Light field cameras are an emerging imaging device for acquiring 3-D information of a scene by capturing a field of light rays traveling in space. As light field cameras become portable, hand-held, and affordable, their potential as a 3-D measurement instrument is growing in many applications, including 3-D evidence imaging in crime scene investigations. We evaluated the lateral resolution of commercially available light field cameras, which is one of the fundamental specifications of imaging instruments. For the evaluation of the camera’s lateral resolution, we imaged Siemens stars under various imaging configurations and experimental conditions, including changes in distance between the camera and the resolution target plate, illumination, zoom level, location of the Siemens star in the camera’s field-of-view, and cameras of the same model. The analysis results from a full factorial experiment showed that (i) when a lower zoom level of the camera was used, the lateral resolution tended not to be affected by distance; however, when a higher zoom level was used, it tended to decrease significantly with respect to the distance, (ii) the center region of the camera’s field-of-view provided a better lateral resolution than the peripheral regions, (iii) a higher zoom level yielded a higher lateral resolution, (iv) the two cameras of the same model used in the study did not show a significant difference in the lateral resolution, and (v) changes in illumination did not affect the lateral resolution of the cameras.

This paper proposes a multiple features fusion method for moving shadows removal mainly based on optical reflection invariant. It assumed that the optical reflection of object surface changed little when casted by shadows. Two techniques are designed for reflection-invariant estimation. One is homomorphic filtering that obtains reflection component from input image directly; the other technique is the proposed maximum channel prior. Based on homomorphic filtering, scale-invariant local ternary pattern is utilized to discriminate shadows from the foreground mask. Based on maximum channel prior, an approximate illumination invariant is designed by local neighboring information calculation. These techniques are utilized for weak or strong shadow detection. For weak shadows, multiple descriptors including color constancy, texture consistency, and reflection invariance at the pixel-level or region-level are combined based on shadow assumptions. For strong shadows, a coarse-to-refine strategy is proposed using maximum channel prior technique. Experimental results on standard datasets consisting of 12 video sequences illustrate the proposed method’s effectiveness compared with some state-of-the-art approaches.

An asymmetric multiple-information cryptosystem based on spherical wave and chaotic random phase mask (CRPM) is proposed. In this security system, each secret color image is decomposed into R, G, and B channels and then independently phase-only encoded. The resulting channel is first illuminated by spherical wave and then modulated by CRPM to obtain an encoded channel. Similarly, each encoded channel of n secret images is obtained and then individually multiplied together to produce a corresponding encoded channel. The final encoded image R, G, and B channels of n secret images are digitally concatenated. The proposed method has mainly three advantages over reported methods. First, extremely sensitive and robust parameters of CRPM and spherical wave are exploited as decryption keys to enhance the security levels. Second, the different secret images are encrypted by different decryption keys, which cannot be accessed by unauthorized users. Moreover, fewer optical components are employed in the proposed scheme. The extracted secret images are free from crosstalk noise effects. The load of the key’s management and transmission is lightened by using CRPM. The encryption and decryption processes can be implemented by optoelectronic setup. Numerical simulation results show the viability and security of the proposed method.

A lidar system based on a photon-counting Geiger-mode avalanche photodiode (GmAPD) array can provide high range resolution three-dimensional imaging. The small number of pixels in the GmAPD array limits the size of measured scenes compared to passive cameras. Here, the performance of rapid panoramic imaging of larger scenes is investigated. Controlled environment indoor measurement at up to 24 m range is used to validate the principle. Outdoor daylight measurements of natural targets at 230 to 340 m range are performed to test the capability of the measurement system. A 20 deg by 1.8 deg scene is imaged with 0.25-mrad lateral resolution in 4 s. The standard deviation of point to plane distances for a flat wall in the outdoor measurement is 10.6 mm, giving an estimate of the achievable relative range accuracy, even if this will depend on the signal-to-noise ratio. A radiometric model is used to estimate the performance of an upgraded optimized system and the necessary laser power for imaging at longer ranges in both day and night conditions.

In digital photoelasticity, fringe pattern analysis is crucial because the photoelastic fringes provide information about direction and magnitudes of the principal stresses at the surface of the inspected object. These fringes exhibit visual properties that depend on the applied load, their spatial location in the inspected object geometry, and the illumination source. Traditional methods for fringe analysis in photoelasticity have limited performance when dealing with noisy or not well contrasted fringes, or if the spatial resolution of the fringes is lost. This work presents an approach for analyzing fringe patterns in photoelasticity images using texture information, in conjunction with machine learning techniques. Stress fields are simulated in multiple spectral bands for two models. Then, different regions of interest in these models are characterized with well-known texture descriptors. Furthermore, feature ranking and five classification schemes are used to describe the texture variations that occur in the models when they undergo diametral compression in the different spectral bands considered. The results show that texture descriptors are suitable tools for describing the stress information provided by photoelastic fringe patterns. Also, it is possible to use machine learning techniques to learn, recognize, and predict the behavior of models subjected to mechanical load in photoelasticity experiments.

A trapezoid (TZOID) pixel array complementary metal oxide semiconductor (CMOS) image sensor, and a pixel-resolution mapping (P-RM) method to extract information from the observed objects without requiring any specific patterns or man-made markings, are proposed for traffic applications. The size of each pixel in the TZOID imager was determined by pixel design equations based on the relationship between the pixel’s physical size and related spatial resolution. The TZOID imager trades detection accuracy for a tolerable data loss to reduce communication bandwidth and computation power. The P-RM method determines the pixels’ actual resolutions in the real world of the captured imaged by both standard rectangular single- and multiresolution TZOID pixel arrays. Both indoor and outdoor experiments were performed using single- and multiresolution imagers. The TZOID imager integrated circuit (IC) was fabricated in a 0.18-μm CMOS process. It was tested, and its performance parameters were compared with an off-the-shelf CMOS image sensor IC. Tests showed that the TZOID generates 98% less data than that of a standard image sensor, trading off only 2% detection accuracy. Results also show that the proposed method provides a general procedure for designing a multiresolution sensor for traffic applications.

The ability of the human visual system to integrate a rapidly oscillating image in time is investigated in this paper. A secret image is encoded into a stochastic geometric moiré grating. The secret is leaked in the form of a pattern of time-averaged moiré fringes when the cover image is oscillated according to a predetermined law of motion. Special encoding algorithms are designed for an effective real-time implementation of the scheme on a digital computer screen. A series of experiments is performed to determine the minimum frequency of oscillations required for the interpretation of the embedded secret image.

Previous research has indicated that coded masks with open fractions <0.5 are optimal for imaging some types of far-field scenes. The open fraction, in this case, refers to the ratio of open elements in the mask, with values <0.5 considered as low open fraction. Research is limited by the sparsity of <0.5 open fractions masks; thus a further 94 lower open fraction arrays are calculated and presented. These include the dilute uniformly redundant array and singer set, along with information on imaging potential, array sizes, and open fractions. Signal-to-noise ratio reveals the 0.5 open fraction modified uniformly redundant array to be the optimal coded mask for near-field x-ray backscatter imaging, over the lower open fraction singer set, dilute uniformly redundant and random array.

We propose an approach to the problem of estimating the distance of a moving object with a laser range-gated imaging (LRGI) sensor so as to keep constantly capturing images of the moving object. The issue of acquiring images of the moving object without interruption is particularly important for the surveillance system equipped with the LRGI sensor since the recognition of the moving object is generally achieved by inspecting the range-gated images. We first show that the distance estimation problem can be converted to the parameter estimation problem by randomly shifting the range gate from the desired position. Then, we present two parameter estimation techniques that are based on maximum-likelihood estimation. We show the performances of the proposed method through numerical simulations.

Due to low density, high specific stiffness, and low thermal expansion, carbon fiber reinforced plastic (CFRP) is believed to be one of the potential material choices for optical mirrors. But CFRP is one of the two-phase materials that cannot be used as optical surface and must be surface modified. To develop one kind of grid-reinforced CFRP mirror, optical replication technology was used to modify and achieve high-precision surface, and theoretical deformation due to replica resin curing and deformation caused by laminates’ manufacturing errors were studied in detail. Optical replication experiment has shown that λ  /  20 root mean square high-precision surface can be achieved for ϕ100-mm grid-reinforced carbon fiber mirrors.

The fusion of infrared and visible images may result in low contrast, which is unsuitable for observation by human eyes. Thus, we propose a contrast-enhanced fusion algorithm with nonsubsampled shearlet transform (NSST) frames, in which the NSST is first employed to decompose each of the source images into one low frequency sub-band and a series of high frequency sub-bands. To improve the fusion performance, we designed two measures for fusion of the low frequency and the high frequency: the low frequency is divided into salient and nonsalient regions in accordance with the human visual system to improve the global contrast by targeted fusion and the high frequency requires a local contrast fusion strategy. Finally, the merged sub-bands are constructed according to the selection principles, and the final fused image is produced by applying the inverse NSST on these merged sub-bands. Experimental results demonstrate the effectiveness and superiority of the proposed method over the state-of-the-art fusion methods in terms of both visual effect and objective evaluation results.

The all-optical differential equation solver has great potential in high-speed and wide-band signal processing, such as special waveform generation and description of the complex dynamics. We propose an all-optical differential equation solver in a silicon microring resonator with tunable constant coefficient k. Using inverse Raman scattering effect, k can be tunable by changing the power of the incident pump light. The influences of the pump power and signal pulse width on the tunable range of k are investigated. It is demonstrated that k is continuously tunable in the range of 0.035  /  ps to 0.130  /  ps with the error of <5  %  .

A measurement datum is crucial in measuring a gear tooth flank by laser interferometry. Manufacturing a physical reference tooth flank as a measurement datum has many drawbacks: many different reference tooth flanks are needed with the changing of the measured gear’s parameters; the reference tooth flank’s manufacturing error lowers the measurement precision; the installation error between the measured and referenced tooth flank lowers the measurement precision also. The drawbacks restrain the practical use of measuring gear by laser interferometry. A construction method of a virtual measurement datum has been proposed to solve the problem. First, a virtual error-free model of the measured tooth flank is built according to its parameters and actual measurement position. Subsequently, the interferogram of the virtual tooth flank is simulated as a measurement datum through the ray tracing. To harvest the actual measurement position of the measured tooth flank, a compensation approach for optical system error and a tracing algorithm for actual installation position are developed. Experimental results prove that a virtual measurement datum can be constructed and used in measuring a gear tooth flank. Furthermore, this research verifies that ray tracing can be used to construct a virtual measurement datum in the relevant comparative measurements processes by laser interferometry.

Absolute measurement is an effective way to obtain high-precision optical surface measurements. This paper describes a convenient absolute testing approach that allows reconstruction of surfaces using Zernike polynomials. This method requires a classical three-flat measurement and a one-rotation measurement before reconstructing the surface. Utilizing a well-established procedure, the absolute surface profile of the testing surface can be reconstructed with more Zernike orders than are provided by Fritz’s method. In particular, simulation of the testing error through recalculation of the test surface profile at a different angle could provide the optimized angle with a minimum testing error. This implies that an additional rotation measurement for the optimized angle can improve testing accuracy. The experimental results of a 100-mm flat surface provided a reflected root mean square (RMS) of 2.6 nm and a residual RMS of 0.1 nm.

Diffuse reflectance standards of known hemispherical reflectance Rh are widely used in optical and imaging studies. We have developed a stochastic surface model to investigate light reflection and roughness dependence. Through Monte Carlo simulations, the angle-resolved distributions of reflected light have been modeled as the results of local surface reflection with a constant reflectance Rs representing the overall ability of a reflectance standard. The surface was modeled by an ensemble of random Gaussian surface profiles parameterized by a mean surface height δ and transverse correlation length a. By decreasing δ  /  a, the calculated reflected light distributions were found to transit from Lambertian to specular reflection regime. Reflected light distributions were measured with three standards by nominal reflectance Rh valued at 10%, 80%, and 99%. The calculated results agree well with the measured data in their angular distributions at different incident angles by setting Rs  =  Rh and δ  =  a  =  3.5  μm.

We used a system that exploits the Mach–Zehnder interferometer structure and Young’s double-pinhole interference principle to measure the three-dimensional topographies of small objects at high precision. Next, we performed phase profilometry to measure small objects and achieve a height measurement within a 10  ×  10-cm area. The accuracy of the measurement system improved by 44.1%, and the measurement time was reduced by 63.2%.

An application of digital holographic interferometry for the measurement of the initial displacement of canine and molar in the human maxilla, under different canine retraction mechanisms, is presented. The objective of this study is to determine and compare the canine and molar displacements with and without transpalatal arch (TPA) on 0.018-in. stainless steel (SS) and 0.019-  ×  0.025-in. SS arch-wires, using three different canine retraction sliding mechanism methods: nickel–titanium (Ni–Ti)-closed coil-spring, active tie-back, and elastomeric chains. The proposed technique is highly sensitive and enables the displacement measurement of the canine and molar in human maxilla under different canine retraction methods, more precisely and accurately compared with its counterparts. The experiment was conducted on a dry human skull without mandible with intact dental arches and aligned teeth. The experimental results reveal that Ni–Ti-closed coil-spring produces maximum initial canine displacement, followed by active-tie back and elastomeric chain. It was also found that initial canine and molar displacements were more on 0.018-in. SS arch-wire as compared with 0.019  ×  0.025-in. SS arch-wire. Further, the initial displacement of the molar was less when TPA was taken as an anchorage in comparison to without anchorage.

Due to the characteristics of spatially inhomogeneous polarization and unlimited spatial coordinates, a concept of hybrid multiplexing and encoding/decoding based on spatial coordinates and mode states of vector beams is proposed for potentially enhancing the channel capacity and spectrum efficiency. For the sake of verifying the potentiality, a new encoding/decoding method by the mode states (i.e., polarization and mode) and spatial coordinates of vector beams is also proposed in this paper. In addition, the hybrid multiplexed vector beams are also utilized as carriers (16 channels) for transmitting data by four spatial coordinates and four mode states. In order to verify the feasibility of the proposed concepts, the relevant experimental setups are elaborated and established in this paper. Meanwhile, a fast mode recognition method based on the lookup table (LUT), image processing, and digital signal processing are employed to decode data when the hybrid vector beams propagate in free space (FS). The experimental results demonstrate that the encoded sequences (i.e., 0, 66, and D9) by the mode states and spatial coordinates can be successfully received and demodulated to the original data without errors after propagating 115 cm when the hybrid multiplexed vector beams are used for encoding/decoding. The total transmission rate of 320  Gbit  /  s can be acquired by combining the 16 channels (four mode states and four spatial coordinates) when the hybrid multiplexed vector beams are used as carriers. Furthermore, bit error rate, constellation diagrams (different transmission distance, transmitting data rate), and crosstalk among different channels or coordinates are employed to evaluate the propagation performances when the hybrid multiplexed vector beams are used as carriers.

A visualization technique was developed to accurately resolve the locations and phases of liquid and frozen water droplets naturally evolving on a subfreezing surface exposed to a humid air stream. The technique utilized the lens-like properties of unfrozen condensate droplets to focus reflected light in a manner that made it possible to reliably distinguish the unfrozen from frozen droplets. By inducing this lensing effect in all visible droplets on the surface prior to freezing, a small, high contrast region within each liquid droplet was produced. When freezing occurred, the higher opacity of the ice phase caused the frozen droplets to suddenly take on a darker appearance than the unfrozen droplets. A reflected light microscope and digital camera apparatus were used to feed data into an image analysis algorithm capable of mapping the time-evolution of the freezing water droplets on the surface. The algorithm processed raw image sequences into binary image stacks from which the light foci associated with the droplets could be reliably isolated into high contrast white pixel clusters on an otherwise black background. An edge detection method was employed within each binary image to locate the coordinates of the pixels residing along the periphery of each droplet’s focal cone. Those data were subsequently used to compute the centroidal coordinates and state of each droplet over time. Compared with more laborious, frame-by-frame analyses with various commercially available software tools, this technique correctly identified the locations and states of the droplets through the entire growth and freezing processes with accuracies in excess of 99%. This technical approach also allowed for simultaneous tracking of all droplets and freezing events present within the region of interest—typically numbering in the hundreds or thousands—over time, paving the way for interdroplet coalescence and freezing interactions to be studied in detail.

We have fabricated a simple and compact scanning laser optical system using microelectromechanical system (MEMS) scanner that can be incorporated into portable health monitoring devices. The two-dimensional (2-D) MEMS scanner used in this system is much smaller and light in weight compared to galvanometer scanners and polygon scanners used in the commercially available ophthalmic devices. MEMS scanners have many advantages and also limitations compared to galvanometer and polygon scanners. An easy to use and compact device is more useful for rapid alignment for measurement and data acquisition. Sensitivity of the system was quantified by measuring signal-to-noise ratio (SNR) for both high and low reflectivity materials. SNR was 10 for the high reflectance materials and about 4 for low reflectance materials, which is sufficient for biological imaging. Distortion generated using large scanning angle of 2-D MEMS scanner was corrected in real time by custom made LabVIEW program. Ocular safety is important to consider when using a laser for ophthalmic devices. We calculated the maximum permissible beam power for thermal damage and photochemical damage considering the specification of our system such as visual angle and wavelength of the laser beam. The measured laser intensity in the front of model eye was 6  μW, which is much smaller than the maximum permissible beam power recommended by the American National Standards Institute.

Chemical mechanical polishing (CMP) is the key application process in the fabrication of large optical flats to achieve global planarity and smooth surface. The large-diameter pad is a significant part and plays a vital role in CMP. To improve the polishing quality and efficiency, high-profile accuracy of the pad, conditioned by a diamond conditioner, is necessary. However, the conventional conditioning method (CCM) has been unable to satisfy the machining requirements for optical flats, and it is a challenge to improve the conditioning accuracy of the large-diameter polyurethane pad. In this study, we propose an advanced conditioning method (ACM) using the idea of subaperture conditioning, which reduces the size of the conditioner and adds the traverse movement to control the removal of different regions. Based on the conditioning density distribution model, the effect of conditioner diameter and process parameters on the pad planarity is investigated. The conditioning accuracy of the pad and polishing quality of the optical flats obtained with ACM are compared with those of a CCM. Experimental results showed that ACM can create the surface shape of the pad more uniformly than CCM. Furthermore, the polishing accuracy of the large optical flats of ACM exceeds that of CCM.

Piezoelectrically actuated microelectromechanical systems (MEMS) lens structures can be composed of a clamped square elastic diaphragm partially covered with a thin piezoelectric film leaving a circular transparent region to form a lens pupil. To model these lenses’ linear static optoelectromechanical performance, the displacement can be approximated by a linear combination of basis functions, e.g., weighted Gegenbauer polynomials that satisfy clamped boundary conditions along the diaphragm edges. However, such a model needs as much as 120 degrees of freedom (DOFs) to provide a good approximation of the lens optical performance. To improve on this, we here consider approximating the deflection by an expansion using piecewise smooth functions that have different forms in the pupil and the actuator regions. We use exact solutions for the elastic plate differential equation over circular and annular subdomains, and weighted Gegenbauer polynomials in the remaining region. The latter enforces the boundary conditions. We have found that the larger the diaphragm area with exact plate solutions is, the lower is the number of DOFs needed to predict mechanical and optical quantities accurately. For example, a model with 10 DOFs achieves accuracies of 5.1% and 2.1% for RMS wavefront error and reciprocal F-number, respectively, for all pupil openings of interest.

The paper describes the approach for identification of the spatial bandwidth of an optical system in the lateral and axial directions. This approach considers application of Huygens–Fresnel principle to obtain the integral for calculation of amplitude distribution near a focal point. The replacement of integration variables leads to identification of the limits of integration in a space of spatial frequencies. These limits are the spatial cutoff frequencies for amplitude distribution. Doubling these values produces the spatial cutoff frequencies for intensity distribution, and it shows the same result as Abbe theory predicts. The proposed approach can be used for mathematical explanation why optical systems have limited spatial resolution and why the spatial harmonics with high frequencies cannot pass through optical systems. The analog of Abbe theory for axial direction is proposed. To identify the spatial bandwidth in axial direction, it has to consider the grating located in the plane with an optical axis and with slits perpendicular to this axis. Then it is possible to follow Abbe theory: to consider the diffraction on this grating, formation of a spatial spectrum, and a grating image with axial orientation in an image space.

We propose an autostereoscopic three-dimensional (3-D) display using a time-multiplexed method. This display consists of a two-dimensional (2-D) display panel, a time-multiplexed liquid crystal parallax barrier (LCPB), and a lenticular sheet. The 2-D display provides four-view synthetic images. The time-multiplexed LCPB located in front of the 2-D display panel is used to project parallax images in different spatial directions. With the time-multiplexed method, 3-D pixels with different locations can be provided for a time sequence, allowing a high 3-D resolution to be achieved. The lenticular lens is located between the 2-D display panel and the LCPB. Because of congregating ray functions of the lenticular sheet, cross talk can be effectively reduced. We describe the principle and parameter calculations for the prototype of the proposed display. Compared with the conventional autostereoscopic 3-D display, the proposed display has a higher resolution and minimal cross talk.

An inline fiber Mach–Zehnder interferometer (MZI) based on a cross-type microchannel in single mode–multimode–single mode fiber structure was proposed. The cross-type microchannel was fabricated in the fiber material directly using femtosecond laser-induced breakdown in water. The horizontal part of the cross-type microchannel along the fiber direction results in enough phase difference for the interference, and the cross-type shape makes the whole structure more robust because a smaller part of fiber material is etched. In addition, the proposed MZI shows high-pressure sensitivity (−14.27  nm  /  MPa), low-temperature cross sensitivity (0.043 nm/°C), and low strain force cross sensitivity (0.257 nm/N), which makes it a promising candidate for applications in gas pressure sensing field.

Device-to-device (D2D) communication has become one of the focuses for future wireless communications in conjunction with rapidly growing smartphone technologies. Smartphone camera-based D2D communication is presented as a favorable solution to supporting indoor D2D communications. As a smartphone has both camera and display, two smartphones can communicate over a relatively short range distance indoors in the form of smartphone display (transmitter) and smartphone camera (receiver) link. The smartphone camera-based wireless communication called optical camera communication (OCC), however, suffers from low data rate that is at most a hundred kilobits per second. To ameliorate this, we propose a high-density modulation that is more efficient in achieving an Mbps rate for smartphone camera-based D2D systems, by exploiting two-dimensional data transmission capability in OCC. Neural network is also implemented to improve the performance of the proposed system. To verify the proposed system, simulations and experiments were performed. It is demonstrated that a 2.66 Mbps rate of the proposed system is achieved with an acceptable bit error rate.

We demonstrate the laser performance of a gadolinium scandium gallium garnet (GSGG) single crystal with 10 at.% Yb^3  +-doping concentration. In the case of continuous-wave operation, the laser wavelength was blueshifted in the range from 1067.1 to 1027.2 nm with increasing the transmission of the output coupler from 0.5% to 30%. The maximum output power produced was 3.2 W with 3% output transmission. By employing a Cr^4  +  :  YAG crystal as the saturable absorber, a stable Q-switched laser beam with 21-ns pulse duration and 38-μJ single-pulse energy was achieved at a 20-kHz repetition rate. This laser crystal should be a promising candidate for nanosecond pulse generation especially in harsh environments, such as outer space, due to its wide absorption and emission spectral bandwidths and strong radiation resistance.

With the development of autonomous cars, the demand for accurate light detection and ranging (LiDAR) systems is increasing. Previous evaluations of LiDAR were mainly based on experiments and lacked theoretical significance. We theoretically evaluate the accuracy and optical output power of LiDAR systems in autonomous cars. We focus on two ranging schemes: the time-of-flight (TOF) method and the quadrature phase detection (QPD) method. Considering the special requirements of autonomous driving, the theoretical limits of ranging accuracy are calculated by deriving the Cramer–Rao bound (CRB). The influence of reflectivity as well as distance on accuracy are discussed. We also determine the relationship between optical output power and essential parameters, and make comparisons between TOF and QPD. It can be concluded that TOF is more efficient under most circumstances. When designing autonomous cars, such theoretical evaluation provides guidance for choosing laser emitters and receivers, justifying the significance of our work to LiDAR development.

In designing photonic-bandgap fibers, the most effective and well-known approach utilized to suppress surface modes is the T  ∼  S  /  2 rule, which translates to using a core surround with only half the thickness of the average cladding strut. We investigate transmission loss in a commercial 7-cell hollow-core photonic-bandgap fiber. To reduce transmission loss at 1550 nm, we apply the T  ∼  S  /  2 rule to suppress the surface modes. While the strut surface modes are suppressed, the rod surface modes (RSMs) are blueshifted to wavelengths near 1550 nm because the round-core diameters of the cladding-layer corners resting against the fiber core are quite small, which leads to the coupling between the fundamental mode and the RSMs. This induces attenuation at 1550 nm, which indicates that the T  ∼  S  /  2 rule is applicable only within certain limits. By optimizing the round-core diameters of the corners resting against the fiber core, we obtain a structure that is free of surface modes, with the subsequent transmission loss at 1550 nm being ∼10  dB  /  km lower than that of the original fiber. Moreover, we propose a simple method to modify the round-core diameters to aid in surface-mode suppression.

A tunable, multiwavelength, Q-switched fiber laser based on a graphene saturable absorber and tapered fiber is shown to solve the problems in existing multiwavelength tunable Q-switched fiber lasers, which are complex, unstable, difficult to tune, and costly. The whole ring cavity includes a main part that is a self-made tuning device with a tapered fiber placed on MgF2 substrate, single-layer graphene, and polydimethyl siloxane film covering the zone of the tapered fiber. It can achieve a stable output and multiple wavelengths, which can be switched into another wavelength by adjusting the parameters of the various changes and polarization controller.

Optical fiber devices and applications in the 2-μm band have been investigated extensively due to its unique advantages, such as eye safety. A fiber sensor based on tilted fiber Bragg grating with a grating plane angle of 2 deg is fabricated and experimentally tested. To the best of the authors’ knowledge, this is the first tilted fiber Bragg grating sensor to realize simultaneous measurements of temperature, axial strain, and a certain range of surrounding refractive index (SRI) at the 2-μm band. This paper uses the wavelength detection method and selects three independent resonant wavelengths as the investigated parameters. Results show that perturbations of temperature, axial strain, and SRI can shift the wavelengths of the core mode resonance and cladding mode resonance to some degree. The temperature sensitivities of the core mode and cladding mode are nearly the same, but their axial-strain sensitivities are different. Furthermore, the core mode is insensitive to the change in SRI. The sensitivities of SRI, temperature, and strain can thus be obtained by experimentally determining the wavelength shifts of the three independent resonance peaks. A 3  ×  3 matrix containing the relationship coefficients between the disturbances of temperature, axial strain, and SRI and wavelength shifts is constructed. By reversely solving the matrix equation, variations in temperature, strain, and SRI can be obtained using the experimental determination of wavelength drifts.

The laser efficiency and temperature distribution in distributed side-coupled cladding-pumped (DSCCP) fiber lasers are investigated, using a precise and complete analytic model. We show that high-order mode pump handling and (N  +  1) (N  ≥  2) DSCCP fibers can achieve a high laser efficiency when the fibers are separated. In terms of multistage pump schemes, a high laser efficiency requires an appropriate pump-node number and arrangement. According to our calculations, the temperature evolution along the signal fiber could give rise to a fluctuation in the case of strong coupling, which could decrease the mode instability threshold. The fluctuation period is usually determined by the DSCCP structure including the pump fiber number, the fiber diameter, and the fiber spacing. It is suggested that the mode instability threshold can be depressed through excellent DSCCP structure design and the selection on an appropriate pump scheme.

A hybrid optical amplifier, using a combination of Er–Yb codoped waveguide amplifier (EYDWA) and Raman amplifier, is presented. The proposed scheme is investigated for 116  ×  10  Gb  /  s dense wavelength-division multiplexed (DWDM) system at 0.2-nm channel spacing. The parameters are optimized using a hybrid genetic algorithm to achieve a flat gain of >25  dB, with a gain ripple of 2.78 dB. This is obtained over a wavelength range of 1539.7 to 1562.7 nm at 1 mW optimum input power per channel, without using any costly gain flattening scheme. An increase in gain ripple from 2.78 to 6.95 dB is observed for higher input powers up to 10 mW. The obtained noise figure (NF) is <6  dB for the given wavelength range. The effects of Er/Yb concentrations on the gain of EYDWA alone and the proposed hybrid amplifier are also investigated. The obtained high and flat gain along with low NF, lesser bit error rate (<10^−  20), and high Q-factor (>10  dB) confirm the feasibility of the proposed amplifier for broadband long-haul DWDM systems.

A molybdenum disulfide (MoS₂) saturable absorber (SA) was fabricated via the liquid-phase exfoliation method. The optical properties of MoS₂ were systematically characterized and the modulation depth was measured to be 6.2%. A laser-diode-end-pumped, passively mode-locked Nd  :  YVO₄  /  PPLN green laser with a SA based on few-layered MoS₂ was successfully demonstrated. The pulse width of the stable continuous-wave mode-locked laser operating at 532.1 nm was 3.5 ps, and the pulse repetition frequency was 87.2 MHz. The maximum average output power of 757 mW was obtained under an absorbed pump power of 8.62 W. The calculated single pulse energy and peak power were 8.7 nJ and 2.49 kW, respectively.

A liquid crystal (LC)-modulated tunable filter is theoretically studied, consisting of an Au nanorod array and an Au film separated by a dielectric indium tin oxide (ITO) glass layer. It is well established that this system can achieve double absorptive peaks resulting from the strong plasmonic coupling between the plasmon-induced transparency effect of the nanorod array and the cavity mode of the Au microcavity structure. The positions of the double absorptive peaks can be tuned dynamically based on the electro-optic effect of LC. The simulation results reveal that a band range of 160 nm has been confirmed at near-infrared wavelengths by altering the driven voltage of LC from 0 to 8 V. The proposed structure is able to filter the two peaks with the reflection coefficients <5  %  . Compared with the existing tunable filter, it has many advantages, such as continuous tunability, low tuning voltage, and great degree of tunability.

In the resonator micro-optic gyro (RMOG) frequency locking loop, digital-to-analog converter quantization error leads to low-accuracy laser tuning, causing frequency locking and rotation sensing dead zones. This paper proposes a planar waveguide resonator (PWR) frequency locking scheme with a piezoelectric (PZT) ring to tune the PWR resonant frequency to track and lock to the laser frequency. The proposed scheme achieved PZT-PWR Q  =  4  ×  10⁷ and frequency tuning coefficient  =  1.124  MHz  /  V. Direct comparison on an RMOG device experimentally verified the proposed scheme reduced frequency locking noise to 0.25 deg  /  s and improved gyro resolution from 1 to <0.05 deg  /  s.

In this report, a microring structure with partial intermediate refractive index cladding is proposed and simulated using finite-difference time-domain. The introduced intermediate refractive index cladding is a transitional layer between the ring waveguide and the environment to achieve a greater detection range of evanescent wave, which is of great benefit to the light–matter interaction. The sensitivity of the modified microring resonator is obtained up to 480  nm  /  RIU, which is eight times the microring resonator without partial intermediate refractive index cladding. This scheme can offer a way to improve the sensitivity of optical microcavity biosensors.

Generation of the color-tunable reddish to magenta upconversion luminescence in Tm^3  +-single-doped glasses under near-infrared (1.1 to 1.3  μm) excitation is reported. Upconversion emission in the ultraviolet, blue- and red-spectral regions due to D₂1-H₆3, D₂1-F₄3, G₄1-H₆3, G₄1-F₄3, and F_2,33-H₆3 transitions, respectively, was observed. The dependence of upconversion emission upon excitation power and Tm^3  + content was also investigated. Adjusting the excitation power and Tm^3  + concentration, the overall fluorescence emission can be tuned across the reddish to magenta color in the international commission on illumination color chromaticity diagram. The CIE-1931 color coordinates move from red toward the bluish region occupying the magenta region with an increase in the excitation power.

This is an erratum to correct a problem with an equation in the original paper.