KEYWORDS: Scanners, LIDAR, Monte Carlo methods, Ranging, Optical engineering, Tolerancing, Unmanned vehicles, Data modeling, Laser scanners, 3D scanning
Signal interference between light detection and ranging (lidar) sensors has the potential to degrade range data integrity, which may challenge multilidar applications such as autonomous vehicles if not adequately considered. Our work proposes a methodology for evaluating lidar interference, analyzes distinct lidar interference phenomena, and validates a previously proposed model of interference between two circularly scanning, pulsed lidar sensors. The comparison between Monte Carlo simulated and experimentally observed interference events suggests that lidar interference may be inferred through geometrical approximations of lidar scanner arrangements. The experimental data quantify the occurrences of at least two modes of lidar interference—direct and scattered interference. When present, direct interference was found to occur one to two orders of magnitude greater in total occurrence than scattered interference. However, direct interference’s effects represented an average range error of <1 m. Alternatively, scattered interference was observed with an average range error two to four orders of magnitude greater than direct interference and proportional to the maximum range of the victim lidar. Further characterizations of interference phenomenon are presented that include the angular distribution of erroneous ranging data, range distributions of errors, loss of in-tolerance points, and potential radiometric influences.
Signal interference between two light detection and ranging (lidar) scanners can occur when the transmitted laser energy from one lidar is coupled into a second lidar either by scattering or direct transmission. By ray tracing lidar transmission paths, it is shown that signal interference can be modeled with the coincidence of intersections between two lidars’ optical axes and a scattering target or medium. A geometric analysis of intersecting transmission paths is presented for circularly scanning lidars from which theoretical interference risks between two lidars may be deduced. Using the geometric rules proposed here for interference, a Monte Carlo simulation is presented that models interference risks and occurrences. This geometric approximation simplifies analysis by ignoring radiometry for scanners within proximity to one another. Simulation results are used to introduce the concept of intersection point density from which one may assess areas of greatest interference risk for two-lidar, in-plane scanning lidar configurations. It is shown from the intersection point density that the risk for signal interference between two lidars exponentially increases as targets are located closer to and between the two scanners. Simulation results are compared to previously published experimental observations of interference to demonstrate the theory’s potential for interference modeling.
Signal interference between two light detection and ranging (lidar) scanners can occur when the transmitted laser energy from one lidar is scattered from a target and returned to a second lidar. By modeling lidar transmission paths via ray tracing, it is shown that signal interference can be modeled by the coincidence of intersection between two lidar transmission paths and a scattering target. The evaluation of experimental observation and an analytical framework of lidar signal interference is presented that compares results of a Monte Carlo simulation to interference observations from circularly scanning lidar sensors. The comparison between simulated and experimentally observed interference events suggests that lidar interference may largely explained by geometry and angular conditions. The model provides preliminary explanation as to the angular distribution of interference events and distinct transitions between occurrences of different interference modes. However, further radiometric refinement is likely needed to best explain the manifestation of some interference events.
A mathematical description of the intersection of a transmitted beam and a beam-like field of view between two stationary, coplanar, circularly scanning light detection and ranging (lidar) devices is presented. This beam intersection information establishes when scattered signal interference between two lidar devices can occur. Such interference may result in erroneous range data. The fraction of time over which the scanners’ beams intersect is analyzed as a function of rotational frequencies and their reference phases. It is shown that the minima and maxima of this fraction are 0 and 1 / 2, with most configurations resulting near 1 / 4. A procedure to adjust the scan rates and phases to minimize this intersection time (including zero intersection) thus reducing the risk of mutual interference is also outlined. Four configurations that produce zero intersection between two scanners are presented. Configurations of three and four scanners are also presented for which no intersections between the scanners’ transmission paths occur.
Pattern-integrated interference lithography (PIIL) combines multibeam interference lithography and projection lithography simultaneously to produce two-dimensional (2-D) and three-dimensional (3-D) periodic-lattice-based microstructures in a rapid, single-exposure step. Using a comprehensive PIIL vector model and realistic photolithographic conditions, PIIL exposures for a representative photonic-crystal (PhC) 90 deg bend waveguide are simulated in the volume of the photoresist film. The etched structures in the underlying substrate are estimated as well. Due to the imperfect integration of the photomask within the interference pattern, the interference pattern is locally distorted, thereby impacting the PhC periodic lattice and potentially the device performance. To mitigate these distortions, a photomask optimization method for PIIL is presented in this work. With an improved photomask, pillar-area and pillar-displacement errors in the vicinity of the waveguide are reduced by factors of 3.3 and 2.7, respectively. Furthermore, calculated transmission spectra show that the performance of the PIIL-produced PhC device is as good as that of its idealized equivalent.
Most quantitative phase microscopy methods require the use of custom-built or modified microscopic configurations which are not typically available to most bio/pathologists. There are, however, phase retrieval algorithms which utilize defocused bright-field images as input data and are therefore implementable in existing laboratory environments. Among these, deterministic methods such as those based on inverting the transport-of-intensity equation (TIE) or a phase contrast transfer function (PCTF) are particularly attractive due to their compatibility with Köhler illuminated systems and numerical simplicity. Recently, a new method has been proposed, called multi-filter phase imaging with partially coherent light (MFPI-PC), which alleviates the inherent noise/resolution trade-off in solving the TIE by utilizing a large number of defocused bright-field images spaced equally about the focal plane. Despite greatly improving the state-ofthe- art, the method has many shortcomings including the impracticality of high-speed acquisition, inefficient sampling, and attenuated response at high frequencies due to aperture effects. In this report, we present a new method, called bright-field quantitative phase microscopy (BFQPM), which efficiently utilizes a small number of defocused bright-field images and recovers frequencies out to the partially coherent diffraction limit. The method is based on a noiseminimized inversion of a PCTF derived for each finite defocus distance. We present simulation results which indicate nanoscale optical path length sensitivity and improved performance over MFPI-PC. We also provide experimental results imaging live bovine mesenchymal stem cells at sub-second temporal resolution. In all, BFQPM enables fast and accurate phase imaging with unprecedented spatial resolution using widely available bright-field microscopy hardware.
Multi-beam interference (MBI) represents a method of producing one-, two-, and three-dimensional submicron periodic optical-intensity distributions for applications including micro- and nano-electronics, photonic crystals, metamaterial, biomedical structures, optical trapping, and numerous other subwavelength structures. Accordingly, numerous optical configurations have been developed to implement MBI. However, these configurations typically provide limited ability to condition the key parameters of each interfering beam. Constraints on individual beam amplitudes and polarizations are systematically considered to understand their effects on lithographically useful MBI periodic patterning possibilities. A method for analyzing parametric constraints is presented and used to compare the optimized optical-intensity distributions for representative constrained systems. Case studies are presented for both square and hexagonal-lattices produced via three-beam interference. Results demonstrate that constraints on individual-beam polarizations significantly impact patterning possibilities and must be included in the systematic design of an MBI system.
Multimode interference (MMI) devices operating at high data rates are important for integrated optics and optical networks. Their 1×N splitting provides a basic functionality in these applications. Ultra-high speed data transmission at 40Gb/s per channel with a total bandwidth of 320Gb/s for all 8 output ports is demonstrated for the first time on a 1 × 8 photo-definable polymer-based MMI power splitter. The transmission integrity is confirmed by the bit-error-rate (BER) testing. To further determine the speed limitations of MMI devices, ultra-short pulse response of these devices is quantified. For example, for 20fs Gaussian input pulses into a 1×8 polymer-based MMI splitter, the output pulses are severely degraded in coupling efficiency (47%) and completely broken up in time and in space primarily due to inter-modal and intra-modal (waveguide) dispersions.
KEYWORDS: Digital video discs, Nano optics, Modulation, Near field, Near field optics, Optical storage, Polarization, Manufacturing, Data modeling, Performance modeling
The promise of near-field (NF) optics to increase the density in optical data storage, whether solid immersion lenses (SILs), apertures, or their variants, has not yet been fulfilled, largely because of the difficulties with the head-media interface at near-field dimensions, especially if a design goal is removable media. We introduce a new approach to NF optical storage, in which the near-field elements, in the form of cylindrical NF nano-optics (nOptics), are integral with the media. Combining the advantages of near- and far-field optical data storage, we obtain double the track density of DVD in removable rewritable phase change media. Potential for in-track density increase is also demonstrated. We briefly report on the modeling, manufacture, and performance of NF nOptic media.
A possible laser device is designed with the use of classically free quasibound electron states. An asymmetric semiconductor electron wave Fabry-Perot interference filter is designed with an upper electron state having much stronger confinement than the lower electron state. This structure also allows for direct current pumping of the upper state and rapid depletion of the lower state under the presence of a field. Spectroscopy experiments demonstrate the existence of the upper quasibound state in a test structure. This laser filter structure, designed for infrared gain with current pumping, is combined with a special injector filter for room temperature narrow energy current injection into the upper lasing state. A stack of 54 periods of this electrically pumped structure is placed within a waveguide geometry. A laser device is fabricated by etching mesa structures from 50 to 100 micrometers wide. End cleaved facets serve as reflectors for mesas from 2 to 5 mm long. Tests are performed on these devices to determine their electrical properties and suitability for lasing.
A review of the rigorous coupled-wave analysis (RCWA) as applied to the diffraction of electromagnetic waves by gratings is presented. The analysis is valid for any polarization, angle of incidence, and for conical diffraction. Cascaded and/or multiplexed gratings as well as material anisotropy and loss can be incorporated under the same formalism. Volume and surface-relief gratings can be analyzed. Convergence analysis is presented for rectangular-groove surface-relief dielectric and metallic gratings. The role of multilevel surface-relief and holographic gratings in the substrate-mode photonic interconnect configuration is investigated. Results obtained using the RCWA are presented for 1-, 2-, 4-, 8-, and 16-level surface-relief gratings and are compared with the predictions of the simple scalar model. Two practical configurations are analyzed: (a) a silicon substrate at a freespace wavelength of 1.3 microns and (b) a glass substrate at a freespace wavelength of 0.84 microns. Equivalent holographic gratings are also designed and compared. Small period rectangular groove gratings can also be modeled using approximately equivalent uniaxial homogeneous layers (effective media). The ordinary and extraordinary refractive indices of these layers depend on the grating filling factor, the refractive indices of the substrate and superstrate, and the ratio of the freespace wavelength to grating period. It is shown how these models result from the eigenvalue equation of the boundary-value rectangular-groove grating problem. Comparisons of the homogeneous effective medium approximations with the rigorous coupled-wave analysis are presented. Antireflection designs (single-layer or multilayer) using the effective medium models are presented and compared.
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