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NASA NTRS1972

Refractive index and birefringence of 2H silicon carbide

The refractive indices of 2H SiC were measured over the wavelength range 435.8 to 650.9 nm by the method of minimum deviation. At the wavelength lambda = 546.1 nm, the ordinary index n sub 0 was 2.6480 and the extraordinary index n sub e was 2.7237. The estimated error (standard deviation) in the measured values is 0.0006 for n sub 0 and 0.0009 for n sub e. The experimental data were curve fitted to the Cauchy equation for the index of refraction as a function of wavelength. The birefringence of 2H SiC was found to vary from 0.0719 at lambda = 650.9 nm to 0.0846 at lambda = 435.8 nm.

Powell, J. A.↗

NASA NTRS1972

Refractive index and birefringence of 2H silicon carbide.

Measurement of the refractive indices of 2H SiC over the wavelength range from 435.8 to 650.9 nm by the method of minimum deviation. A curve fit of the experimental data to the Cauchy dispersion equation yielded, for the ordinary index, n sub zero = 2.5513 + 25,850/lambda squared + 8.928 x 10 to the 8th power/lambda to the 4th power and, for the extraordinary index, n sub e = 2.6161 + 28,230/lambda squared + 11.490 x 10 to the 8th power/lambda to the 4th power when lambda is expressed in nm. The estimated error (standard deviation) in these values is plus or minus 0.0006 for n sub zero and plus or minus 0.0009 for n sub e. The birefringence calculated from these expressions is about 20% less than previously published values.

Powell, J. A.↗

NASA NTRS1974

Birefringing arising from the reorientation of the polarizability anisotropy of molecules in collisionless gases

The refractive index change in a collisionless gas is evaluated from the Stark shifts of the rotational energy levels that arise from the polarizability anisotropy. For the limit of an extremely short-duration excitation, a multilevel coherent effect resulted in delayed refractive index bursts. Both stationary and transient responses of this birefringence to an optical field were considered for symmetric top molecules, with particular emphasis on the special case of linear molecules.

Lin, C. H.↗

NASA NTRS1976

Birefringence arising from the reorientation of the polarizability anisotropy of molecules in collisionless gases

The refractive-index change in a collisionless gas is evaluated from the Stark shifts of the rotational energy levels that arise from the polarizability anisotropy. In the limit of an extremely-short-duration excitation, a multilevel coherent effect results in delayed refractive-index bursts. Both stationary and transient responses of this birefringence to an optical field are considered for symmetric-top molecules, with particular emphasis on the special case of linear molecules.

Lin, C. H.↗

NASA NTRS1976

Improvement in birefringent filters. IV - The alternate partial polarizer filter

The design and performance of a birefringent filter with alternate partial polarizers are analyzed. The properties of several filter configurations with imperfect intermediate polarizers are examined. It is shown that such filters have significant advantages in transmission and profile shape over both the standard Lyot and the contrast element Lyot filters. These theoretical advantages are demonstrated by the measured properties of an actual filter using the alternate partial polarizer (APP) design. This filter is a four-module eight-crystal APP device which has been built using Polaroid HN-38 for the perfect polarizers and two laminated sheets of HN-55 for the partial polarizers. The measured characteristics of the filter are found to be in good agreement with theory.

Title, A. M.↗

NASA NTRS1976

Recent advances in birefringent filters

In the past several years, important developments have been made in the construction and theory of birefringent filters. Tunable filters that operate from 4500 to 7000 A with full widths at half maximum of 0.05 A and maximum transmissions of almost 20 percent have been constructed. Inexpensive large-aperture (at least 15 cm) achromatic waveplates have been fabricated which allow full use of resolution-solid angle products that are 100 times that of Fabry-Perots and 10,000 times that of grating systems.

Title, A. M.↗

NASA NTRS1976

Development of birefringent filters for spaceflight

The critical problem for flight of a birefringent filter is the shock mounting of the calcite. The design presented here bonds the calcite block with silicon rubbers to the calcite holder. The calcite together with its all necessary polarizers and rotating achromatic plates are mounted together in units called a filter module. By using a set of modules containing calcite crystals of differing lengths, a filter can be produced. A description of the modules is given. Also described is a container for the filter modules, which can be used both to hermetically seal the system or contain an index matching oil. The response of a filter element while being controlled by the Lockheed Temperature Control is described and the determination of the wavelength sensitivity to temperature of calcite is explained. Operation of the filter using a software control algorithm instead of a hardware temperature controller is shown. Some radiation considerations of filter systems are given.

Title, A. M.↗

NASA NTRS1980

Tunable birefringent networks

Consideration is given to mechanically tuned birefringent filters for the spectral range 4500-7500 A with bandpasses as narrow as 50 mA and stability of better than 5 mA. Two tuning techniques are described: optical path change and waveplate tuning. The tuning techniques are mechanically simple and require minimal demands on encoders and drive electronics. Tuning to a random wavelength can be accomplished in less than one second while adjacent wavelengths can be reached in a fraction of a second.

Title, A. M.↗

NASA NTRS1982

Extension of the hole-drilling method to birefringent composites

A complete stress analysis and reliable failure criteria are essential for important structural applications of composites in order to fully utilize their unique properties. The inhomogeneity, anisotropy and inelasticity of many composites make the use of experimental methods indispensable. Among the experimental techniques, transmission photoelasticity has been extended to birefringent composites in recent years. The extension is not straight-forward, in view of the complex nature of the photoelastic response of such model materials. This paper very briefly reviews the important developments in the subject and then describes the theoretical basis for a new method of determining the individual values of principal stresses in composite models. The method consists in drilling very small holes at points where the state of stress has to be determined. Experiments are then described which verify the theoretical predictions. The limitations of the method are pointed out 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 NTRS1984

Fabrication of Multi-Ply Birefringent Fibrous Composite Laminates

Fabrication method produces unidirectional, multi-ply, transparent birefringent fibrous composite laminates for use in macromechanical stress analysis conducted by means of anisotropic photoelasticity. New laminates glass-fiber-reinforced plastics for which matrix and fibers have same index of refraction. Method utilized in structural applications of composites.

Daniel, I.↗

NASA NTRS1984

Small-scale variations in the galactic magnetic field - The rotation measure structure function and birefringence in interstellar scintillations

The structure function of rotation measures of extragalactic sources and birefringence in interstellar scintillations are used to investigate variations in the interstellar magnetic field on length scales of about 0.01-100 pc and 10 to the 11th cm, respectively. Model structure functions are derived for the case of a power-law power spectrum of irregularities in the quantity (n(e)B), and an estimate for the structure function is computed for several regions of the sky using data on extragalactic sources. The results indicate an outer angular scale for rotation measure (RM) variations of not less than about 5 deg (a linear scale of about 9-90 pc at a distance of 0.1-1 kpc). There is also evidence for RM variations on angular scales as small as 1 arcmin, but it cannot be determined whether these are intrinsic to the source or caused by the interstellar medium. The effect of a random, Faraday-active medium on the diffraction of radio waves is derived, and an upper limit to the variations in n(e)B on a length scale of 10 to the 11th cm is obtained from available observations.

Simonetti, J. H.↗

NASA NTRS1988

Spectral contents readout of birefringent sensors

The objective of the research performed was to establish the feasibility of using spectral contents analysis to measure accurately, strains and retardation in birefringent sensors, and more generally, on transparent materials.

Redner, Alex S.↗

NASA NTRS1990

Improved Design For Birefringent Filter

Highly selective laser tuning achieved without thin plates of questionable optical quality. Improved birefringent filter developed for use with broad-band-emission laser. Provides improved narrow-band operation and wavelength selectivity. New filter design improves traditional design by providing method of increasing wavelength separation between highly transmitted peaks. Such broad-band lasers becoming popular in scientific laboratories and useful in military applications and separation of isotopes.

Bair, Clayton H.↗

NASA NTRS1992

Spectral-Content Readout Of Stress-Induced Birefringence

Spectrum of transmitted light indicates stress in sensor or specimen. Photoelastic apparatus demonstrates feasibility of analysis of spectrum of transmitted light to quantify birefringence in transparent specimen. By augmenting conventional photoelastic analysis with spectral sensors and automating it with computer control and processing of data, technique made more versatile and useful. Potential uses include measurement of stresses in optical fibers and transparent materials in general.

Redner, Alex S.↗

NASA NTRS1992

Designing Birefringent Filters For Solid-State Lasers

Mathematical model enables design of filter assembly of birefringent plates as integral part of resonator cavity of tunable solid-state laser. Proper design treats polarization eigenstate of entire resonator as function of wavelength. Program includes software modules for variety of optical elements including Pockels cell, laser rod, quarter- and half-wave plates, Faraday rotator, and polarizers.

Monosmith, Bryan↗

NASA NTRS1993

Spectral-Content Readout Of Birefringent Sensor

Stresses and strains measured optically at temperatures up to 2,000 degrees F. Spectrum of light processed through polarizer, sensor, analyzer, and associated optical components measured and processed to extract amount of retardation and, equivalently, stress-induced birefringence in sensor. Method offers same advantages as electrical strain sensors, including acquisition of data from multiple locations and under dynamic conditions. Additional advantage: immunity of fiber-optic transmission to interference by electromagnetic interference at radio and lower frequencies.

Redner, Alex S.↗

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↗