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NASA NTRS1978

Reflection polarizers for the vacuum ultraviolet using Al + MgF2 mirrors and an MgF2 plate

Consideration is given to the design and operation of a three-mirror reflecting polarizer where one of the reflecting surfaces is an MgF2 plate, the other surfaces are Al + MgF2 coatings, and one reflection occurs at or near the true Brewster angle. It is found that the polarizer is most efficient in the 1200-2000 A wavelength region, and that by optimum selection of the angle of incidence on the MgF2 plate, polarization values of 100 and over are yielded from 900 to 3000 A. The polarizer may be used at wavelengths as short as 500 A, although it is observed that at such wavelengths the polarization value decreases to about 10. It is noted that all reflecting polarizers operating in the vacuum ultraviolet wavelength may manifest changing characteristics as their mirrors become contaminated, and that polarization must therefore be occasionally remeasured.

Hass, G.↗

NASA NTRS1981

Gas-laser power monitor

Device attaches simply to front of laser housing for continuous monitoring of power output. Monitor is calibrated to read either total output or power generated in test volume. It is fabricated from four black-anodized aluminum parts; crown glass positioned at Brewster angle reflects 0.33 percent of beam onto photodiode calibrated for electrical output proportional to laser power. Unlike conventional calorimeter, monitor does not interrupt laser beams, and fast-response diode allows instantaneous tracking of power fluctuations.

Russ, C. E., Jr.↗

NASA NTRS1980

Characterization of acoustic disturbances in linearly sheared flows

Inviscid fluctuations in a compressible linearly sheared, but otherwise homogeneous, parallel two-dimensional flow are considered. The equation describing the plane wave propagation (PWP), the stability, or the rectangular duct mode characteristics in such a flow is shown to be reducible to Whittaker's equation. The exact solutions are applied to problems of PWP and stability in linearly sheared flows as limiting cases in which the speed of sound goes to infinity (incompressible limit) or the shear layer thickness, or wave number, goes to zero (vortex sheet limit). With respect to the PWP it is shown that the shear layer possesses no resonances and no Brewster angles, while with regard to the problem of the stability of a finite thickness shear layer with a linear velocity profile, it is shown that the thin layer is unstable to long wavelength disturbances for all Mach numbers.

Koutsoyannis, S. P.↗

NASA NTRS1983

Scattering of electromagnetic waves from a half space of densely distributed dielectric scatterers

The scattering of a plane wave obliquely incident on a half space of densely distributed spherical dielectric scatterers is studied. The quasi-crystalline approximation is applied to truncate the hierarchy of multiple scattering equations, and the Percus-Yevick and the Verlet-Weis results are used to represent the pair distribution function. The coherent reflected wave is studied with these approximations. The incoherent scattered wave is calculated with the distorted Born approximation. In the low-frequency limit, closed-form expressions are obtained for the effective propagation constants, the coherent reflected wave, and the bistatic scattering coeficients. Results at higher frequencies are calculated numerically. The advantage of the present approach is that, in the low-frequency limit, it reproduces the effects of specular reflection, Fresnel reflection coefficient, Brewster angle, and Clausius-Mosotti relation. In addition to the classical results, the bistatic scattering coefficients are also calculated. The theory is also applied to match backscattering data from dry snow at microwave frequencies.

Tsang, L.↗

NASA NTRS1984

Optical constants of organic tholins produced in a simulated Titanian atmosphere - From soft X-ray to microwave frequencies

The determination of the real (n) and imaginary (k) parts of the complex refractive index of thin films of the dark reddish organic solids called tholins, produced by continuous D.C. discharge through a 0.9 N2/0.1 CH4 gas mixture at 0.2 mb, is reported. For the 250 A-1000 micron wavelength range, n and k have been determined from a combination of transmittance, specular reflectance, interferometric, Brewster angle, and ellipsometric polarization measurements. Experimental uncertainties are estimated at + or -0.05 for n and + or -30 percent for k. The results in the visible range are consistent with deductions made by ground-based and spacecraft observations of Titan, in whose atmosphase tholins are thought to be present.

Khare, B. N.↗

NASA NTRS1988

Electromagnetic wave propagation through a dielectric-chiral interface and through a chiral slab

The reflection from and transmission through a semiinfinite chiral medium are analyzed by obtaining the Fresnel equations in terms of parallel- and perpendicular-polarized modes, and a comparison is made with results reported previously. The chiral medium is described electromagnetically by the constitutive relations D = (epsilon)E+i(gamma)B and H = i(gamma)E+(1/mu)B. The constants epsilon, mu and gamma are real and have values that are fixed by the size, the shape, and the spatial distribution of the elements that collectively compose the medium. The conditions are obtained for the total internal reflection of the incident wave from the interface and for the existence of the Brewster angle. The effects of the chirality on the polarization and the intensity of the reflected wave from the chiral half-space are discussed and illustrated by using the Stokes parameters. The propagation of electromagnetic wave through an infinite slab of chiral medium is formulated for oblique incidence and solved analytically for the case of normal incidence.

Bassiri, S.↗

NASA NTRS1988

Synchronously pumped mode-locked Ti:Al2O3 lasers

Synchronously pumped mode-locked laser operation in a Ti:Al2O3 ring laser is demonstrated. The laser pump source was a frequency-doubled Nd:YAG laser-amplifier system producing a 60-micron-long macropulse that comprised a wavetrain of mode-locked 70-ps micropulses with a 10-ns spacing (100 MHz). The Ti:Al2O3 laser consisted of a 1-cm-long crystal with faces cut at the Brewster angle and placed in a ring laser cavity configuration with a 2 percent output coupling mirror. The cavity was adjusted for a 5-ns round trip transit time, which was close to half the temporal spacing of the pump pulse. When the crystal was pumped synchronously at 532 nm with a 30-mJ macropulse, which was approximately an order of magnitude above the laser threshold, mode-locked lasing at approximately 200 MHz was obtained. This macropulse pump energy corresponds to an average micropulse pump energy of about 5 microJ at the Ti:Al2O3 crystal face. The mode-locked operation of the Ti:Al2O3 was observed only after several microseconds into the macropulse, indicating a long build-up process over many cavity trips. During this time, relaxation oscillations were observed. When mode locking started, the relaxation oscillation frequency increased by an order of magnitude. Synchronous pumping has been limited to operation with dye lasers and used either a CW or pulsed mode-locked pump source. Recent work has demonstrated CW mode-locked operation of a Ti:Al2O3 laser. The result raises the possibility that pulsed synchronous pumping may be used to obtain significantly narrowed laser pulses in Ti:Al2O3.

Source record↗

NASA NTRS1991

Imaging polarimeters for solar extreme ultraviolet astronomy

Accounts are given of EUV/FUV polarimetric instrument concepts for solar research which observe linear polarization in the spectral lines which originate in the outer solar atmosphere. The coronagraph/polarimeter instruments discussed employ all-reflective optical systems using ultrasmooth, low-scatter normal incidence mirrors and reflective polarization analyzers. The reflecting polarization analyzers operate at the Brewster angle.

Hoover, Richard B.↗

NASA NTRS1993

Polarized and specular reflectance variation with leaf surface features

The linearly polarized reflectance from a leaf depends on the characteristics of the leaf surface. In the present study the leaf reflectance of a number of plant species with varying surface characteristics was measured at the Brewster angle with a polarization photometer having 5 visible and near-infrared wavelength bands. We found that all leaf surfaces polarized incident light. Differences among species could be explained by variation in surface features. The results support our hypothesis that the polarized light is reflected by the leaf surface, not by its interior. Two mechanisms appeared responsible for the linearly polarized reflectance: (1) specular reflectance and (2) surface particle scattering. In most cases, large values of linearly polarized reflectance could be attributed to specular reflectance from the leaf surface. Attribution required knowledge of the optical dimensions of features on the leaf surface.

Grant, Lois↗

NASA NTRS2002

Methods for LWIR Radiometric Calibration and Characterization

The utility of a remote sensing system increases with its ability to retrieve surface temperature or radiance accurately. Research applications, such as sea temperature and power plant discharge, require a 0.2 C resolution or better for absolute temperature retrievals. Other applications, including agriculture water stress detection, require at least a 1 C resolution. To achieve these levels of accuracy routinely, scientists must perform laboratory and onboard calibration, as well as in-flight vicarious radiometric characterization. A common approach used for in-flight radiometric characterization incorporates a well-calibrated infrared radiometer that is mounted on a bouy and placed on a uniform water body. The radiometer monitors radiant temperature along with pressure, humidity, and temperature measurements of an associated column of atmosphere. On very still waters, however, a buoy can significantly distrub these measurements. Researchers at NASA's Stennis Space Center (SSC) have developed a novel approach of using an uncooled infrared camera mounted on a boom to quantify buoy effects. Another critical aspect of using buoy-mounted infrared radiometers is the need for extensive laboratory characterization of the instruments' radiometric sensitivity, field of view, and spectral response. Proper surface temperature retrieval also requires detailed knowledge of both the upward emission and the reflected sky emission. Recent work at SSC has demonstrated that the use of a polarization-based radiometer operating at the Brewster angle can greatly simplify temperature retrieval as well as improve overall accuracy.

Ryan, Robert↗

NASA NTRS1999

Semi-monolithic cavity for external resonant frequency doubling and method of performing the same

The fabrication of an optical cavity for use in a laser, in a frequency doubling external cavity, or any other type of nonlinear optical device, can be simplified by providing the nonlinear crystal in combination with a surrounding glass having an index of refraction substantially equal to that of the nonlinear crystal. The closed optical path in this cavity is formed in the surrounding glass and through the nonlinear crystal which lies in one of the optical segments of the light path. The light is transmitted through interfaces between the surrounding glass in the nonlinear crystal through interfaces which are formed at the Brewster-angle to minimize or eliminate reflection.

Hemmati, Hamid↗

NASA NTRS2016

Measurement of Antenna Bore-Sight Gain

The absolute or free-field gain of a simple antenna can be approximated using standard antenna theory formulae or for a more accurate prediction, numerical methods may be employed to solve for antenna parameters including gain. Both of these methods will result in relatively reasonable estimates but in practice antenna gain is usually verified and documented via measurements and calibration. In this paper, a relatively simple and low-cost, yet effective means of determining the bore-sight free-field gain of a VHF/UHF antenna is proposed by using the Brewster angle relationship.

Fortinberry, Jarrod↗

NASA NTRS2016

Measurement of Antenna Bore-Sight Gain

Fortinberry, Jarrod↗

NASA NTRS2014

Is There Spectral Variation in the Polarized Reflectance of Leaves?

The light scattered by plant canopies depends in part on the light scattering/absorbing properties of the leaves and is key to understanding the remote sensing process in the optical domain. Here we specifically looked for evidence of fine spectral detail in the polarized portion of the light reflected from the individual leaves of five species of plants measured at Brewsters angle over the wavelength range 450 to 2300nm. Our results show no strong, unambiguous evidence of narrow band spectral variation of the polarized portion of the reflectance factor.

quasi specular reflection↗

NASA NTRS2014

Is there Spectral Variation in the Polarized Reflectance of Leaves?

Leaf reflectance↗

NASA NTRSDate not supplied

Linear Polarizer Design for Application in the Far-Ultraviolet Spectral Range

New mission concepts that are under consideration by NASA call for the design and implementation of Far Ultraviolet (FUV) polarizer technologies that have not been developed yet. A team that includes members from the NASA Goddard Space Flight Center (GSFC), Arizona State University (ASU), and Woodruff Consulting, worked on the design and development of a polarizer design that produce very high extinction ratios in the FUV spectral range (100-200 nm). This polarizer consists of reflecting light through a series of mirrors from a combination of two silicon carbide (SiC) and two lithium fluoride (LiF) crystals positioned at angles of incidence (relative to surface normal) close to the average LiF Brewster’s angle in the FUV. The output is a highly linearly polarized beam. This polarizer concept was fabricated and tested in the existing McPherson 225 Vacuum Ultraviolet (VUV) spectrometer located in the Optics Branch at NASA-GSFC. Initial testing using a MgF2 crystal at the Brewster’s angle as an analyzer has shown that this design can produce state-of-the-art extinction ratios at the Hydrogen Lyman-Alpha (Ly-α) wavelength of 121.6 nm, and that the measured extinction ratio of the two crossed polarizers, ≈114, is mostly limited by the MgF2 analyzer. A polarizer with such a performance at this wavelength has never been reported and it signifies a breakthrough in FUV polarization technology. The levels of effectiveness paired with the polarizer’s compact design allows for a new polarizer capability that would one day be implemented in a future FUV spectropolarimetry space mission.

Far Ultraviolet (FUV)↗

NASA NTRS1986

Polarization photometer to measure bidirectional reflectance factor R(55 deg, 0 deg, 55 deg, 180 deg) of leaves

A polarization photometer has been developed for rapidly determining the bidirectional, polarized, and diffuse light-scattering properties of individual leavs illuminated and measured 'in vivo' at an angle of 55 deg (approximately Brewster's angle) in six wavelength bands in the visible and near-IR wavelength regions. The optical performance and data quality of the system are evaluated to estimate the magnitude of the variation in the data attributable to the instrument system and to measurement procedures. The potential to discriminate between three species of oak is demonstrated using data acquired by the polarization photometer. The instrument is being used to increase understanding of the radiation transfer process in plant canopies and specifically to determine how agronomic information of the physical and chemical processes in leaves, plants, and plant canopies is expressed in these light-scattering properties.

Vanderbilt, V. C.↗

NASA NTRSDate not supplied

A New Linear Polarizer Design for Application in the Far-Ultraviolet Spectral Range

New mission concepts that are under consideration by NASA (such as the Polstar MIDEX mission concept) call for the design and implementation of Far Ultraviolet (FUV) polarizer technologies that have not been developed yet. A team that includes members from the NASA Goddard Space Flight Center (GSFC), Arizona State University (ASU), and Woodruff Consulting, worked on the design and development of a polarizer design that may produce extinction ratios higher than have ever been reported before in the FUV spectral range (100-200 nm). This polarizer consists of transmitting linearly polarized light through a series of reflections from a combination of two silicon carbide (SiC) and two lithium fluoride (LiF) crystals positioned at angles of incidence (relative to surface normal) close to the LiF Brewster’s angle. The performance of this polarizer concept was fabricated and tested with an existing McPherson 225 Vacuum Ultraviolet (VUV) spectrometer located in the Optics Branch at NASA-GSFC. Initial testing has shown that in the FUV spectral range, this design can produce state-of-the-art extinction ratios at the Hydrogen Lyman-Alpha (Ly-α) wavelength of 121.6 nm. A polarizer with such a performance has never been reported and it signifies a breakthrough in FUV polarization technology. The levels of effectiveness paired with the design’s compact design allows for a new polarizer capability that would one day be implemented in a future spectropolarimetry space mission. In addition, this polarizer concept could potentially be used to characterize the optical properties of new mirror coatings that will be used in a future Habitable World Observatory (HWO) mission concept as proposed by the 2020 Decadal Survey.2

Far Ultraviolet (FUV), polarimetry, instrumentatio↗