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NASA NTRS1983

Far-field distributions of semiconductor phase-locked arrays with multiple contacts

Experimental results on far-field patterns of semiconductor laser arrays with multiple contacts are reported. It is found that, by tailoring the distribution of the currents through the array elements, narrow single-lobe patterns, which are more useful in most applications, can be obtained from arrays that usually operate in a double-lobe mode. A diffraction-limited 1.8 deg-wide far field pattern was obtained from a three-element array.

Katz, J.↗

NASA NTRS1984

Detection of extrasolar planets by the large deployable reflector

The best wavelength for observing Jupiter-size planetary companions to stars other than the Sun is one at which a planet's thermal emission is strongest; typically this would occur in the far-infrared region. It is assumed that the orbiting infrared telescope used is diffraction-limited so that the resolution of the planet from the central star is accomplished in the wings of the star's Airy pattern. Proxima Centauri, Barnard's Star, Wolf 359, and Epsilon Eridani are just a few of the many nearest main-sequence stars that could be studied with the large deployable relfector (LDR). The detectability of a planet improves for warmer planets and less luminous stars; therefore, planets around white dwarfs and those young planets which have sufficient internal gravitational energy release so as to cause a significant increase in their temperatures are considered. If white dwarfs are as old as they are usually assumed to be (5-10 billion yr), then only the nearest white dwarf (Sirius B) is within the range of LDR. The Ursa Major cluster and Perseu cluster are within LDR's detection range mainly because of their proximity and young age, respectively.

Hollenbach, D. J.↗

NASA NTRS1983

Large Deployable Reflector (LDR) - A concept for an orbiting submillimeter-infrared telescope for the 1990s

The history and background of the Large Deployable Reflector (LDR) are reviewed. The results of the June 1982 Asilomar (CA) workshop are incorporated into the LDR science objectives and telescope concept. The areas where the LDR may have the greatest scientific impact are in the study of star formation and planetary systems in the own and nearby galaxies and in cosmological studies of the structure and evolution of the early universe. The observational requirements for these and other scientific studies give rise to a set of telescope functional requirements. These, in turn, are satisfied by an LDR configuration which is a Cassegrain design with a 20 m diameter, actively controlled, segmented, primary reflector, diffraction limited at a wavelength of 30 to 50 microns. Technical challenges in the LDR development include construction of high tolerance mirror segments, surface figure measurement, figure control, vibration control, pointing, cryogenics, and coherent detectors. Project status and future plans for the LDR are discussed.

Swanson, P. N.↗

NASA NTRS1984

Science with the solar optical telescope

The Solar Optical Telescope (SOT) is designed to provide the solar physics community with the data necessary for solving several fundamental problems in the energetics and dynamics of the solar atmosphere. Among these problems are questions on the origin and evolution of the sun's magnetic field, heating of the outer solar atmosphere, and sources of the solar wind in the lower lying regions of the outer atmosphere. The SOT will be built under the management of NASA's Goddard Space Flight Center, with science instruments provided by teams led by Principal Investigators. The telescope will be built by the Perkin-Elmer Corporation, and the science instruments selected for the first flight will be provided by the Lockheed Palo Alto Research Laboratory (LPARL) and the California Institute of Technology, with actual construction of a combined science instrument taking place at the LPARL. The SOT has a 1.3-meter-diameter primary mirror that will be capable of achieving diffraction-limited viewing in the visible of 0.1 arc-second. This dimension is less than a hydrodynamic scale-height or a mean-free-path of a continuum photon in the solar atmosphere. Image stability will be achieved by a control system in the telescope, which moves both the primary and tertiary mirrors in tandem, and will be further enhanced by a correlation tracker in the combined science instrument. The SOT Facility is currently scheduled for its first flight on Spacelab at the beginning of the 1990's.

Jordan, S. D.↗

NASA NTRS1984

Nonstandard representations of aspheric surfaces in a telescope design

Space-based telescopes for sub-mm astronomy may require large, segmented primary mirrors. While spherical segments are the easiest to manufacture and test, a fast sphere produces large aberrations which must be corrected by one or more aspheric surfaces. Attention is presently given to an f/10 system with an f/0.7 spherical primary mirror and a field of view of + or - 1.5 min of arc. The third and fourth mirrors are conics, and the secondary is a 24th-degree power series on a base sphere. For sub-mm astronomy, a reasonable target for this design is a diffraction-limited image at 30 microns. In an effort to improve performance, the Wassermann-Wolf (1949) approach to aspheric design was applied.

Rodgers, J. M.↗

NASA NTRS1984

Ultranarrow linewidth, magnetically switched, long pulse, xenon chloride laser

A spectral linewidth of less than 70 fm and diffraction-limited beam divergence have been obtained from a long-pulse, electric-discharge xenon chloride laser with intracavity Fabry-Perot etalons. A gain duration of 100 ns provided for multipass operation of the etalons, significantly improving both contrast and finesse. The electrical-discharge circuit required to produce this long gain duration was comprised of a pulse-forming network, a saturable-inductor magnetic switch, and a tapered constant-impedance interface transmission line.

Pacala, T. J.↗

NASA NTRS1984

Chirped arrays of diode lasers for supermode control

Nonuniform structures of phase-locked diode lasers, which make it possible to discriminate efficiently against all the higher order array supermodes (lateral modes) are proposed. In these nonuniform arrays, the effective mode index in each channel varies across the array. Consequently, the envelopes of the various supermodes, including the highest order one, differ significantly from each other. Thus, by proper tailoring of the gain distribution across the array, one can conveniently select the fundamental supermode. Such fundamental supermode oscillation is essential in order to obtain single lobe, diffraction limited beams and minimal spectal spread from phase-locked laser arrays.

Kapon, E.↗

NASA NTRS1984

Single longitudinal mode operation of an XeCl laser

Single longitudinal mode output from a directly tuned, long pulse, magnetically switched XeCl laser is reported for the first time. Mode control was achieved using spatial apertures and intracavity etalons. The narrow bandwidth laser output was diffraction limited with an optical pulse width of 30 ns, which would yield a transform limited bandwidth of 30 MHz.

Pacala, T. J.↗

NASA NTRS1984

Single contact tailored gain phased array of semiconductor lasers

A single contact tailored gain-guided array is demonstrated in which the gain profile across the array is made strongly asymmetric by varying the width of the contact stripes. A proton isolated arary of six (GaAl)As lasers with 5-micron separations and widths varying linearly between 3 and 8 microns had a single lobed far field 2 deg wide, close to the diffraction limit for a single supermode. Fabrication of this device is simple, and suited to large-scale processing techniques. It is also shown that in such an asymmetric gain-guided array the fundamental mode is favored over higher order modes, and that higher order modes can have single lobed far-field patterns differing only slightly from that of the fundamental.

Lindsey, C. P.↗

NASA NTRS1983

NASA astrophysics - Optical systems to explore the universe

Major and minor NASA astrophysical research efforts in the near-term are outlined, together with projections of direction for future projects. The Space Telescope is being readied for a 1986 launch and will feature an f/24, 2.4 m aperture, an MgF2 mirror with better than 1/60 wavelength accuracy and will be diffraction-limited in the UV. Pointing accuracy is designed to be 0.007 arcsec for 24 hr. Optical, spectrometric, and photometric equipment will be included. Around 1990, Shuttle-based missions will include an IR telescope and a subarcsec solar surface imaging device. A free-flying X-ray observatory (AXAF) is planned and will include a sensitivity that exceeds that of the HEAO-2 spacecraft by two orders of magnitude. Instruments are under development for higher resolution UV, gamma-ray, and IR studies. In-orbit interferometry is being studied and will depend on in-orbit assembly and servicing of stable structures with segmented optics.

Pellerin, C. J., Jr.↗

NASA NTRS1984

The Structure of the Uranian Rings and the Search for Rings Around Neptune

The nine narrow rings of Uranus, presently the only confirmed features of this system, have been observed with the diffraction-limited resolution (3.5 km) of ground-based occultations since their discovery in 1977. These data have been used to establish an orbit model, from which the five Keplerian orbit parameters for each ring, the pole of the mean ring plane, and the gravitational harmonic coefficients J sub 2 and J sub 4 have been determined. The rings are typically a few kilometers wide with eccentricities of about 0.001 and inclinations of a few hundredths of a degree, although a few have no measurable eccentricity or inclination. Interesting Voyager investigations would include probing the structure of the rings at higher spatial resolution, searching for new rings in the system (including inter-ring material), locating the postulated shepherd satellites, and searching for satellites inside the orbit of Miranda that might have dynamical influence on the rings. The high precision (approx 2 km) of the ring orbits might prove useful for spacecraft navigation. For Neptune, occultation searches have revealed no rings to a limit of a few hundreths optical depth, within a few hundred kilometers from the top of the planet's atmosphere (for equatorial rings).

Elliot, J. L.↗

NASA NTRS1983

Restoration of multichannel microwave radiometric images

A constrained iterative image restoration method is applied to multichannel diffraction-limited imagery. This method is based on the Gerchberg-Papoulis algorithm utilizing incomplete information and partial constraints. The procedure is described using the orthogonal projection operators which project onto two prescribed subspaces iteratively. Some of its properties and limitations are also presented. The selection of appropriate constraints was emphasized in a practical application. Multichannel microwave images, each having different spatial resolution, were restored to a common highest resolution to demonstrate the effectiveness of the method. Both noise-free and noisy images were used in this investigation.

Chin, R. T.↗

NASA NTRS1984

Study of alternate optical and fine guidance sensor designs for the space infrared telescope facility (SIRTF)

A unique optical design was developed that compensates for the coma degraded images caused by field chopping in SIRTF. The conic constants of a Cassegrain telescope were altered to compensate for the coma induced by the secondary mirror tilt. The modulation transfer function is essentially independent of secondary mirror tilt, and diffraction limited image quality is maintained over a several arcminute field during chopping. With an untilted secondary mirror, the coma compensated (CC) design has a smaller field than the unchopped Ritchey-Chretien design; but use of relay optics, such as the inverted Cassegrain design developed for the fine guidance sensor (FGS), can increase the CC telescope's field size. A reactionless secondary mirror chopper mechanism that uses superconducting magnets was studied. The heart producing elements are confined to a reaction plate that is not directly viewed by the IR focal plane. A design was also developed for a low moment of inertia, reticulated HIP beryllium secondary mirror consistent with blank fabrication technology and optical finishing requirements.

Wissinger, A.↗

NASA NTRS1984

SIRTF: The next step

The scientific and technical background and prospects for the space infrared telescope facility (SIRTF) are described. This facility is a superfluid-helium-cooled, 0.85-meter infrared telescope to be placed in orbit in 1993. It is designed to carry out photometry over the wavelength range 2 to 700 micrometers, and diffraction-limited imagery in either broad or narrow spectral bands over the range 1.8 to 200 micrometers. It is proposed that SIRTF measure spectra in the range 2.5 to 200 micrometers with resolving power between 50 and 1000 and that the focal plane contain about 20,000 detector elements, both discrete and in arrays. The SIRTF observatory is designed to be a long-lived facility providing opportunities for general investigations by the entire scientific community. For following up the all-sky survey carried out by the Infrared Astronomical Satellite (IRAS), SIRTF is ideal. It can do a deep survey to flux levels 5000 times fainer than IRAS and can obtain spectra of even the faintest IRAS sources.

Gillett, F. C.↗

NASA NTRS1985

Stokes parameters modulator for birefringent filters

The Solar Birefringent Filter (Filter Polarisiant Solaire Selectif FPSS) of Meudon Observatory is presently located at the focus of a solar refractor with a 28 cm lens directly pointed at the Sun. It produces a diffraction limited image without instrumental polarization and with a spectral resolution of 46,000 in a field of 6 arc min. diameter. The instrument is calibrated for absolute Doppler velocity measurements and is presently used for quantitative imagery of the radial velocity motions in the photosphere. The short period oscillations are recorded. Work of adapting the instrument for the imagery of the solar surface in the Stokes parameters is discussed. The first polarizer of the birefringent filter, with a reference position angle 0 deg, is associated with a fixed quarter wave plate at +45 deg. A rotating quarter wave plate is set at 0 deg and can be turned by incremented steps of exactly +45 deg. Another quarter wave plate also initially set at 0 deg is simultaneously incremented by -45 deg but only on each even step of the first plate. A complete cycle of increments produces images for each of the 6 parameters I + or - Q, I + or - U and I + or - V. These images are then subtracted by pairs to produce a full image in the three Stokes parameters Q, U and V. With proper retardation tolerance and positioning accuracy of the quarter wave plates, the cross talk between the Stokes parameters was calculated and checked to be minimal.

Dollfus, A.↗

NASA NTRS1984

Fundamental mode oscillation of a buried ridge waveguide laser array

An eight-element phase-locked array of index-guided separate confinement ridge AlGaAs diode lasers is fabricated. In this array the absorption of light in the region between lasers is negligible and the gain profile across the array is nearly uniform. Unlike most other arrays, this array oscillates in its fundamental mode. Stable radiation patterns of near diffraction-limited single narrow beam with 1.6 deg width are obtained. The beam width approaches the theoretical limit for the present array structure.

Mukai, S.↗

NASA NTRS1983

Deployable reflector configurations

Both the theoretical reasons for considering a non-circular format for the Large Deployable Reflector, and a potentially realizable concept for such a device, are discussed. The optimum systems for diffraction limited telescopes with incoherent detection have either a single filled aperture, or two such apertures as an interferometer to synthesize a larger aperture. For a single aperture of limited area, a reflector in the form of a slot can be used to give increased angular resolution. It is shown how a 20 x 8 meter telescope can be configured to fit the Space Shuttle bay, and deployed with relatively simple operations. The relationship between the sunshield design and the inclination of the orbit is discussed. The possible use of the LDR as a basic module to permit the construction of supergiant space telescopes and interferometers both for IR/submm studies and for the entire ultraviolet through mm wave spectral region is discussed.

Meinel, A. B.↗

NASA NTRS1984

Perception for a large deployable reflector telescope

Optical science and technology concepts for a large deployable reflector for far-infrared and submillimeter astronomy from above the earth's atmosphere are discussed. Requirements given at the Asilomar Conference are reviewed. The technical challenges of this large-aperture (about 20-meter) telescope, which will be diffraction limited in the infrared, are highlighted in a brief discussion of one particular configuration.

Breckinridge, J. M.↗