in the feedback cavity. Furthermore, when the flipping position moves to the edge of each fringe, this optical fringe becomes promising for use in precision measurement. The theoretical analysis based on the compound cavity model and the Floch rotation flipping mechanism is presented, and it agrees well with the experimental results.

In a holographic optical tweezers setup, although the use of noniterative algorithms can result in the fast generation of multiple traps array, the performance of these algorithms is often inferior compared to iterative types of algorithms. Particularly in the case of symmetric trap arrays, the performance of noniterative algorithms is very poor. Suitability of the use of a noniterative superposition algorithm for generating symmetric trap arrays has been investigated after introducing small position disorders for the individual traps. It could be seen that the introduction of small disorders in the positions of the individual traps can significantly improve the quality of the generated trap array pattern over the case when an ideal symmetric pattern is targeted.

This paper proposes an absolute phase unwrapping method for three-dimensional measurement that uses two cameras and one projector. On the left camera image, each pixel has one wrapped phase value, which corresponds to multiple projector candidates with different absolute phase values. We use the geometric relationship of the system to map projector candidates into the right camera candidates. By applying a series of candidate rejection criteria, a unique correspondence pair between two camera images can be determined. Then, the absolute phase is obtained by tracing the correspondence point back to the projector space. Experimental results demonstrate that the proposed absolute phase unwrapping algorithm can successfully work on both complex geometry and multiple isolated objects measurement.

We show a method to form radially and azimuthally polarized beams as well as higher order polarization singularities by superposition of optical vortices with opposite topological charges obtained by optical parametric amplification. The proposed method could find applications in optical trapping of particles, nonlinear optics experiments, and laser material processing.

Initial alignment is one of the most prominent and vital issues in fiber-optic gyro strapdown inertial navigation systems (SINS). In most research, the standard Kalman filter (KF) and its various versions have been used to accomplish the initial alignment of SINSs. A robust alignment approach is presented based on a generalized proportional–integral–derivative filter. The proposed inertial frame-based alignment approach outperforms the standard KF-based alignment methods and achieves a robust and accurate solution for marine fiber-optic SISN alignment. Experimental results also verify the prominent performance of the presented approach compared to the conventional standard KF-based alignment method.

Photographic fisheye lenses with fixed focal length for cameras with different sensor formats have been well developed for decades. However, photographic fisheye lenses with variable focal length are rare on the market due in part to the greater design difficulty. This paper presents a large aperture zoom fisheye lens for DSLR cameras that produces both circular and diagonal fisheye imaging for 35-mm sensors and diagonal fisheye imaging for APS-C sensors. The history and optical characteristics of fisheye lenses are briefly reviewed. Then, a 9.2- to 16.1-mm F/2.8 to F/3.5 zoom fisheye lens design is presented, including the design approach and aberration control. Image quality and tolerance performance analysis for this lens are also presented.

A laser with a wavelength of 660 nm was focused by microsized tapered glass tubes with different diameters of the exit. By using the 3-μm optical fiber and micrometer displacement stages, we measured the light intensity distribution around the focal spot, the focal distance, and the transmission coefficient of the light transmitted through these tubes. The focusing effect for the glass tubes with smaller outlet diameters of the exit was found to be much stronger than those with larger diameters of the exit. Furthermore, the dependence of the size and distance and the maximum intensity of the focal spot on the tubes’ diameter of exit are obtained.

We report a multifunctional microwave photonic signal processor based on dual-parallel Mach–Zehnder modulator and stimulated Brillouin scattering. The signal processor acts as a microwave photonic filter (MPF) and microwave photonic phase shifter (MPS) simultaneously. The MPF and MPS can be tuned separately. Experimental results demonstrate that the central frequency of the bandpass MPF is tunable from 3 to 18 GHz while the MPS in the passband of the MPF is continuously adjustable over 360 deg.

The communication between data-intensive applications in data centers often involves a collection of parallel flows, usually referred to as a coflow. A coflow between two groups of machines can capture diverse communication patterns observed in data centers. The static coflow routing and spectrum assignment (CofRSA) problem in optical orthogonal frequency division multiplexed data center networks is investigated, when the coflow traffic demands are given. The static CofRSA problem considers the spectrum constraints of the flows between different coflows and those within the same coflow. We formulate the static CofRSA problem as an integer linear programming (ILP) model. The objective of the ILP model is to minimize the used spectrum slots. However, the ILP model cannot achieve an optimal solution within tolerable time for large networks. To solve the problem, two highly efficient heuristic algorithms, the most cofSize first (MCSF) ordering algorithm and the greedy inserting (GI) algorithm, are proposed to achieve suboptimal solutions. The simulation results indicate that ILP provides an optimal solution for small networks, whereas GI and MCSF yield suboptimal solutions in large networks. The results also show that GI provides more efficient solutions than MCSF.

Based on rapid prototyping and laser cladding, laser material deposition (LMD) can be used to produce a near-net shape component in which the deposition quality and geometry characteristics of the previous layer, especially the first layer, determines the deposition quality and process stability of LDM-ed parts. A theoretical model for the first deposition layer in the LMD process has been developed to estimate the clad geometry (width, depth, and height) depending on the process parameters (laser power, powder feed rate, and scanning speed). This model contains two constants that are calculated based on the experimental results. The theoretical results are discussed and compared with the experimental data from the perspective of mass energy and line mass. The theoretical results are in accordance with the experimental results with reasonable accuracy that indicates the capability of predicting the clad geometry.

A digital signal process enabled dual-drive Mach–Zehnder modulator (DD-MZM)-based spectral converter is proposed and extensively investigated to realize dynamically reconfigurable and high transparent spectral conversion. As another important innovation point of the paper, to optimize the converter performance, the optimum operation conditions of the proposed converter are deduced, statistically simulated, and experimentally verified. The optimum conditions supported-converter performances are verified by detail numerical simulations and experiments in intensity-modulation and direct-detection-based network in terms of frequency detuning range-dependent conversion efficiency, strict operation transparency for user signal characteristics, impact of parasitic components on the conversion performance, as well as the converted component waveform are almost nondistortion. It is also found that the converter has the high robustness to the input signal power, optical signal-to-noise ratio variations, extinction ratio, and driving signal frequency.

We propose a flexible and reconfigurable wavelength-division multiplexing (WDM) multicast scheme supporting downstream emergency multicast communication for WDM optical access network (WDM-OAN) via a multicast module (MM) based on four-wave mixing (FWM) in a semiconductor optical amplifier. It serves as an emergency measure to dispose of the burst, large bandwidth, and real-time multicast service with fast service provisioning and high resource efficiency. It also plays the role of physical backup in cases of big data migration or network disaster caused by invalid lasers or modulator failures. It provides convenient and reliable multicast service and emergency protection for WDM-OAN without modifying WDM-OAN structure. The strategies of an MM setting at the optical line terminal and remote node are discussed to apply this scheme to passive optical networks and active optical networks, respectively. Utilizing the proposed scheme, we demonstrate a proof-of-concept experiment in which one-to-six/eight 10-Gbps nonreturn-to-zero-differential phase-shift keying WDM multicasts in both strategies are successfully transmitted over single-mode fiber of 20.2 km. One-to-many reconfigurable WDM multicasts dealing with higher data rate and other modulation formats of multicast service are possible through the proposed scheme. It can be applied to different WDM access technologies, e.g., time-wavelength-division multiplexing-OAN and coherent WDM-OAN, and upgraded smoothly.

DFT-S-orthogonal frequency division multiplexing (OFDM) and single-carrier (SC) modulation are two typical modulation formats in radio-over-fiber (RoF) systems. They may have respective advantages and disadvantages in different scenarios. Therefore, bit error ratio comparison results of these two modulation formats will be useful for designing and optimizing the practical RoF system. We experimentally compare these two modulation formats in a long wireless distance RoF system at W-band. It can be concluded that DFT-S-OFDM and SC modulation have similar performances in a RoF system with transmission distance over 80-km fiber and 224-m wireless link.

Benefiting from the high spectral efficiency and low peak-to-average power ratio, constant envelope orthogonal frequency division multiplexing (OFDM) is a promising technique in coherent optical communication. Polarization-division multiplexing (PDM) has been employed as an effective way to double the transmission capacity in the commercial 100  Gb/s PDM-QPSK system. We investigated constant envelope OFDM together with PDM. Simulation results show that the acceptable maximum launch power into the fiber improves 10 and 6 dB for 80- and 320-km transmission, respectively (compared with the conventional PDM OFDM system). The maximum reachable distance of the constant envelope OFDM system is able to reach 800 km, and even 1200 km is reachable if an ideal erbium doped fiber amplifier is employed.

We propose a joint estimation scheme for fast, accurate, and robust frequency offset (FO) estimation along with phase estimation based on modified adaptive Kalman filter (MAKF). The scheme consists of three key modules: extend Kalman filter (EKF), lock detector, and FO cycle slip recovery. The EKF module estimates time-varying phase induced by both FO and laser phase noise. The lock detector module makes decision between acquisition mode and tracking mode and consequently sets the EKF tuning parameter in an adaptive manner. The third module can detect possible cycle slip in the case of large FO and make proper correction. Based on the simulation and experimental results, the proposed MAKF has shown excellent estimation performance featuring high accuracy, fast convergence, as well as the capability of cycle slip recovery.

As the best candidate for wireless-access networks, radio-over-fiber (RoF) technology can carry a variety of business. It is necessary to provide differentiated services for different users, so the network needs to produce signals with different modulation formats and different frequencies. A reconfigurable RoF system based on a switch and tunable optical filter that can realize modulation format conversion and multiple frequency signal switching functions is designed. It has a good performance in terms of bit error rate and an eye diagram. The design can help to use radio frequency resources efficiently and make dynamic bandwidth resources controllable.

The peak-to-average power ratio (PAPR) of a next generation data center optoelectronic communication system is analyzed. A new challenge of ultrabroadband signals transmission over coupled printed transmission lines (PTLs) is introduced for the case of separated design of the optical module and on-board electronics. PAPR enhancement models are developed for transmission of ultrabroadband signals over coupled PTLs, due to enhanced coupling impairments. High instantaneous peak power values may lead to compression and nonlinear distortions due to the limited dynamic range of the communication system components. The analysis, which is verified by Monte Carlo simulations, reveals that the instantaneous PAPR increases significantly.

We report on a near-infrared, multiwavelength-stimulated Raman scattering (SRS) laser source based on the PbWO4 crystal, pumped by a pico-second Nd:YAG laser. At a high pump level, simultaneous two phonon modes SRSs are obtained, including 901 and 323  cm−1 Raman shifts, presenting six Raman spectral lines distributing in 1- to 1.5-μm waveband. The largest Raman output energy is 0.72 mJ, corresponding to an optical conversion efficiency of 37.9% and a slope efficiency of 80.4%. This work manifests that, as a low cost, large size, and high damage threshold crystal, PbWO4 is an efficient, convenient SRS material to enrich near-infrared laser wavelengths.

The relative impact of in-band crosstalk and amplified spontaneous emission (ASE) noise on a system’s performance has been investigated with a differentially phase-shift keying signal. I have first measured an ASE noise-induced penalty without any in-band crosstalk and then estimated the system’s penalty with a simple addition of the measured ASE noise-induced penalty and the calculated in-band crosstalk-induced penalty. Using this approach, the estimated penalty agreed well with a measured system penalty when the Q value was 3 or an optical signal-to-noise ratio (OSNR) of the signal was higher than 30 dB at Q=6. To estimate the system penalty with a low OSNR level at Q=6, an addition of signal-ASE and signal-in-band crosstalk beat noises were added with a weighting factor. Based on this approach, a discrepancy between the estimated and the measured penalties was reduced drastically with an OSNR of 20 dB at Q=6. However, a small discrepancy was still observed even with the weighted addition of two beat noises. Thus, I have confirmed that the effect of in-band crosstalk–in-band crosstalk beat noise should be taken into account for the proper estimation of a system penalty with an OSNR of <30  dB at Q=6.

The effect of displacement currents due to dielectric relaxation of majority carriers in the charge-neutral region of a semiconductor photodiode is discussed. The dielectric relaxation is often neglected when treating the response time of photodiodes. We show that this component may dominate the slow response of not fully depleted photodiodes and has to be taken into account for correct analysis of silicon photodiode response to a brief laser pulse. A phenomenological expression for the photodiode response time that accounts for the displacement current effects is proposed and used to compare with the experimental results.

With the introduction of phosphorescent and thermally activated delayed fluorescence emitter materials, organic light-emitting diodes (OLEDs) with internal quantum yields of up to 100% can be realized. Still, light extraction from the OLED stack is a bottleneck, which hampers the availability of OLEDs with large external quantum efficiencies. Many different strategies to enhance the outcoupling of the light have been suggested, for instance, the use of collective lattice resonances induced by arrays of plasmonic nanodiscs. Here, we investigate the usability of these nanodisc arrays to tune the emission color of an organic blue-emitting material. By means of extinction and photoluminescence spectroscopy, we show a correlation of the sharp features observed in extinction with a selective fluorescence enhancement. At the same time, the nanodisc array also modifies the microcavity of an OLED stack. For one exemplarily lattice constant of an aluminum nanodisc array directly integrated into an OLED stack, we show that a combination of these effects allows the modification of the emission color from CIE1931 (x,y) chromaticity coordinates of (0.149, 0.225) to (0.152, 0.352). Importantly, the OLED exhibited a similar emission color modification under optical as well as electrical excitation.

As a high-precision angular sensor, the interferometric fiber-optic gyroscope (FOG) usually shows high sensitivity to disturbances of the environmental temperature. To research the related influencing factors of influencing the thermal-induced rate error of an FOG is essential to enhance precision and environmental suitability. This paper starts with the factors neglected in past research to derive the thermal-induced error model of a fiber coil including various factors of equivalent radius, asymmetry of fiber tail, cross-layer leap, and so on in detail, and then translates this error into the inner product form of penalty factor matrix and temperature field matrix. Then, the mathematical model and the three-dimensional temperature field model of the fiber coil with the quadrupolar winding pattern is built, which includes the optic core, coating, glue, packing paper, and accurate temperature boundary conditions. The penalty factor matrix and temperature field matrix can be obtained from these models. Finally, the advancement of this revised the thermal-induced rate error model has been verified through simulation and experimental comparison.

By using in situ temperature-dependent Raman spectroscopy, we systematically study the annealing properties of photosensitive single-mode fibers and report on microstructural changes in Ge:SiO2 fiber glass as a function of time for thermal cycle between ambient temperature and 1000°C. We clearly observe the fiber core-structural relaxation during annealing as well as the temperature dependence of the vibrational band obtained with peak deconvolution, and it thereby enables one to explain thermal expansion effect independently, with the remaining possible challenge to extract the thermo-optic coefficient. This is particularly interesting for a detailed, microscopic understanding and improving optical properties of glass fibers at an elevated temperature.

This errata describes a change to the corresponding author.