This PDF file contains the editorial “Editorials and Commentary” for OE Vol. 54 Issue 06

Bundle adjustment (BA) refines the estimation of three-dimensional (3-D) structures and viewing parameters in multiview 3-D reconstruction. It is crucial to the accuracy of the reconstruction. Generic bundle adjustment (GBA) fails to make full use of a priori knowledge of the structure of the objects. We propose a new constrained BA algorithm for 3-D reconstruction of a cylinder surface. In this algorithm, 3-D point positions are parameterized as a combination of the parameters of the cylinder and their two-dimensional locations on the cylinder surface. A random-sample-consensus-based fitting step is used to initialize the parameters of the cylinder surface from more than nine points. Parameters of cameras, the cylinder, and 3-D point locations are then refined by solving the problem of minimizing the reprojection error. Experiments based on synthesized data and real images are performed. Experiment results with synthesized data show that the proposed algorithm achieved a 50% lower reconstruction error than the GBA. Experiment results with real images show that the proposed algorithm is more accurate than the GBA approach when the number of matched point pairs is limited.

Enhancement of vehicular night vision thermal infrared images is an important problem in intelligent vehicles. We propose to create a colorful three-dimensional (3-D) display of infrared images for the vehicular night vision assistant driving system. We combine the plane parameter Markov random field (PP-MRF) model-based depth estimation with classification-based infrared image colorization to perform colored 3-D reconstruction of vehicular thermal infrared images. We first train the PP-MRF model to learn the relationship between superpixel features and plane parameters. The infrared images are then colorized and we perform superpixel segmentation and feature extraction on the colorized images. The PP-MRF model is used to estimate the superpixel plane parameter and to analyze the structure of the superpixels according to the characteristics of vehicular thermal infrared images. Finally, we estimate the depth of each pixel to perform 3-D reconstruction. Experimental results demonstrate that the proposed method can give a visually pleasing and daytime-like colorful 3-D display from a monochromatic vehicular thermal infrared image, which can help drivers to have a better understanding of the environment.

We present our recent studies on a set of three different type-II InAs/GaSb superlattice interband cascade infrared (IR) photodetectors. Electroluminescence and x-ray diffraction measurements suggest that all the grown structures had comparable material qualities. Two of these detectors were two- and three-stage structures with regular-illumination configurations and the other was a two-stage structure with a reverse-illumination configuration. The 100% cutoff wavelength for these detectors was 6.2  μm at 78 K, extending to 8  μm at 300 K. At T=125  K and higher temperatures, we were able to observe the benefits of the three-stage detector over the two-stage device in terms of lower dark current and higher detectivity. We conjecture that the imperfections from the device growth and fabrication had a substantial effect on the low-temperature device performance and were responsible for the unexpected behavior at these temperatures. We also found that the zero-bias photoresponse increased with temperatures up to 200 K, which was indicative of efficient collection of photogenerated carriers at high temperatures. These detectors were able to operate at temperatures up to 340 K with a cutoff wavelength longer than 8  μm. This demonstrates the advantage of the interband cascade structures to achieve high-temperature operation for long-wave IR photodetectors.

A kinematic description of a star spot in the focal plane is presented for star sensors under dynamical conditions, which involves all necessary parameters such as the image motion, velocity, and attitude parameters of the vehicle. Stars at different locations of the focal plane correspond to the slightly different orientation and extent of motion blur, which characterize the space-variant point spread function. Finally, the image motion, the energy distribution, and centroid extraction are numerically investigated using the kinematic model under dynamic conditions. A centroid error of eight successive iterations <0.002 pixel is used as the termination criterion for the Richardson-Lucy deconvolution algorithm. The kinematic model of a star sensor is useful for evaluating the compensation algorithms of motion-blurred images.

The microbolometric exact theoretical output responsivity expression for a focal plane array is obtained for the first time. Based on the physical thermal model for a microbolometer, the temperature change behavior of the microbolometer is analyzed and discussed to get its theoretical temperature change expression under pulse voltage bias. With the temperature change expression and infrared response analysis of the microbolometer, the theoretical output responsivity expression is successfully derived. The theoretical responsivity expression reveals some key factors that affect and determine the infrared sensibility of the microbolometric array, which can be utilized to guide the design optimization for a microbolometric focal plane array.

An efficient and low-latency pixel data transmission module for the adaptive optics wavefront processor is presented. A fiber-based custom real-time pixel data transfer protocol is developed, which has the characteristics of long transmission distance, low-latency, and low-protocol overhead. The hardware part of the suite has been verified on different circuit boards with different field-programmable gate arrays. Obtained results demonstrate that the transmission and protocol processing delay are only 413.5 ns, the transmission bandwidth is 3.125 Gbps, and the error rate is less than 10−¹². Under the same conditions of 156.25 MHz clock frequencies and length of transmission line, compared with the serial front panel data port protocol adopted by the thirty meter telescope and the european southern observatory, the transmission delay is significantly reduced by 2.82 times and has a remarkably low logic occupancy rate through our solution. Also, the method has been applied in several actual projects.

Human action recognition is an active and interesting research topic in computer vision and pattern recognition field that is widely used in the real world. We proposed an approach for human activity analysis based on motion energy template (MET), a new high-level representation of video. The main idea for the MET model is that human actions could be expressed as the composition of motion energy acquired in a three-dimensional (3-D) space-time volume by using a filter bank. The motion energies were directly computed from raw video sequences, thus some problems, such as object location and segmentation, etc., are definitely avoided. Another important competitive merit of this MET method is its insensitivity to gender, hair, and clothing. We extract MET features by using the Bhattacharyya coefficient to measure the motion energy similarity between the action template video and the tested video, and then the 3-D max-pooling. Using these features as input to the support vector machine, extensive experiments on two benchmark datasets, Weizmann and KTH, were carried out. Compared with other state-of-the-art approaches, such as variation energy image, dynamic templates and local motion pattern descriptors, the experimental results demonstrate that our MET model is competitive and promising.

An alignment method of a 2CCD camera to build pixel-to-pixel correspondence between the infrared (IR) CCD sensor and the visible CCD sensor by using the absolute phase data is presented. Vertical and horizontal sinusoidal fringe patterns are generated by software and displayed on a liquid crystal display screen. The displayed fringe patterns are captured simultaneously by the IR sensor and the visible sensor of the 2CCD camera. The absolute phase values of each pixel at IR and visible channels are calculated from the captured fringe pattern images by using Fourier transform and the optimum three-fringe number selection method. The accurate pixel corresponding relationship between the two sensors can be determined along the vertical and the horizontal directions by comparing the obtained absolute phase data in IR and visible channels. Experimental results show the high accuracy, effectiveness, and validity of the proposed 2CCD alignment method. By using the continuous absolute phase information, this method can determine the pixel-to-pixel correspondence with high resolution.

A differential interferometric technique for the measurement of surface roughness of plane optical surfaces is proposed. A polarization Sagnac interferometer, placed in the image space of a reflection microscope system with linearly polarized, spatially coherent, quasimonochromatic incident beam illumination, splits the image-forming beam from a plane test surface (TS) into laterally sheared (LS) linear orthogonal components. The LS components interfere when brought to the same state of polarization. The optical path difference (OPD) variation along the direction of lateral shear is a measure of the slope variation on the TS. Polarization phase shifting interferometry has been applied to extract the OPD variation and thus the slope distribution, slope errors, and, subsequently, height errors. System error contribution is eliminated by combining height error data obtained for parallel sections of the TS that lie along a section of the image field parallel to the direction of lateral shear, as the TS is shifted in a direction normal to the lateral shear direction between the data capture. Results obtained for an aluminized plane polished TS are presented.

In phase measuring deflectometry (PMD), the inspection accuracy of the defects and height of the specular surface are related to the level of phase errors. The usage of numeric integration in reconstructing the shape and the defocusing capture of the fringe pattern, which will amplify the phase errors, make error discussion more significant in PMD than other shape measurement techniques. Phase error analysis and reduction in PMD are presented. The random noises, nonlinear response function, the nontelecentric imaging of the charge-coupled device camera, and the nonlinear response function of the liquid crystal display screen are the main phase error sources in PMD. The analytical relation between the random phase error and its influence factors in PMD is deduced. From the relation formulation, the influence factors of random phase error are analyzed, and the results are proven by the simulation and experiment. A possible phase error-reduction method, which integrates several methods for congeneric errors in fringe projection profilometry, is investigated to reduce phase errors in PMD. This composite method is proven to have a good performance by a plane mirror experiment.

A high-quality ultrasmooth surface is demanded in short-wave optical systems. However, the existing polishing methods have difficulties meeting the requirement on spherical or aspheric surfaces. As a new kind of small tool polishing method, active feed polishing (AFP) could attain a surface roughness of less than 0.3 nm (RMS) on spherical elements, although AFP may magnify the residual figure error or mid-frequency error. The purpose of this work is to propose an effective algorithm to realize uniform removal of the surface in the processing. At first, the principle of the AFP and the mechanism of the polishing machine are introduced. In order to maintain the processed figure error, a variable pitch spiral path planning algorithm and the dwell time-solving model are proposed. For suppressing the possible mid-frequency error, the uniformity of the synthesis tool path, which is generated by an arbitrary point at the polishing tool bottom, is analyzed and evaluated, and the angular velocity ratio of the tool spinning motion to the revolution motion is optimized. Finally, an experiment is conducted on a convex spherical surface and an ultrasmooth surface is finally acquired. In conclusion, a high-quality ultrasmooth surface can be successfully obtained with little degradation of the figure and mid-frequency errors by the algorithm.

For a gas turbine blade working in a narrow space, the accuracy of blade temperature measurements is greatly impacted by environmental irradiation. A reflection model is established by using discrete irregular surfaces to calculate the angle factor between the blade surface and the hot adjacent parts. The model is based on the rotational angles and positions of the blades, and can correct for measurement error caused by background radiation when the blade is located at different rotational positions. This method reduces the impact of reflected radiation on the basis of the turbine’s known geometry and the physical properties of the material. The experimental results show that when the blade temperature is 911.2 ± 5 K  and the vane temperature ranges from 1011.3 to 1065.8 K, the error decreases from 4.21 to 0.75%.

With increasing concerns about light pollution and energy waste in general lighting, efficient and effective lighting techniques have become imperative. We present the design of a kind of dedicated oblique illumination (OI) system to solve the problems of light pollution and the waste of energy in general lighting. The OI system consists of an extended (actual) light source, a collimator, and a freeform reflector array. Two different layouts of the OI system are proposed for different lighting situations, and design models of the two layouts are also given by use of the Monge–Ampère equation method. Two typical lighting situations, street lighting and advertising board lighting, are given here to illustrate the proposed OI system, and characteristics of the OI system are explored. The results show the light distribution is well controlled with a pretty good optical performance and the OI system has a large tolerance to the change of intensity distribution of the light source and to the size of the light source.

A solid–liquid mixed tunable lens with multilayered structure is proposed. The designed lens utilizes a solid-state elastic polymer, optical liquid, and glass as the optical medium, and adjusts the focus by changing the surface curvature of the elastic polymer. The integrated structure of the tunable lens is presented, as well as detailed descriptions of the lens materials, fabrication, and assembling process. Images captured through the tunable lens under different displacement loads are presented, and the relationship among the displacement load, curvature radius, and effective focal length is analyzed. Additionally, the optical property of the tunable lens is simulated using the ZEMAX software. A change in focal length from 14.8 mm to 30 mm is demonstrated within the tiny 0.12 mm variation of the displacement load. Numerical analyses show that the lens distortion is less than 2%, and the modulation transfer function reaches 67 line pairs per mm. The solid–liquid mixed tunable lens shows the potential for developing a compact, low-aberration, and stable optical system.

An all-fiber sensor for simultaneous measurement of temperature and microdisplacement is presented and demonstrated. The sensor head is fabricated by a peanut structure Mach–Zehnder interferometer (MZI) cascaded with a fiber Bragg grating (FBG). Experimental results show that the temperature sensitivities of the MZI (dip1) and the FBG (dip_FBG) are 0.0909 and 0.0121  nm/°C, respectively. The microdisplacement sensitivities are −0.0233  and 0.0122  nm/μm, respectively. The simultaneous measurement of the temperature and microdisplacement is demonstrated based on the sensitive matrix. With the advantages of low cost and easy fabrication, this sensor has potential applications in security, construction, and energy.

We design a seven-core photonic crystal fiber with specifically designed dispersion and group velocity profile which is optimized for high-power visible supercontinuum (SC) generation pumped by ∼1-μm pulsed lasers. The fiber has both a large air-filling fraction and a large effective mode field area. Additionally, the in-phase supermode of this fiber exhibits an even field distribution after mode modification. The simulation results suggest that it has a great potential to generate a high-power SC extending to 400 nm, which is highly desirable in biological applications.

Multiple-input single-output systems are employed in free-space optical links to mitigate the degrading effects of atmospheric turbulence. We formulate the power scintillation as a function of transmitter and receiver coordinates in the presence of weak atmospheric turbulence by using the extended Huygens–Fresnel principle. Then the effect of the receiver–aperture averaging is quantified. To get consistent results, parameters are chosen within the range of validity of the wave structure functions. Radial array beams and a Gaussian weighting aperture function are used at the transmitter and the receiver, respectively. It is observed that the power scintillation decreases when the source size, the ring radius, the receiver–aperture radius, and the number of array beamlet increase. However, increasing the number of array beamlets to more than three seems to have negligible effect on the power scintillation. It is further observed that the aperture averaging effect is stronger when radial array beams are employed instead of a single Gaussian beam.

A configuration based on phase difference on a double-sideband pump wave is proposed to detect the differential variation of temperature or strain in single-mode optical fibers. In our configuration, a probe wave only experiences a differential Brillouin gain contributed by the perturbation of temperature or strain in the sensing fiber. As a result, the power limitation of the probe wave can be alleviated and the photodetector in our configuration does not easily become saturated in the case of a longer sensing range. The spatial resolution is determined by the duration of the phase difference on the two sidebands and the signal-to-noise of our system is nearly twice as high as that of a differential pulse-width pair Brillouin optical time domain analysis sensor since a π-phase shift on the pump wave is employed. The properties and performances of our method are also theoretically derived and experimentally validated.

A possible means of optical sensing, based on a porous hyperbolic material that is infiltrated by a fluid containing an analyte to be sensed, was theoretically investigated. The sensing mechanism relies on the observation that extraordinary plane waves propagate in the infiltrated hyperbolic material only in directions enclosed by a cone aligned with the optic axis of the infiltrated hyperbolic material. The angle this cone subtends to the plane perpendicular to the optic axis is θ_c. The sensitivity of θ_c to changes in the refractive index of the infiltrating fluid, namely n_b, was explored; also considered were the permittivity parameters and porosity of the hyperbolic material, as well as the shape and size of its pores. Sensitivity was gauged by the derivative dθ_c/dn_b. In parametric numerical studies, values of dθ_c/dn_b in excess of 500 deg per refractive index unit were computed, depending upon the constitutive parameters of the porous hyperbolic material and infiltrating fluid and the nature of the porosity. In particular, it was observed that exceeding large values of dθ_c/dn_b could be attained as the negative-valued eigenvalue of the infiltrated hyperbolic material approached zero.

An open-path spectrometer for fast spatial detection and identification of gaseous plumes in a realistic environmental conditions is presented. Gases are released in a 500  m³ hall; detection and identification is performed by spectroscopic means—measuring the light spectral absorption (at 8 to 10  μm) by shining an external-cavity quantum cascade laser beam through the inspected volume. Real-time identification is demonstrated for gas plumes of CH₂FCF₃ (R134a) and CHF₃ at a distance of 30 m round trip with a minimum identification level of 0.2 ppm (response times of 2 to 10 s). The relatively wide spectral coverage allows a high probability of detection (PD) and low probability for a false alarm to be obtained in these realistic conditions. It is also demonstrated that the use of several lines-of-sight improves PD as gas spreading in the hall in these conditions is slow and unpredictable.

The main objective of this study is to optimize an optical component considered as an essential building block in wavelength division multiplexing applications. The work presented here focuses on the design of an optimum 1×8 compact splitter based on a two-dimensional (2-D) photonic crystal (PhC) in triangular unit cells exhibiting high transmission. It generates a contribution to the 2-D planar PhCs in the integrated optics field. These new materials may prohibit the propagation of light in certain directions and energies. We also optimize the splitter topologies in order to integrate them in optoelectronic systems as division components. To do so, the 2-D finite-difference time-domain method is employed to characterize the transmission properties. Simulation results show that total transmissions of about 86%, 78%, and 86% for the 1×2, 1×4, and 1×8 Y splitters, respectively, at output ports are obtained around the wavelength 1.55  μm widely used in optical telecommunications. It is demonstrated numerically that the corresponding total insertion losses for the three splitters are, respectively, about 0.65, 1.08, and 0.65 dB. The simulation results are presented and discussed.

The effect of AlN addition on the phase structures and luminescence properties of SrAl₂O₄:Eu²⁺,Dy³⁺ phosphor has been discussed. SrAl_2+xO₄N_x:0.02Eu²⁺,0.03Dy³⁺ (x=0 to 0.3) phosphors have been synthesized by a solid-state reaction. X-ray powder diffraction shows that the phase structures of as-prepared SrAl_2+xO₄N_x:0.02Eu²⁺,0.03Dy³⁺ remain invariable with the SrAl₂O₄ phase when the content of AlN is below 0.3. With the addition of AlN, photoluminescence intensities of SrAl_2+xO₄N_x:0.02Eu²⁺,0.03Dy³⁺ phosphors decrease with increasing x; however, the afterglow phosphorescence intensities obviously increased. The mechanism of the enhanced phosphorescence properties was also discussed.

Optical planar waveguides in Nd³+-doped phosphate glasses are fabricated by a 6.0-MeV carbon ion implantation with a dose of 6.0×10¹⁴ ions/cm² and a 6.0-MeV oxygen ion implantation at a fluence of 6.0×10¹⁴ ions/cm², respectively. The guided modes and the corresponding effective refractive indices were measured by a modal 2010 prism coupler. The refractive index profiles of the waveguides were analyzed based on the stopping and range of ions in matter and the RCM reflectivity calculation method. The near-field light intensity distributions were measured and simulated by an end-face coupling method and a finite-difference beam propagation method, respectively. The comparison of optical properties between the carbon-implanted waveguide and the oxygen-implanted waveguide was carried out. The microluminescence and Raman spectroscopy investigations reveal that fluorescent properties of Nd³+ ions and glass microstructure are well preserved in the waveguide region, which suggests that the carbon/oxygen-implanted waveguide is a good candidate for integrated photonic devices.

We propose a new approach for efficient detection of terahertz (THz) radiation in biomedical imaging applications. A double-layered absorber consisting of a 32-nm-thick aluminum (Al) metallic layer, located on a glass medium (SiO₂) of 1 mm thickness, was fabricated and used to design a fine-tuned absorber through a theoretical and finite element modeling process. The results indicate that the proposed low-cost, double-layered absorber can be tuned based on the metal layer sheet resistance and the thickness of various glass media. This can be done in a way that takes advantage of the diversity of the absorption of the metal films in the desired THz domain (6 to 10 THz). It was found that the composite absorber could absorb up to 86% (a percentage exceeding the 50%, previously shown to be the highest achievable when using single thin metal layer) and reflect <1% of the incident THz power. This approach will enable monitoring of the transmission coefficient (THz transmission fingerprint) of the biosample with high accuracy, while also making the proposed double-layered absorber a good candidate for a microbolometer pixel’s active element.

This study presents an alternative approach for the nondestructive assessment of fruit quality parameters with the use of a simplified optical fiber red–green–blue system (OF-RGB). The optical sensor system presented in this work is designed to rapidly measure the firmness, acidity, and soluble solid content of an intact Sala mango on the basis of color properties. The system consists of three light-emitting diodes with peak emission at 635 (red), 525 (green), and 470 nm (blue), as well as a single photodetector capable of sensing visible light. The measurements were conducted using the reflectance technique. The analyses were conducted by comparing the results obtained through the proposed system with those measured using two commercial spectrometers, namely, QE65000 and FieldSpec 3. The developed RGB system showed satisfactory accuracy in the measurement of acidity (R²=0.795) and firmness (R²=0.761), but a relatively lower accuracy in the measurement of soluble solid content (R²=0.593) of intact mangoes. The results obtained through OF-RGB are comparable with those measured by QE65000 and FieldSpec 3. This system is a promising new technology with rapid response, easy operation, and low cost with potential applications in the nondestructive assessment of quality attributes.

Dyakonov–Tamm (DT) waves are highly sensitive to the constitutive properties of the partnering materials near the interface. DT waves are excited at the interface of two dielectric materials of which at least one is anisotropic and periodically nonhomogeneous normal to their interface. Sculptured nematic thin film (SNTF) is a good candidate for the periodically nonhomogeneous dielectric partner for optical sensing of a fluid due to its porosity. The nanoscale parameters of an uninfiltrated SNTF obtained from the inverse Bruggeman homogenization formalism were used in the forward Bruggeman homogenization formalism to determine the constitutive parameters for the infiltrated SNTF. The sensitivity of DT waves to the refractive index was analyzed for two possible sensing modalities and it was found that the sensitivity was comparable to that of the chiral sculptured thin films (STFs) made of the same material as of the SNTF. This implies that the sensing with DT waves is robust, is independent of the morphology of the partnering nonhomogeneous dielectric material and could make the sensing easier since SNTFs are easier to fabricate than the chiral STFs.

Gold–silver bimetallic nanoparticles with broad plasmon response were fabricated on soft substrates using a laser-induced transfer technique. The bimetallic nanostructures were fabricated with centimeter scale. By careful design, an approximately 200-nm broad plasmon response can be obtained by gold–silver alloy nanoparticles, which can be attributed to the electromagnetic interaction between gold and silver nanoparticles. This nanofabrication technique provides an annealing-free approach for the fabrication of flexible bimetallic nanostructures with a broad plasmon response with low cost.

A monolithically integrated optical receiver in 0.6  μm bipolar complementary metal oxide semiconductor (BiCMOS) technology with 45 channels, each working at a data rate of 3.125  Gbit/s, is described. The optical receiver uses integrated pin photodiodes with a diameter of 90  μm. This parallel optical silicon receiver is capable of operating at 100°C. The parallel optical receiver consumes less than 950 mW in total and each of its channels achieve an optical sensitivity of −17.5  dBm at 850 nm wavelength and a bit error ratio of 10−⁹.

We analytically investigated the influence of grating shape on polarization characteristics of the emission from a GaN-based light-emitting diode with a low-contrast subwavelength grating (SWG), such as SiO₂-SWG. The electromagnetic field distribution, calculated using the finite-difference time-domain method, predicted that the polarization characteristics strongly depend on the grating side slope. A trapezoid SiO₂-SWG was fabricated on the GaN-based-LED using electron-beam lithography. The optical characteristics of the electroluminescence agreed with those theoretically predicted, and we succeeded in demonstrating the influence of grating shape on the polarization of LED emission.

We report on Y-branch superficial depressed-cladding waveguides fabricated by femtosecond laser writing of MgO:LiTaO₃ crystal. The cladding waveguides with a rectangular cross-section are single mode for both transverse electric and transverse magnetic polarization, and show good transmission properties at a telecommunication wavelength of 1.55  μm. Divergence angles as large as 2.6 deg are successfully achieved in the splitters with nearly equalized splitting ratios (1:1). The fabricated shallow structures are excellent photonic elements for optoelectronic applications.