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Single pixel imaging can be the preferred method over traditional 2D-array imaging in spectral ranges where conventional cameras are not available. However, when it comes to real-time video imaging, single pixel imaging cannot compete with the framerates of conventional cameras, especially when high-resolution images are desired. Here we evaluate the performance of an imaging approach using two detectors simultaneously. First, we present theoretical results on how low SNR affects final image quality followed by experimentally determined results. Obtained video framerates were doubled compared to state of the art systems, resulting in a framerate from 22 Hz for a 32 × 32 resolution to 0.75 Hz for a 128 × 128 resolution image. Additionally, the two detector imaging technique enables the acquisition of images with a resolution of 256 × 256 in less than 3 s.

This paper describes the design, modeling, construction, and testing of a low-cost and compact (80 mm×50 mm) 1×5 wavelength-selective switch. The core beam-deflecting element of the switch is a nematic liquid crystal on silicon spatial light modulator. The switch is designed for coarse wavelength-division multiplexing wavelengths in order to bring the benefit of a low-cost, compact, and robust switching design toward the customer end in the access network. During the system development stage, a single optomechanical assembly was designed and prototyped using the three-dimensional printing technology. The experimental results show an insertion loss of -13.8±1.4 dB and a worst-case scenario crosstalk level of -24.8 dB. Approaches for enhancing the performance of the switch are analyzed and discussed.

The measured distance error caused by double peaks in the BOTDRs (Brillouin optical time domain reflectometers) system is a kind of Brillouin scattering spectrum (BSS) deformation, discussed and simulated for the first time in the paper, to the best of the authors' knowledge. Double peak, as a kind of Brillouin spectrum deformation, is important in the enhancement of spatial resolution, measurement accuracy, and crack detection. Due to the variances of the peak powers of the BSS along the fiber, the measured starting point of a step-shape frequency transition region is shifted and results in distance errors. Zero-padded short-time-Fourier-transform (STFT) can restore the transition-induced double peaks in the asymmetric and deformed BSS, thus offering more accurate and quicker measurements than the conventional Lorentz-fitting method. The recovering method based on the double-peak detection and corresponding BSS deformation can be applied to calculate the real starting point, which can improve the distance accuracy of the STFT-based BOTDR system.

Atmospheric lidar techniques for the measurement of wind, temperature, and optical properties of aerosols rely on the exact knowledge of the spectral line shape of the scattered laser light on molecules. We report on spontaneous Rayleigh–Brillouin scattering measurements in the ultraviolet at a scattering angle of 90 °o n N 2 and on dry and moist air. The measured line shapes are compared to the Tenti S6 model, which is shown to describe the scattering line shapes in air at atmospheric pressures with small but significant deviations. We demonstrate that the line profiles of N2 and air under equal pressure and temperature conditions differ significantly, and that this difference can be described by the S6 model. Moreover, we show that even a high water vapor content in air up to a volume fraction of 3:6 vol:% has no influence on the line shape of the scattered light. The results are of relevance for the future spaceborne lidars on ADM-Aeolus (Atmospheric Dynamics Mission) and EarthCARE (Earth Clouds, Aerosols, and Radiation Explorer). © 2010 Optical Society of America OCIS codes: 280.3640, 280.1310, 290.5830, 300.6390.

We propose a fringing-effect model based on the experimentally measured phase response across the phase transition region of a liquid crystal on silicon (LCOS) device. The measured phase profile in the phase transition region is characterized by a scaled error function of the flyback width. The flyback width can be determined by a cubic function of the phase depth between neighboring pixels. This dependence of the flyback width on the phase depth is explained by a linear rotation model of the liquid crystal director. The simulated diffraction efficiency based on the fringing-effect model shows a close agreement with the experimental measurement.

A theory is presented for the application of Hill's matrix method to the calculation of the reflection and transmission spectra of multitone holographic interference filters in which the permittivity is modulated by a sum of repeating functions of arbitrary period. Such filters are important because they may have two or more independent reflection bands. Guidelines are presented for accurately truncating the Hill matrix, and numerical methods are described for finding the exponential coefficient and the coefficients of the Floquet-Bloch waves within the filter. The latter calculation is performed by use of a computational technique known as inverse iteration. The Hill matrix for such problems is sparse, and thus, even though the matrix can be quite large, it may be efficiently stored and processed by a desktop computer. It is shown that the results of using Hill's matrix method are in close agreement with numerical calculations based on thin-film decomposition, a transfer-matrix technique. An important result of this research is the demonstration that Hill's matrix method may, in principle, be used to analyze any multiperiodic problem, so long as the periods are known to finite precision.

A subpixel area-based evaluation method for an improved slanted lenticular film that minimizes the crosstalk in a quasi-three-dimensional (Q3D) display is proposed in this paper. To identify an optimal slant angle of the film, a subpixel area-based measurement is derived to evaluate the crosstalk among viewing regions of the intended subpixel and adjacent unintended subpixel by taking the real subpixel shape and black matrix into consideration. The subpixel mapping, which corresponds to the optimal slant angle of the film, can then be determined. Meanwhile, the viewing zone characteristics are analyzed to balance the light intensity in both right and left eye channels. A compact and portable Q3D system has been built and appropriate experiments have been applied. The results indicate that significant improvements in both crosstalk and resolution can be obtained with the proposed technique.

Compact all-pass and add-drop microring resonators (radius=10 μm) integrated with grating couplers working at 2 μm wavelength are designed, fabricated, and characterized on a commercial 340-nm-thick-top-silicon silicon-on-insulator platform. They are suitable for high-volume integrated optical circuits at 2 μm wavelength as the fabrication process involved are uncomplicated and complementary metal-oxide-semiconductor (CMOS)-process compatible, thus making them more convenient to be utilized. The performance of the grating couplers, based on four most important parameters, has been simulated and optimized. The simulation and experimental results of grating couplers show the lowest coupling loss of 4.5 dB and 6.5 Db, respectively. By utilizing the grating couplers to couple light in and out from the chip, the designed microring resonators have been tested. The experimental results of microring resonators show that an extinction ratio of 12 dB and a quality factor of 11,200 can be achieved. To the best of our knowledge, this is thus far the smallest microring resonator ever demonstrated at this wavelength.

This paper presents a novel method that leverages the stereo geometric relationship between projector and camera for absolute phase unwrapping on a standard one-projector and one-camera structured light system. Specifically, we use only one additional binary random image and the epipolar geometric constraint to generate a coarse correspondence map between projector and camera images. The coarse correspondence map is further refined by using the wrapped phase as a constraint. We then use the refined correspondence map to determine a fringe order for absolute phase unwrapping. Experimental results demonstrated the success of our proposed method.

The demonstration of an all-optical, ultra-high-speed, time-domain signal correlator based on spatial-spectral holographic (SSH) technology is described. The fully programmable signal correlator demonstration operates asynchronously and continuously on signals with up to 32 GHz of bandwidth and correlative filter length exceeding a time-bandwidth product of 104, for the equivalent of teraflop-scale processing. Experimental demonstrations are presented that show both digital and analog correlation capability using phase-shift keyed modulation formats to search plain text ASCII data sources for arbitrary phrases at continuous line rate throughputs up to 200 Gbps with minimal latency. These high-bandwidth demonstrations were enabled by improvements in the photonic supporting components and cryogenic SSH for RF and microwave signal processing methods. Potential application of the SSH real-time correlator for high-bandwidth analog or multi-level format signals is discussed.

Applications based on photonic crystal fibers depend strongly on their dispersion properties that might differ from the desired specifications due to deficiencies in the manufacturing process. Since dispersion characteristics might also be affected by the placement of the fiber, in this paper the effect of various placements on the chromatic dispersion properties of a commercially available HC-800-02 photonic crystal fiber was investigated between 760 and 870 nm with Fourier-transform spectral interferometry. To test the scaling of dispersion with fiber length, samples of different lengths ranging from 10 to 97 cm were used in the measurements. It was found that the dispersion properties of the orthogonal directions were different. The dispersion parameter showed small dependence on the placement and fiber length. The polarization-mode dispersion (PMD) of the fiber was measured using an indirect and a direct technique. To retrieve the PMD directly in the case of the shorter fibers where the fringes were too sparse for the Fourier method, the so-called minima-maxima method was employed. The precision was comparable with both techniques; however, the direct approach proved to be more accurate when longer samples were measured, and the indirect method seemed to be more reliable in the case of shorter fibers.

In this paper we investigate the integral version of the localized approximation (ILA)-a powerful technique for evaluating the beam shape coefficients in the framework of the generalized Lorenz-Mie theory-as applied to ideal scalar Bessel beams (BBs). Originally conceived for arbitrary shaped beams with a propagating factor exp(±ikz), it has recently been shown that care must be taken when applying the ILA for the case of ideal scalar BBs, since they carry a propagating factor exp(±ikz cos α), with α being the axicon angle, which cannot be smoothly accommodated into its mathematical formalism. Comparisons are established between the beam shape coefficients calculated from both ILA and exact approaches, assuming paraxial approximation and both on- and off-axis beams. Particular simulations of radiation pressure forces are provided based on the existing data in the literature. This work helps us in elucidating that ILA provides adequate beam shape coefficients and descriptions of ideal scalar BBs up to certain limits and, even when it fails to do so, reliable information on the physical optical properties of interest can still be inferred, depending on specific geometric and electromagnetic aspects of the scatterer.

© 2017 Optical Society of America. In this work, we propose the design of a sharp bend in a two-dimensional optical waveguide which has superellipse curve boundaries and a gradient refractive index structure in its core. Numerical simulations are presented to show the efficient light propagation in the waveguide bend, as well as the efficient light coupling between the proposed waveguide bend and a straight waveguide, for TE0 and TM0 modes. The proposed design strategy is also useful for designing other compact optical and photonic components.

The inter-satellite laser link is the backbone of the next inter-satellite information network, and ranging and communication are the main functions of the inter-satellite laser link. This study focuses on the inter-satellite laser ranging based on the pseudo-code correlation technology. In this paper, several typical laser-ranging methods have been compared and we determined that the laser pseudo-code ranging architecture is more suitable for the inter-satellite laser communication link. The pseudo-code ranging system is easy to combine with a digital communication system, and we used it to calculate integer ambiguity by modulating the time information. The main challenge of the ranging system is range precision, which is the main focus of this paper. First, the framework of the pseudo-code ranging system is introduced; the ranging architecture of dual one-way ranging is used to eliminate the clock error between the two transceivers, and then the uncertainty of the phase detector is analyzed. In the analysis, the carrier to noise ratio and the ranging code rate are constrained by the laser communication link margin and the electronic hardware limitation. Therefore, the relationship between the sampling depth and the phase detector uncertainty is verified. A series of optical fiber channel laser pseudo-code ranging experiments demonstrated the effects of sampling depth on the ranging precision. By adjusting the depth of storage, such as the depth of 1.6 Mb, we obtained a pseudo-range measurement precision of 0.33 mm (1σ), which is equivalent to 0.0001 times code subdivision of 10.23 Mcps pseudo-code. This paper has achieved high precision in a pseudo-range measurements, which is the foundation of the inter-satellite laser link.

The Cassini spacecraft orbiting Saturn carries the composite infrared spectrometer (CIRS) designed to study thermal emission from Saturn and its rings and moons. CIRS, a Fourier transform spectrometer, is an indispensable part of the payload providing unique measurements and important synergies with the other instruments. It takes full advantage of Cassini’s 13-year-long mission and surpasses the capabilities of previous spectrometers on Voyager 1 and 2. The instrument, consisting of two interferometers sharing a telescope and a scan mechanism, covers over a factor of 100 in wavelength in the mid and far infrared. It is used to study temperature, composition, structure, and dynamics of the atmospheres of Jupiter, Saturn, and Titan, the rings of Saturn, and surfaces of the icy moons. CIRS has returned a large volume of scientific results, the culmination of over 30 years of instrument development, operation, data calibration, and analysis. As Cassini and CIRS reach the end of their mission in 2017, we expect that archived spectra will be used by scientists for many years to come.

In this paper, a scheme for optical modulation format conversion from one 20 Gbps quadrature phase-shift keying (QPSK) signal to one 20 Gbps binary phase-shift keying (BPSK) signal with information integrity is proposed and verified by simulation. The theory of degenerate phase-sensitive amplifier (PSA) employed as a phase de-multiplexer is derived in detail and used to decompose the in- (I) and quadrature- (Q) phase components of QPSK. Then the I and Q components are parallel-to-series converted into one BPSK. The constellations show that the phase noise of the original signal is effectively restrained by the conversion system through use of the PSA. The error vector magnitude and bit-error rate (BER) of the QPSK, converted BPSK, and a back-to-back BPSK are measured and compared with each other. We find that the BER performance of the converted BPSK is better than QPSK and maintains the original information integrity with different input signal quality. Some potential issues are also discussed as to practical implementation of the scheme. This modulation-format-conversion scheme has potential applications in improving the signal BER performance and flexible transmitters and receivers in software-defined networks.

In this work, we analyze the diffraction produced by Ronchi gratings where the fill factor is not constant, but presents random fluctuations around its nominal value. This effect can be produced while developing the grating with etchers since the process can be slightly unpredictable. We obtain the theoretical formalism to describe the intensity produced by the grating at near and far field, showing that smoothing of the self-images is produced at the near field and, consequently, cancellation of higher diffraction orders is obtained at the far field. In addition, different nominal fill factors produce different diffraction behaviors in terms of the randomness. We corroborate the analytical formalism using a direct integration method based on the Rayleigh-Sommerfeld formula and conclude that the numerical results are in high agreement with the theoretical predictions.

The control of surface errors as a function of spatial frequency is critical during the fabrication of modern optical systems. A large-scale surface figure error is controlled by a guided removal process, such as computer-controlled optical surfacing. Smaller-scale surface errors are controlled by polishing process parameters. Surface errors of only a few millimeters may degrade the performance of an optical system, causing background noise from scattered light and reducing imaging contrast for large optical systems. Conventionally, the microsurface roughness is often given by the root mean square at a high spatial frequency range, with errors within a 0.5×0.5 mm local surface map with 500×500 pixels. This surface specification is not adequate to fully describe the characteristics for advanced optical systems. The process for controlling and minimizing mid- to high-spatial frequency surface errors with periods of up to ∼2-3 mm was investigated for many optical fabrication conditions using the measured surface power spectral density (PSD) of a finished Zerodur optical surface. Then, the surface PSD was systematically related to various fabrication process parameters, such as the grinding methods, polishing interface materials, and polishing compounds. The retraceable experimental polishing conditions and processes used to produce an optimal optical surface PSD are presented.

The objective of applying transmission holograms in automotive headlamp systems requires the adaptation of holograms to divergent and polychromatic light sources like light-emitting diodes (LEDs). In this paper, four different options to describe the scalar light waves emitted by a typical automotive LED are regarded. This includes a new approach to determine the LED's wavefront from interferometric measurements. Computer-generated holograms are designed considering the different LED approximations and recorded into a photopolymer. The holograms are reconstructed with the LED and the resulting images are analyzed to evaluate the quality of the wave descriptions. In this paper, we show that our presented new approach leads to better results in comparison to other wave descriptions. The enhancement is evaluated by the correlation between reconstructed and ideal images. In contrast to the next best approximation, a spherical wave, the correlation coefficient increased by 0.18% at 532 nm, 1.69% at 590 nm, and 0.75% at 620 nm.

Hadamard spectroscopy is a method of taking spectroscopic measurements suited to domains such as far infrared, where the sensor noise is dominant. Unfortunately, the signal-to-noise ratio of such a system scales poorly with the resolution. In this paper, we analyze a technique called microscanning that attempts to solve this problem by combining several lower resolution "preview" spectra to form a higher resolution spectrum. We provide a theoretical analysis of the method concerning the noise reduction and blurring. We conduct simulations based on the measured far infrared spectra of minerals in order to determine if the noise reduction can outweigh the loss in accuracy caused by the blurring effect, and we demonstrate the microscanning method in experimental conditions. We conclude that the microscanning method provides a noise reduction by a factor proportional to the number of previews and that it can be useful in far infrared spectroscopy.

In this work, a mobile laser-induced breakdown spectroscopy (LIBS) system has been successfully applied to in situ analysis of heavy metals in soil samples. The LIBS system had two working methods, which were a fixed measuring method and a handheld method. For the fixed measuring method, a simple sample pretreatment was needed to reduce the soil matrix effect generated by moisture and porosity. Experiments proved that this method could be used to semi-quantitatively detect heavy metals when combined with the traditional calibration curve method. The limits of detection for copper, lead, and zinc were all below 10 mg/kg, which satisfied the need of heavy metal detection in soil. Principal component analysis was used for soil classification, which helped to build appropriate calibration curves. On the basis of soil classification, accurate and rapid detection of heavy metals in soil is feasible. For the handheld method, spectrum intensity and stability decreased significantly compared with the fixed measuring method. However, by using the internal standard method, the stability of LIBS data was improved significantly to 6%. For soil samples with serious heavy-metal pollution, the measurement errors were less than 12%, which indicated that handheld LIBS was effective to monitor heavy-metal pollution in soil. The research results provide application support for rapid and on-site monitoring of heavy metals in soil.

Recently, a new noncontact reflection-mode imaging modality called photoacoustic remote sensing (PARS) microscopy was introduced providing optical absorption contrast. Unlike previous modalities, which rely on interferometric detection of a probe beam to measure surface oscillations, the PARS technique detects photoacoustic initial pressures induced by a pulsed laser at their origin by monitoring intensity modulations of a reflected probe beam. In this paper, a model describing the temporal evolution from a finite excitation pulse is developed with consideration given to the coherence length of the interrogation beam. Analytical models are compared with approximations, finite-difference time-domain (FDTD) simulations, and experiments with good agreement.

We demonstrated the fabrication of a chiral long-period grating (CLPG) by twisting a two-mode fiber (TMF) when a CO2 laser beam was sweeping along the fiber axis. The torsion, temperature, and surrounding refractive index characteristics of the fabricated TMF-CLPG were investigated experimentally. The fabricated TMF-CLPG has a high torsion sensitivity [0.7768 nm/(rad/m)], and can measure the twist rate and twist direction simultaneously. This kind of CLPG would have great potential applications in high-sensitivity optical sensors.

Mode conversion based on adiabatical mode evolution in a two-core configuration is investigated. The configuration can convert all the launching modes to higher-order modes from one port and convert all the launching modes to lower-order modes from another port. Mode conversion between the two degenerated high-order modes is also demonstrated numerically. The mode conversion feature is only dependent on the relationship between the effective mode indices of the two cores in the configuration, which shows the characteristics of high flexibility and large fabrication tolerance.

For this paper, we employ the Monte Carlo scene (MCScene) radiative transfer code to elucidate the underlying physics giving rise to the structure and colors of the antitwilight, i.e., twilight opposite the Sun. MCScene calculations successfully reproduce colors and spatial features observed in videos and still photos of the antitwilight taken under clear, aerosol-free sky conditions. Through simulations, we examine the effects of solar elevation angle, Rayleigh scattering, molecular absorption, aerosol scattering, multiple scattering, and surface reflectance on the appearance of the antitwilight. We also compare MCScene calculations with predictions made by the MODTRAN radiative transfer code for a solar elevation angle of +1°.

Solid-immersion lens (SIL) microscopy requires the dimensions of the SIL to be precisely fabricated, otherwise spherical aberrations will be encountered [1]. Uncorrected spatial aberrations not only degrade the lateral and axial resolutions of a sub-surface microscopy system, but also couple into the time domain to stretch the pulses beyond their minimum transform-limited durations [2]. Chromatic aberration remaps the arrival times at the focal plane of the different colors in the pulse, while spherical aberration changes the position of the best focal plane for each frequency of the pulse. Therefore, time-domain measurements can be applied to diagnose the presence of spatial aberrations, and even to inform an experimental strategy of minimizing their impact. Here we describe an investigation of this approach in the context of sub-surface SIL microscopy of integrated circuits.

Light-emitting diodes (LEDs) are causing big changes in the automotive lighting field. Optical systems with LEDs can use a plastic lens, which means almost all shapes can be made through plastic injection molding. Already, some car manufacturers are adopting this technology in their designs to express their own identities to customers. However, to use this technology, an important defect has to be overcome, which is color aberration near the cutoff line of the low-beam. In this paper, the criteria for measuring the color coordinates are established and simulations for axial color aberration in the plastic lens are carried out.

With the help of the vector theory of reflection and refraction, a novel emitting Cassegrain antenna with a semitransparent secondary mirror has been proposed and analyzed for a distant point source. Based on the absorptivity valued at 3.00% and the reflectivity valued at 0.10%, this new emitting antenna can increase the transmission efficiency from 63.65% to 93.85%. In addition, an off-axis parabolic receiving antenna corresponding to the emitting antenna is designed and the 3D ray-trace simulation result is given. According to the simulation result, this receiving antenna can nicely converge the rays from the emitting antenna.

Results of investigations of 98.0Li2B4O7−2.0Ag2O and 97.0Li2B4O7−2.0Ag2O−1.0Gd2O3 glasses with Ag nanoparticles (Ag NPs), formed by thermal treatment in vacuum and in air, are presented. Intensive plasmon absorption bands, connected with Ag NPs, were observed in their optical transmission spectra. It is ascertained that in volumes of both glasses there is formed a small number of Ag NPs, whereas their main mass is concentrated near the surface of samples. The mechanism of Ag NPs formation is proposed. A conclusion is drawn that annealing in vacuum does not necessarily require the presence of reducing ions, whereas formation of nanoparticles at annealing in air is impossible without reducing agents. Structural defects play a decisive role in the Ag NPs nucleation process. Radii of formed Ag NPs are estimated by the half-width of plasmon bands, and by means of small-angle x-ray scattering.

This paper describes a multi-static interferometric synthetic aperture ladar (IFSAL) for high-resolution, high-precision 3D imaging. Code division multiple access apertures with periodic, pseudorandom noise waveforms are used to create aperture diversity and overcome the ambiguity associated with the aperture separation requirements for interferometric synthetic aperture ladar. The basic theory for mapping relative aperture phase to a high-precision elevation profile is derived for a multi-static IFSAL system and subsequent processing steps are presented. An analytic model and computer simulation are developed to demonstrate the 3D imaging capability of an IFSAL system.

A Fabry-Perot ultrasound sensor with nonhygroscopic dielectric mirrors made out of Ta2O5 and SiO2 for use in photoacoustic tomography is described. The sensor offers flat frequency response up to 36 MHz, low noise-equivalent pressure (70 Pa), and near-omnidirectional response up to 20 MHz as well as optical transparency for near-infrared illumination. A numerical model was developed to predict its frequency response, and the results were validated experimentally. An image of the human palm was acquired to demonstrate in vivo imaging capabilities.

We report on a simple scheme to generate broadband, μJ pulses centered at 2.1 μm with an intrinsic carrier-envelope phase (CEP) stability from the output of a Yb:YAG regenerative amplifier delivering 1-ps pulses with randomly varying CEP. To the best of our knowledge, the reported system has the highest optical-to-optical efficiency for converting 1-ps, 1 μm pulses to CEP stable, broadband, 2.1 μm pulses. The generated coherent light carries an energy of 5.4 μJ, at 5 kHz repetition rate, that can be scaled to higher energy or power by using a suitable front end, if required. The system is ideally suited for seeding broadband parametric amplifiers and multichannel synthesizers pumped by picosecond Yb-doped amplifiers, obviating the need for active timing synchronization. Alternatively, this scheme can be combined with high-power oscillators with tens of μJ energy to generate CEP stable, multioctave supercontinua, suitable for field-resolved and time-resolved spectroscopy.

The rotational magnetic field of radial magnetic bearings characterizes remarkable time and spatial nonlinearity due to the eddy current and induced electromagnetic field. It is significant to experimentally obtain the features of the rotational magnetic field of the radial magnetic bearings to validate the theoretical analysis and reveal the discipline of a rotational magnetic field. This paper developed thin-slice fiber Bragg grating-giant magnetostrictive material (FBG-GMM) magnetic sensors to measure air-gap flux density of a radial magnetic bearing with a rotating rotor; a radial magnetic bearing test rig was constructed and the rotational magnetic field with different rotation speed was measured. Moreover, the finite element method (FEM) was used to simulate the rotational magnetic field; the measurement results and FEM results were investigated, and it was concluded that the FBG-GMM sensors were capable of measuring the radial magnetic bearing's air gap flux density with a rotating rotor, and the measurement results showed a certain degree of accuracy.

We report on a passively Q-switched Yb-doped all-fiber (YDF) ring laser based on stimulated Brillouin scattering (SBS) feedback in a 20-m single-mode fiber (SMF). The Q-switched pulses are generated from the Stokes pulses of SBS and amplified in the YDF. The 10-ns self-Q-switched pulses with ∼1 kW maximum peak power are obtained. The repetition frequency of the pulse train is tuned from 4 to 12.6 kHz by changing the pump power in the experiment. A bandpass filter inserted in the laser cavity was used to suppress the time jitter of the Q-switched pulses.

In order to meet the requirements of uniform illumination for optical palm/fingerprint instruments and overcome the shortcomings of the poor uniform illumination on the working plane of the optical palm/fingerprint prism, a novel secondary optical lens with a free-form surface, compact structure, and high uniformity is presented in this paper. The design of the secondary optical lens is based on emission properties of the near-infrared light-emitting diode (LED) and basic principles of non-imaging optics, especially considering the impact of the thickness of the prism in the design. Through the numerical solution of Snell's law in geometric optics, we obtain the profile of the free-form surface of the lens. Using the optical software TracePro, we trace and simulate the illumination system. The results show that the uniformity is 89.8% on the working plane of the prism, and the test results show that the actual uniformity reaches 85.7% in the experiment, which provides an effective way for realizing a highly uniform illumination system with high-power near-infrared LED.

Bicontinuous structures are an important subset of three-dimensional periodic structures. In multibeam interference structures, the conditions for bicontinuity depend on the beam parameters and the exposure dose. As described in the present work, these conditions can be applied to establish the range of bicontinuity for any multibeam-interference-produced structure. In addition to the bicontinuity range, the analysis yields the volume fraction of the constituent materials and the normalized interface areas. This analysis has been performed for rhombohedral and woodpile lattices as well as their cubic structure limiting cases. A sphere-at-each-lattice-site model for each of the cubic cases has also been developed for comparison. The multibeam interference structures were investigated for representative media and for various incident polarizations.

Internal stray radiation is a key factor that influences infrared imaging systems, and its suppression level is an important criterion to evaluate system performance, especially for cryogenic infrared imaging systems, which are highly sensitive to thermal sources. In order to achieve accurate measurement for internal stray radiation, an approach is proposed, which is based on radiometric calibration using a spherical mirror. First of all, the theory of spherical mirror design is introduced. Then, the calibration formula considering the integration time is presented. Following this, the details regarding the measurement method are presented. By placing a spherical mirror in front of the infrared detector, the influence of internal factors of the detector on system output can be obtained. According to the calibration results of the infrared imaging system, the output caused by internal stray radiation can be acquired. Finally, several experiments are performed in a chamber with controllable inside temperatures to validate the theory proposed in this paper. Experimental results show that the measurement results are in good accordance with the theoretical analysis, and demonstrate that the proposed theories are valid and can be employed in practical applications. The proposed method can achieve accurate measurement for internal stray radiation at arbitrary integration time and ambient temperatures. The measurement result can be used to evaluate whether the suppression level meets the system requirement.

Core-shell structured SiO2@YVO4:Yb3+, Er3+, Eu3+ nanospheres were prepared by a simple solgel method followed by a subsequent heat treatment. The as-prepared composites were characterized by a scanning electron microscope, high-resolution transmission electron microscope, x-ray diffraction, and photoluminescence spectra. The influences of different coating numbers were also studied. To sum up, the composites are spherical shaped with an average diameter of 200 nm, and the YVO4:Yb3+, Er3+, Eu3+ luminescent particles are successfully coated on the surface of SiO2 nanospheres. Under the near-infrared irradiation at 980 nm, the composites can emit strong green lights (at 525 nm, 550 nm) attributed to the H11/22→I15/24, S3/24→I15/24 transitions of Er3+ ions, and slight red lights (at 590 nm, 620 nm) attributed to the D05→F17, D05→F27 transitions of Eu3+ ions. Given the above, due to the regular core-shell structure, the uniform distribution of nanoparticles, and the colorful emissions, the SiO2@YVO4:Yb3+, Er3+, Eu3+ nanospheres may have great potential for some biological applications, such as biological tracers, bio-labeling, and so on.

An improved shift-rotation method for the absolute testing of spherical surfaces is developed to obtain pixel-level spatial resolution and a low noise propagation ratio. The absolute testing process includes multiple rotational tests and two lateral shifting tests with large shifts. A wavefront reconstruction algorithm based on subaperture division and least squares fitting is proposed to reconstruct the surface figure of the test optics. Numerical simulation results show that the method reveals high-frequency figures missed in the traditional Zernike-based shift-rotation method. The algorithm error is lower than 0.4%, and the noise propagation ratio can be reduced by 70% using large shifts. The absolute testing of spherical optics is carried out to verify this method. One spherical surface was tested with the presented absolute testing method and the method using a point diffraction interferometer. The difference of the measurement results based on the two methods showed that the testing uncertainty reached 0.19 nm root mean square (RMS), which indicated that the presented method has potential subnanometer testing uncertainty.

We numerically simulate Kerr comb generation in an anomalous dispersion microcavity by modal expansion method and demonstrate that the initiation of comb generation is affected by the change of local dispersion possibly caused by avoided mode crossings. We also quantitatively analyze the instantaneous phase matching of different modes and reveal the characteristics of energy distribution in different modes in the dynamics of comb generation. We demonstrate that the local dispersion change can control the Kerr comb to transform between Type I and Type II combs. We also find that local dispersion is closely related to the stability of the power of Kerr comb lines, something that can change the dynamical state of the system near the Hamiltonian-Hopf bifurcation under an anomalous dispersion regime from a quasi-periodic oscillation state to a periodic state (Turing patterns).

Filtering off speckle noise from a fringe image is one of the key tasks in electronic speckle pattern interferometry (ESPI). In general, ESPI fringe images can be divided into three categories: low-density fringe images, high-density fringe images, and variable-density fringe images. In this paper, we first present a general filtering method based on variational image decomposition that can filter speckle noise for ESPI fringe images with various densities. In our method, a variable-density ESPI fringe image is decomposed into low-density fringes, high-density fringes, and noise. A low-density fringe image is decomposed into low-density fringes and noise. A high-density fringe image is decomposed into high-density fringes and noise. We give some suitable function spaces to describe low-density fringes, high-density fringes, and noise, respectively. Then we construct several models and numerical algorithms for ESPI fringe images with various densities. And we investigate the performance of these models via our extensive experiments. Finally, we compare our proposed models with the windowed Fourier transform method and coherence enhancing diffusion partial differential equation filter. These two methods may be the most effective filtering methods at present. Furthermore, we use the proposed method to filter a collection of the experimentally obtained ESPI fringe images with poor quality. The experimental results demonstrate the performance of our proposed method.

Three scenarios that produce colored thunderstorms are simulated. In Scenario #1, the thunderstorm's sunlit face exhibits a color gradient from white or yellow at top to red at base when the sun is near the horizon. It is simulated with a second-order scattering model as a combination of sunlight and skylight reflected from the cloud face that is attenuated and reddened by Rayleigh and Mie scattering over the long optical path near sunset that increases from cloud top to base. In Scenario #2, the base of the precipitation shaft appears luminous green-blue when surrounded by a much darker arcus cloud. It is simulated as multiply scattered light transmitted through the precipitation shaft using a Monte Carlo model that includes absorption by liquid water and ice. The color occurs over a wide range of solar zenith angles with large liquid water content, but the precipitation shaft is only bright when hydrometeors are large. Attenuation of the light by Rayleigh and Mie scattering outside the precipitation shaft shifts the spectrum to green when viewed from a distance of several kilometers. In Scenario #3, the shaded cloud face exhibits a "sickly" yellow-green color. It is simulated with a second-order scattering model as the result of distant skylight that originates in the sunlit region beyond an opaque anvil of order 40 km wide but is attenuated by Rayleigh and Mie scattering in its path to the cloud and observer.

The comment by Vorontsov and Weyrauch [Appl. Opt.55, 9950 (2016)APOPAI0003-693510.1364/AO.55.009950] is aimed at rebutting the critiques in Sprangle et al. [Appl. Opt.54, F201 (2015)APOPAI0003-693510.1364/AO.54.00F201] and Nelson et al. [Appl. Opt.55, 1757 (2016)APOPAI0003-693510.1364/AO.55.001757]. In the comment, Vorontsov and colleagues describe their experiments aimed at demonstrating the feasibility of coherent combining of lasers on a distant target, using relatively low-power lasers and a cooperative retro-reflective target. The Naval Research Laboratory has demonstrated the capability to project high power on a distant target by making use of an incoherent combining architecture. The proof-of-concept experiments were performed in a realistic environment without employing cooperative targets and without sophisticated adaptive optics instrumentation.

A simple lock-in detection technique especially suited for digital phase-generated carrier (PGC) demodulators is proposed in this paper. It mixes the interference signal with rectangular waves whose Fourier expansions contain multiple odd or multiple even harmonics of the carrier to recover the quadrature components needed for interference phase demodulation. In this way, the use of a multiplier is avoided and the efficiency of the algorithm is improved. Noise performance with regard to light intensity variation and circuit noise is analyzed theoretically for both the proposed technique and the traditional lock-in technique, and results show that the former provides a better signal-to-noise ratio than the latter with proper modulation depth and average interference phase. Detailed simulations were conducted and the theoretical analysis was verified. A fiber-optic Michelson interferometer was constructed and the feasibility of the proposed technique is demonstrated.

The aero-optical environment around a hemisphere-on-cylinder turret with both flat and conformal windows was studied experimentally in flight using the Airborne Aero-Optical Laboratory-Transonic for a range of subsonic and transonic Mach numbers between 0.5 and 0.8. Above M=0.6, the local shock appeared near the top of the turret, causing additional aero-optical distortions at side-looking angles. Using time-resolved wavefronts, instantaneous shock locations were extracted and analyzed. The mean shock location was found to be near a viewing angle, α=80 deg for both window types at M=0.7 and 0.8. For M=0.8, the shock has a single frequency peak at StD=0.15, the same as for the unsteady separation line, indicating a lock-in mechanism between the shock and the separated wake region. Analysis of aero-optical distortions in the wake indicated that the wake dynamics were beginning to be affected by the shock only at high transonic speed of M=0.8 for the conformal-window turret.

This publisher's note amends the author list and Acknowledgments in Appl. Opt.56, 2589 (2017)APOPAI0003-693510.1364/AO.56.002589.

Due to the similarity of Brillouin optical time domain analyzer (BOTDA) signals, image denoising could be utilized to remove the noise. However, the performance can be much degraded due to inaccurate noise level estimation. By numerical and experimental study, we compare the noise level estimation of three different methods for BOTDA: calculating the standard deviation (STD) of the measurements, a filter-based estimation algorithm, and a patch-based estimation algorithm proposed in this paper, which selects weak textured patches of BOTDA signal and then estimates noise level using principal component analysis (W-PCA). The results show that W-PCA and the mean of STD can accurately estimate the noise level, while the filter-based method overestimates the noise level. Nevertheless, for BOTDA with distributed amplification, the STD has huge fluctuation along the length, while the W-PCA is relatively robust for its global consideration. Experimental results of an ultra-long-distance BOTDA prove that the non-local means denoising processing based on W-PCA effectively removes the noise of a sensing system without signal distortion.

A single-layer graphene saturable absorber is employed for passive Q-switching of an Er, Yb:GdAl3(BO3)4 (Er,Yb:GdAB) compact laser, representing the first Er-doped oxoborate laser Q-switched by graphene. This laser is based on a c-cut 1.8 at. % Er3+, 15 at. % Yb3+:GdAB crystal diode-pumped at 0.976 μm. It generates a maximum average output power of 360 mW at 1.55 μm with a slope efficiency of 23% (with respect to the incident power). Stable ∼1 μJ/130 ns pulses are achieved at a repetition rate of 400 kHz. This result represents, to the best of our knowledge, the shortest pulse duration ever achieved in bulk Er lasers Q-switched by 2D materials. Graphene is a promising material for generating nanosecond pulses at high repetition rates (MHz range) in Er-doped oxoborate lasers emitting in the eye-safe range at 1.5–1.7 μm.

Speckle pattern-based characteristics of digital image correlation (DIC) restrict its application in engineering fields and nonlaboratory environments, since serious decorrelation effect occurs due to localized sudden illumination variation. A simple and efficient speckle pattern adjusting and optimizing approach presented in this paper is aimed at providing a novel speckle pattern robust enough to resist local illumination variation. The new speckle pattern, called neighborhood binary speckle pattern, derived from original speckle pattern, is obtained by means of thresholding the pixels of a neighborhood at its central pixel value and considering the result as a binary number. The efficiency of the proposed speckle pattern is evaluated in six experimental scenarios. Experiment results indicate that the DIC measurements based on neighborhood binary speckle pattern are able to provide reliable and accurate results, even though local brightness and contrast of the deformed images have been seriously changed. It is expected that the new speckle pattern will have more potential value in engineering applications.

This work describes noncontact temperature measurements using wavelength shifts of CdSe quantum dot (QD) doped poly(methyl methacrylate) microfiber. The sensor is fabricated using a drawing method by bridging two tapered single mode fibers with a polymer microfiber (PMF) approximately 3 μm in diameter. A set of a PMF section with and without the doping of the CdSe-ZnS core-shell QD was applied as sensing probes and used to measure temperatures over the range of 25°C-48°C. The experimental results show that the doped PMF is able to achieve a higher performance with a reasonably good sensitivity of 58.5 pm/°C based on the wavelength shifting, which is about 18 times that of the undoped PMF temperature sensitivity. The proposed sensor showed a linear temperature sensing range that matches well with the physiologically relevant temperatures. Moreover, these results open the way for long-term and high-stability realization of temperature sensing optical fibers.