We extend an earlier second-order characterization of photodiode nonlinearities to third order and present the data as a contour plot over the feasible photodiode operation points. Using this data, we calculate the additional penalty in a nonlinear (i.e., using a Mach–Zehnder modulator) optical link due to the photodiode nonlinearity and illustrate its utility in systems calculations.

We propose a scheme based on a type of continuous-phase encoding grating for the generation of multiple curved Bessel-like beams simultaneously. It is shown that multiple identical curved Bessel-like beams that diverge from a common center can be generated by overlapping a specially designed phase with a continuous-phase grating, and multiple high-order curved Bessel-like beams can be generated when another spiral phase is embedded. As an example, a 5×5 symmetric continuous-phase grating embedded with the special designed phase and a spiral phase is demonstrated based on a spatial light modulator. The experimental results show that 5×5 square arrays of bright- and dark-center curved Bessel-like beams were well generated. The proposed method provides an interesting method for obtaining simultaneously multiple curved Bessel-like beams, which should be of high interest for its promising applications in parallel optical manipulation, optical guiding, laser machining or laser surgery, particle acceleration, etc.

Classical photometric stereo requires uniform collimated light, but point light sources are usually employed in practical setups. This introduces errors to the recovered surface shape. We found that when the light sources are evenly placed around the object with the same slant angle, the main component of the errors is the low-frequency deformation, which can be approximately described by a quadratic function. We proposed a postprocessing method to correct the deviation caused by the nonuniform illumination. The method refines the surface shape with prior information from calibration using a flat plane or the object itself. And we further introduce an optimization scheme to improve the reconstruction accuracy when the three-dimensional information of some locations is available. Experiments were conducted using surfaces captured with our device and those from a public dataset. The results demonstrate the effectiveness of the proposed approach.

We present a lens distortion model based on the Gaussian function. The model is a potential source function of the popular Brown distortion model and requires the practical estimation of one Boolean and one real parameter. We also present a general technique for lens distortion identification, which consists of three steps: image data acquisition, localization and indexation/matching of image features, and the estimation of distortion parameters. The method uses a structured light technique, in which bar patterns are indexed by a Gray code. Acquired images are automatically processed using a multistep approach that localizes and indexes calibration points. An iterative analysis of differences between localizations of distorted points and their undistorted counterparts is proposed to estimate distortion model parameters. To compute undistorted localizations, the method estimates a homography matrix that is based on both undistorted data and iterative processing of distorted coordinates, which improves compensation accuracy. Experiments with three cameras show that the indexing strategy significantly decreases compensation error (from 0.26 to 0.09 pixels). The newly introduced Gaussian model is shown to be slightly more accurate and considerably more stable than the popular Brown model.

This study introduces a depth-encoding positron emission tomography (PET) detector inserting a horizontal-striped glass between the pixilated scintillation crystal layers. This design allows light spreading so that scintillation photons can travel only through the X direction and allows alteration in the light distribution so that it can generate a unique pattern diagram of the two-dimensional (2-D) flood histogram that identifies depth position as well as X-Y position of γ-ray interaction. A Monte Carlo simulation was conducted for the assessment of the depth of interaction (DOI)-PET detector. The traced light distribution for each event was converted into the 2-D flood histogram. Light loss caused by inserting the horizontal-striped glass between the crystal layers was estimated. Applicable weighting factors were examined for each DOI-PET detector. No considerable degradation of light loss was observed. The flood histogram, without overlapping of each crystal position, can be generated for the DOI detector based on each crystal block by inserting the horizontal-striped glass with a thickness of >1  mm and the modified resistive charge division networks with applicable weighting factors. This study demonstrated that the proposed DOI-PET detector can extract the three-dimensional γ-ray interaction position without considerable performance degradations of the PET detector from the 2-D flood histogram.

Existing methods for evaluating the performance of head trackers usually rely on publicly available face databases, which contain facial images and the ground truths of their corresponding head orientations. However, most of the existing publicly available face databases are constructed by assuming that a frontal head orientation can be determined by compelling the person under examination to look straight ahead at the camera on the first video frame. Since nobody can accurately direct one’s head toward the camera, this assumption may be unrealistic. Rather than obtaining estimation errors, we present a method for computing the covariance of estimation error rotations to evaluate the reliability of head trackers. As an uncertainty measure of estimators, the Schatten 2-norm of a square root of error covariance (or the algebraic average of relative error angles) can be used. The merit of the proposed method is that it does not disturb the person under examination by asking him to direct his head toward certain directions. Experimental results using real data validate the usefulness of our method.

This paper presents an effective technique to calibrate the binocular system. A collinear geometry transformation is derived and introduced into the calibration process. First, we construct a virtual binocular system with every two point pair in the same image, and then an analytic method based on the virtual binocular system is proposed to calculate the internal and external parameters of left and right cameras. The structure parameters are solved from the calibrated extrinsic parameters of each camera. All the parameters of the binocular system are calculated with only one image pair. Furthermore, an iterative refinement by minimizing the metric distance error between the reconstructed point and the real point in a three-dimensional measurement coordinate system is applied to enhance the calibration accuracy. Simulations and real experiments have been carried out. The calibration accuracy is compared with the traditional calibration method. The results show that the proposed calibration methods are efficient in improving the calibration accuracy and simply equipped.

Hyperspectral imaging polarimetry enables both the spectrum and its spectrally resolved state of polarization to be measured. This information is important for identifying material properties for various applications in remote sensing and agricultural monitoring. We describe the design and performance of a ruggedized, field deployable hyperspectral imaging polarimeter, designed for wavelengths spanning the visible to near-infrared (450 to 800 nm). An entrance slit was used to sample the scene in a pushbroom scanning mode across a 30 deg vertical by 110 deg horizontal field-of-view. Furthermore, athermalized achromatic retarders were implemented in a channel spectrum generator to measure the linear Stokes parameters. The mechanical and optical layout of the system and its peripherals, in addition to the results of the sensor’s spectral and polarimetric calibration, are provided. Finally, field measurements are also provided and an error analysis is conducted. With its present calibration, the sensor has an absolute polarimetric error of 2.5% RMS and a relative spectral error of 2.3% RMS.

This paper presents a three-dimensional (3-D) pose estimation algorithm based on monocular vision. The algorithm relies on the circle target whose radius is known, with the scale condition given, the depth information of the circle can be recovered incompletely, and finally the pose of the target can be estimated by single projection only. First, the circle target was mapped to be an upright elliptic cone in the pinhole imaging model. Second, radius constraint was applied to recover partial depth of the circle target based on the upright elliptic cone. Experimental work concerning the validity and accuracy of this method is presented; furthermore, an application case for robot-aided positioning is introduced.

Considering the increasing amount of data for communication and infotainment applications, fabrication of optical networks and bus systems is a challenging task for production engineering. A two-step manufacturing process for polymer optical waveguides is presented. By improving the highly efficient flexographic printing technology by laser functionalization of the printing tool in combination with a subsequent spray application, high-quality waveguides are accomplished. By adjusting the resulting surface energy of the foil substrate in the first fabrication process, the spray application achieved high-aspect ratio waveguides with a low attenuation of 0.2  dB/cm at 850 nm.

We discuss the design, realization, and characterization of a miniaturized focus-tunable camera objective featuring a gravity-neutral, liquid-tunable aspherical lens and compare its performance to an equivalent system optimized for a conventional tunable lens. In addition to the innovative component, the objective design features three fixed elements and an aperture, which are all assembled to form a 5×5×13  mm system. With an image sensor size of 3 mm, the objective provides a viewing angle of 40 deg. By tuning the lens in its gravity-neutral range, the object distance can be shifted from 5 mm to infinity, with image-side cutoff frequency remaining above 84  lp/mm across the entire range. Through ray-tracing simulations and experimental results, we demonstrate that for optical trains of identical complexity, tunable aspherical lenses provide substantially better imaging quality over the object distance range of interest, compared to conventional, spherical tunable lenses.

A subpixel displacement measurement method based on the combination of particle swarm optimization (PSO) and gradient algorithm (GA) was proposed for accuracy and speed optimization in GA, which is a subpixel displacement measurement method better applied in engineering practice. An initial integer-pixel value was obtained according to the global searching ability of PSO, and then gradient operators were adopted for a subpixel displacement search. A comparison was made between this method and GA by simulated speckle images and rigid-body displacement in metal specimens. The results showed that the computational accuracy of the combination of PSO and GA method reached 0.1 pixel in the simulated speckle images, or even 0.01 pixels in the metal specimen. Also, computational efficiency and the antinoise performance of the improved method were markedly enhanced.

Acquiring the three-dimensional (3-D) surface geometry of objects with a full-frame resolution is of great concern in many applications. This paper reports a 3-D measurement scheme based on single-frame pattern projection in the combination of random binary encoding and color encoding. Three random binary encoding patterns generated by a computer embedded in three channels of a color pattern lead to a color binary encoding pattern. Two color cameras with a stereo-vision arrangement simultaneously capture the measured scene under the proposed encoding structured illumination. From captured images, three encoding images are extracted and analyzed using the extended spatial–temporal correlation algorithm for 3-D reconstruction. Theoretical explanation and analysis concerning the encoding principle and reconstruction algorithm, followed by experiments for reconstructing 3-D geometry of stationary and dynamic scenes show the feasibility and practicality of the proposed method.

One of the approaches that is being tested for the integration of the mirror modules of the advanced telescope for high-energy astrophysics x-ray mission of the European Space Agency consists in aligning each module on an optical bench operated at an ultraviolet wavelength. The mirror module is illuminated by a plane wave and, in order to overcome diffraction effects, the centroid of the image produced by the module is used as a reference to assess the accuracy of the optical alignment of the mirror module itself. Among other sources of uncertainty, the wave-front error of the plane wave also introduces an error in the position of the centroid, thus affecting the quality of the mirror module alignment. The power spectral density of the position of the point spread function centroid is here derived from the power spectral density of the wave-front error of the plane wave in the framework of the scalar theory of Fourier diffraction. This allows the defining of a specification on the collimator quality used for generating the plane wave starting from the contribution to the error budget allocated for the uncertainty of the centroid position. The theory generally applies whenever Fourier diffraction is a valid approximation, in which case the obtained result is identical to that derived by geometrical optics considerations.

The high-precision detection for surface defect of ceramic balls, widely used in precise bearings, is a challenging task. Due to the fact that some defects cannot be directly detected by the gray-level of images, we propose a method for surface defect detection based on fringe reflection. Based on specular characteristics of ceramic balls, this method utilizes flat screens with fringes drawn on them. If a ceramic ball is nondefective, the image, formed by reflection on its surface, presents even fringes. The distortion of fringes designed by a reverse exact ray-tracing method occurs at the defective region of ceramic balls. During experiments conducted with silicon nitride (Si3N4) ceramic balls, images are captured by a CMOS camera with high resolution and processed by specific algorithms. Experimental results demonstrate the feasibility of this method, which can be applied for high-precision detection for surface defect of ceramic balls and other objects with similar characteristics.

In high precision and large-scale coordinate measurement, one commonly used approach to determine the coordinate of a target point is utilizing the spatial trigonometric relationships between multiple laser transmitter stations and the target point. A light receiving device at the target point is the key element in large-scale coordinate measurement systems. To ensure high-resolution and highly sensitive spatial coordinate measurement, a high-performance and miniaturized omnidirectional single-point photodetector (OSPD) is greatly desired. We report one design of OSPD using an aspheric lens, which achieves an enhanced reception angle of −5  deg to 45 deg in vertical and 360 deg in horizontal. As the heart of our OSPD, the aspheric lens is designed in a geometric model and optimized by LightTools Software, which enables the reflection of a wide-angle incident light beam into the single-point photodiode. The performance of home-made OSPD is characterized with working distances from 1 to 13 m and further analyzed utilizing developed a geometric model. The experimental and analytic results verify that our device is highly suitable for large-scale coordinate metrology. The developed device also holds great potential in various applications such as omnidirectional vision sensor, indoor global positioning system, and optical wireless communication systems.

Measurement of objects with complex geometry and many self-occlusions is increasingly important in many fields, including additive manufacturing. In a fringe projection system, the camera and the projector cannot move independently with respect to each other, which limits the ability of the system to overcome object self-occlusions. We demonstrate a fringe projection setup where the camera can move independently with respect to the projector, thus minimizing the effects of self-occlusion. The angular motion of the camera is tracked and recalibrated using an on-board inertial angular sensor, which can additionally perform automated point cloud registration.

Turbulence affects optical propagation, and, as a result, the intensity is attenuated along the path of propagation. The attenuation becomes significant when the turbulence becomes stronger. Transmittance is a measure indicating how much power is collected at the receiver after the optical wave propagates in the turbulent medium. The on-axis transmittance is formulated when a flat-topped optical beam propagates in a marine atmosphere experiencing anisotropic non-Kolmogorov turbulence. Variations in the transmittance are evaluated versus the beam source size, beam number, link distance, power law exponent, anisotropy factor, and structure constant. It is found that larger beam source sizes and beam numbers yield higher transmittance values; however, as the link distance, power law exponent, anisotropy factor, or structure constant increase, transmittance values are lowered. Our results will help in the performance evaluations of optical wireless communication and optical imaging systems operating in a marine atmosphere.

Based on the process by which the spatial depth clue is obtained by a single eye, a monocular stereo vision to measure the depth information of spatial objects was proposed in this paper and a humanoid monocular stereo measuring system with two degrees of freedom was demonstrated. The proposed system can effectively obtain the three-dimensional (3-D) structure of spatial objects of different distances without changing the position of the system and has the advantages of being exquisite, smart, and flexible. The bionic optical imaging system we proposed in a previous paper, named ZJU SY-I, was employed and its vision characteristic was just like the resolution decay of the eye’s vision from center to periphery. We simplified the eye’s rotation in the eye socket and the coordinated rotation of other organs of the body into two rotations in the orthogonal direction and employed a rotating platform with two rotation degrees of freedom to drive ZJU SY-I. The structure of the proposed system was described in detail. The depth of a single feature point on the spatial object was deduced, as well as its spatial coordination. With the focal length adjustment of ZJU SY-I and the rotation control of the rotation platform, the spatial coordinates of all feature points on the spatial object could be obtained and then the 3-D structure of the spatial object could be reconstructed. The 3-D structure measurement experiments of two spatial objects with different distances and sizes were conducted. Some main factors affecting the measurement accuracy of the proposed system were analyzed and discussed.

Establishing a highly accurate phase-to-height mapping relationship is very important in fringe projection profilometry, which guarantees the accuracy of final three-dimensional reconstruction. The influence coming from lens distortion, random noises, and the nontelecentric projecting and imaging of the measurement system is analyzed in detail, followed by the exhaustive discussion of a more accurate phase-to-height mapping method. The mapping tabulation between absolute phase and height information is set up by the piecewise linear fitting method within the whole measurement range for per-pixel. Our method is compared with the previously used methods, such as linear fitting (LF), quadratic fitting (QF), and cubic fitting (CF) methods. Computer simulations and experiments verify that the reconstructed height distribution employing our method is more accurate than either LF or QF methods when the random noise is obvious. In addition, if the random noise can be controlled to low level and the lens distortion is considered, the reconstruction accuracy of our method is better than that of the CF method.

Chemical etching experiments are carried out to expose and change the morphology of multiple scratches and single scratches in a set of K9 glasses, during which the transmission of some glasses is measured. The results show that brittle scratches have an influence on the transmission, and the transmission decreases at first and then increases obviously with the increase of etching time for the glass with a high density of cracks. The scattered field distribution around trailing indent crack(s) and the transmission of the glass with a scratch etched deeply (adjacent cracks have coalesced with each other) are simulated under different parameters. Using finite-difference time-domain (FDTD) algorithm, the light intensification effect around two single-evolving scratches and two adjacent cracks with different parameters are simulated, respectively. The results indicate that the position with the maximum light intensity changes for the glass with a scratch and its light intensity enhancement factors (LIEFs) will generally decrease as the etching process progresses. Compared with the morphology evolution of a scratch during the etching process, the changing mechanisms of transmission and LIEFs are revealed. This work will contribute to the improvement of service performance of optical glass by chemical etching process.

The use of dual cameras in parallax in order to detect and create 3-D images in mobile devices has been increasing over the last few years. We propose a concept where the second camera will be operating in the short-wavelength infrared (SWIR—1300 to 1800 nm) and thus have night vision capability while preserving most of the other advantages of dual cameras in terms of depth and 3-D capabilities. In order to maintain commonality of the two cameras, we propose to attach to one of the cameras a SWIR to visible upconversion layer that will convert the SWIR image into a visible image. For this purpose, the fore optics (the objective lenses) should be redesigned for the SWIR spectral range and the additional upconversion layer, whose thickness is <1  μm. Such layer should be attached in close proximity to the mobile device visible range camera sensor (the CMOS sensor). This paper presents such a SWIR objective optical design and optimization that is formed and fit mechanically to the visible objective design but with different lenses in order to maintain the commonality and as a proof-of-concept. Such a SWIR objective design is very challenging since it requires mimicking the original visible mobile camera lenses’ sizes and the mechanical housing, so we can adhere to the visible optical and mechanical design. We present in depth a feasibility study and the overall optical system performance of such a SWIR mobile-device camera fore optics design.

The electricity consumption in houses and commercial buildings generates about 18% of greenhouse gas emission. A critical issue of building energy consumption is heat and cooling loss through the window. Low-emissivity windows control thermal radiation through glass without decreasing the intensity of visible light. They are made up of optical filter coatings grown on a flat glass surface. Solar filters based on Ag/IZO multilayer films are grown and simulated on glass substrate. The targeted structure designs are grown by a sputtering system and characterized by scanning electron microscopy and x-ray diffraction techniques. To accurately simulate transmission spectrum, silver (Ag) and IZO optical constants were estimated by fitting ellipsometric data at different thicknesses. Transmission spectrum shows a good agreement among experiment and simulation. In addition, optical constant curves strongly show layer thickness dependence in both materials. In particular, the ultrathin Ag layer displays a percolation threshold in the vicinity of 15 nm, which leads to surface plasmon resonance with absorption at about 450 nm. These types of optical filter coatings would have potential applications as low-emission windows.

This article demonstrates the design of dielectric terahertz (THz) attenuators comprising of periodically placed x-cut and z-cut ion-sliced lithium niobate dielectric layers. Changes introduced in the propagating wave due to alternating refractive indices of a ferroelectric material have been exploited for the design of an effective attenuator. The electrical and optical properties gathered from experimental investigations have been used to study the influence of different crystalline orientations on the design. The conduit comprising the periodically placed dielectric slabs has been configured as a tristate switch by modulating the amplitude of the traversing THz wave by altering its angle of incidence. Full-wave finite-element simulations have been conducted over a series of parametric configurations to come up with the optimized design. A modulation depth as high as 94.76% is achieved. The proposed low-cost, easily configurable THz dielectric attenuators operating around 0.625 THz can be potentially used as an external modulator for THz quantum cascade lasers.

To respond to current demands of nano- and microtechnologies, e.g., miniaturization and integration, different bottom-up strategies have been developed. These strategies are based on picking, placing, and assembly of multiple components to produce microsystems with desired features. This paper covers the fabrication of arbitrary-shaped microcomponents by two-photon polymerization and the trapping, moving, and aligning of these structures by the use of a holographic optical tweezer. The main focus is on the assembly technique based on a cantilever microsnap-fit. More precisely, mechanical properties are characterized by optical forces and a suitable geometry of the snap-fit is designed. As a result of these investigations, a fast and simple assembly technique is developed. Furthermore, disassembly is provided by an optimized design. These findings suggest that the microsnap-fit is suitable for the assembly of miniaturized systems and could broaden the application opportunities of bottom-up strategies.

We present a simple method to design apochromat and superachromat objectives. The chromatic focal shift is used to determine glass combinations that yield three and four crossings in the chromatic focal shift curve. The method can be extended to design superachromats with more than four crossings.

A fiber chirped-pulse amplification system with pulse energy as high as 105  μJ is achieved at 200-kHz repetition rate using the rod-type photonic crystal fiber. The whole system’s nonlinearity accumulated in the fiber amplification is effectively suppressed, and the compressed pulse duration of 808 fs is obtained. A 500-kHz high-repetition rate KTiOPO₄ Pockels cell is also applied to make the ultrafast laser pulse selection for generating pulse trains with controllable pulse number and pulse splitting without changing the pulse energy. The demonstrated pulse selection and splitting method are useful for processing of different materials and parallel processing. The pulse selection efficiency of the Pockels cell is as high as 96%.

The digital micromirror device (DMD) is the key device in maskless lithography. However, because of the machinery manufacturing limit of DMDs, the gap between the micromirrors may destroy the continuity of the graphic. This work presents a simple way to fill the imaging crack by controlling the partial coherence factor σ of the light source. A crack can be regarded as the image of a dark space. By considering the resolving power for such cracks under partially coherent illumination, the images of such dark spaces can be covered, preventing them from being imaged on the substrate. By using mathematical derivations of the light intensity distribution exposed to the substrate, and by utilizing the diffraction effect induced by the finite aperture of the optical projection system, an appropriate σ value can be determined for eliminating the image of the crack in an actual scene. The numerical simulation results demonstrate that this method can ensure the continuity of the graphic at the critical partial coherence factor σc regardless of the shape of the target graphic.

An indoor positioning algorithm based on visible light communication (VLC) is presented. This algorithm is used to calculate a three-dimensional (3-D) coordinate of an indoor optical wireless environment, which includes sufficient orders of multipath reflections from reflecting surfaces of the room. Leveraging the global optimization ability of the genetic algorithm (GA), an innovative framework for 3-D position estimation based on a modified genetic algorithm is proposed. Unlike other techniques using VLC for positioning, the proposed system can achieve indoor 3-D localization without making assumptions about the height or acquiring the orientation angle of the mobile terminal. Simulation results show that an average localization error of less than 1.02 cm can be achieved. In addition, in most VLC-positioning systems, the effect of reflection is always neglected and its performance is limited by reflection, which makes the results not so accurate for a real scenario and the positioning errors at the corners are relatively larger than other places. So, we take the first-order reflection into consideration and use artificial neural network to match the model of a nonlinear channel. The studies show that under the nonlinear matching of direct and reflected channels the average positioning errors of four corners decrease from 11.94 to 0.95 cm. The employed algorithm is emerged as an effective and practical method for indoor localization and outperform other existing indoor wireless localization approaches.

This paper proposes a frequency-tunable optoelectronic oscillator (OEO) based on a single bandpass microwave photonic filter (MPF), which is implemented by a polarization division multiplexing emulator (PDME) with a tunable time delay difference between the two polarization states, a polarization modulator (PolM), a broadband optical source (BOS), and a linearly chirped fiber Bragg grating (LCFBG). The center frequency of the MPF can be shifted by adjusting the time delay difference between the two polarization states of the PDME. By incorporating the MPF into an OEO, the oscillation frequency of OEO is tuned. The proposed OEO is experimentally demonstrated. Frequency-tunable oscillation signals from 2.45 to 12.72 GHz are generated with their performance being investigated.

A self-heated diode-pumped alkali laser (SDPAL) with a microfabricated alkali cell is proposed. Based on Beach’s model and finite-element analysis theory, the output characteristics of a cesium self-heated laser are studied. The results indicate that an SDPAL with a cell length of 2 mm is feasible. The output power of a typical SDPAL is ∼Watt level. Rapid heat convection around the mini cell can increase the output power. At the same time, the utilization ratio of the pumping light will decrease. A heating experiment is also conducted to validate the theoretical model. When pumping power of 0.69 W is illuminated on the light absorber, the cell temperature can reach 76.4°C with a single-side heated structure. The results show that with a mini vapor cell, SDPAL can be portable and competitive when ∼Watt-level laser with wavelength of alkali D1 line is required.

We experimentally demonstrate a 2×2 optical multiple-inputs multiple-outputs (MIMO) visible light communications system based on the modified orthogonal frequency-division multiplexing/offset quadrature amplitude modulation scheme. The adjacent subcarrier frequency-domain averaging (ASFA) with the full-loaded (FL) and half-loaded (HL) preamble structures is proposed for demultiplexing and mitigating the intrinsic imaginary interference (IMI) effect. Compared with the conventional channel estimation (CE) method, ASFA offers improved transmission performance. With the FL method, we obtain more accurate MIMO CE to mitigate the IMI effect and the optical noise compared to the HL method.

We propose and experimentally demonstrate four-level pulse-amplitude-modulation (PAM-4) signal delivery in a radio-over-fiber system for the first time. Over 8-Gbit/s PAM-4 signals have been transmitted over 20-km single-mode fiber-28 and 1-m wireless distance. The signal after transmission is detected directly by an envelope detector at the receiver side. The maximal bit rate could be increased if the bandpass amplifier and envelope detector have more bandwidth.

For an optical satellite link, the error sources that influence the pointing accuracy of the terminals include the attitude estimation error, ephemeris accuracy, and the boresight alignment error. It means an uncertainty cone (UC) exists for the spatial acquisition process. The acquisition process is a continuous adjustment for the visual axis of the two communication terminals, to enable them to detect the light signal from the other end, and finally realize the light closed loop. Beaconless acquisition algorithm and the corresponding acquisition process, modeling for spiral scan pattern and the simulation results of staring-scanning mode are presented. The UC scan range is 5 mrad, through the overlap with a spot divergence angle of 80  μrad, the total acquisition time is 98 s.

A multiple-input multiple-output visible light communication (VLC) system based on disorder dispersion components is presented. Instead of monochromatic sources and large size photodetectors used in the traditional VLC systems, broadband sources with different spectra act as the transmitters and a compact imaging chip sensor accompanied by a disorder dispersion component and a calculating component serve as the receivers in the proposed system. This system has the merits of small size, more channels, simple structure, easy integration, and low cost. Simultaneously, the broadband sources are suitable to act as illumination sources for their white color. A regularized procedure is designed to solve a matrix equation for decoding the signals at the receivers. A proof-of-concept experiment using on–off keying modulation has been done to prove the feasibility of the design. The experimental results show that the signals decoded by the receivers fit well with those generated from the transmitters, but the bit error ratio is increased with the number of the signal channels. The experimental results can be further improved using a high-speed charge-coupled device, decreasing noises, and increasing the distance between the transmitters and the receivers.

An upgradable radio-over-fiber (RoF) transport system is proposed based on a vertical-cavity surface-emitting laser (VCSEL)-composed wavelength selector. The passband window of the wavelength selector can be electrically adjusted to select a specific lightwave from multiple injected optical carriers and attenuate the others. By adding the VCSEL-composed wavelength selector installed inside each base station (BS), the proposed transport system can remain as the original tree topology, but its overall network capacity can be flexibly extended (reduced) by adding (removing) an additional optical carrier into (out of) the optical network. Each BS can dynamically receive data from one of the multiple injected optical carriers. This method is useful because the bandwidth requirement of each BS, which is serviced by an RoF transport system, may significantly change with the visitor flow rate. Experimental results prove that the BSs in the proposed upgradable RoF transport system can be reallocated to multiple logical RoF links and can be dynamically switched to receive data from any one of the logical RoF signals without modifying the original network structure. The transmission performances of these optical carriers are ensured by low bit error rate, clear eye, and optical/electrical spectra diagrams.

The performance of a free-space optical communication system is highly affected by the atmospheric turbulence in terms of scintillation. An optical communication system based on intensity-modulation direct-detection was built with 1-km transmission distance to evaluate the bit error rate (BER) performance over real atmospheric turbulence. 2.5-, 5-, and 10-Gbps data rate transmissions were carried out, where error-free transmission could be achieved during over 37% of the 2.5-Gbps transmissions and over 43% of the 5-Gbps transmissions. In the rest of the transmissions, BER deteriorated as the refractive-index structure constant increased, while the two measured items have almost the same trend.

We report on a fiber-optic delay-based quasidistributed temperature sensor with high precision. The device works by detecting the delay induced by the temperature instead of the spectrum. To analyze the working principle of this sensor, the thermal dependence of the fiber-optic delay was theoretically investigated and the delay-temperature coefficient was measured to be 42.2  ps/km°C. In this sensor, quasidistributed measurement of temperature could be easily realized by dense wavelength-division multiplexing and wavelength addressing. We built and tested a prototype quasidistributed temperature sensor with eight testing points equally distributed along a 32.61-km-long fiber. The experimental results demonstrate an average error of &lt;0.1°C. These results prove that this quasidistributed temperature sensor is feasible and that it is a viable option for simple and economic temperature measurements.

We propose and demonstrate a compact and portable-size 84-GHz passive mode-locked fiber laser, in which a dual-fiber coupled fused-quartz microresonator is employed as the intracavity optical comb filter as well as the optical nonlinear material for optical frequency comb generation. About eight coherent optical tones can be generated in the proposed fiber laser. The 20-dB bandwidth is larger than 588 GHz. The full-width half-maximum pulse-width of the proposed laser is 2.5 ps. We also demonstrate the feasibility of using the proposed passive mode-locked fiber laser to carry a 5-Gbit/s on-off-keying signal and transmit over 20-km standard single mode fiber. A 7% forward error correction requirement can be achieved, showing the proposed fiber laser can be a potential candidate for fiber-wireless applications.

We investigate the three-coupled Hirota system, which is applied to model the long distance communication and ultrafast signal routing systems governing the propagation of light pulses. With the aid of the Darboux dressing transformation, composite rogue wave solutions are derived. Spatial–temporal structures, including the four-petaled structure for the three-coupled Hirota system, are exhibited. We find that the four-petaled rogue waves occur in two of the three components, whereas the eye-shaped rogue wave occurs in the other one. The composite rogue waves can split up into two or three single rogue waves. The corresponding conditions for the occurrence of such phenomena are discussed and presented. We find that the relative position of every single rogue wave is influenced by the ratios of certain parameters. Besides, the linear instability analysis is performed, and our results agree with those from the baseband modulation instability theory.

A power efficient optical frequency comb (OFC) is highly desired for applications with limited power supply, e.g., microwave photonics applications in moving vehicles. An ultraflat OFC generation is demonstrated with 0.19-dB flatness, 15 comb lines, and 18.3-dBm total RF power consumption. The scheme is based on spectral convolution. A five-line comb with 0.06-dB flatness and 4.0-GHz spacing is first generated in an intensity modulator. Then the signal is injected to a phase-modulator driven by ×5 frequency of 20.0 GHz to obtain the ultraflat 15-line comb with 0.19-dB flatness. The comb spacing is adjustable by controlling the input frequency of microwave source within the bandwidth of phase-modulation. Our work demonstrates a power efficient approach to obtain ultraflat comb and may benefit microwave photonics applications requiring low-power consumption.

Polarization-interleave-multiplexed (PIM) with single-sideband orthogonal frequency-division multiplexing (SSB-OFDM) based on direct detection is proposed for short-reach applications transmitted up to 80 km in which the guard band can be shared for the two SSB signals with interleave electrical center frequencies. Based on two dual-drive Mach–Zehnder modulators with one single-end photodetector (PD), 100-Gb/s PIM-SSB-OFDM transmission over a 80-km standard single-mode fiber is successfully demonstrated. After 80-km transmission, the optical signal-to-noise ratio requirement is 29.1 dB with respect to the bit error rate threshold of 7% hard decision-forward error correction overhead.

A downconversion scheme using a pilot-aided blind phase search (PA-BPS) algorithm for phase noise compensation is proposed and experimentally evaluated in coherent optical orthogonal frequency-division multiplexing systems based on optical comb and heterodyne detection. The proposed method can increase the systems bandwidth utilization efficiency and reduce the receiver complexity. Experimental results show that, based on the method, a receiver sensitivity as low as −27  dBm can be achieved in the transmission systems considered. Meanwhile, in each PA-BPS calculation, the method only requires 2 pilots and 10 testing phases.

Optical signal processing is expected to be applied in network nodes. In photonic routers, label recognition is one of the important functions. We have studied different kinds of label recognition methods so far for on–off keying, binary phase-shift keying, quadrature phase-shift keying, and 16 quadrature amplitude modulation-coded labels. We propose a method based on waveguide circuits to recognize an optical eight quadrature amplitude modulation (8QAM)-coded label by simple passive optical signal processing. The recognition of the proposed method is theoretically analyzed and numerically simulated by the finite difference beam propagation method. The noise tolerance is discussed, and bit-error rate against optical signal-to-noise ratio is evaluated. The scalability of the proposed method is also discussed theoretically for two-symbol length 8QAM-coded labels.

We propose a method for developing small all-fiber vehicle laser rangefinders that is based on pulse position modulation (PPM) and data integration and present a theoretical study on its performance. Compared with spatial coupling, which is employed by most of the current commercial vehicle laser rangefinders, fiber coupling has the advantage that it can guide laser echoes into the interior of a car, so the electronic components following the photodiode can operate in a moderate-temperature environment. However, optical fibers have numerical apertures (NAs), which means that a laser beam from a receiving lens cannot be coupled into an optical fiber if its incident angle exceeds the critical value. Therefore, the effective size of the receiving lens is typically small since it is limited by its focal length and the NA of the fiber, causing the power of the laser echoes gathered by the receiving lens to be insufficient for performing target identification. Instead of increasing the peak transmitting laser power unrestrictedly, PPM and data integration effectively compensate for the low signal-to-noise ratio that results from the effective receiving lens size reduction. We validated the proposed method by conducting numerical simulations and performance analysis. Finally, we compared the proposed method with pseudorandom noise (PN) code modulation and found that, although the two methods perform equally well in single-target measurement scenarios, PPM is more effective than PN code modulation for multitarget measurement. In addition, PPM enables the transmission of laser beams with higher peak powers and requires less computation than PN code modulation does.

Midinfrared (MIR) wavelength conversion based on degenerate four-wave mixing is theoretically investigated in hydrogenated amorphous silicon (a-Si:H) waveguides. The broadband phase mismatch is achieved in the normal group-velocity dispersion regime. The conversion bandwidth is extended to 900 nm, and conversion efficiency of up to −14  dB with a pump power of 70 mW in a 2-mm long a-Si:H rib waveguides is obtained. This low-power on-chip wavelength converter will have potential for application in a wide range of MIR nonlinear optic devices.

The influence of centrifugal forces on angular velocity sensors that measure a spectral shift of whispering-gallery modes (WGMs) is investigated. Spherical WGM resonators of different materials are considered the sensing elements. The study is based on the results of the simulation in OOFELIE::Multiphysics software.

Integrated electric-field (E-field) sensors are commonly used devices in E-field sensing. However, distortion in the modulated signal due to high half-voltage ( Vπ) and obtaining a low-frequency response are challenging issues in low-frequency AC E-field sensors. The aim of this study is to investigate a modification by adding a Si layer beneath a segmented slot waveguide (SSW) and optimizing the hybrid SSW as the core of a sensor to determine the sensor features in terms of the frequency response and sensitivity. The results of reducing the Si-layer thickness and segment width with high periodicity revealed a high modulation efficiency for very low-frequency AC E-field sensors and simultaneously expanded the minimum limit of detection by incorporating sensors in very small AC E-fields. This study validated the feasibility and efficiency of using a hybrid SSW as the core of highly sensitive low-frequency AC E-field sensors.

We numerically investigated the plasmon-induced transparency (PIT) effect in a three-dimensional plasmonic metamaterial composed of three identical rings. It is illustrated that the PIT effect appears as a result of the destructive interference between the electric dipole and the quadrupole resonance mode. By tuning gap distance, radius or rotation angle of the metamaterial, the required transmission spectra with a narrow sharp transparency peak can be realized. In particular, it is found that an on-to-off amplitude modulation of the PIT transparency window can be achieved by moving or rotating the horizontal ring. Two dips move to high frequency and low frequency regions, respectively, in the transmission spectra by moving the horizontal ring, namely, the width of transmission peak becomes larger. With the rotation of horizontal ring, both width and position of transmission peak are kept invariant. Our designed structure achieved a maximum group index of 352 in the visible frequency range, which has a significant slow light effect. Moreover, the PIT effect is explained based on the classical two-oscillator theory, which is in well agreement with the numerical results. It indicates our proposed structure and theoretical analysis may open up avenues for the tunable control of light in highly integrated optical circuits.

An electrically tunable grating coupler is designed and numerically demonstrated. With a lateral p-i-n diode embedded, the optical spectrum of coupling efficiency can be tuned with the applied voltage. To simulate the coupling spectra response with bias voltage, the optical simulation and electrical simulation are carried out with the commercial software Lumerical Finite-Difference Time-Domain Solutions and Synopsys Sentaurus TCAD. Due to the dual effect of spectrum shift and optical loss, the coupling efficiency spectrum can be greatly modulated. With a bias voltage of 2 V, the resulting spectrum shift is 47.5 nm and the peak coupling efficiency at the designed wavelength center can be modulated from 52% to 10%. In addition, the electrical tuning can be used for compensation of postassembly spectrum shift. The effects of the incident angle error and epoxy curing process are discussed. According to our simulation results, tuning voltages of 1 and 2 V are enough to compensate for the incident angle error of 2.5 deg and 3.5 deg, respectively. For the spectrum shift caused by epoxy bonding, the required tuning voltage is as low as 0.82 V. Though it brings additional optical loss, the tuning technique shows interesting prospects in postassembly coupling optimization or channel equalization.

A dual-parameter optical fiber sensor, which is fabricated by sandwiching a segment of few-mode fiber (FMF) with two down-tapers between two segments of standard single-mode fibers (SMFs), is investigated theoretically and experimentally. The two down-tapers on the FMF can enhance the evanescent field, making the sensor more sensitive to changes in the external environment. The refractive index (RI) and temperature are measured simultaneously using the different sensitivities of the two dips in this experimental interference spectrum. The measured temperature sensitivities are 0.097 and 0.114  nm/°C, and the RI sensitivities are −97.43 and −108.07  nm/RIU, respectively. Meanwhile, the simple SMF-FMF-SMF structure is also measured. By comparing the experimental results of the two structures, the sensitivities of the proposed structure based on the dual-taper FMF are significantly improved. In addition, the sensor is easy to fabricate and cost effective.

We present a simple analysis of the design of a passive miniature resonant optical gyroscope. By combining the requirements on the angular random walk and the bias stability, we end up with simple expressions of the minimum diameter of the ring waveguide cavity and the maximum power that should be used to probe it. Using state-of-the-art performances of photonic integrated circuit and whispering gallery mode technologies in terms of propagation losses and mode size, we show that tactical grade gyroscope performances can be achieved with a diameter of a few cm provided the detrimental influence of the Kerr effect is mitigated using, for instance, an active control of the unbalance in the intensities. We further extend the analysis to medium performance gyroscope and give some hints on the efforts to be made to potentially demonstrate a miniature resonant optical gyroscope with this level of performance.

A method for reconstructing the vibration waveform from the optical time-domain backscattering pulses in the distributed optical fiber sensing system (DOFSS) is proposed, which allows for extracting and recovering the external vibration signal from the tested pulses by analog signal processing, so that can obtain vibration location and waveform simultaneously. We establish the response model of DOFSS to the external vibration and analyze the effects of system parameters on the operational performance. The main parts of the DOFSS are optimized, including delay fiber length and wavelength, to improve the sensitivity of the system. The experimental system is set up and the vibration amplitudes and reconstructed waveforms are fit well with the original driving signal. The experimental results demonstrate that the performance of vibration waveform reconstruction is good with SNR of 15 dB whenever the external vibrations with different intensities and frequencies exert on the sensing fiber.

Cr,Er:YSGG ( Y3Sc2Ga3O12) crystals with 30 at. % Er3+ and two different concentrations of Cr3+ ions were grown by the Czochralski method. The spectra show the absorption coefficients at 450 and 654 nm and the fluorescence intensity at 2794 nm for 3 at. % Cr,Er:YSGG which are larger than those of 2 at. % Cr,Er:YSGG. A maximum pulse energy 1151.0 mJ operated at 5 Hz and 2.79  μm is obtained on the 3 at. % Cr,Er:YSGG crystal, corresponding to electrical-to-optical efficiency of 1.40%, slope efficiency of 1.71%, and threshold of 8.6 J. Under the same conditions, the values are 1029.8 mJ, 1.23%, 1.50%, and 12.6 J for 2 at. % Cr,Er:YSGG, respectively. Therefore, the 3 at. % Cr,Er:YSGG exhibits a larger output energy, higher laser efficiency, and lower pumping threshold. These results suggest that the laser performance of the Cr,Er:YSGG crystal can be improved by further optimizing the doping ions concentration and pumping parameters.