This study presents designs for developing two-dimensional optical matrices exhibiting low cross correlation and excellent orthogonality. The key scheme is to use original one-dimensional temporal codes and a particular mapping technique. A two-dimensional temporal/spatial optical code-division multiple-access network using the proposed optical matrices was also constructed. This study subsequently analyzed the effect of multiuser interference on code performance. The analysis result reveals that some families of optical codes applied to the network theoretically provided a favorable code performance level.

Many long-wave infrared spectroscopic imaging applications are limited by the portability and cost of detector arrays. We present a characterization of a newly available, low-cost, uncooled vanadium oxide microbolometer array, the Seek Compact, in accordance with common infrared detector specifications: noise-equivalent differential temperature (NEDT), optical responsivity spectra, and Allan variance. The Compact’s imaging array consists of 156×206  pixels with a 12-μm pixel pitch, 93% of the pixels yield useful temperature readings. Characterization results show optical response between λ=7.4 and 12  μm with an NEDT of 148 mK (at ≈7  fps). Comparing these results to a research-grade camera, the Seek Compact exhibits a 4× and 48× reduction in weight (2.0/0.5  lbs) and cost ($12,000/$250) but takes 93× longer to achieve the same NEDT (1.55  s/16.6  ms for 45 mK). Additionally, a proof-of-concept spectral imaging experiment of SiN thin films is conducted. Leveraging this price reduction and spectroscopic imaging capability, the Seek Compact has potential in enabling field-deployable and distributed active midinfrared spectroscopic imaging, where cost and portability are the dominate inhibitors and high frame rates are not required.

We propose an image upsampling algorithm to increase the depth image resolution while preserving edge information and protecting from the texture copy problem. The proposed upsampler selectively combines the spatial, depth range, and color range weight functions for cost calculation according to the weight selection algorithm. In particular, a color range weight function is proposed to reduce the texture copy problem caused by distinct patterns in the color image. Simulation results show that the proposed algorithm outperforms other conventional upsamplers.

This PDF file contains the editorial “Special Section Guest Editorial: Optical Computational Imaging” for OE Vol. 56 Issue 04

In contrast with traditional extended depth-of-field approaches, we propose a depth-based deconvolution technique that realizes the depth-variant nature of the point spread function of an ordinary fixed-focus camera. The developed technique brings a single blurred image to focus at different depth planes which can be stitched together based on a depth map to output a full-focus image. Strategies to suppress the deconvolution’s ringing artifacts are implemented on three levels: block tiling to eliminate boundary artifacts, reference maps to reduce ringing initiated by sharp edges, and depth-based masking to mitigate artifacts raised by neighboring depth-transition surfaces. The performance is validated numerically for planar and multidepth objects.

Polarization is a phenomenon that cannot be observed by the human eye, but it provides rich information regarding scenes. The proposed method estimates the surface normal of black specular objects through polarization analysis of reflected light. A unique surface normal cannot be determined from a polarization image observed from a single viewpoint; thus, we observe the object from multiple viewpoints. To analyze the polarization state of the reflected light at the corresponding points when observed from multiple viewpoints, the abstract shape is predetermined using a space carving technique. Unlike a conventional photometric stereo or multiview stereo, which cannot estimate the shape of a black specular object, the proposed method estimates the surface normal and three-dimensional coordinates of black specular objects via polarization analysis and space carving.

Stitching is used to reduce incomplete infusion of T-joint core (dry-core) and reinforce T-joint structure. However, it may cause new types of flaws, especially submillimeter flaws. Thermographic approaches including microvibrothermography, microlaser line thermography, and microlaser spot thermography on the basis of pulsed and lock-in techniques were proposed. These techniques are used to detect the submillimeter porosities in a stitched T-joint carbon fiber reinforced polymer composite specimen. X-ray microcomputed tomography was used to validate the thermographic results. Finally an experimental comparison of microlaser excitation thermography and microultrasonic excitation thermography was conducted.

We exploit the two different versions of Kinect, v1 and v2, for the calculation of microimages projected onto integral-imaging displays. Our approach is based on composing a three-dimensional (3-D) point cloud from a captured depth map and RGB information. These fused 3-D maps permit to generate an integral image after projecting the information through a virtual pinhole array. In our analysis, we take into account that each of the Kinect devices has its own inherent capacities and individualities. We illustrate our analysis with some imaging experiments, provide the distinctive differences between the two Kinect devices, and finally conclude that Kinect v2 allows the display of 3-D images with very good resolution and with full parallax.

Compressive sensing matrices are traditionally based on random Gaussian and Bernoulli entries. Nevertheless, they are subject to physical constraints, and their structure unusually follows a dense matrix distribution, such as the case of the matrix related to compressive spectral imaging (CSI). The CSI matrix represents the integration of coded and shifted versions of the spectral bands. A spectral image can be recovered from CSI measurements by using iterative algorithms for linear inverse problems that minimize an objective function including a quadratic error term combined with a sparsity regularization term. However, current algorithms are slow because they do not exploit the structure and sparse characteristics of the CSI matrices. A gradient-based CSI reconstruction algorithm, which introduces a filtering step in each iteration of a conventional CSI reconstruction algorithm that yields improved image quality, is proposed. Motivated by the structure of the CSI matrix, Φ, this algorithm modifies the iterative solution such that it is forced to converge to a filtered version of the residual Φ^Ty, where y is the compressive measurement vector. We show that the filtered-based algorithm converges to better quality performance results than the unfiltered version. Simulation results highlight the relative performance gain over the existing iterative algorithms.

Flame chemiluminescence tomography (FCT), which combines computerized tomography theory and multidirectional chemiluminescence emission measurements, can realize instantaneous three-dimensional (3-D) diagnostics for flames with high spatial and temporal resolutions. One critical step of FCT is to record the projections by multiple cameras from different view angles. For high accuracy reconstructions, it requires that extrinsic parameters (the positions and orientations) and intrinsic parameters (especially the image distances) of cameras be accurately calibrated first. Taking the focus effect of the camera into account, a modified camera calibration method was presented for FCT, and a 3-D calibration pattern was designed to solve the parameters. The precision of the method was evaluated by reprojections of feature points to cameras with the calibration results. The maximum root mean square error of the feature points’ position is 1.42 pixels and 0.0064 mm for the image distance. An FCT system with 12 cameras was calibrated by the proposed method and the 3-D CH* intensity of a propane flame was measured. The results showed that the FCT system provides reasonable reconstruction accuracy using the camera’s calibration results.

In the first part of this work, we present two methods for improving the shape-threat detection performance of x-ray computed tomography. Our work uses a fixed-gantry system employing 25 x-ray sources. We first utilize Kullback–Leibler divergence and Mahalanobis distance to determine the optimal single-source single-exposure measurement. The second method employs gradient search on Bhattacharyya bound on error rate (P_e) to determine an optimal multiplexed measurement that simultaneously utilizes all available sources in a single exposure. With limited total resources of 10⁶ photons, the multiplexed measurement provides a 41.8× reduction in P_e relative to the single-source measurement. In the second part, we consider multiple exposures and develop an adaptive measurement strategy for x-ray threat detection. Using the adaptive strategy, we design the next measurement based on information retrieved from previous measurements. We determine both optimal “next measurement” and stopping criterion to insure a target P_e using sequential hypothesis testing framework. With adaptive single-source measurements, we can reduce P_e by a factor of 40× relative to the measurements employing all sources in sequence. We also observe that there is a trade-off between measurement SNR and number of detectors when we study the performance of systems with reduced detector numbers.

The snapshot colored compressive spectral imager (SCCSI) is a recent compressive spectral imaging (CSI) architecture that senses the spatial and spectral information of a scene in a single snapshot by means of a colored mosaic FPA detector and a dispersive element. Commonly, CSI architectures allow multiple snapshot acquisition, yielding improved reconstructions of spatially detailed and spectrally rich scenes. Each snapshot is captured employing a different coding pattern. In principle, SCCSI does not admit multiple snapshots since the pixelated tiling of optical filters is directly attached to the detector. This paper extends the concept of SCCSI to a system admitting multiple snapshot acquisition by rotating the dispersive element, so the dispersed spatio-spectral source is coded and integrated at different detector pixels in each rotation. Thus, a different set of coded projections is captured using the same optical components of the original architecture. The mathematical model of the multishot SCCSI system is presented along with several simulations. Results show that a gain up to 7 dB of peak signal-to-noise ratio is achieved when four SCCSI snapshots are compared to a single snapshot reconstruction. Furthermore, a gain up to 5 dB is obtained with respect to state-of-the-art architecture, the multishot CASSI.

The image reconstruction in diffuse optical tomography (DOT) is a typical inverse problem; therefore, regularization techniques are essential to obtain a reliable solution. The most general form of regularization is Tikhonov regularization. With any Tikhonov regularized reconstruction algorithm, one of the crucial issues is the selection of the regularization parameter that controls the trade-off between the regularized solution and fidelity to the given sets of data. Automatic methods such as L-curve, generalized cross-validation, minimal residual method, projection error method, and model function method have been introduced to select the regularization parameter over the years. However, little investigation of comparison of all the algorithms has been reported in DOT. The performance of the five methods for choosing regularization parameter is comprehensively compared, and advantages and limitations are discussed.

A compressive imaging model is proposed that multiplexes segments of the field of view (FOV) onto an infrared focal plane array (FPA). Similar to compound imaging, our model is based on combining pixels from a surface comprising of the different parts of the FOV. We formalize this superposition of pixels in a global multiplexing process reducing the number of detectors required for the FPA. We present an analysis of the signal-to-noise ratio for the full rank and compressive collection paradigms for a target detection and tracking scenario. We then apply automated target detection algorithms directly on the measurement sequence for this multiplexing model. We extend the target training and detection processes for the application directly on the encoded measurements. Optimal measurement codes for this application may imply abandoning the ability to reconstruct the actual scene in favor of reconstructing the locations of interesting objects. We present a simulated case study as well as real data results from a visible FOV multiplexing camera.

Compressive sensing theory was proposed to deal with the high quantity of measurements demanded by traditional hyperspectral systems. Recently, a compressive spectral imaging technique dubbed compressive sensing miniature ultraspectral imaging (CS-MUSI) was presented. This system uses a voltage controlled liquid crystal device to create multiplexed hyperspectral cubes. We evaluate the utility of the data captured using the CS-MUSI system for the task of target detection. Specifically, we compare the performance of the matched filter target detection algorithm in traditional hyperspectral systems and in CS-MUSI multiplexed hyperspectral cubes. We found that the target detection algorithm performs similarly in both cases, despite the fact that the CS-MUSI data is up to an order of magnitude less than that in conventional hyperspectral cubes. Moreover, the target detection is approximately an order of magnitude faster in CS-MUSI data.

Conventional sensors measure the light incident at each pixel in a focal plane array. Compressive sensing (CS) involves capturing a smaller number of unconventional measurements from the scene, and then using a companion process to recover the image. CS has the potential to acquire imagery with equivalent information content to a large format array while using smaller, cheaper, and lower bandwidth components. However, the benefits of CS do not come without compromise. The CS architecture chosen must effectively balance between physical considerations, reconstruction accuracy, and reconstruction speed to meet operational requirements. Performance modeling of CS imagers is challenging due to the complexity and nonlinearity of the system and reconstruction algorithm. To properly assess the value of such systems, it is necessary to fully characterize the image quality, including artifacts and sensitivity to noise. Imagery of a two-handheld object target set was collected using an shortwave infrared single-pixel CS camera for various ranges and number of processed measurements. Human perception experiments were performed to determine the identification performance within the trade space. The performance of the nonlinear CS camera was modeled by mapping the nonlinear degradations to an equivalent linear shift invariant model. Finally, the limitations of CS modeling techniques are discussed.

In a multiplexed image, multiple fields-of-view (FoVs) are superimposed onto a common focal plane. The attendant gain in sensor FoV provides a new degree of freedom in the design of an imaging system, allowing for performance tradeoffs not available in traditional optical designs. We explore design choices relating to a shift-encoded optically multiplexed imaging system and discuss their performance implications. Unlike in a traditional imaging system, a single multiplexed image has a fundamental ambiguity regarding the location of objects in the image. We present a system that can shift each FoV independently to break this ambiguity and compare it to other potential disambiguation techniques. We then discuss the optical, mechanical, and encoding design choices of a shift-encoding midwave infrared imaging system that multiplexes six 15×15  deg FoVs onto a single one megapixel focal plane. Using this sensor, we demonstrate a computationally demultiplexed wide FoV video.

Infrared images have shortcomings of background noise, few details, and fuzzy edges. Therefore, noise suppression and detail enhancement play crucial roles in the infrared image technology field. To effectively enhance details and eliminate noises, an infrared image processing algorithm based on multiscale feature prior is proposed. First, the maximum a posterior model estimating optimal free-noise results is constructed and discussed. Second, based on the extended 16 high-order differential operators and multiscale features, we propose a structure feature prior that is immune to noises and depicts infrared image features more precisely. Third, with the noise-suppressed image, the final image is enhanced by the improved multiscale unsharp mask algorithm, which enhances details and edges adaptively. Finally, testing infrared images in different signal-to-noise ratio scenes, the effectiveness and robustness of the proposed approach is analyzed. Compared with other well-established methods, the proposed method shows the evident performance in terms of noise suppression and edge enhancement.

A passive, millimeter wave (MMW) and terahertz (THz) dual-band imaging system composed of 94 and 250 GHz single-element detectors was used to investigate preprocessing and fusion algorithms for dual-band images. Subsequently, an MMW and THz image preprocessing and fusion integrated algorithm (MMW-THz IPFIA) was developed. In the algorithm, a block-matching and three-dimensional filtering denoising algorithm is employed to filter noise, an adaptive histogram equalization algorithm to enhance images, an intensity-based registration algorithm to register images, and a wavelet-based image fusion algorithm to fuse the preprocessed images. The performance of the algorithm was analyzed by calculating the SNR and information entropy of the actual images. This algorithm effectively reduces the image noise and improves the level of detail in the images. Since the algorithm improves the performance of the investigated imaging system, it should support practical technological applications. Because the system responds to blackbody radiation, its improvement is quantified herein using the static performance parameter commonly employed for thermal imaging systems, namely, the minimum detectable temperature difference (MDTD). An experiment was conducted in which the system’s MDTD was measured before and after applying the MMW-THz IPFIA, verifying the improved performance that can be realized through its application.

Nonideal stereo videos do not hinder viewing experience in stereoscopic displays. However, for autostereoscopic displays nonideal stereo videos are the main cause of reduced three-dimensional quality causing calibration artifacts and multiview synthesis artifacts. We propose an efficient multiview rendering algorithm for autostereoscopic displays that takes uncalibrated stereo as input. First, the epipolar geometry of multiple viewpoints is analyzed for multiview displays. The uncalibrated camera poses for multiview display viewpoints are then estimated by algebraic approximation. The multiview images of the approximated uncalibrated camera poses do not contain any projection or warping distortion. Finally, by the exploiting rectification homographies and disparities of rectified stereo, one can determine the multiview images with their estimated camera poses. The experimental results show that the multiview synthesis algorithm can provide results that are both temporally consistent and well-calibrated without warping distortion.

Phase diversity (PD) is the special case of multiframe blind deconvolution algorithm, where an object is observed with a stack intentionally defocused image. The known defocused information between phases is used to estimate the object and aberration function. This paper presents a parallel three-dimensional (3-D) wavefront aberration estimation using blind deconvolution. We used multiplane PD for widefield fluorescence microscopy images of 3-D objects. Parallel multiplane PD yields a speedup factor of 4.33 with respect to the counterpart sequential algorithm for an image size of 256×256. We used the outcome of the PD algorithm to generate 3-D point cloud data from images from one view.

Existing methods for tracking three-dimensional (3-D) eye positions with a monocular color camera mostly rely on a generic 3-D face model and a certain face database. However, the performance of these methods is susceptible to the variations of head poses. For this reason, existing methods for estimating 3-D eye position from a single two-dimensional face image may yield erroneous results. To improve the accuracy of 3-D eye position trackers using a monocular camera, we present a compensation method as a postprocessing technique. We address the problem of determining an optimal registration function for fitting 3-D data consisting of the inaccurate estimates from the eye position tracker and their corresponding ground truths. To obtain the ground truths of 3-D eye positions, we propose two different systems by combining an optical motion capture system and checkerboards, which construct the form of the hand-eye and robot-world calibration. By solving a least-squares optimization problem, we can determine the optimal registration function in an affine form. Real experiments demonstrate that the proposed method can considerably improve the accuracy of 3-D eye position trackers using a single color camera.

X-ray fluorescence computed tomography (XFCT) was performed on a high-intensity synchrotron radiation source or a pencil beam with a long exposure time due to the low emission and detection efficiency of x-ray fluorescence photons. For the first time, the feasibility and experimental results of a full-field fan-beam XFCT with a photon-counting detector array are presented. This full-field fan-beam XFCT consists of a conventional low-intensity x-ray tube, an energy-sensitive photon-counting detector array, and a tungsten pinhole collimator. A phantom containing gadolinium solution (Kα, 42.74 keV) was scanned for 30 min using a polychromatic x-ray fan beam with a third-generation computed tomography (CT) geometry. After scattering and attenuation corrections, experimental results showed that XFCT had better accuracy and performance than spectral CT. Full-field XFCT is a promising modality for biomedical imaging of exogenous molecular probes containing nanoparticles of high atomic number.

In visual tracking, deep learning with offline pretraining can extract more intrinsic and robust features. It has significant success solving the tracking drift in a complicated environment. However, offline pretraining requires numerous auxiliary training datasets and is considerably time-consuming for tracking tasks. To solve these problems, a multiscale sparse networks-based tracker (MSNT) under the particle filter framework is proposed. Based on the stacked sparse autoencoders and rectifier linear unit, the tracker has a flexible and adjustable architecture without the offline pretraining process and exploits the robust and powerful features effectively only through online training of limited labeled data. Meanwhile, the tracker builds four deep sparse networks of different scales, according to the target’s profile type. During tracking, the tracker selects the matched tracking network adaptively in accordance with the initial target’s profile type. It preserves the inherent structural information more efficiently than the single-scale networks. Additionally, a corresponding update strategy is proposed to improve the robustness of the tracker. Extensive experimental results on a large scale benchmark dataset show that the proposed method performs favorably against state-of-the-art methods in challenging environments.

A low-loss microstructure fiber is numerically investigated for convenient transmission of polarization maintaining terahertz (THz) waves. The dual-hole units (DHUs) are used inside the core of the kagome lattice microstructure to achieve high birefringence and low effective material loss (EML). It is demonstrated that by rotating the axis of orientation of the DHUs, it is possible to obtain low EML of 0.052  cm−1, low confinement loss of 0.01  cm−1, and high birefringence of 0.0354 at 0.85 THz. It is also reported that the transmission properties of the proposed microstructure fiber are varied with rotation angle, core diameter, and operating frequencies. Other guiding characteristics, such as single-mode propagation, power fraction, and dispersion, are also discussed thoroughly.

The morphology of the human ear presents rich and stable information embedded on the curved 3-D surface and has as a result attracted considerable attention from forensic scientists and engineers as a biometric recognition modality. However, recognizing a person’s identity from the morphology of the human ear in unconstrained environments, with insufficient and incomplete training data, strong person-specificity, and high within-range variance, can be very challenging. Following our previous work on ear recognition based on local texture descriptors, we propose to use anatomical and embryological information about the human ear in order to find the autonomous components and the locations where large interindividual variations can be detected. Embryology is particularly relevant to our approach as it provides information on the possible changes that can be observed in the external structure of the ear. We experimented with three publicly available databases, namely: IIT Delhi-1, IIT Delhi-2, and USTB-1, consisting of several ear benchmarks acquired under varying conditions and imaging qualities. The experiments show excellent results, beyond the state of the art.

Charged-couple devices (CCD) and complementary metal oxide semiconductor (CMOS) image sensors, in conjunction with the second moment radius analysis method, are effective tools for determining the radius of a laser beam. However, the second moment method heavily weights sensor noise, which must be dealt with using a thresholding algorithm and a software aperture. While these noise reduction methods lower the random error due to noise, they simultaneously generate systematic error by truncating the Gaussian beam’s edges. A scale factor that is invariant to beam ellipticity and corrects for the truncation of the Gaussian beam due to thresholding and the software aperture has been derived. In particular, simulations showed an order of magnitude reduction in measured beam radius error when using the scale factor—irrespective of beam ellipticity—and further testing with real beam data demonstrated that radii corrected by the scale factor are independent of the noise reduction parameters. Thus, through use of the scale factor, the accuracy of beam radius measurements made with a CCD or CMOS sensor and the second moment are significantly improved.

We demonstrated a data-processing method based on multiple beam interference and Fresnel equations that simultaneously gave the refraction index and absorption coefficient from the raw data of terahertz (THz) time-domain spectroscopy (TDS), which laid the foundation for obtaining the dielectric parameters. This method was independent of phase processing, and complete material information was reserved without having to cut the time-domain signal. The optical coefficients including refractive indices and absorption coefficients of white polyethylene and quartz samples at different thicknesses were obtained. The applicability and accuracy of this method were discussed and verified by comparison with the traditional data-processing method of THz TDS.

Speckle interferometry is a useful optical deformation measurement method for objects with rough surfaces. This method can measure not only the out-of-plane but also the in-plane deformation of these objects. Recently, the method has been used to measure three-dimensional (3-D) deformations at a high resolution. Many kinds of 3-D measurement methods based on speckle interferometry have already been proposed. However, the parameters in the sensitivity matrices of these conventional methods are not generally the same in each of the 3-D. Methods for which the sensitivity is not the same in the 3-D cannot be employed as an exact 3-D measurement method because the measurement uncertainty is different in each direction. This paper proposes a method where the sensitivity is the same in 3-D. An optical system was set up based on the concept of the proposed method. A method based on vector analysis is also proposed for detecting the deformation in an arbitrary section of the object. Finally, the 3-D deformation from the buckling of a mechanical beam was measured using in new speckle interferometry.

We show how a sinusoidal fringe pattern can be obtained by using a single cube beam splitter based on the Gates’ interferometer configuration. When an expanded and collimated laser beam hits the binding edge of a nonpolarizing cube beam splitter parallel to the splitter coating, it generates interference fringes at the exit due to the internal reflections and refractions of the laser beam passing through the cube. Unlike common digital projection systems, the proposed optical arrangement generates a perfectly sinusoidal and continuous fringe pattern, minimizing the problems associated with the discretization of a synthetic digital signal. The fast Fourier transform and phase-shifting techniques are used to demodulate the captured fringe patterns. Experimental results are presented for the three-dimensional shape reconstruction of the relief of a coin and of a spherical indentation on a piece of aluminum with a maximum height of about 150  μm. In addition, we evaluate the accuracy and resolution of the proposed measuring device: shape reconstruction accuracy is about 1.4% and axial resolution is 0.15  μm. Due to its simple and compact setup, the proposed system is particularly suited to be miniaturized.

Carbon dioxide (CO2) is considered a precursor gas of volcanic eruptions by volcanologists. Monitoring the anomalous release of this parameter, we can retrieve useful information for the mitigation of volcanic hazards, such as for air traffic security. From a dataset collected during the Stromboli volcano field campaign, an assessment of the wind speed, in both horizontal and vertical paths, performing a fast tracking of this parameter was retrieved. This was determined with a newly designed shot-per-shot differential absorption LiDAR system operated in the near-infrared spectral region due to the simultaneous reconstruction of CO2 concentrations and wind speeds, using the same sample of LiDAR returns. A correlation method was used for the wind speed retrieval in which the transport of the spatial inhomogeneities of the aerosol backscattering coefficient, along the optical path of the system, was analyzed.

We experimentally demonstrate spatial light modulation based on graded-index plasma channels induced by femtosecond pulses. The spatial profile of the probe beam can be conveniently controlled by adjusting the intensity distribution of the pump beam or by changing the relative position between pump beam and probe beam. We also show that the modulation depth of the probe beam can be controlled by adjusting the power of the pump beam.

Fiber-optic gyro strapdown inertial navigation system is typically physically separated from the log giving an aiding velocity. This results in a velocity difference under angular motions, otherwise known as the outer lever arm effect. The mathematical relationship between the velocity difference and the outer lever arm is derived, and the effect of the outer lever arm on angular velocity compensation and harmful acceleration compensation is analyzed using the structure of in-motion gyrocompass alignment. Based on this analysis, a method for outer lever arm correction is given to counteract the outer lever arm effect on the performance of the alignment. Simulation and trial tests are then used to verify the analysis results.

Three dimensional (3-D) modeling is important in applications ranging from manufacturing to entertainment. Multiview registration is one of the crucial steps in 3-D model construction. The automatic establishment of correspondences between overlapping views, without any known initial information, is the main challenge in point clouds registration. An automatic registration algorithm is proposed to solve the registration problem of rigid, unordered, scattered point clouds. This approach is especially suitable for registering datasets that are lacking in features or texture. In general, the existing techniques exhibit significant limitations in the registration of these types of point cloud data. The presented method automatically determines the best coarse registration results by exploiting the statistical technique principal component analysis and outputs translation matrices as the initial estimation for fine registration. Then, the translation matrices obtained from coarse registration algorithms are used to update the original point cloud and the optimal translation matrices are solved using an iterative algorithm. Experimental results show that the proposed algorithm is time efficient and accurate, even if the point clouds are partially overlapped and containing large missing regions.

A composite method combining energy and intensity mapping is proposed to address the issue of surface error caused by the irregular sampling phenomenon in freeform illumination lens design. In the combined method, the central region of the freeform lens is designed by the intensity mapping method, whereas the peripheral region is designed by the energy-mapping method. Furthermore, an iterative feedback optimization is added to scale out the application in extended light sources. As an evaluating example, a freeform lens with a 120-deg viewing angle, fitted with an appropriate number of points is designed by the proposed combined method. Compared with that designed by the energy method, the lens forms a more uniform illumination on the target surface without the appearance of a hot spot in the central region. The proposed method also exhibits superiority in extended-source design, where only a one-time optimization is needed to achieve the preset uniformity with the proper choice of coefficients.

One of the key optical transmission components is optical amplifiers. Studies on the amplification properties of the 1310-nm optical amplifiers are presented. The evaluated optical amplifiers are semiconductor optical amplifier (SOA) and praseodymium-doped fiber amplifier (PDFA). The study is aimed at the dynamic operation in single- and multiwavelength domains with high rate signals. The maximum obtained gain was 25.0 dB for SOA and 20.9 dB for PDFA. For the SOAs, the minimum achieved value of the receiver sensitivity was −11.5  dBm for a single channel and −11.5  dBm for a dense wavelength division multiplexing case while for PDFA those values were −11.0  dBm and −10.9, respectively. The main advantage of the PDFA in comparison to the measured SOAs is its higher saturation power. The SOAs proved to be viable candidates for high-speed amplification in the 1310-nm wavelength domain.

We develop a rigorous procedure for the optimum design of few-mode erbium (Er)-doped fiber amplifiers, which is tackled as a multiobjective optimization problem, in an approach based on the combination of the topology optimization and genetic algorithm techniques. We demonstrated that the usual ring-like doping distributions are necessarily the best choices only if the pump intensity shows no azimuthal dependence. Additionally, in general, the optimum doping distribution will be a function of the signal and pump azimuthal mode numbers. For the particular case of the LP11 pump, we also provide a triple-ring Er-doping profile that maximizes modal equalization for seven-group modes over the whole C-band, the highest modal count proposed in the literature so far.

Taking into account the influence both from the far-field divergent angle of the eigenmode and the slight nonplanar effect in a total reflection prism (TRP) ring resonator simultaneously, we analyze the effect of linear stress birefringence of prism on the eigenmode polarization and null drift in the resonator based on the theory of Jones matrix and the condition of eigenmode self-consistency. The evolution of the polarization ellipticity for both clockwise and counterclockwise beams versus the linear stress birefringence of prisms has been quantitatively established, and the minimum and maximum influences from the direction of linear stress birefringence on eigenmode polarization and null drift have also been found for the first time as far as we know. The results should be useful for designing and optimizing the structure of super high-precision TRP ring laser gyroscopes.

We propose and experimentally demonstrate an all-fiber optical mode switching structure supporting independent switching, exchanging, adding, and dropping functionalities in which each mode can be switched individually. The mode switching structure consists of cascaded mode selective couplers (MSCs) capable of exciting and selecting specific higher order modes in few-mode fibers with high efficiency and one multiport optical switch routing the independent spatial modes to their destinations. The data carried on three different spatial modes can be switched, exchanged, added, and dropped through this all-fiber structure. For this experimental demonstration, optical on-off-keying (OOK) signals at 10-Gb/s carried on three spatial modes are successfully processed with open and clear eye diagrams. The mode switch exhibits power penalties of less than 3.1 dB after through operation, less than 2.7 dB after exchange operation, less than 2.8 dB after switching operation, and less than 1.6 dB after mode adding and dropping operations at the bit-error rate (BER) of 10−3, while all three channels carried on three spatial modes are simultaneously routed. The proposed structure, compatible with current optical switching networks based on single-mode fibers, can potentially be used to expand the switching scalability in advanced and flexible short-reach mode-division multiplexing-based networks.

A single-longitudinal-mode (SLM) double-pulse injection-seeded neodymium-doped yttrium aluminium garnet (Nd:YAG) laser was established utilizing an RbTiOPO4 electro-optic crystal to modulate the optical path of the slave resonator for generating a resonance condition. The Q-switcher was fired twice during every pump period. This enabled the laser to emit a pair of SLM laser pulses with a time separation of 200  μs. Each pulse had a pulse energy of 13 mJ at 50-Hz repetition rate, pulse duration of 20±0.5  ns, and linewidth of 30±0.3  MHz (within 2 min). The beam quality factor of M2 was <1.22. A frequency jitter of 1.4 MHz was obtained within 2 min.

We present simple and complete empirical relations to predict the angle of beam divergence in terms of normalized frequency and aspect ratio of a single-mode trapezoidal index fiber. This is done for the far-field characterization over a long range of normalized frequencies without the calculation of normalized spot sizes. On comparison, we observe an excellent match of our results with exact values establishing the validity of our formulation. The formulation should attract attention as a simple alternative to the rigorous methods of estimating the angle of beam divergence for such fibers. It can be widely used by system users and developers for much better control over far-field related calculations and experiments.

Soliton self-frequency shift (SSFS) is a nonlinear effect that generates wavelength-shifted laser pulses in optical fibers. Based on SSFS, we report the generation of ultrashort pulses emitting at 1.7-μm band in a common dispersion shifted fiber (DSF) pumped by an ultrafast Er-doped fiber laser. Through optimizing the DSF length, sub-200 fs pulses in the range from 1.70 to 1.74  μm have been achieved with 76% conversion efficiency. The maximum output power is 26.8 mW and the single pulse energy is up to 0.7 nJ. Numerically, simulations have been carried out to map the SSFS pulse evolution in DSF. The simulation results are in a good agreement with the experiment results, giving the feasibility of this method.

An all-fiber type, CW, linearly polarized thulium-doped fiber laser is reported. Highly linear polarization was achieved by a special management of fiber Bragg gratings, which performs as the laser cavity reflectors. The laser generated 28 W signal output at 1949 nm with a slope efficiency of 47.3%. The polarization extinction ratio of the laser was measured to be around 20 dB. The beam quality of the laser was near diffraction-limited, with M2 of 1.1. The laser’s output features make it to be a potential light source for some important applications such as for pumping holmium-doped solid state lasers.

We propose and experimentally demonstrate a multicast-enabled survivable architecture for wavelength-division-multiplexed passive optical networks. By manipulating the sinusoidal control clock and the wavelength of the light source for each downstream wavelength channel at the optical line terminal, different optical subcarriers are generated to provide subscribers with not only multicast data transmission but also survivability against fiber failure in either distribution or feeder fibers. The simultaneous transmissions of 10-Gb/s unicast data and 10-Gb/s multicast data under normal working mode, as well as the 10-Gb/s transmissions for unicast data under protection mode, are experimentally demonstrated.

Multicolor light emission and energy-transfer processes were examined in Dy3+/Tb3+-codoped LaF3 nanocrystals. The phosphor was synthesized, and the light emission feature was examined under UV (375 and 385 nm) excitation. Emission around 484, 573, 663, and 754 nm due to dysprosium and 488, 545, 585, 620, and 690 nm owing to terbium ions was observed and analyzed as a function of the dysprosium and terbium contents and excitation wavelength. A dysprosium-terbium energy-transfer process was also observed and analyzed. The excitation spectrum was examined and showed resonances around 385 nm for the Dy (573 nm emission) and 375 nm for the Tb (540 nm emission). Results indicate that the polychromatic light-emitting phosphor Dy3+/Tb3+-codoped LaF3 nanocrystals herein reported generated light with a color tone tunable either by excitation wavelength or active ions content and suggest that it is a promising contender for LED-based illumination.

Circular dichroism (CD) spectra of complexes based on ZnS:Mn/ZnS and CdSe/ZnS quantum dots (QDs) and chlorin e6 (Ce6) molecules in dimethyl sulfoxide (DMSO) and in aqueous solutions at different pH levels were investigated. The changes in CD spectra of Ce6 upon its bonding in complex with semiconductor QDs were analyzed. CD spectroscopy allowed us to obtain the CD spectrum of a luminescent Ce6 dimer and to identify a nonluminescent Ce6 aggregate, which is thought to be a tetramer. The dissymmetry factor of the tetramer is 40 times larger than that of the Ce6 monomer. The analysis of the obtained data showed that in complexes with QDs Ce6 can be either in the monomer form or in the form of a nonluminescent tetramer. The interaction of the relatively unstable luminescent Ce6 dimer with QDs leads to its partial monomerization and the formation of complexes with Ce6 in the monomer form. On the basis of time-dependent density functional theory calculations, we performed a geometry model of Ce6 dimer form with corresponding calculated absorption and CD spectra, which are in a qualitative agreement with the experimental data.

Most safety problems of architectural structures are caused by structural deformation, and the structures usually deform in more than one direction. So it is important and necessary to collect the safety monitoring data from all directions. Conventional fiber Bragg grating (FBG) sensors cannot fully meet the requirements of a modern safety monitoring system in practical application. Therefore, the research of a three-dimensional (3-D) force sensor that can expand the application range of fiber optic sensing technology is necessary and significant. A 3-D force sensor based on multiplexed FBG strain sensors is proposed, which can be used to measure 3-D force on a structure under test, force distribution, and the trend of relative microdeformation. The sensor that has an integral structure with a design has been described in detail, and its sensing principle has been investigated. The results of calibration experiments show that it can accurately and effectively realize the 3-D force measurement with good linearity, repeatability, and consistency. Experimental and analytical results both demonstrate its feasibility. It can work in harsh environments due to its good stability and anti-interference ability. The sensor proposed in this paper has great engineering application value and application prospects in the field of structure health monitoring.

It is experimentally shown that silver-containing silicate photosensitive glasses can be used for multilevel optical information recording using UV nanosecond laser pulses. Information can be recorded by the creation of luminescent centers, which are neutral silver molecular clusters, or silver nanoparticles with plasmon resonance using additional thermal treatment. It is shown that silver nanoparticles formed in glass have spheroidal shapes and have dielectric shells. Multilevel optical information recording can be performed by modulation of luminescence or absorption intensity varying UV laser irradiation dose. The effect of dopants (Ce, Sb, and Cl) on the recording process is shown. The described methods allow to record optical information in octal and hexadecimal number systems.

Functional dyes allow optical applications of polymers in optical fiber technology. This paper presents a fabrication process and spectroscopic characterization of poly(methyl methacrylate) (PMMA) fiber doped by Spirooxazine (SO). The fabricated specimens at concentrations 0.4 to 0.8 wt. % have been characterized. The reversible absorption band with a maximum at 602 nm was observed under 365 nm exposition. The optical properties of PMMA-doped functional dye (photochromic and fluorescence) are presented in this paper. The kinetics of photochromic phenomenon is also investigated. The properties of polymeric fiber doped by SO are also presented since reabsorption effect changes the luminescence spectrum shape of SO.

A temperature sensor formed by a cascaded sphere and an abrupt taper, together in a standard single-mode fiber, was developed. The dip of the measured spectrum signal shifted obviously when the surrounding temperature changed. Measurement sensitivity to 18.36  pm/°C was shown with the surrounding temperature ranging from 35°C to 395°C. Due to its compact size and all-fiber configuration, the proposed sensor has the advantages of simplicity and low-cost fabrication, thus the device would find potential applications in sensing fields.

Applying flux conserving properties between the entrance and exit pupils of a lens system, as well as ideas concerning irradiance magnification in the low numerical aperture limit, it is shown that some basic radiometric equations may be recast into an expression for relative illumination that reveals effects from image distortion and a lesser known quantity called “differential distortion.” The expression presented offers a rapid means for estimating the effects of image distortion on relative illumination, which may be useful for basic analysis prior to applying the more comprehensive formulary for relative illumination that has been provided by Reshidko and Sasian.

This PDF file contains the errata for “OE Vol. 56 Issue 04 Paper OE-2017-0322-ERR” for OE Vol. 56 Issue 04