Results of experiments with the laser guide star adaptive optics system on the 3-meter Shane telescope at Lick Observatory have demonstrated a factor of 4 performance improvement over previous results. Stellar images recorded at a wavelength of 2 micrometers were corrected to over 40 percent of the theoretical diffraction-limited peak intensity. For the previous two years, this sodium-layer laser guide star system has corrected stellar images at this wavelength to approximately 10 percent of the theoretical peak intensity limit. After a campaign to improve the beam quality of the laser system, and to improve calibration accuracy and stability of the adaptive optics system using new techniques for phase retrieval and phase-shifting diffraction interferometry, the system performance has been substantially increased. The next step will be to use the Lick system for astronomical science observations, and to demonstrate this level of performance with the new system being installed on the 10-meter Keck II telescope.
A phase-shifting diffraction interferometer (PSDI) has been integrated into an adaptive optics (AO) system developed by LLNL for use on the three meter Shane telescope at Lick Observatory. The interferometer is an all fiber optic design, which is extremely compact. It is useful for calibrating the control sensors, measuring the aberrations of the entire AO optical train, and measuring the influence functions of the individual actuators on the deformable mirror. The PSDI is particularly well suited for this application because it measures converging, quasi-spherical wavefronts, such as are produced by an AO imaging system. Thus, a PSDI can be used to measure the aberrations of the entire AO system, in-situ and without errors introduced by auxiliary optics. This provides an extremely accurate measurement of the optical properties of the AO system.
Any adaptive optics system must be calibrated with respect to internal aberrations in order for it to properly correct the starlight before it enters the science camera. Typical internal calibration consists of using a point source stimulus at the input to the AO system and recording the wavefront at the output. Two methods for such calibration have been implemented on the adaptive optics system at Lick Observatory. The first technique, Phase Diversity, consists of taking out of focus images with the science camera and using an iterative algorithm to estimate the system wavefront. A second technique sues a newly installed instrument, the Phase-Shifting Diffraction Interferometer, which has the promise of providing very high accuracy wavefront measurements. During observing campaigns in 1998, both of these methods were used for initial calibrations. In this paper we present results and compare the two methods in regard to accuracy and their practical aspects.
The electrostatic comb finger drive has become an integral design for microsensor and microactuator applications. This paper reports on utilizing the levitation effect of comb fingers to design vertical-to-the-substrate actuation for interferometric applications. For typical polysilicon comb drives with 2 micrometers gaps between the stationary and moving fingers, as well as between the microstructures and the substrate, the equilibrium position is nominally 1-2 micrometers above the stationary comb fingers. This distance is ideal for many phase shifting interferometric applications. Theoretical calculations of the vertical actuation characteristics are compared with the experimental result, and a general design guideline is derived from these result. The suspension flexure stiffness, gravity forces, squeeze film damping, and comb finger thicknesses are parameters investigated which affect the displacement curve of the vertical microactuator. By designing a parallel plate capacitor between the suspended mass and the substrate, in situ position sensing can be used to control the vertical movement, providing a total feedback-controlled system. Fundamentals of various capacitive position sensing techniques are discussed. Experimental verification is carried out by a Zygo distance measurement interferometer.
The features and operation of an electro-optically switched binary optical time delay system are discussed. THe system based on polarization switching using the low cost ferro- electric liquid crystal and polarizing beam splitters provides compactness, low complexity, low insertion loss and arbitrary time delay. We present the design, component selection, fabrication, testing, and evaluation of a prototype.
It is well known that gratings with high spatial frequency (spacings <EQ (lambda) /2) show form birefringence. Therefore, gratings formed in dielectric materials can act as a wave plate when the grating spacing is smaller than, or near to, half of the incident beam wavelength. Previous researchers used high frequency surface relief structures or sinusoidal gratings formed in photoresists to produce this effect. In this paper we show the performance of a volume holographic quarterwave plate formed in DCG operating at 632.8 nm. To our knowledge this is the first demonstration of a retardation plate exhibiting this magnitude of phase delay in a volume material. The fabrication techniques required to realize this element are also presented.
Daylighting techniques are an effective means of reducing both lighting and cooling costs; however, many of the standard techniques have flaws which reduce their effectiveness. Daylighting holograms are an efficient and effective method for diffracting sunlight up onto the ceiling, deep in a room, without diffracting the light at eye-level. They need only cover the top half of a window to produce significant energy savings. They may be used as part of a new glazing system or as a retrofit to existing windows. These holograms are broadband and are able to passively track the movement of the sun across the sky, throughout the day and year.
Zeroth-order effective medium theory is applied to analyze the properties of volume holographic gratings that have periods much smaller than the illumination wavelength. These holograms, called zeroth order gratings, can be modeled as negative uniaxial films using the quasistatic field approximation. Exact analytic expressions for the form birefringence of gratings with sinusoidal, rectangular, and triangular permittivity profiles are obtained and compared. Approximate equations are derived that indicate a quadratic dependence of the form birefringences on the permittivity modulation. It is determined that the optimum rectangular profile yields the greatest form birefringence.
The rigorous coupled-wave model is used to determine the polarization of diffracted waves from two superimposed volume gratings with their grating vectors in the same plane. The phase relations used in computing the phase shift of diffracted beams are derived. The relative phase difference between orthogonal components of the + 1 diffracted order is computed for a single grating and two superimposed gratings. The level of induced ellipticity in polarization is greater for light diffracted from a multiplexed grating than from a single grating. Experimental and theoretical results show that the phase shift of a beam diffracted from two superimposed gratings has both positive and negative values.
The refractive index of holographic emulsions is an important parameter needed for designing holographic optical elements (HOEs). In this paper theoretical calculations of the accuracy required for the refractive index and thickness of emulsions needed to meet predetermined Bragg angle conditions are presented. A modified interferometric method is used to find average refractive index of the unexposed and the developed dichromated gelatin holographic films. Slanted transmission HOEs are designed considering the index and thickness variations, and used to verify the index measurement results. The Brewster angle method is used to measure surface index of the unexposed and the developed films. The differences between average index and surface index are discussed. Theoretical calculation of the effects of index variation on diffraction efficiency, and experimental results for index modulation variation caused by process changes are also presented.