The components in optical projectors are becoming increasingly smaller due to the need for increased output resolution and the desire for small form-factor devices. One such component is Liquid Crystal (LC) panels, that utilize periodic micro-lens arrays which become more sensitive to diffractive effects as the period becomes near/sub wavelength. This paper explores the diffraction effects within these systems through numerical modeling. Traditionally Ray tracing techniques have been used for analyzing projection systems and has led to significant improvements in illumination uniformity and efficiency. However, increasingly complex projector designs that incorporate smaller geometric features like micro/nano lens arrays, including coherent diffraction and interference effects arising from such structures, cannot be handled by ray-tracing approaches alone. Rigorous electromagnetic (EM) wave optics based techniques, such as finite-difference time-domain (FDTD) and rigorous coupled wave analysis (RCWA) which solve Maxwell’s equations must be used. These rigorous EM techniques, however, have difficulty in analyzing the larger projector structures due to computational resource limitations. We use a mixedlevel optical simulation methodology which unifies the use of rigorous EM wave-level and ray-level tools for analyzing projector performance. This approach uses rigorous EM wave based tools to characterize the LC panel through a Bidirectional Scattering Distribution function (BSDF) file. These characteristics are then incorporated into the ray-tracing simulator for the illumination and imaging system design and to obtain the overall performance. Such a mixed-level approach allows for comprehensive modeling of the optical characteristic of projectors, including coherent effects, and can potentially lead to more accurate performance than that from individual modeling tools alone.
This paper presents a case study in using the CODE V optical design software to model an astronomical adaptive system end to end. Such models can be useful in predicting system performance and in garnering a better understanding of the process of adaptive correction. Described in this paper is the model itself and the custom macros that are extensively used throughout to measure the wavefront aberration; drive the adaptive system and collect the resulting data. Of special interest is the algorithm used to create the turbulence screen. The turbulence created by this sine wave summation algorithm is in keeping with Kolmogorov statistics that are commonly used to describe atmospheric turbulence at astronomical sites.
We report on new results of simultaneous measurements of sodium layer column density and the absolute return flux from laser guide stars created by a monochromatic approximately 1 W CW laser, tuned to the peak of the sodium D2 hyperfine structure. The return was measured at the MMT while the sodium abundance was measured at the CFA 60 inch telescope, about 1 km away, with the Advanced Fiber Optic Echelle spectrograph. The laser frequency stability, which can greatly affect the return flux, was monitored at the same time in order to improve the measurement accuracy. After the correction for laser frequency jitter and atmospheric transmission, the absolute flux return above the atmosphere for circularly polarized light is 1.2 X 106 photons s-1 m-2 per watt launched above the atmosphere, per unit column density, which we taken as our measured mean over the year of N(Na) equals 3.7 X 109 cm-2 at Tucson. The solidification of a final well-determined relationship between the sodium laser guide star brightness and sodium layer column density is pivotal in the design of the next generation laser guide star adaptive optics systems. We also report the measurements and analysis of the relationship between the projected beam waist of the sodium laser and the resultant spot size on the sodium layer under typical atmospheric conditions. Since wavefront measured error is proportional to spot size, and also to 1/(root) power, minimum spot size is crucial for lowest laser power requirement. By projecting the laser through diffraction limited optics of 0.5 m diameter, roughly 3 r0, we have achieved the smallest artificial beacon yet recorded, about 0.8 arcsec.
The FASTTRAC II adaptive optics instrument has been used at the Multiple Mirror Telescope (MMT) for the past 2 years to provide improved image resolution in the near infrared. Results have been obtained using both natural guide stars and an artificial sodium laser beacon. With the imminent closure of the MMT prior to its conversion to a single-mirror 6.5 m telescope, FASTTRAC II has come to the end of its life. The instrument has been to the telescope for a total of 8 runs, and during that time it has been of enormous value both as a learning aid, demonstrating the requirements of its successor on the 6.5 m, and as a scientific tool. At this meeting, we present a selection of astrophysical data derived from FASTTRAC II, including the first closed-loop demonstration of an adaptive optics system using a sodium laser beacon. The sodium laser has been used to obtain near diffraction-limited near-infrared images of the core of M13, allowing the construction of a color-magnitude diagram to below the main sequence turnoff. Results have also been obtained from several gravitationally lensed quasars, and the cores of nearby galaxies in the local group. We also summarize work characterizing atmospheric conditions at the site. These studies have proceeded in two areas - understanding the behavior of the phase perturbation with field angle and time, and characterizing the return from the sodium resonance beacon.
The first images of astronomical objects have been obtained with a telescope exploiting wavefront compensation with adaptive optics where the reference beacon was generated by laser excitation of mesospheric sodium. This was done using the FASTTRAC II low-order adaptive optics system at the multiple mirror telescope (MMT). FASTTRAC II is a prototype for a full-scale adaptive optics system under construction for the 6.5 m telescope that will replace the MMT in late 1997. The 6.5 m system is designed to provide correction to the diffraction limit of resolution in the near infrared (1 - 5 micrometer) with high Strehl ratio and excellent sky coverage. This paper describes the new system and its expected performance in view of the achieved performance of FASTTRAC II.
A new adaptive optics system has been constructed for moderately high resolution in the near infrared at the Multiple Mirror Telescope (MMT). The system, called FASTTRAC II, has been designed to combine the highest throughput with the lowest possible background emission by making the adaptive optical element be an existing and necessary part of the telescope, and by eliminating all warm surfaces between the telescope and the science camera's dewar. At present, only natural guide stars are supported, but by the end of 1995, we will add the capability to use a single sodium resonance beacon derived from a laser beam projected nearly coaxially with the telescope. In this paper, we present a description of FASTTRAC II, and show results from its first test run at the telescope in April 1995.
A sodium guide star has been used to sense and correct atmospheric aberration during two runs at the Multiple Mirror Telescope (MMT). For the first run in 1993 May, the artificial star was created by a 0.5 W beam from a continuous- wave dye laser tuned to the D2 resonance line, projected from a telescope centered and coaxial with the main array of six 1.8 m mirrors. Scattering by the mesospheric sodium layer produced an artificial beacon equivalent in brightness to a natural star of visual magnitude 12.5, and of angular extent 1'.2 full width at half maximum (FWHM). During the second run in 1994 February, a 1.7 W dye laser was used to generate an artificial guide star of visual magnitude 10.4, and 1'.1 FWHM. In each case, the beacon was used by the MMT adaptive optics system to compensate in real time for atmospherically- induced differential image motion between the six mirror elements, at correction rates of up to 76 Hz. In the latter experiment, global wavefront tilt correction using a natural reference star was added, giving complete adaptive control. Simultaneously recorded images of a natural star coincident with the laser beacon show significantly reduced width and an increase in Strehl ratio of almost a factor of two.
We have obtained the first measurements with a sodium laser beacon of focus anisoplanatism over large aperture, at the Multiple Mirror Telescope. In complementary studies, the atmospheric turbulence at the high altitude and on the large scale responsible for the measured focus anisoplanatism was explored by observations of binary stars of different separations. We confirm the predictions of Kolmogorov theory, and derive an effective height for the turbulence of 5050 m above the telescope. These results confirm that the sodium laser guide star planned for use with the 6.5 m telescope conversion of the MMT in 1996 will allow diffraction limited infrared observations in the H and K bands.
Adaptive correction of infrared images formed by large telescopes will be possible with the aid of a single laser guide star projected along the telescope axis. The fundamental limit set by focus anisoplanatism of a sodium beacon allows correction to the diffraction limit in the 1.65 and 2.2 micron bands under typical conditions at a good site. A 1 arcsec `star' of V magnitude approximately equals 9 is then required to reduce photon noise to negligible level. In this paper we report on recent tests of continuous wave dye lasers at wavelength 589 nm, used to create sodium beacons above the Multiple Mirror Telescope. A coherent ring dye laser operating at 2.5 W average power and projected as circularly polarized light yielded a beacon of mv equals 9.85, and a Livermore-designed standing wave laser operating at 1.7 W and projected as linearly polarized light yielded mv equals 10.4. Beacon sizes close to 1 arcsec were achieved. The 9th magnitude goal should be achieved by using more advanced cw dye lasers with 20% conversion efficiency of a 25 W argon ion pump laser, and from improvements in beam projection efficiency. We describe a refractive beam expander under construction for the 6.5 m conversion of the MMT.
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