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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Measurement of birefringence for optical recording disk substrates

The birefringence of bare and coated substrates for magneto-optical recording is experimentally investigated using ellipsometry at the wavelengths of 632.8 nm and 780 nm. The rotation and ellipticity of the polarization state of the reflected or transmitted light is measured for different incident angles and different orientations of the incident linear polarization. The measured data is then fitted by a computer program which solves the Maxwell equations for the plane-wave propagation in a multilayer structure and minimizes the error between the measured and calculated data by adjusting the unknown parameters of the multilayer. This approach enables us to determine orientations of the three principal axes in the substrate and the corresponding refractive indices. A special feature of our ellipsometers is that a glass hemisphere is placed in contact with the substrate, which eliminates the refraction of the incident light and enables a maximum propagation angle of 70 degrees (with respect to the normal) in the substrate. This increases the sensitivity of the measurement. Certain anomalies were observed, which we believe are associated with the presence of grooves on these substrates.

Fu, Hong↗
NASA NTRS1996

Switched-Polarization Birefringent Optical Delay Lines

Controllable birefringent optical delay lines proposed for use in photonically controlling phases of signals in radiating elements of phased-array antennas. Features low optical insertion losses and made very compact, enabling high packing densities where multiple delay channels needed. Polarization switched in segments to adjust overall delay. Furthermore, two or more delay lines cascaded increases delay-adjustment range.

Yao, Xiaotian Steve↗
NASA NTRS2005

Photonic variable delay devices based on optical birefringence

Optical variable delay devices for providing variable true time delay to multiple optical beams simultaneously. A ladder-structured variable delay device comprises multiple basic building blocks stacked on top of each other resembling a ladder. Each basic building block has two polarization beamsplitters and a polarization rotator array arranged to form a trihedron; Controlling an array element of the polarization rotator array causes a beam passing through the array element either going up to a basic building block above it or reflect back towards a block below it. The beams going higher on the ladder experience longer optical path delay. An index-switched optical variable delay device comprises of many birefringent crystal segments connected with one another, with a polarization rotator array sandwiched between any two adjacent crystal segments. An array element in the polarization rotator array controls the polarization state of a beam passing through the element, causing the beam experience different refractive indices or path delays in the following crystal segment. By independently control each element in each polarization rotator array, variable optical path delays of each beam can be achieved. Finally, an index-switched variable delay device and a ladder-structured variable device are cascaded to form a new device which combines the advantages of the two individual devices. This programmable optic device has the properties of high packing density, low loss, easy fabrication, and virtually infinite bandwidth. The device is inherently two dimensional and has a packing density exceeding 25 lines/cm2. The delay resolution of the device is on the order of a femtosecond (one micron in space) and the total delay exceeds 10 nanosecond. In addition, the delay is reversible so that the same delay device can be used for both antenna transmitting and receiving.

Yao, X. Steve↗
NASA NTRS1973

Light-direction sensor based on birefringency

Optical system consisting of polarizer, analyzer, quarterwave retarder converts incident light beam to one which has an intensity related to the extent the incident beam is off axis.

Johnston, A. R.↗
NASA NTRS1973

Contrast elements in birefringent filters.

Calculations were made to determine the effects which contrast elements of various thicknesses have in Lyot filters. A filter which is significantly narrower than the line at which it is looking produces the best results. Therefore, if the filter is broad the addition of as thick an element as possible is desired. However, if the filter is already narrow, a contrast element whose thickness equals that of the second Lyot element will produce the best performance.

Schoolman, S. A.↗
NASA NTRS1975

Wide-field birefringent elements

Birefrigent array consists of two plates with retardation properties nearly independent of direction of incident light over unlimited range of wavelengths. Array can be used as birefrigent color filter. Optical properties of plates may also be changed electro-optically rather than thermally.

Miller, A.↗
NASA NTRS1981

Determination of principal stress in birefringent composites by hole-drilling method

The application of transmission photoelasticity to stress analysis of composite materials is discussed.The method consists in drilling very small holes at points where the state of stress has to be determined. Experiments are described which verify the theoretical predicitons. The limitations of the method are discussed and it is concluded that valuable information concerning the state of stress in a composite model can be obtained through the suggested method.

Prabhakaran, R.↗
NASA NTRS1983

Simultaneous elastic and photoelastic calibration of birefringent orthotropic model materials

Transmission photoelastic analysis of composite models has been shown in recent years to be a viable stress analysis method for composite structures. However, difficulties in the fabrication of the model materials require the development of efficient mechanical and optical calibration procedures. In this paper, the methods for measuring elastic and photoelastic constants of composites are briefly reviewed. Then a new method, utilizing a relatively small calibration specimen, is described. In this method, electrical resistance strain gauges are mounted at selected points of a half-plane model and photoelastic measurements and strain readings are simultaneously taken. The data are processed by a least-squares procedure to yield the elastic and photoelastic constants. Comparison of results obtained by the proposed method with the results from conventional tests shows good agreement.

Prabhakaran, R.↗
NASA NTRS1983

Reduced thermal focusing and birefringence in zig-zag slab geometry crystalline lasers

The present investigation is concerned with interferometric measurements of the thermally induced optical effects in laser crystals, taking into account crystals cut in both the zig-zag slab and the conventional rod geometries. The measurements conducted demonstrate the significant advantages obtained by replacing the rod in a Nd:YAG laser with a slab. A description is provided of simple measures designed to eliminate end and edge effects, and efficiency and threshold data for the Nd:YAG slab and the rod it replaced are presented. The only disadvantage is a small increase in cost due to the more difficult fabrication of the slab and the greater amount of Nd:YAG which is needed to allow the ends to be unpumped.

Kane, T. J.↗
NASA NTRS1989

Automated measurement of birefringence - Development and experimental evaluation of the techniques

Traditional photoelasticity has started to lose its appeal since it requires a well-trained specialist to acquire and interpret results. A spectral-contents-analysis approach may help to revive this old, but still useful technique. Light intensity of the beam passed through the stressed specimen contains all the information necessary to automatically extract the value of retardation. This is done by using a photodiode array to investigate the spectral contents of the light beam. Three different techniques to extract the value of retardation from the spectral contents of the light are discussed and evaluated. An experimental system was built which demonstrates the ability to evaluate retardation values in real time.

Voloshin, A. S.↗
NASA NTRS1995

Polarization errors associated with birefringent waveplates

Although zero-order quartz waveplates are widely used in instrumentation that needs good temperature and field-of-view characteristics, the residual errors associated with these devices can be very important in high-resolution polarimetry measurements. How the field-of-view characteristics are affected by retardation errors and the misalignment of optic axes in a double-crystal waveplate is discussed. The retardation measurements made on zero-order quartz and single-order 'achromatic' waveplates and how the misalignment errors affect those measurements are discussed.

West, Edward A.↗
NASA NTRS2007

Compensation for Phase Anisotropy of a Metal Reflector

A method of compensation for the polarization- dependent phase anisotropy of a metal reflector has been proposed. The essence of the method is to coat the reflector with multiple thin alternating layers of two dielectrics that have different indices of refraction, so as to introduce an opposing polarization-dependent phase anisotropy. The anisotropy in question is a phenomenon that occurs in reflection of light at other than normal incidence: For a given plane wave having components polarized parallel (p) and perpendicular (s) to the plane of incidence, the phase of s-polarized reflected light differs from the phase p-polarized light by an amount that depends on the angle of incidence and the complex index of refraction of the metal. The magnitude of the phase difference is zero at zero angle of incidence (normal incidence) and increases with the angle of incidence. This anisotropy is analogous to a phase anisotropy that occurs in propagation of light through a uniaxial dielectric crystal. In such a case, another uniaxial crystal that has the same orientation but opposite birefringence can be used to cancel the phase anisotropy. Although it would be difficult to prepare a birefringent material in a form suitable for application to the curved surface of a typical metal reflector in an optical instrument, it should be possible to effect the desired cancellation of phase anisotropy by exploiting the form birefringence of multiple thin dielectric layers. (The term "form birefringence" can be defined loosely as birefringence arising, in part, from a regular array of alternating subwavelength regions having different indices of refraction.)

Hong, John↗
NASA NTRS1978

Partial polarizer filter

A birefringent filter module comprises, in seriatum. (1) an entrance polarizer, (2) a first birefringent crystal responsive to optical energy exiting the entrance polarizer, (3) a partial polarizer responsive to optical energy exiting the first polarizer, (4) a second birefringent crystal responsive to optical energy exiting the partial polarizer, and (5) an exit polarizer. The first and second birefringent crystals have fast axes disposed + or -45 deg from the high transmitivity direction of the partial polarizer. Preferably, the second crystal has a length 1/2 that of the first crystal and the high transmitivity direction of the partial polarizer is nine times as great as the low transmitivity direction. To provide tuning, the polarizations of the energy entering the first crystal and leaving the second crystal are varied by either rotating the entrance and exit polarizers, or by sandwiching the entrance and exit polarizers between pairs of half wave plates that are rotated relative to the polarizers. A plurality of the filter modules may be cascaded.

Title, A. M.↗
NASA NTRS2014

Self-Calibration of BICEP1 Three-Year Data and Constraints on Astrophysical Polarization Rotation

Cosmic microwave background (CMB) polarimeters aspire to measure the faint B-mode signature predicted to arise from inflationary gravitational waves. They also have the potential to constrain cosmic birefringence, rotation of the polarization of the CMB arising from parity-violating physics, which would produce nonzero expectation values for the CMB's temperature to B-mode correlation (TB) and E-mode to B-mode correlation (EB) spectra. However, instrumental systematic effects can also cause these TB and EB correlations to be nonzero. In particular, an overall miscalibration of the polarization orientation of the detectors produces TB and EB spectra which are degenerate with isotropic cosmological birefringence, while also introducing a small but predictable bias on the BB spectrum. We find that BICEP1 three-year spectra, which use our standard calibration of detector polarization angles from a dielectric sheet, are consistent with a polarization rotation of alpha = −2.77deg +/- 0.86deg (statistical) +/- 1.3deg (systematic). We have revised the estimate of systematic error on the polarization rotation angle from the two-year analysis by comparing multiple calibration methods. We also account for the (negligible) impact of measured beam systematic effects. We investigate the polarization rotation for the BICEP1 100 GHz and 150 GHz bands separately to investigate theoretical models that produce frequency-dependent cosmic birefringence. We find no evidence in the data supporting either of these models or Faraday rotation of the CMB polarization by the Milky Way galaxy's magnetic field. If we assume that there is no cosmic birefringence, we can use the TB and EB spectra to calibrate detector polarization orientations, thus reducing bias of the cosmological B-mode spectrum from leaked E-modes due to possible polarization orientation miscalibration. After applying this "self-calibration" process, we find that the upper limit on the tensor-to-scalar ratio decreases slightly, from r < 0.70 to r < 0.65 at 95% confidence.

three-year↗
NASA NTRS1973

Study of self-focusing and self-phase-modulation in the picosecond-time regime.

Study of the propagation of a single picosecond pulse of a mode-locked Nd-glass laser in CS2 nitrobenzene and toluene by observations of the birefringence and spectral changes in self-focused laser light. Multiple second-harmonic probing beams are used for birefringence measurements in a single IR pulse in various portions of the active cell. The orientational Kerr effect is found to be the prime factor of induced birefringence in nitrobenzene and toluene. The results for CS2 are also consistent with this mechanism and show that the duration of an IR pulse in the focal volume is less than that of the incident pulse. The properties of beams which are focused within the cell by an external lens are also investigated.

Reintjes, J.↗