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.

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 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.

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.% Yb3  +  -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 Cr4  +    :  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.

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.