Bruce R. Peters

Cryogenic optical testing results for the Subscale Beryllium Mirror Demonstrator (SBMD)

An Optical Testing System (OTS) has been developed to measure the figure and radius of curvature of Next Generation Space Telescope (NGST) developmental mirrors in a vacuum, cryogenic environment using the X-Ray Calibration Facility (XRCF) at Marshall Space Flight Center (MSFC). The OTS consists of a WaveScope Shack-Hartmann sensor from Adaptive Optics Associates as the main instrument and a Leica Disto Pro distance measurement instrument. Testing is done at the center of curvature of the test mirror and at a wavelength of 632.8 nm. The error in the figure measurement is <EQ(lambda) /13 peak-to-valley (PV). The error in radius of curvature is less than 5 mm. The OTS has been used to test the Subscale Beryllium Mirror Demonstrator (SBMD), a 0.532-m diameter spherical mirror with a radius of curvature of 20 m. SBMD characterization consisted of three separate cryogenic tests at or near 35 K. The first two determined the cryogenic changes in the mirror surface and their repeatability. The last followed cryo-figuring of the mirror. This paper will describe the results of these tests. Figure results will include full aperture results as well as an analysis of the mid-spatial frequency error results. The results indicate that the SBMD performed well in these tests with respect to the requirements of (lambda) /4 PV (full aperture), (lambda) /10 PV (mid-spatial, 1-10 cm), and +/- 0.1 m for radius of curvature after cryo-figuring.

AMSD test error budget sensitivity analysis

The successful augmentation of NASAs X-Ray Cryogenic Facility (XRCF) at the Marshall Space Flight Center (MSFC) to an optical metrology testing facility for the Sub-scale Beryllium Mirror Development (SBMD) and NGST Mirror Sub-scale Development (NMSD) programs required significant modifications and enhancements to achieve useful and meaningful data. In addition to building and integrating both a helium shroud and a rugged and stable platform to support a custom sensor suite, the sensor suite was assembled and integrated to meet the performance requirements for the program. The subsequent evolution from NMSD and SBMD testing to the Advanced Mirror System Demonstrator (AMSD) program is less dramatic in some ways, such as the reutilization of the existing helium shroud and sensor support structure. However, significant modifications were required to meet the AMSD programs more stringent test requirements and conditions resulting in a substantial overhaul of the sensor suite and test plan. This overview paper will discuss the instrumentation changes made for AMSD, including the interferometer selection, null optics, and radius of curvature measurement method. The error budgeting process will be presented, and the overall test plan developed to successfully carry out the tests will be discussed.

Compact optical interconnect module

The results of a design study for the development of an eye- safe (near-infrared wavelength), compact, multichannel optical interconnect system appropriate for integration with electronics and to be used for short distance communication are discussed. There are potential advantages to using optical interconnects instead of current hardwire interconnections for data transmission over short distances. This technology also has potential applications to data transmission for computing applications. This design study focused on the development of an optical interconnect module to function much like a conventional data cable. The module must be rugged, small, easily integrated into current data transfer, and must have the potential to be produced in volume and at lost cost. The desired system level performance of the optical interconnects was evaluated and design specifications were determined for the optical design. Trade studies involving current technologies were performed to determine suitable hardware configurations. These requirements pointed toward the application of microfabrication technology and micro-optics in order to accomplish the design goals. A pseudo-monolithic silicon-based optical system has been proposed involving diffractive and microrefractive optics along with integrated sensors and emitters. The device emphasizes the use of existing technologies gathered from different disciplines and integrated into one system.

Compact infrared zoom lens for the 3- to 5-um spectral band

The results of a comprehensive design study for the development of a compact infrared zoom lens suitable for use in guided munitions are discussed. The continuously variable zoom of the lens offers significant operational performance benefits to weapon systems using fixed or switchable FOV optics. Two practical zoom lens systems were designed that showed potential to meet typical guided munitions system requirements by utilizing in the first system conventional surfaces and a combination of conventional and diffractive surfaces in the second system. Significant weight savings, enhanced optical performance, and excellent athermalization over conventional lenses were realized. The optical performance over the entire 4:1 zoom range and 5-20 degrees field-of-view is near-diffraction limit while maintaining a constant F-number.

Lightweight lidar telescopes for space applications

NASA is intent on exploiting the unique perspective of space-based remote optical instruments to observe and study largescale environmental processes. Emphasis on smaller and more affordable missions continues to force the remote sensing instruments to find innovative ways to reduce the size, weight, and cost of the sensor package. This is a challenge because many of the proposed instruments incorporate a high quality meter-class telescope that can be a significant driver of total instrument costs. While various methods for telescope weight reduction have been achieved, many of the current approaches rely on exotic materials and specialized manufacturing techniques that limit availability or substantially increase costs. A competitive lightweight telescope technology that is especially well suited to space-based coherent Doppler wind lidar has been developed through a collaborative effort involving NASA Marshall Space Flight Center (MSFC) through the Global Hydrology and Climate Center (GHCC) and the University of Alabama in Huntsville (UAH) at the Center for Applied Optics (CAO). The new lightweight optics using metal alloy shells and surfaces (LOMASS) fabrication approach is suitable for high quality metal mirrors and meter-class telescopes. Compared to alternative materials and fabrication methods the new approach promises to reduce the areal density of a meter-class telescope to less than 15-kg/m2; deliver a minimum ?/1O-RMS surface optical quality; while using commercial materials and equipment to lower procurement costs. The final optical figure and finish is put into the mirrors through conventional diamond turning and polishing techniques. This approach is especially advantageous for a coherent lidar instrument because the reduced telescope weight permits the rotation of the telescope to scan the beam without requiring heavy wedges or additional large mirrors. Ongoing investigations and preliminary results show promise for the LOMASS approach to be successful in demonstrating a novel alternative approach to fabricating lightweight mirrors with performance parameters comparable with the Space Readiness Coherent Lidar Experiment (SPARCLE). Development and process characterization is continuing with the design and fabrication of mirrors for a 25-cm telescope suitable for a lidar instrument.

Preliminary investigation of an active PLZT lens

The design analysis and preliminary testing of a prototype AFOCL is described. The AFOCL is an active optical component composed of solid state lead lanthanum-modified zirconate titanate (PLZT) ferroelectric ceramic with patterned indium tin oxide (ITO) transparent surface electrodes that modulate the refractive index of the PLZT to function as an electro- optic lens. The AFOCL was developed to perform optical re- alignment and wavefront correction to enhance the performance of Ultra-Lightweight Structures and Space Observatories. The AFOCL would be an active optical component within a larger optical system. Information from a wavefront sensor would be processed to provide input to the AFOCL to drive the sense4d wavefront tot he desired shape and location. While offering variable and rapid focusing capability similar to liquid crystal based spatial light modulators, the AFOCL offers some potential advantages because it is a solid-stat, stationary, low-mass, rugged, and thin optical element that can produce wavefront quality comparable to the solid refractive lens it replaces. The AFOCL acts as a positive or negative lens by producing a parabolic phase-shift in the PLZT material through the application of a controlled voltage potential across the ITO electrodes. To demonstrate the technology, a 4 mm diameter lens was fabricated to produce 5-waves of optical power operating at 2.051 micrometers wavelength. Optical metrology was performed on the device to measure focal length, optical quality, and efficiency for a variety of test configurations. Preliminary data was analyzed and compared to idealized performance available from computer-based models of the AFOCL.

Primary mirror manufacturing considerations for a space-based coherent lidar

The measurement of winds from a space borne platform is of significant scientific importance to both weather prediction and climate research. One of the key technologies embodied in coherent detection of winds from space is the use of large aperture, compact, lightweight, high-quality wavefront, photon-efficiency optics. This paper discusses the optical design, the mechanical design, material preference, diamond turning issues, polishing requirements, and coating selections for the primary mirror of a 25X afocal beam expander intended for use in space-based coherent lidar systems.

Thermal considerations for the SPARCLE optical system

The SPAce Readiness Coherent Lidar Experiment (SPARCLE) is the first demonstration of a coherent Doppler wind lidar in space. Coherent lidars can accurately measure the wind velocity by extracting the Doppler frequency shift in the back-scattered signal from the atmosphere through optical heterodyne (coherent) detection. Coherent detection is therefore highly sensitive to aberrations in the signal phase front, and to relative alignment between the signal and the local oscillator beams. The telescope and scanning optics consist of an off-axis Mersenne telescope followed by a rotating wedge of silicon and a window of fused silica. The wedge is in very close proximity to the experiment window, and is essentially in contact with the scanner motor/encoder system. The can environment temperature is nominally 20 degrees Celsius, the window ranges from -20 degrees Celsius to 0 degrees Celsius, and the scanner motor/encoder system alone could generate temperatures as high as 35 degrees Celsius. This thermal environment, coupled with the relatively large sensitivity of silicons refractie index to temperature, has required careful thermal design and compensation techniques. This paper discusses the optical issues of these thermal effects and a variety of methods used to ameliorate them.

Characterization of an optical subsystem for 2-μm coherent lidars

This paper presents the test results on a compact, off-axis telescope which is the precursor projector/receiver for a NASA Shuttle-based coherent lidar system operating at a wavelength of 2 microns to measure atmospheric wind profiles. The afocal telescope has an entrance pupil diameter of 25 cm, and an angular magnification of 25x. To determine the transmitted and returned optical wavefront quality, the telescope was tested in a Twyman-Green configuration at the operational wavelength. Interferograms were obtained via an infrared camera, and analyzed using a digitizing tablet and WYKO WISP software. Interferograms were obtained with and without an 11.7 degree wedged silicon window located in the entrance pupil. This window, which rotates orthogonal to the telescope optical axis, serves as the lidar system scanner. The measured wavefront information from the interferometer was used in a GLAD heterodyne receiver model to predict the effect of the optical system on the lidar performance. The experimental setup and procedures will be described, and the measurement results of the coherent lidar optical subsystem will be presented in this paper.

Optomechanical design of a multi-axis stage for the SPARCLE telescope

A critical component in the 2-micrometer coherent spaced-based lidar system (SPARCLE) is the compact, off-axis, 25-cm aperture telescope. The stressing optical performance demanded from this telescope coupled with the difficulty associated with aligning such a fast, off-axis system; has created the need for a multiple-axis alignment stage for the secondary mirror. Precision micrometer kinematic mounts were used in the laboratory to demonstrate the ability to successfully align the telescope. For the flight configuration, a more robust and considerably smaller stage (both in size and weight) had to be designed in order to fit within the space shuttle packaging constraints. The new stage operates with multiple degrees of freedom of motion to achieve micrometer precision alignment and then uses a mechanical multiple point support to lock-in the alignment and provide stability. The optomechanical design of the flight stage is described.