KEYWORDS: Modulation transfer functions, Data modeling, Wavefronts, Tolerancing, Image quality, Lenses, Point spread functions, Monte Carlo methods, Imaging systems
Ruda designs high performance imaging systems to meet difficult mission requirements, but these nearly diffraction limited systems often have small margins between the image quality of the nominal design and the required performance of the as-built system. Due to this we may spend significant resources designing and operating specialized test setups to ensure that results of MTF and Ensquared Energy (EE) measurements are well-calibrated and accurate. Alternatively, wavefront measurements – like those captured by wavefront sensors and interferometers – can be taken of the system to characterize the quality of the as-built system. Wavefront measurements are typically higher resolution, faster to setup, and quicker to measure than image quality metrics, making them particularly attractive for use when validating as-built system quality. Since the wavefront is related to the point spread function, and thereby the image quality, different wavefront measurements can contain information about the system MTF and EE. Thus, if the relationship between the wavefront and image quality metrics of interest can be established for an as-built system, it is possible to supplement or fully validate MTF and EE requirements from wavefront measurements. To investigate this relationship, we used Zemax OpticStudio to generate toleranced Monte Carlo trials of two nearly diffraction limited imaging systems designed by Ruda. The Monte Carlo models were then analyzed to form large data sets for statistical analysis. For wavefront data, the simulation produces single pass and double pass wavefront Zernike decompositions as well as wavefront root mean squared error over a range of object fields and visible wavelengths. For image quality data, the MTF at three spatial frequencies and the EE at two integration lengths are computed for the same fields and wavelengths as the wavefront data. These data sets are then processed to demonstrate that high degrees of correlation can exist between wavefront data and image quality metrics in toleranced high performance imaging systems, even when there is a difference in wavelength between the metrics. Sources of noise in these correlations are identified, and paths for supplementing or validating image quality requirement with correlated wavefront measurement data through machine learning are discussed.
Threaded mounts are one of the most common interfaces between optical systems and commercial-off-the-shelf cameras. Popular examples include the established C-mount, as well as the newer TFL-mount which accommodates for larger sensor formats such as the APS-C detector. In all cases, the thread is used to adjust for focus by clocking the optical system with respect to a fixed camera assembly or vice versa. For this reason, the alignment between the datum axis of the optical system and the array detector plane inside the camera depends on both the allowances and tolerances of the thread interface, and on the manufacturing tolerances of the mount components. To highlight how the stack up of these tolerances can affect image quality of an optical system, we first perform an inverse sensitivity analysis to determine the detector alignment specification as a function of system F/#, field of view, and chief ray angle. We then calculate the misalignment contributions of the thread between the optical system and the lock ring that sets the camera axial position for best focus. This optomechanical analysis allows us to determine if thread mounts are appropriate for the specifications of the optical system under consideration and to specify the tolerances of the thread interface when this is the case.
Passively athermalized optical systems produce high quality images over a large thermal range without actively adjusting focus. This athermalization is achieved through careful selection of the glass for each lens and metal for each mount. For drop-in systems, the material combination for best optical performance often leads to a lens stack with an overall coefficient of thermal expansion (CTE) that is different from the CTE of the barrel that holds it together. Since bulk glass and metal are relatively stiff, this CTE mismatch results in large variations of the preload force retaining the lens stack in compression over the optical system’s survival thermal range. For this reason, compliant spacers are commonly added to the lens stack in an effort to attenuate these preload force variations. However, the effect of these compliant spacers on the athermalization of optical systems is seldom analyzed. We perform a first-order calculation of the effective CTE of compliant spacers to assess their impact on optical performance and introduce an optomechanical design approach to reduce the amount of compliance needed by matching the overall CTE of the lens stack to the CTE of the barrel.
Optical systems are often athermalized over large temperature ranges through the proper choice of glasses and mounting materials. However, variations in the coefficients of thermal expansion (CTE) and thermo-optical coefficients that govern thermal behavior are seldom included in the tolerance analysis. Manufacturers rarely provide these material tolerances and we can only account for their effects through custom macros in lens design software. We demonstrate that a first-order sensitivity analysis on the change in focus position at each environmental condition accurately predicts the degradation of the system performance. We verified this correlation by creating a custom catalog of identical glasses with perturbed thermal parameters and evaluating the RMS wavefront error for each material substitution.
Adaptive optics systems and their laboratory test environments call for a number of unusual optical components. Examples include lenslet arrays, pyramids, and Kolmogorov phase screens. Because of their specialized application, the availability of these parts is generally limited, with high cost and long lead time, which can also significantly drive optical system design. These concerns can be alleviated by a fast and inexpensive method of optical fabrication. To that end, we are exploring direct-write lithographic techniques to manufacture three different custom elements. We report results from a number of prototype devices including 1, 2, and 3 wave Multiple Order Diffractive (MOD) lenslet arrays with 0.75 mm pitch and phase screens with near Kolmogorov structure functions with a Fried length r0 around 1 mm. We also discuss plans to expand our research to include a diffractive pyramid that is smaller, lighter, and more easily manufactured than glass versions presently used in pyramid wavefront sensors. We describe how these components can be produced within the limited dynamic range of the lithographic process, and with a rapid prototyping and manufacturing cycle. We discuss exploratory manufacturing methods, including replication, and potential observing techniques enabled by the ready availability of custom components.
Measuring masses of long-period planets around F, G, and K stars is necessary to characterize exoplanets and assess their habitability. Imaging stellar astrometry offers a unique opportunity to solve radial velocity system inclination ambiguity and determine exoplanet masses. The main limiting factor in sparse-field astrometry, besides photon noise, is the non-systematic dynamic distortions that arise from perturbations in the optical train. Even space optics suffer from dynamic distortions in the optical system at the sub-μas level. To overcome this limitation we propose a diffractive pupil that uses an array of dots on the primary mirror creating polychromatic diffraction spikes in the focal plane, which are used to calibrate the distortions in the optical system. By combining this technology with a high-performance coronagraph, measurements of planetary systems orbits and masses can be obtained faster and more accurately than by applying traditional techniques separately. In this paper, we present the results of the combined astrometry and and highcontrast imaging experiments performed at NASA Ames Research Center as part of a Technology Development for Exoplanet Missions program. We demonstrated 2.38x10-5 λ/D astrometric accuracy per axis and 1.72x10-7 raw contrast from 1.6 to 4.5 λ/D. In addition, using a simple average subtraction post-processing we demonstrated no contamination of the coronagraph field down to 4.79x10-9 raw contrast.
We have developed a hybrid lithography process necessary to fabricate a vertical optical coupler and an array of waveguide structures using the same buffer coat material on a single substrate. A virtual vernier scale built into the process enables precise alignment of both structures.
A polymer-based flat, flexible and parallel optical interconnect has become an attractive approach for short-range data
transfer. For such a device, a low cost fabrication technique is required for light couplers to redirect light from source to
waveguides. Recently, we demonstrated a mask-less gray scale lithography process, which used a CMOS compatible
polymer for a 45-degree mirror coupler. Polymer materials such as epoclad and AP2210B can be used to fabricate
flexible substrates and waveguides, respectively. We propose an all-photopolymer lithography process to fabricate the
flexible and parallel optical interconnect in conjunction with the mirror couplers. In the process, a buried polymer
structure is used to precisely align the mirror coupler to waveguides, which make it possible to avoid an additional
metallization process. However, the contrast of such buried fiducial mark is low since such the structure is a phase
structure. As a result, it is not feasible to use the buried polymer structure as an alignment mark with conventional
amplitude based imaging modalities. To increase the contrast of these buried alignment marks, we propose a feature
specific alignment system for which the shape and depth of the buried alignment marks are optimized for phase-based
imaging such as phase contrast and Schlieren imaging. Our results show that an optimized alignment mark provides a
significant contrast enhancement while using a phase contrast imaging system compared to that of a conventional
imaging system. In addition, we have fabricated an optimized alignment mark specifically for use with a Schlieren
imaging system.
We report a CMOS compatible fabrication and optical characterization of the micrometer scale optical coupler, a 45°
mirror-based optical coupler for inter-layer optical coupling. A newly proposed mask-based and mask-less hybrid
lithography process enables accurate surface profile of the micrometer sized 45° mirror by using a CMOS compatible
buffer coat material. Surface profile inspected by an optical interferometry agrees well with SEM based inspection
results. Experimental and theoretical results for routing and coupling of laser beam in 90° will be discussed.
We show the design for a laser scanning microscopy defect detection system based upon the idea that the light can reflect off a photoresist-laden fused-silica sample containing defects, allowing height and depth information to be obtained through changes in light intensity. Image registration using predefined points is employed. Image processing techniques involving median and deconvolution filtering are used. Results show that the 2.1-μm resolution of these defects is obtainable, and receiver operating characteristic curves are used for quantifying results. Discriminabilities of 0.73 are achieved. Preliminary results for larger-array patterns through stitching processes are also shown.
We utilized a hybrid lithography technique in the fabrication of a 45 degree micro mirror coupler to be used for a 3D
optical circuit. The hybrid process combines traditional mask-based lithography techniques with mask-less methods.
The result is a CMOS compatible process that can be used for fabrication of integrated micro-optics.
Combining high-contrast imaging and astrometry in a single space mission would enable efficient detection and characterization of single- and multiple- planetary systems around nearby stars, allowing determination of planetary mass, composition, atmosphere, and system architecture. These science goals can be achieved using a 2m wide-field (>0.1deg2) class telescope equipped with two instruments: a high-performance coronagraph to perform direct imaging, and a wide field camera to achieve sub-microarcsecond astrometric accuracy. However, these measurements are only possible if there are no relative distortion changes between astrometric observations. At sub-microarcsecond accuracy regime, even space optics suffers from dynamic distortions in the optical system and dominates the error budget. We propose to utilize a diffractive pupil, in which an array of dots on the primary mirror generates polychromatic diffraction spikes in the focal plane to calibrate the dynamic distortions of the optical system. According to simulations, this technique would allow to obtain 0.2microarcsecond single-visit precision astrometric accuracy. In this paper we present the laboratory results that demonstrate the diffractive pupil concept on wide-field images. We also discuss simulations and experiments performed at the NASA Ames ACE test bed, demonstrating that the diffractive pupil does not affect the coronagraph performance down to 2x10-7. Finally, we assess the compatibility of a diffractive pupil telescope with a general astrophysics mission, showing that the spikes do not impact wide-field observations.
We demonstrated a 45 degree micro mirror by a direct laser writing method. A flat, smooth and clearly defined mirror surface has been fabricated despite of the finite size and long tail of the point spread function of the exposure tool.
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