Radiation pressure forces on a right-triangular metallic coated prism grating due to solar illumination from 100 to 600 [THz] are determined via FDTD simulation. The maximum transverse force efficiency is evaluated for prism angles [5° − 45°].
Experimental measurements related to the opto-mechanical stability of thin film diffractive beam-riders are discussed. Our theoretical predictions of radiation pressure forces indicate that these structures allow a perturbed laser-driven light sail to remain in the beam path. Radiation pressure forces of both a liquid crystal polymer bi-grating and an etched photoresist axicon diffraction grating will be described. Our experiments made use of a vacuum torsion oscillator having sub-nano-Newton sensitivity. The parametric damping of both systems will also be described. Our measurements validate the technical feasibility of a laser-driven light sail based on diffractive thin films.
The nearly 100 year-old paradigm of navigating the heavens by means of radiation pressure on a reflective solar sail is being challenged by the advent of advanced diffractive films that offer efficient propulsion and navigation. Other advantages include photon recycling when the sail is transmissive, and non-mechanical (e.g., electro-optic) navigation protocols. Unlike optical tweezers, the transverse force on a solar sail is afforded by the angular deviation of light away from the sunline, rather than a gradient force. Whereas a metal-coated film makes use of the law of reflection, the deviation of light from a diffractive film may be described by use of the grating equation. In the latter case, the grating momentum is, in fact, a mechanical phenomenon that we have observed in the laboratory by means of a vacuum torsion oscillator. Following a brief history of solar sailing I will describe how diffractive sails may enable the placement of a constellation of solar polar orbiters for monitoring the entire surface of the Sun. Laser-driven sails provide another opportunity for in-space propulsion, provided a stable “beam rider” can be invented. I will report our progress on the demonstration of a diffractive beam rider (patent pending).
A Point Spread Function (PSF) engineered imaging system provides functionality at the expense of image distortion. Deconvolution and other post-processing techniques may partially restore the image if the PSF is known. We compare how various phase mask functions (e.g., vortex, axicon, cubic, and circular harmonic) may functionally protect a sensor from a coherent beam, and we discuss the subsequent trade-off between peak irradiance and the integrated modulation transfer function (Strehl ratio). Both experimental and numerical examples demonstrate that the peak irradiance may be suppressed by orders of magnitude without intolerable loss of image fidelity. The design of an optimal phase mask that accomplishes this task is made difficult by the nonlinear relationship between peak irradiance and Strehl. Results from experimental and numerical optimization schemes like simulated annealing, differential evolution, and Nelder-Mead will be compared.
Solar sailcrafts make use of radiation pressure to propel a payload through space. Modern diffractive structures such as broadband single order gratings, polarization diffraction gratings, and related metamaterials offer the potential to replace reflective sails with efficient diffractive sails for either solar or laser driven space travel. We have experimentally verified the radiation pressure force on a transmissive diffraction grating, demonstrating a large component of force parallel to the surface of the grating. This component is important for orbit-raising types of maneuvers. Unlike a reflective sail with force components only normal to the surface, we also measure a near-vanishing normal force component around the Littrow angle
KEYWORDS: Point spread functions, Imaging systems, Modulation transfer functions, Spatial light modulators, Optical transfer functions, Computational imaging, Diffraction, Deconvolution, Optical filters, Signal to noise ratio
A phase-only filter is placed in the pupil plane of an imaging system to engineer a new point spread function with a low peak intensity. Blurred detected images are then reconstructed in post-processing through Wiener Deconvolution. A Differential Evolution algorithm is implemented to optimize these filters for high SNR across the MTF. These filters are tested experimentally using a reflective Spatial Light Modulator (SLM) in the pupil of a system and successfully show the peak intensity reduced 100 times the diffraction limit. Results are compared to expected performance.
Vortex and axicon phase masks are introduced to the pupil plane of an imaging system, altering both the point spread function and optical transfer function for monochromatic and broadband coherent and incoherent light. Each phase mask results in the reduction of the maximum irradiance of a localized coherent laser source, while simultaneously allowing for the recovery of the incoherent background scene. We describe the optical system, image processing, and resulting recovered images obtained through this wavefront encoding approach for laser suppression.
A means to digitally generate a partially coherent beam with orbital angular momentum is presented. Our approach is based on encoding the randomness of broadband light passing through a spiral phase plate in a spatial light modulator. We illustrate the technique by generating partially coherent beams with orbital angular momentum content and different coherence lengths, with no moving optical elements. We study the cross correlation spectra which yields to good agreement with theory.
In this paper, we present some ideas regarding the optics and imaging aspects of granular spacecraft. Granular spacecraft are complex systems composed of a spatially disordered distribution of a large number of elements, for instance a cloud of grains in orbit. An example of this application is a spaceborne observatory for exoplanet imaging, where the primary collecting aperture is a cloud of small particles instead of a monolithic aperture.
We present methods for optimizing pupil and focal plane optical elements that improve the performance of vortex coronagraphs on telescopes with obstructed or segmented apertures. Phase-only and complex masks are designed for the entrance pupil, focal plane, and the plane of the Lyot stop. Optimal masks are obtained using both analytical and numerical methods. The latter makes use of an iterative error reduction algorithm to calculate "correcting" optics that mitigate unwanted diffraction from aperture obstructions. We analyze the achieved performance in terms of starlight suppression, contrast, off-axis image quality, and chromatic dependence. Manufacturing considerations and sensitivity to aberrations are also discussed. This work provides a path to joint optimization of multiple coronagraph planes to maximize sensitivity to exoplanets and other faint companions.
We experimentally explored the reconstruction of the image of two point sources using a sequence of
random aperture phase masks. The speckled intensity profiles were combined using an improved shift-and-add and
multi-frame blind deconvolution to achieve a near diffraction limited image for broadband light (600-670 nm). Using
a numerical model we also explored various algorithms in the presence of noise and phase aberration.
A means to measure orbital angular momentum in a partially coherent beam is demonstrated by using a wavefront
folding interferometer. This interferometer allows us to study the cross correlation function of a partially coherent
vortex beam. It is shown that the cross correlation function possesses ring dislocations which are related to the
topological charge of the partially coherent vortex, exhibiting a one to one correspondence between the number of
rings and the value of the topological charge, thus providing a direct measure of the orbital angular momentum.
A new modality for optical micromanipulation is under investigation. Optical wings are shaped refractive objects that experience a force and torque owing to the reflection and transmission of uniform light at the object surface. We present wing designs that provide a restoring torque that returns the wing to a source facing orientation while preserving efficient thrust from radiation pressure. The torsional stiffness and orbital period of a set of optical wing cross-sectional shapes are determined from numerical ray-tracing analyses. These results demonstrate the potential to develop an efficient optomechanical device for applications in microbiology and space flight systems.
In a recent article [Swartzlander et al. Nature Photonics, 5, 4851 (2010)], the optical analogue of conventional, aerodynamic
lift was experimentally demonstrated. When exposed to quasi-plane wave illumination, a dielectric hemicylinder rotates
into a stable configuration in which its cylindrical axis is perpendicular to the direction of propagation and its flat surface
angled to it. In this configuration the body forces experienced by the particle contain a component perpendicular to the
momentum flux of the incident field. This phenomenon can be meaningfully termed "optical lift", and the hemicylinder acts
as a "light foil". Here, we present rigorous, full wave vector simulations of this effect for light foils of varying dimensions
and composition. We investigate the general form of the forces and torques experienced by light foils, as a function of their
orientation. The influence of the linear dimensions and the refractive indices of the hemicylinders is also investigated.
Laser beam shaping is an active discipline in optics owing to its importance to both illumination and detection processes. The formation of single or multiples optical vortices in a laser beam has taken on recent interest in areas ranging from electron and atom optics to astronomy. An optical vortex is characterized as a point node of destructive interference around which the phase varies by an integer multiple of 2 times π. Here we describe our efforts to create localized vortex cores using only the interference of several Gaussian profile laser beams, a method that may be particularly suited to the application of vortex modes to intense femtosecond laser pulses.
Optical singularities serve as scientific landmarks in the topological landscape of light patterns. These curious
features exhibit conservation properties and unique diffraction patterns that are finding increasing importance
in many branches of modern optics. Caustics are singularities in ray optics that are markedly different from
those in wave optics owing to the inclusion of phase and polarization in the latter case. Optical vortices and
polarization singularities are examples of singular patterns in electromagnetic fields. There is some evidence that
single photons can also exhibit singular attributes. This talk will describe examples of both naturally occurring
and man-made optical singularities, how they may be put to use, and future directions in this field.
We have constructed a high-speed image stabilization system, BESSEL, that is capable of performing
wavefront correction at a rate exceeding 1 kHz. BESSEL achieved on-sky Strehl ratios of 98-99% at 800 nm
as we approached the inner scale of atmospheric turbulence when the refractor telescope aperture was stopped
down to 25.4 mm (~r0/2). This is better than expected from Kolmogorov theory, indicating that at D ~r0/2 we
are within the inner scale of turbulence. Utilizing high Strehls and the technique of roll subtraction enabled
BESSEL to resolve the binary, ADS 10418AB, with separation of only 0.71 λ/D and a delta magnitude of ~3 mags at 800 nm. BESSEL's capability to produce high Strehls ratios means that the instrument can be used to test the performance of interference/phase coronagraphs on-sky for the first time. Integrated
with an optical vortex coronagraph, BESSEL is capable of nulling the first airy ring of Betelgeuse by more
then a factor of ten.
The splitting of a fundamental vortex beam into four separate beams is theoretically and experimentally described for the
propagation of light through a uniaxial crystal. This novel phenomenon opens new opportunities for higher order the
entanglement of and correlation of photons.
Previous work has shown that single-beam gradient traps are unable to trap high index particles in fluids
where the index contrast is large. However, by changing the refractive index of the surrounding medium to
more closely match the index of refraction of the particle, trapping of high index particles is possible. We
report preliminary efforts to trap high index glass particles having indices of about 1.9. The experimental
trap stiffness data of polystyrene beads with radius 1.8 and 10 µm suspended in water is presented. The
next step is to trap high index particles as well as determine the trap stiffness for those particles having
diameters in the 3-10 μm range suspended in fluids having refractive indices in the range of 1.4 to 1.6.
The optical vortex coronagraph is a promising scheme for achieving high contrast low loss imaging
of exoplanets as close as 2λ/D from the parent star. We describe results using a high precision
vortex lens that was fabricated using electron-beam lithography. We also report demonstrations of
the coronagraph on a telescope employing a tip-tilt corrector.
The goal of the Terrestrial Planet Finder Mission is to detect and characterize Earth-like planets. Detection of these faint objects, which appear very close to their parent stars, requires a coronagraph capable of achieving better than 10-10 starlight suppression within a few Airy rings of the stellar image. The coronagraph is also required to maintain this high stellar extinction over a 100nm spectral bandwidth. To ease requirements on the telescope, a high planet light throughput and low sensitivity to wave front aberrations are also desirable features. An optical vortex coronagraph is a promising candidate architecture, which makes use of a spiral phase plate placed in an intermediate image plane to null out the stellar signal. This architecture has the advantage of high stellar extinction, high planet light throughput, and low sensitivity to wave front aberrations. Here we report the high contrast performance of an optical vortex coronagraph limited by the manufacturability of the spiral phase plate.
Writing-to-learn principles and peer-learning were integrated into a third-year physical optics course for undergraduate students. By writing about the life and research of optics faculty, student benefits extended beyond the course material. Peer tutors recruited from the class helped enhanced the overall writing quality. Students published their papers on the open-access web.
In response to a need for interactive learning, we designed a physical optics course that incorporates writing-to-learn principles and allows students to meld in-class theory with real-world practice. Students "published" their papers on the open-access web. This exercise necessitated that students understand principles sufficiently to simplify complex topics for a broad general audience.
As our continuous effort to develop a package for modeling of beam propagation in nonlinear optical devices, we use
different means to improve its user-friendliness, availability and capability. We have extended our model to include pulse
propagation, i.e. 4-dimensional propagation of an optical beam. Currently, we have developed a few models for intensitydependent
and fluence-dependent propagation of nonlinear wave, including various nonlinear absorption and refractive
mechanisms such as thermal diffusion and reverse saturation absorption (RSA). These models can provide significant insight
into the underlying optical processes which occur in nonlinear optical devices such as optical limiters. Here we will concentrate
our discussion on thermal diffusion and reverse saturable absorption. To improve user-friendliness, availability and
capability of the package, we have implemented two graphical user interfaces, a Internet version based on Hypertext Markup
Language HTML/pen script and a standalone version based on TcIITk script. The two interfaces can be executed in a variety
of computers (Macintosh, workstation or PC) while the actual simulation can be performed in a more powerful computer.
The two interfaces have their own merits. The Tcl!Fk version can be easily modified and installed in a computer that has no
access to the Internet. On the other hand, the web based version makes the package available to more users via world-wide
web (WWW). The layouts of the interfaces are almost the same. They generate simulation results in text files for plotting as
well as animation sequences which can be viewed with a free software, available from National Center for Supercomputing
Applications.
A numerical software package for (2 plus 1)-dimensional simulation of optical limiters has been developed. The purpose of this effort is to provide the sensor protection community with a means to develop, understand, and optimize optical limiters. A graphics interface has been implemented to guide the user in both running the program and interpreting the results. Scientific visualization tools have been integrated with the package to facilitate a physical understanding of the numerical results. We have used it successfully to perform (2 plus 1)-dimensional nonlinear propagation of a focused optical beam with arbitrary intensity profile, such as Gaussian or tophat profile, in a f/5 system. The package can handle various nonlinear mechanisms, including intensity dependent refraction and absorption. Non-local nonlinearities such as thermal effects may be included as a modular subprogram. Temporal effects may also be included by simulating the integrated spatio-temporal evolution of the light and material properties, although this approach is memory- intensive and often requires the use of a super-computer.
The so-called "defocusing limiter" is one of the best configurations for achieving laser radiation protection over a broadband spectrum in a low fnumber system. Available nonlinear materials, however, can not provide maximum permissible exposure levels over a large dynamic range. On the other hand, we believe that novel nonlinear optical engineering techniques can be used to overcome these difficulties. We have recently discovered a rich class of nonlinear optical phenomena in defocusing media, which opens new opportunities to enhance the performance of defocusing limiters and to explore completely new limiting schemes. Here we show that a nonlinear mask can be used to improve the device performance by reducing the throughput power by 90% in an ff5 optical system. We have numerically modeled a generic nonlinear optical limiter to account for these effects. The code allows us to vary the parameters of the optical system and the nonlinçar medium. In addition to obtaining the transmission data, we also determine the intensity distribution in the final focal plane.
In this paper, we discuss recent results on the propagation of dark spatial solitons (DSS). Dark spatial solitons are particular solutions of the nonlinear Schroedinger (NLS) equation modeling propagation of light beams in optical Kerr media. Experimental results are presented for three systems, including sodium vapor, various thermally nonlinear liquids, and the bulk semiconductor system ZnSe. The results of these investigations indicate that experimental dark spatial solitons obey the conservation laws of the NLS equation, possess collision properties characteristic of the theoretical DSS solutions, and are stable to external perturbations induced by the experiment. In addition, through an interferometric technique, we investigate the phase profile of the dark spatial solitons and show that it is in good agreement with the NLS solution. In addition to the fundamental DSS, we have performed experiments where nonfundamental DSS are excited in pairs by making use of an even initial field profile as originally discussed by Zakharov and Shabat. The transverse velocities of the solitons excited in the is manner are measured and found to be in good agreement with those predicted theoretically.
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