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Techniques and devices using whispering gallery mode (WGM) optical resonators, where the optical materials of the WGM resonators exhibit an electro-optical effect to perform optical modulation. Examples of actively mode-locked lasers and other devices are described.
We are developing high-energy grazing incidence shell optics for hard-x-ray telescopes. The resolution of a mirror shells depends on the quality of cylindrical mandrel from which they are being replicated. Mid-spatial-frequency axial figure error is a dominant contributor in the error budget of the mandrel. This paper presents our efforts to develop a deterministic cylindrical polishing process in order to keep the mid-spatial-frequency axial figure errors to a minimum. Simulation software is developed to model the residual surface figure errors of a mandrel due to the polishing process parameters and the tools used, as well as to compute the optical performance of the optics. The study carried out using the developed software was focused on establishing a relationship between the polishing process parameters and the mid-spatial-frequency error generation. The process parameters modeled are the speeds of the lap and the mandrel, the tool s influence function, the contour path (dwell) of the tools, their shape and the distribution of the tools on the polishing lap. Using the inputs from the mathematical model, a mandrel having conical approximated Wolter-1 geometry, has been polished on a newly developed computer-controlled cylindrical polishing machine. The preliminary results of a series of polishing experiments demonstrate a qualitative agreement with the developed model. We report our first experimental results and discuss plans for further improvements in the polishing process. The ability to simulate the polishing process is critical to optimize the polishing process, improve the mandrel quality and significantly reduce the cost of mandrel production
Next-generation pushbroom sensors for earth observation require high-performance optics that provide high spatial resolution over wide fields of view. Specifically, blur diameters on the order of 10 to 15 microns are needed over 5 to 15 deg fields. In addition to this fundamental level of optical performance, other characteristics, such as spatial coregistration of spectral bands, flat focal plane, telecentricity, and workable pupil location are significant instrument design considerations. The detector-assembly design, optical line-of-sight pointing method and sensor packaging all hinge on these secondary attributes. Moreover, the need for broad spectral coverage, ranging from 0.4 to 12.5 microns, places an additional constraint on optical design. This paper presents alternative design forms that are candidates for wide-field pushbroom sensors, and discusses the instrument-design tradeoffs that are linked to the selection of these alternate optical approaches.
The James Web Space Telescope (JWST) is a large, infrared-optimized space telescope scheduled for launch in 2011. This is a continuation of a series of papers on modeling activities for JWST. The structural-thermal-optical, often referred to as STOP, analysis process is used to predict the effect of thermal distortion on optical performance. The benchmark STOP analysis for JWST assesses the effect of an observatory slew on wavefront error. Temperatures predicted using geometric and thermal math models are mapped to a structural finite element model in order to predict thermally induced deformations. Motions and deformations at optical surfaces are then input to optical models, and optical performance is predicted using either an optical ray trace or a linear optical analysis tool. In addition to baseline performance predictions, a process for performing sensitivity studies to assess modeling uncertainties is described.
The aero-optical effects associated with propagating a laser beam through both an aircraft turbulent boundary layer and artificially generated shear layers are examined. The data present comparisons from observed optical performance with those inferred from aerodynamic measurements of unsteady density and correlation lengths within the same random flow fields. Using optical instrumentation with tens of microsecond temporal resolution through a finite aperture, optical performance degradation was determined and contrasted with the infinite aperture time averaged aerodynamic measurement. In addition, the optical data were artificially clipped to compare to theoretical scaling calculations. Optical instrumentation consisted of a custom Q switched Nd:Yag double pulsed laser, and a holographic camera which recorded the random flow field in a double pass, double pulse mode. Aerodynamic parameters were measured using hot film anemometer probes and a five hole pressure probe. Each technique is described with its associated theoretical basis for comparison. The effects of finite aperture and spatial and temporal frequencies of the random flow are considered.
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The James Webb Space Telescope (JWST) consists of an infrared-optimized Optical Telescope Element (OTE) that is cooled down to 40 degrees Kelvin. A second adjacent component to the OTE is the Integrated Science Instrument Module, or ISIM. This module includes the electronic compartment, which provides the mounting surfaces and ambient thermally controlled environment for the instrument control electronics. Dissipating the 200 watts generated from the ISIM structure away from the OTE is of paramount importance so that the spacecraft's own heat does not interfere with the infrared light detected from distant cosmic sources. This technical challenge is overcome by a thermal subsystem unit that provides passive cooling to the ISIM control electronics. The proposed design of this thermal radiator consists of a lightweight structure made out of composite materials and low-emittance metal coatings. In this paper, we will present characterizations of the coating emittance, bidirectional reflectance, and mechanical structure design that will affect the performance of this passive cooling system.
Phillips Laboratory's Long Duration Exposure Facility (LDEF) optical experiment is designed to determine the adverse effects of the natural space environment on laser optical component and coating materials. The experiment consists of 10 sample sets, each containing six different material samples. The materials were chosen because of their common use in laser optical components. Sample characterization is divided into three phases. Phase 1 testing is limited to visual and optical performance evaluation. Phase 2 tests investigate the fundamental causes of the performance degradation quantified in Phase 1. During Phase 3, selected samples will be cleaned and some Phase 1 measurements repeated to determine if acceptable optical performance can be restored, and laser damage testing will be performed on a small number of samples. Performance deteriorations will be correlated to exposure duration, sample location on the LDEF, atomic oxygen levels, and other space environment conditions. Preliminary results obtained on the optical samples are discussed.
Due to its simplicity and robustness against wavefront distortion, pulse position modulation (PPM) with photon counting detector has been seriously considered for long-haul optical wireless systems. This paper evaluates the dual-pulse case and compares it with the conventional single-pulse case. Analytical expressions for symbol error rate and bit error rate are first derived and numerically evaluated, for the strong, negative-exponential turbulent atmosphere; and bandwidth efficiency and throughput are subsequently assessed. It is shown that, under a set of practical constraints including pulse width and pulse repetition frequency (PRF), dual-pulse PPM enables a better channel utilization and hence a higher throughput than it single-pulse counterpart. This result is new and different from the previous idealistic studies that showed multi-pulse PPM provided no essential information-theoretic gains than single-pulse PPM.
A Cassegrainian solar concentrator, using a 7-meter diameter primary reflector, was analyzed in three forms: (1) an unmodified Cassegrainian, (2) the Ritchey-Chretien configuration, and (3) the unmodified Cassegrainian with a non-imaging tertiary reflector. Optical performance was not significantly improved with the Ritchey-Chretien; however, the tertiary resulted in significant improvement in intercept factor and optical efficiency. The effects of misalignment of the secondary and tertiary reflectors on the optical performance of the collector were also analyzed.
During cryogenic vacuum testing of the James Webb Space Telescope (JWST) Integrated Science Instrument Module (ISIM), the global alignment of the ISIM with respect to the designed interface of the JWST optical telescope element (OTE) will be measured through a series of optical characterization tests. These tests will determine the locations and orientations of the JWST science instrument projected focal surfaces and entrance pupils with respect to their corresponding OTE optical interfaces. If any optical performance non-compliances are identified, the ISIM will be adjusted to improve its performance. In order to understand how to manipulate the ISIM's degrees of freedom properly and to prepare for the ISIM flight model testing, a series of optical-mechanical analyses have been completed to develop and identify the best approaches for bringing a non-compliant ISIM element into compliance. In order for JWST to meet its observatory-level optical requirements and ambitious science goals, the ISIM element has to meet approximately 150 separate optical requirements. Successfully achieving many of those optical requirements depends on the proper alignment of the ISIM element with respect to the OTE. To verify that the ISIM element will meet its optical requirements, a series of cryogenic vacuum tests will be conducted with an OTE Simulator (OSIM). An optical Ray Trace and Geometry Model tool was developed to help solve the multi-dimensional alignment problem. The tool allows the user to determine how best to adjust the alignment of the JWST ISIM with respect to the ideal telescope interfaces so that the approximately 150 ISIM optical performance requirements can be satisfied. This capability has not existed previously.
Traditionally, evaluating the optical performance of mirrors derived from finite element (FEA), with third-party software requires cumbersome manual effort – which impacts efficient design optimization. This paper presents a process that seamlessly integrates FEA results with wavefront analysis software, significantly simplifying performance assessment and enabling rapid design iteration. We leverage the 4D technology interferometer and "4Sight" wavefront analysis software for performance measurement and comparison with FEA predictions. Recognizing the need for efficient data exchange, we developed an application that converts FEA data into a format compatible with 4Sight. The software takes two user-defined inputs: optical surface position and deformation information, typically provided as CSV or TXT files. Within a second, it generates an output file suitable for direct import into 4Sight, enabling immediate visualization of key optical performance metrics.
Avalanche photodiodes (APDs) operating in the Geiger mode can provide a significantly improved single-photon detection sensitivity over conventional photodiodes. However, the quenching circuit required to remove the excess charge carriers after each photon event can introduce an undesirable dead time into the detection process. The effect of this detector dead time on the performance of a binary pulse-position-modulated (PPM) channel is studied by analyzing the error probability. It is shown that, when background noise is negligible, the performance of the detector with dead time is similar to that of a quantum-limited receiver. For systems with increasing background intensities, the error rate of the receiver starts to degrade rapidly with increasing dead time. The power penalty due to detector dead time is also evaluated and shown to depend critically on badkground intensity as well as dead time. Given the expected background strength in an optical channel, therefore, a constraint must be placed on the bandwidth of the receiver to limit the amount of power penalty due to detector dead time.
Avalanche photodiodes (APDs) operating in the Geiger mode can provide a significantly improved single-photon detect ion sensitivity over conventional photodiodes. However, the quenching circuit required to remove the excess charge carriers after each photon event can introduce an undesirable dead time into the detection process. The effect of this detector dead time on the performance of a binary pulse-position-modulted (PPM) channel is studied by analyzing the error probability. It is shown that, when back- ground noise is negligible, the performance of the detector with dead time is similar to that o f a quantum-limited receiver. For systems with increasing background intensities, the error rate of the receiver starts to degrade rapidly with increasing dead time. The power penalty due to detector dead time is also evaluated and shown to depend critically on background intensity as well as dead time. Given the expected background strength in an optical channel, therefore, a constraint must be placed on the bandwidth of the receiver to limit the amount of power penalty due to detector dead time.
The Geostationary Operational Environmental Satellite (GOES) Telescope is designed to be passively temperature compensated so that focus requirements will be met over a broad range of temperatures. Concerns over the effects of temperature gradients on the optical performance of the telescope and the repeatability of the 'pointing error' of the telescope spawned the need for a detailed thermal/optical test. The telescope temperature compensation system, the thermal environment in which it must work and the test setup used to measure optical performance under varying temperature conditions are discussed in this paper.
The comparative advantages of synthetic aperture telescopes (SATs) of segmented primary mirror and common secondary mirror type, on the one hand, and on the other those employing an array of independent telescopes, are discussed. The diffraction-limited optical performance of both redundant and nonredundant subaperture configurations are compared in terms of point spread function characteristics and encircled energy plots. Coherent imaging with afocal telescope SATs involves a pupil-mapping operation followed by a Fourier transform one. A quantitative analysis of the off-axis optical performance degradation due to pupil-mapping errors is presented, together with the field-dependent effects of residual design aberrations of independent telescopes.
The Lucy Jupiter-Trojan asteroid mission launched in November 2021. Its original mission concept included six small-body encounters over its 12-year primary mission. In the fall of 2022, an additional target of opportunity encounter was proposed to be executed in the fall of 2023. The encounter with (152830) Dinkinesh (previously 1999 VD57) presented myriad imaging, navigation, engineering, and planning challenges, as well as a chance to exercise and further refine the Optical NavigationSystem concept of operations, interfaces, and tools. Dinkinesh would be the smallest and dimmest target Lucy would encounter, with a higher uncertainty in these physical parameters than for other targets. While the Op Nav system and instruments carried a high amount of heritage from the New Horizons and OSIRIS-REx missions, this would be the first use of these systems on Lucy for navigation purposes. Despite these additional challenges, the Lucy Dinkinesh encounter was a resounding success throughout which the navigational system exceeded requirements.1Optical Navigation was successfully performed and fed into the orbit determination and trajectory maneuver activities up to the final knowledge update. The Dinkinesh encounter also proved to be greatly scientifically interesting, if not additionally challenging, as the Dinkinesh system was discovered to be a binary system through imaging during closest approach, and the secondary body was itself found to be a contact binary. This added complexity notwithstanding, the OpNav and OD teams were able to re-construct the close-approach trajectory of Dinkinesh in cooperation and concert with the Lucy Science Team’s shape modelling efforts.
Candidate technologies for a lightweight primary mirror for the SOFIA telescope are evaluated for both mirror blank fabrication and polishing. Two leading candidates for the type mirror blank are considered: the frit-bonded, structured form, and the thin meniscus form. The feasible mirror is required to be very lightweight with an areal density of approximately 100 kg/sq m, have an f/ratio near 1.0, and have surface quality that permits imaging in the visible as well as the infrared. Also considered are the results of a study conducted to assess the feasibility of designing a suitable mounting system for the primary mirror. The requirements for the mount design are given both in terms of the environmental conditions and the expected optical performance. PATRAN and NASTRAN programs are used to model mirror and mounting. The sandwich-type mirror made of ultra low expansion silica with square cells in the core, is modeled using equivalent solid elements for the core. The design study produces primary mirror surface deflections in 1g as a function of mirror elevation angles. The surface is analyzed using an optical analysis program, FRINGE, to give a prediction of the mirror optical performance. Results from this analysis are included.