scispace - formally typeset
Search or ask a question

Showing papers in "Applied Optics in 2011"


Journal ArticleDOI
TL;DR: Up-to-date see-through 3D display is a concept close to the ultimate goal of presenting seamless virtual images, although it is still far from practical use, many efforts have been made to resolve issues such as occlusion problems.
Abstract: Recent trends in three-dimensional (3D) display technologies are very interesting in that both old-fashioned and up-to-date technologies are being actively investigated together. The release of the first commercially successful 3D display product raised new research topics in stereoscopic display. Autostereoscopic display renders a ray field of a 3D image, whereas holography replicates a wave field of it. Many investigations have been conducted on the next candidates for commercial products to resolve existing limitations. Up-to-date see-through 3D display is a concept close to the ultimate goal of presenting seamless virtual images. Although it is still far from practical use, many efforts have been made to resolve issues such as occlusion problems.

331 citations


Journal ArticleDOI
TL;DR: A detailed comparison of classical, two-section, and stack-chain guiding strategies are chosen for comparison and the classical guiding strategy with a data structure of indexed interwoven linked list is the one to use.
Abstract: Quality-guided phase unwrapping is a widely used technique with different quality definitions and guiding strategies reported. It is thus necessary to do a detailed comparison of these approaches to choose the optimal quality map and guiding strategy. For quality maps, in the presence of noise, transform-based methods are found to be the best choice. However in the presence of discontinuities, phase unwrapping is itself unresolved and hence quality-guided phase unwrapping is not sufficient. For guiding strategies, classical, two-section, and stack-chain guiding strategies are chosen for comparison. If accuracy is the foremost criterion then the classical guiding strategy with a data structure of indexed interwoven linked list is best. If speed is of essence then the stack-chain guiding strategy is the one to use.

245 citations


Journal ArticleDOI
TL;DR: The use of high-intensity femtosecond pulses for flow tagging allows the accurate determination of velocity profiles with a single laser system and camera.
Abstract: Time-accurate velocity measurements in unseeded air are made by tagging nitrogen with a femtosecond-duration laser pulse and monitoring the displacement of the molecules with a time-delayed, fast-gated camera. Centimeter-long lines are written through the focal region of a ∼1 mJ, 810 nm laser and are produced by nonlinear excitation and dissociation of nitrogen. Negligible heating is associated with this interaction. The emission arises from recombining nitrogen atoms and lasts for tens of microseconds in natural air. It falls into the 560 to 660 nm spectral region and consists of multiple spectral lines associated with first positive nitrogen transitions. The feasibility of this concept is demonstrated with lines written across a free jet, yielding instantaneous and averaged velocity profiles. The use of high-intensity femtosecond pulses for flow tagging allows the accurate determination of velocity profiles with a single laser system and camera.

176 citations


Journal ArticleDOI
Zhu Zheng1, Baifu Zhang1, Hao Chen1, Jianping Ding1, Hui-Tian Wang1 
TL;DR: Experiment on optical tweezers demonstrates that the focused Airy beams can create multiple traps for two-dimensional confining particles, and the stable traps exist in the vicinity of the main intensity lobes in the focused beams.
Abstract: Airy beams are attractive owing to their two intriguing properties—self-bending and nondiffraction—that are particularly helpful for optical manipulation of particles. We perform theoretical and experimental investigations into the focusing property of Airy beams and provide insight into the trapping ability of tightly focused Airy beams. Experiment on optical tweezers demonstrates that the focused Airy beams can create multiple traps for two-dimensional confining particles, and the stable traps exist in the vicinity of the main intensity lobes in the focused beams. The trapping pattern can be varied with changes in the cross section of the focused beam. The focused Airy beam offers a novel way of optically manipulating microparticles.

161 citations


Journal ArticleDOI
TL;DR: The HICO is an innovative prototype sensor that builds on extensive experience with airborne sensors and makes extensive use of commercial off-the-shelf components to build a space sensor at a small fraction of the usual cost and time.
Abstract: The Hyperspectral Imager for the Coastal Ocean (HICO) is the first spaceborne hyperspectral sensor designed specifically for the coastal ocean and estuarial, riverine, or other shallow-water areas The HICO generates hyperspectral images, primarily over the 400-900 nm spectral range, with a ground sample distance of ≈90 m (at nadir) and a high signal-to-noise ratio The HICO is now operating on the International Space Station (ISS) Its cross-track and along-track fields of view are 42 km (at nadir) and 192 km, respectively, for a total scene area of 8000 km(2) The HICO is an innovative prototype sensor that builds on extensive experience with airborne sensors and makes extensive use of commercial off-the-shelf components to build a space sensor at a small fraction of the usual cost and time Here we describe the instrument's design and characterization and present early images from the ISS

158 citations


Journal ArticleDOI
TL;DR: An expression for the dark-field effect in terms of the sample material's complex refractive index is obtained, which can be verified experimentally without fitting parameters.
Abstract: In grating-based x-ray phase sensitive imaging, dark-field contrast refers to the extinction of the interference fringes due to small-angle scattering. For configurations where the sample is placed before the beamsplitter grating, the dark-field contrast has been quantified with theoretical wave propagation models. Yet when the grating is placed before the sample, the dark-field contrast has only been modeled in the geometric optics regime. Here we attempt to quantify the dark-field effect in the grating-before-sample geometry with first-principle wave calculations and understand the associated particle-size selectivity. We obtain an expression for the dark-field effect in terms of the sample material’s complex refractive index, which can be verified experimentally without fitting parameters. A dark-field computed tomography experiment shows that the particle-size selectivity can be used to differentiate materials of identical x-ray absorption.

157 citations


Journal ArticleDOI
TL;DR: A photon-counting, single-pixel, laser radar camera for 3D imaging where transverse spatial resolution is obtained through compressive sensing without scanning is experimentally demonstrated.
Abstract: We experimentally demonstrate a photon-counting, single-pixel, laser radar camera for 3D imaging where transverse spatial resolution is obtained through compressive sensing without scanning. We use this technique to image through partially obscuring objects, such as camouflage netting. Our implementation improves upon pixel-array based designs with a compact, resource-efficient design and highly scalable resolution.

147 citations


Journal ArticleDOI
TL;DR: Experimental results predict a redshift in the resonance wavelength with the increase in the refractive index of the sensing layer for a given thickness of the silicon layer and the experimental results obtained on sensitivity match qualitatively with the theoretical results obtained using the N-layer model and the ray approach.
Abstract: We have experimentally studied the surface plasmon resonance (SPR)-based fiber-optic refractive index sensor incorporating a high-index dielectric layer using the wavelength interrogation method. Silver and gold have been used as SPR active metals followed by a high-index dielectric layer of silicon. Experimental results predict a redshift in the resonance wavelength with the increase in the refractive index of the sensing layer for a given thickness of the silicon layer. Further, as the thickness of the silicon layer increases, the sensitivity of the sensor increases. The upper limit of the silicon film thickness for the enhancement of the sensitivity has been found to be around 10 nm. The experimental results obtained on sensitivity match qualitatively with the theoretical results obtained using the N-layer model and the ray approach. The increase in sensitivity is due to the increase in the electric field intensity at the silicon-sensing-region interface. In addition to an increase in sensitivity, the silicon layer can be used to tune the resonance wavelength and can protect the metal layer from oxidation and hence can improve the durability of the probe.

146 citations


Journal ArticleDOI
Di Wu1, Tao Zhu1, Ming Deng1, Dewen Duan1, Leilei Shi1, Jun Yao1, Yun-Jiang Rao1 
TL;DR: A highly sensitive refractive index (RI) sensor based on three cascaded single-mode fiber tapers, in which a weak taper is sandwiched between the two tapers to improve the sensitivity of the sensor.
Abstract: We report a highly sensitive refractive index (RI) sensor based on three cascaded single-mode fiber tapers, in which a weak taper is sandwiched between the two tapers to improve the sensitivity of the sensor. Experimental results show that the sensitivity of the device is 0.286 nm for a 0.01 RI change, which is about four times higher than that of the normal two-cascaded-taper-based Mach–Zehnder interferometer. In addition, the sensitivity of the device could be enhanced by tapering a longer and thinner middle weak taper. Such kinds of low-cost and highly sensitive fiber-optic RI sensors would find applications in chemical or biochemical sensing fields.

144 citations


Journal ArticleDOI
TL;DR: This work proposes a noniterative method using the jigsaw transformation (JT) in a single step, with improved security because the inverse JT of these masks, along with correct permutation keys that are necessary to decrypt the original image are provided.
Abstract: Interference-based optical encryption schemes have an inherent silhouette problem due to the equipollent nature of the phase-only masks (POMs) generated using an analytical method. One of the earlier methods suggested that removing the problem by use of exchanging process between two masks increases the computational load. This shortcoming is overcome with a noniterative method using the jigsaw transformation (JT) in a single step, with improved security because the inverse JT of these masks, along with correct permutation keys that are necessary to decrypt the original image. The stringent alignment requirement of the POMs in two different arms during the experiment is removed with an alternative method using a single spatial light modulator. Experimental results are provided to demonstrate the decryption process with the proposed method.

137 citations


Journal ArticleDOI
TL;DR: Airborne measurements of CO(2) column abundance conducted during two 2009 campaigns using a 2.05 μm laser absorption spectrometer provide insight into the capabilities of the IPDA method for both airborne measurements and future global-scale CO( 2) measurements from low Earth orbit pertinent to the NASA Active Sensing of CO (2) Emissions over Nights, Days, and Seasons mission.
Abstract: We report airborne measurements of CO2 column abundance conducted during two 2009 campaigns using a 2.05 μm laser absorption spectrometer. The two flight campaigns took place in the California Mojave desert and in Oklahoma. The integrated path differential absorption (IPDA) method is used for the CO2 column mixing ratio retrievals. This instrument and the data analysis methodology provide insight into the capabilities of the IPDA method for both airborne measurements and future global-scale CO2 measurements from low Earth orbit pertinent to the NASA Active Sensing of CO2 Emissions over Nights, Days, and Seasons mission. The use of a favorable absorption line in the CO22 μm band allows the on-line frequency to be displaced two (surface pressure) half-widths from line center, providing high sensitivity to the lower tropospheric CO2. The measurement repeatability and measurement precision are in good agreement with predicted estimates. We also report comparisons with airborne in situ measurements conducted during the Oklahoma campaign.

Journal ArticleDOI
TL;DR: The influence of experimental parameters on the classical hologram reconstruction methods is assessed, offering guidelines for optimal image rendering regarding the hologram recording conditions.
Abstract: Holographic rendering of off-axis intensity digital holograms is discussed. A review of some of the main numerical processing methods, based either on the Fourier transform interpretation of the propagation integral or on its linear system counterpart, is reported. Less common methods such as adjustable magnification reconstruction schemes and Fresnelet decomposition are presented and applied to the digital treatment of off-axis holograms. The influence of experimental parameters on the classical hologram reconstruction methods is assessed, offering guidelines for optimal image rendering regarding the hologram recording conditions.

Journal ArticleDOI
TL;DR: A white-light snapshot channeled linear imaging (CLI) polarimeter is demonstrated by utilizing polarization gratings (PGs) to enable polarimetric image data to be acquired at shorter exposure times in daylight conditions, making it more appealing over the quasi-monochromatic channeled imaging polarimeters previously described in the literature.
Abstract: A white-light snapshot channeled linear imaging (CLI) polarimeter is demonstrated by utilizing polarization gratings (PGs). The CLI polarimeter is capable of measuring the two-dimensional distribution of the linear Stokes polarization parameters by incorporating two identical PGs, in series, along the optical axis. In this configuration, the general optical shearing functionality of a uniaxial crystal-based Savart plate is realized. However, unlike a Savart plate, the diffractive nature of the PGs creates a linear dependence of the shear versus wavelength, thus providing broadband functionality. Consequently, by incorporating the PG-based Savart plate into a Savart plate channeled imaging polarimeter, white-light interference fringes can be generated. This enables polarimetric image data to be acquired at shorter exposure times in daylight conditions, making it more appealing over the quasi-monochromatic channeled imaging polarimeters previously described in the literature. Furthermore, the PG-based device offers significantly more compactness, field of view, optical simplicity, and vibration insensitivity than previously described white-light CLI polarimeters based on Sagnac interferometers. Included in this paper are theoretical descriptions of the linear (S0, S1, and S2) and complete (S0, S1, S2, and S3) channeled Stokes imaging polarimeters. Additionally, descriptions of our calibration procedures and our experimental proof of concept CLI system are provided. These are followed by laboratory and outdoor polarimetric measurements of S0, S1, and S2.

Journal ArticleDOI
TL;DR: Combining the new algorithms for estimating the laser phase and updating C value, the paper finally presents a novel technique for displacement measurement with improved accuracy performance in contrast to existing techniques.
Abstract: Approaches that are, to our knowledge, novel, are proposed in this paper to improve the accuracy performance of self-mixing interferometry (SMI) for displacement measurement First, the characteristics associated with signals observed in SMI systems are studied, based on which a new procedure is proposed for achieving accurate estimation of the laser phase The studies also revealed the reasons for the inherent errors associated with the existing SMI-based techniques for displacement measurement Then, this paper presents a new method for estimating the optical feedback level factor (denoted by C) in real time Combining the new algorithms for estimating the laser phase and updating C value, the paper finally presents a novel technique for displacement measurement with improved accuracy performance in contrast to existing techniques The proposed technique is verified by both simulation and experimental data

Journal ArticleDOI
TL;DR: This research experimentally studied a large depth range of defocused fringe patterns, from near-binary to near-sinusoidal, and analyzed the associated phase errors and established a mathematical phase error function in terms of the wrapped phase and the depth z.
Abstract: This paper analyzes the phase error for a three-dimensional (3D) shape measurement system that utilizes our recently proposed projector defocusing technique. This technique generates seemingly sinusoidal structured patterns by defocusing binary structured patterns and then uses these patterns to perform 3D shape measurement by fringe analysis. However, significant errors may still exist if an object is within a certain depth range, where the defocused fringe patterns retain binary structure. In this research, we experimentally studied a large depth range of defocused fringe patterns, from near-binary to near-sinusoidal, and analyzed the associated phase errors. We established a mathematical phase error function in terms of the wrapped phase and the depth z. Finally, we calibrated and used the mathematical function to compensate for the phase error at arbitrary depth ranges within the calibration volume. Experimental results will be presented to demonstrate the success of this proposed technique.

Journal ArticleDOI
TL;DR: Nitric oxide planar laser-induced fluorescence (NO PLIF) imaging at repetition rates as high as 1 MHz is demonstrated in the NASA Langley 31 in.
Abstract: Nitric oxide planar laser-induced fluorescence (NO PLIF) imaging at repetition rates as high as 1MHz is demonstrated in the NASA Langley 31 in. Mach 10 hypersonic wind tunnel. Approximately 200 time-correlated image sequences of between 10 and 20 individual frames were obtained over eight days of wind tunnel testing spanning two entries in March and September of 2009. The image sequences presented were obtained from the boundary layer of a 20° flat plate model, in which transition was induced using a variety of different shaped protuberances, including a cylinder and a triangle. The high-speed image sequences captured a variety of laminar and transitional flow phenomena, ranging from mostly laminar flow, typically at a lower Reynolds number and/or in the near wall region of the model, to highly transitional flow in which the temporal evolution and progression of characteristic streak instabilities and/or corkscrew-shaped vortices could be clearly identified.

Journal ArticleDOI
TL;DR: The proposed scheme can provide a real-time solution for measuring three-dimensional objects having a high abrupt height difference with moderate accuracy and can be used as a fast polarization imaging measurement tool.
Abstract: We describe what we believe to be a new digital holographic configuration that can be utilized for both single-shot, dual-wavelength, off-axis geometry and imaging polarimetry. To get the feasibility of the single-shot, dual-wavelength, off-axis geometry, a sample with a nominal step height of 1.34 μm is used. Undesirable noises that strongly affect the measurement have been suppressed successfully by using a modified flat fielding method for the dual-wavelength scheme. And also, the experiment is conducted on a nanopattern sample on the basis of a single image acquisition to show the imaging polarimetry capability. The proposed scheme can provide a real-time solution for measuring three-dimensional objects having a high abrupt height difference with moderate accuracy. Furthermore, it can be used as a fast polarization imaging measurement tool.

Journal ArticleDOI
TL;DR: A ternary nonmechanical beam steering device based on polarization gratings that allows for a unique three-way (ternary) steering design with substantially high throughput is introduced and demonstrated.
Abstract: We introduce and demonstrate a ternary nonmechanical beam steering device based on polarization gratings (PGs). Our beam steering device employs multiple stages consisting of combinations of PGs and wave plates, which allows for a unique three-way (ternary) steering design. Ultrahigh efficiency (∼100%) and polarization sensitive diffraction of individual PGs allow wide steering angles (among three diffracted orders) with extremely high throughput. We report our successful demonstration of the three-stage beam steerer having a 44° field of regard with 1.7° resolution at 1550 nm wavelength. A substantially high throughput of 78%–83% is observed that is mainly limited by electrode absorption and Fresnel losses.

Journal ArticleDOI
TL;DR: A load for surface indentation by abrasive particles is provided at their interaction near the wall with heavier basic (magnetic) particles, which fluctuate (due to collision) in the shear flow of concentrated suspension.
Abstract: A concept of material removal based on the principle of conservation of particles momentum in a binary suspension is applied to analyze material removal in magnetorheological finishing and magnetorheological jet processes widely used in precision optics fabrication. According to this concept, a load for surface indentation by abrasive particles is provided at their interaction near the wall with heavier basic (magnetic) particles, which fluctuate (due to collision) in the shear flow of concentrated suspension. The model is in good qualitative and quantitative agreement with experimental results.

Journal ArticleDOI
TL;DR: This IOP-centered system can be used to derive IOPs from angular Rrs and offers an alternative to the system centered on the concentration of chlorophyll and is applicable to oceanic and coastal waters as well as to multiband and hyperspectral sensors.
Abstract: Remote-sensing reflectance (R(rs)), which is defined as the ratio of water-leaving radiance (L(w)) to downwelling irradiance just above the surface (E(d)(0⁺)), varies with both water constituents (including bottom properties of optically-shallow waters) and angular geometry. L(w) is commonly measured in the field or by satellite sensors at convenient angles, while E(d)(0⁺) can be measured in the field or estimated based on atmospheric properties. To isolate the variations of R(rs) (or L(w)) resulting from a change of water constituents, the angular effects of R(rs) (or L(w)) need to be removed. This is also a necessity for the calibration and validation of satellite ocean color measurements. To reach this objective, for optically-deep waters where bottom contribution is negligible, we present a system centered on water's inherent optical properties (IOPs). It can be used to derive IOPs from angular Rrs and offers an alternative to the system centered on the concentration of chlorophyll. This system is applicable to oceanic and coastal waters as well as to multiband and hyperspectral sensors. This IOP-centered system is applied to both numerically simulated data and in situ measurements to test and evaluate its performance. The good results obtained suggest that the system can be applied to angular R(rs) to retrieve IOPs and to remove the angular variation of R(rs).

Journal ArticleDOI
TL;DR: A distributed fiber optic vibration sensor is described, in which two Michelson interferometers are used as phase detectors and two 3×3 couplers are deployed to demodulate the time-varying phase change caused by vibration.
Abstract: A distributed fiber optic vibration sensor is described, in which two Michelson interferometers are used as phase detectors and two 3×3 couplers are deployed to demodulate the time-varying phase change caused by vibration. The two interferometers are separated by four wavelength division multiplexers. The position of the vibration is obtained by signal correlation, which can be used as a perimeter security sensor to locate the intruder. The experimental results with a 4012 m fiber sensor are discussed.

Journal ArticleDOI
TL;DR: A novel optical method to display a complex Fresnel hologram using a single spatial light modulator (SLM) and the degree of freedom of the synthesized complex hologram display can be extended by involving more than three amplitude holograms.
Abstract: We propose a novel optical method to display a complex Fresnel hologram using a single spatial light modulator (SLM). The method consists of a standard coherent image processing system with a sinusoidal grating at the Fourier plane. Two or three position-shifted amplitude holograms displayed at the input plane of the processing system can be coupled via the grating and will be precisely overlapped at the system's output plane. As a result, we can synthesize a complex hologram that is free of the twin image and the zero-order light using a single SLM. Because the twin image is not removed via filtering, the full bandwidth of the SLM can be utilized for displaying on-axis holograms. In addition, the degree of freedom of the synthesized complex hologram display can be extended by involving more than three amplitude holograms.

Journal ArticleDOI
TL;DR: Stimulated Brillouin scattering (SBS) has become a favorable underlying mechanism in many demonstrations of all-optical variable delay in standard fibers, often referred to as slow and fast light as discussed by the authors.
Abstract: Stimulated Brillouin scattering (SBS) has become a favorable underlying mechanism in many demonstrations of all-optical variable delay in standard fibers, often referred to as slow and fast light. Over 100 journal papers and numerous conference sessions have been dedicated to SBS slow light since 2005. In this paper, recent research in this area is reviewed. Following a short introduction to the topic, several specific trends in contemporary work are highlighted: the optimization of the SBS pump spectrum for extended slow light delay and reduced pulse distortion; SBS slow light demonstrations in nonstandard, highly nonlinear fibers; applications of SBS slow light to the delay of analog waveforms; and the role of polarization. Finally, a brief concluding perspective is provided.

Journal ArticleDOI
TL;DR: The proposed MZIs have lower temperature sensitivities compared with normal fiber sensors, which is a desirable merit for RI sensors to reduce the cross sensitivity caused by thermal drift.
Abstract: A taper-based Mach–Zehnder interferometer (MZI) embedded in a thinned optical fiber is demonstrated as a highly sensitive refractive index (RI) sensor. A RI sensitivity of 2210.84 nm/RIU (refractive index unit) is obtained at the external RI of 1.40, which is ten times higher than that of normal taper- and long-period fiber grating (LPFG)-based sensors. The sensitivity can be further improved by decreasing the diameter of the thinned fiber and increasing the interferometer length of the MZI. The proposed MZIs have lower temperature sensitivities compared with normal fiber sensors, which is a desirable merit for RI sensors to reduce the cross sensitivity caused by thermal drift.

Journal ArticleDOI
TL;DR: A wavelength-locked laser source that rapidly steps through six wavelengths distributed across a 1572.335 nm carbon dioxide absorption line to allow precise measurements of atmospheric CO(2) absorption is demonstrated.
Abstract: We demonstrate a wavelength-locked laser source that rapidly steps through six wavelengths distributed across a 1572.335 nm carbon dioxide (CO(2)) absorption line to allow precise measurements of atmospheric CO(2) absorption. A distributed-feedback laser diode (DFB-LD) was frequency-locked to the CO(2) line center by using a frequency modulation technique, limiting its peak-to-peak frequency drift to 0.3 MHz at 0.8 s averaging time over 72 hours. Four online DFB-LDs were then offset locked to this laser using phase-locked loops, retaining virtually the same absolute frequency stability. These online and two offline DFB-LDs were subsequently amplitude switched and combined. This produced a precise wavelength-stepped laser pulse train, to be amplified for CO(2) measurements.

Journal ArticleDOI
TL;DR: The three most common profiles of scanning functions for galvanometer-based scanners (GSs): the sawtooth, triangular and sinusoidal functions are analyzed and certain rules of thumb are deduced to make the best out of the galvoscanner.
Abstract: We analyze the three most common profiles of scanning functions for galvanometer-based scanners (GSs): the sawtooth, triangular and sinusoidal functions. They are determined experimentally with regard to the scan parameters of the input signal (i.e., frequency and amplitude). We study the differences of the output function of the GS measured as the positional error of the oscillatory mirror from the ideal function given by the input signal of the device. The limits in achieving the different types of scanning functions in terms of duty cycle and linearity are determined experimentally for the possible range of scan parameters. Of particular importance are the preservation of an imposed duty cycle and profile for the sawtooth function, the quantification of the linearity for the sinusoidal function, and the effective duty cycle for the triangular, as well as for the other functions. The range of scan amplitudes for which the stability of the oscillatory regime of the galvo mirror is stable for different frequencies is also highlighted. While the use of the device in certain scanning regimes is studied, certain rules of thumb are deduced to make the best out of the galvoscanner. Finally, the three types of scanning functions are tested with a Fourier domain optical coherence tomography (FD OCT) setup and the conclusions of the study are demonstrated in an imaging application by correlating the determined limits of the scanning regimes with the requirements of OCT.

Journal ArticleDOI
TL;DR: The optical design and testing of a new instrument to measure the absorption spectrum from 400 to 750 nm wavelength of the particles collected on filters using an "integrating-sandwich" configuration is described and the measured absorption is shown to be unaffected by scattering of light from the deposited particulates.
Abstract: Light absorption by particulate impurities in snow and ice can affect the surface albedo and is important for the climate. The absorption properties of these particles can be determined by collecting and melting snow samples and extracting the particulate material by filtration of the meltwater. This paper describes the optical design and testing of a new instrument to measure the absorption spectrum from 400 to 750 nm wavelength of the particles collected on filters using an “integrating-sandwich” configuration. The measured absorption is shown to be unaffected by scattering of light from the deposited particulates. A set of calibration standards is used to derive an upper limit for the concentration of black carbon (BC) in the snow. The wavelength dependence of the absorption spectra from 450 to 600 nm is used to calculate an absorption Ångstrom exponent for the aerosol. This exponent is used to estimate the actual BC concentration in the snow samples as well as the relative contributions of BC and non-BC constituents to the absorption of solar radiation integrated over the wavelength band 300 to 750 nm.

Journal ArticleDOI
TL;DR: A simple and versatile approach for fabricating terahertz lenses based on compression molding of micropowder polymer materials in a tabletop hydraulic press, confirming the excellent imaging qualities of the compression molded lenses.
Abstract: We present a simple and versatile approach for fabricating terahertz lenses based on compression molding of micropowder polymer materials in a tabletop hydraulic press. To demonstrate the feasibility of this approach, a biconvex lens shape is calculated using a ray-tracing algorithm and lenses based on two different micropowders are fabricated. As the powder materials have different refractive indices, the resulting lenses share the same geometric shape but differ in their respective focal length. The focusing properties of the lenses are evaluated by transversal and sagittal beam profile measurements in a fiber-coupled terahertz time-domain spectroscopy system, confirming the excellent imaging qualities of the compression molded lenses.

Journal ArticleDOI
Yajun Li1
TL;DR: The exact solution for the problem of precision pointing is generalized to investigate the synthesis of the scan pattern, i.e., to create a desirable scan pattern on some plane perpendicular to the optical axis of the system by controlling the circular motion of the two prisms.
Abstract: Nonparaxial ray tracing through Risley prisms of four different configurations is performed to give the exact solution of the inverse problem arisen from applications of Risley prisms to free space communications. Predictions of the exact solution and the third-order theory [Appl. Opt.50, 679 (2011)APOPAI0003-693510.1364/AO.50.000679] are compared and results are shown by curves for systems using prisms of different materials. The exact solution for the problem of precision pointing is generalized to investigate the synthesis of the scan pattern, i.e., to create a desirable scan pattern on some plane perpendicular to the optical axis of the system by controlling the circular motion of the two prisms.

Journal ArticleDOI
TL;DR: In this paper, a passively mode-locked figure-eight fiber laser scheme was proposed to generate noiselike optical pulses, or subns wave packets with a fine inner structure of subps pulses presenting random amplitudes and durations.
Abstract: We propose and study experimentally and numerically a passively mode-locked figure-eight fiber laser scheme generating noiselike optical pulses, or subns wave packets with a fine inner structure of subps pulses presenting random amplitudes and durations. The particular design of the nonlinear optical loop mirror (NOLM) used in this laser, relying on nonlinear polarization rotation, allows adjusting the switching power through input polarization control. Experimental results show stable pulsed operation over a limited range of the NOLM input polarization angle. Interestingly, the spectral and temporal characteristics of these pulses are observed to be widely variable over that range. In particular, the spectral width varies from 16 to 52 nm and this spectral variation is associated with an inverse evolution in the durations of the bunch and of the inner ultrashort pulses. Simulation results are in good agreement with the experiment. They confirm the strong dependence of the pulse properties on the value of the NOLM switching power, although NOLM switching is not alone responsible for the appearance of the noiselike pulsing mode.