We begin with the assumption that lens design and the manufacture of optical elements are mature technologies and that further improvements in quality will be small. This leaves assembly and alignment as the only areas for making improvements in precision optical system performance. To this end, we emphasize the importance of considering methods of alignment early in the design phase of a system and explore some of the considerations of the optomechanical design that can lead to easier and more precise optical alignment of systems, particularly systems that are more complex than the one-dimensional case of lenses in a barrel.

The Point Source Microscope (PSM) is used to locate the apex of retroreflecting prisms in 3 degrees of translational freedom with a precision of less than 1 micron. The process is easily explained for right angle prisms, as will be done in this paper, but the explanation is valid for cube corner retroreflectors such as those mounted in spherical balls, spherically mounted retroreflectors, or SMRs, for use with laser trackers. With suitable, simple fixturing, the measurements for all 3 directions are made to a precision of < 1 μm in less than 1 minute.

This paper introduces a new type of drop-in technique used to center passively and accurately lenses in optical mounts. This novel lens mounting method is called edge contact mounting and uses the edge at the intersection of the cylindrical and optical surface of the lens as the mounting interface. By providing a spherical mounting seat for the lens on a simple standard threaded ring, it is possible to center accurately lenses of different geometries, diameters and radius of curvatures. The method allows to relax some manufacturing tolerances compared to rim contact drop-in and is not subject to a minimum clamping angle as for the surface contact mounting. This innovative lens mounting method allows to extend the centering accuracy offered by passive lens centering methods to a next level without compromise on cost and complexity.

As optical designs require increasingly more aspheric elements to achieve high-resolution, high-quality imaging from smaller, more cost-effective systems, active alignment solutions to center and de-tilt aspheres in-situ during assembly are becoming paramount. Traditional active alignment systems, i.e. autocollimators, determine lens orientation from center of curvature (CC) positions of each surface and are consequently limited when measuring aspheres. Such tools measure the paraxial CC; therefore, measurement is inherently blind to orientation of the aspheric edge, leading to centered yet tilted aspheric surfaces and, thus, degraded image quality from the final optical assembly. Current market solutions to measure aspheric tilt consist of external probes directly measuring the aspheric edge; however, this approach involves addition and alignment of multiple precision stages and secondary sensors, adding to the complexity and cost of alignment systems. This paper demonstrates a novel solution for accurately measuring aspheric tilt during the assembly process utilizing the existing capabilities of the Lens Alignment Station (LAS). The LAS is an active alignment tool whose specialized design extends measurement ability beyond the requirement of confocal reflection. Aspheric measurement begins with standard vertex centration using the LAS at confocal position, but for tilt, we image away from confocal position and flood the surface with a high NA objective sampling the aspheric edge. Beyond confocal reflection, the LAS detects retroreflected concentric fringes that trace an orbit as a tilted asphere rotates. The orbit radius is proportional to tilt magnitude, such that simple software calculations accurately yield aspheric tilt measurements without requiring expensive external hardware.

The WFIRST wide field instrument (WFI) includes a slitless spectrometer, which plays an important role in the WFIRST mission for the survey of emission-line galaxies. WFI is building engineering design and environmental test (EDU and ETU) units to reduce risk for the flight grism unit. We report here on successful build and test of the EDU grism. The four-element EDU grism consists of two prism elements and two diffractive elements that provide R700 dispersion. The elements were fabricated with alignment fiducials and integral flats to allow opto-mechanical alignment in six-degrees of freedom. Each element in turn, was installed onto a hexapod and positioned to its nominal orientation relative to the grism deck, then bonded into individual cells. Alignment measurements were performed in situ using theodolites to set tip/tilt and a Micro-vu non-contact Multisensor Measurement System was used to set despace, decenter and clocking of each element using the hexapod. After opto-mechanical alignment, the grism wavefront was measured using an Infrared ZYGO interferometer at various field points extending over a 20 by 14- degree (local) field of view. Using modeled alignment sensitivities, we determined the alignment correction required on our Element 2 prism compensator and successfully minimized the field dependent wavefront error and confocality. This paper details the alignment and testing of the EDU grism at ambient and cold operating temperatures.

Helical drilling and helical cutting using a rotating Dove-prism is a highly advanced manufacturing technology for high-precision laser micro processing. This optical approach is often combined with an ultra-short pulsed laser source to enable the process capability of almost all materials due to the vapor dominated ablation process. The optical system shall be designed and coated for the use of multiple wavelengths based on the wavelength- dependent absorption behavior of different materials. The designed length of the Dove-prism is a function of the wavelength. Therefore the use of more than one wavelength results in an average solution. Moreover the manufacturing deviations in length and angle have to be taken into account. The collimated deviations from the ideal state result in a misalignment causing the optical path to take the shape of a Limacon of Pascal. This effect needs to be minimized to a maximum deviation between the two circles of 1µm. Additional optical elements and degrees of freedom in adjusting the position and angle of the dove prism relatively to the rotation axis are needed. To solve this issue the balancing holder has been expanded to hold four optical elements that each can be rotated around an axis that either lies in a horizontal plane or perpendicular to this. The optical elements are fixed after adjustment in a way that they do not perform any movement even under the load of high speed rotation of up to 15.000RPM. This paper will present the adjusting process for three independent systems to show the overall tolerance that is needed to achieve accuracies of at least 1µm.

NASA’s Global Ecosystem Dynamics Investigation (GEDI) instrument was launched Dec. 5, 2018, and installed on the International Space Station 419 km from Earth. The GEDI is a Light Detection and Ranging (LIDAR) instrument; measuring the time of flight of transmitted laser beams to the Earth and back to determine altitude for geospatial mapping of forest canopy heights. The need for very dense cross track sampling for slope measurements of canopy height is accomplished by using three individual laser transmitter systems, where each beam is split into two beams by a birefringent crystal. Furthermore, one transmitter is equipped with a diffractive optical element splitting the two beams into four, for a total of 8 beams. The beams are reflected off of the features and imaged by an 800 mm diameter Receiver Telescope Assembly, composed of a Ritchey-Chrétien telescope, a refractive aft optics assembly and focal plane array which collects and focuses the light from the 8 probe beams into the 8 science fibers, each with a field of view on the Earth subtending 300 μrad. The dense cross-track sampling mandated a custom designed dual-fiber interface. The science fibers had to be aligned to the nominal, projected laser spots. The alignment was highly dependent on optimization and co-positioning of the fibers pair-wise due to mechanical constraints. This paper presents the end-to-end alignment and metrology of the full optical system from transmitter elements through receiver telescope, aft-optics, focal plane and receiver fibers performed at NASA Goddard Space Flight Center.

The optical performance of refractive plano-convex microlenses is mainly related to the quality of their (a)spherical surface. An efficient tolerancing of this surface is a key step towards the manufacturing of high quality microlenses. However, we demonstrate that the intuitive approach based on geometric parameters such as radius of curvature, conic constant and irregularity is ineffective. We thus propose to use common optical figures of merit, RMS spot size and wavefront aberration, to specify the surface. Not much complexity is added since both quantities can be expressed as analytical functions of the surface in the case of plano-convex microlenses. Such surface specification via direct evaluation of the optical performance offers a better control of the microlens quality.

The wafer-level production of Fused Silica microlens arrays is limited by systematic process non-uniformities. The common molten resist-reflow process with subsequent dry-etching allows for manufacturing of microlens arrays on 200 mm wafers. A thorough process review yielded one free parameter. By switching from the geometrical lens description via radius of curvature and conic constant to a functional assessment via the optical design figure of merit we can describe microlens via their optical quality for the intended application with one parameter for a wide variety of cases. Leveraging these points we show improvements on the uniformity of microlens arrays by a factor of 2 for Fused Silica microlens arrays bigger than 100 mm by 100 mm.

When assembling optical systems, uncertainties of the positioning system and overall mounting tolerances lead to the deterioration of performance due to resulting misaligned optical components. In this paper, we present a novel methodology for the correction-less assembly of optical systems based on predictive tolerance bands. By running a simulation model in parallel to the assembly process, performance predictions can be made during the assembly that take into account the uncertainties of the positioning system. Typically, optical performance can be assessed by a variety of criteria. In this paper, we utilize the Marechal criterion based on the root mean square (RMS) error as it allows to verify if the optical system is defraction-limited. The extension with Monte Carlo methods enables the prediction of mean values and standard deviations for the chosen metric. This is done for the entire optical system yet to be assembled by integrating uncertainties of the positioning system within the simulation framework. Before assembly, a desired threshold (here the RMS value derived from the Marechal criterion) can be specified which is predicted and monitored throughout the assembly process. For verification, we analyze a two-lens system in simulation to demonstrate our proposed framework.

Optical fibers are widely used for collecting and monitoring light signals in modern optical metrology systems. The use of fibers helps reduce the size of optical system and makes the interconnection between systems convenient. But on the other side, the design and analysis of systems containing fibers often go beyond traditional ray-optics modeling. Because the interaction between light and fibers, with core diameter only several micrometers, requires electromagnetic field solvers. In this work, we present a physical-optics-based modeling technique for the complete optical system, including large-scale lenses and micro-scaled optical fibers. We investigate the coupling of electromagnetic fields into fibers using different lens systems, and especially, we perform tolerance analysis of given system with respect to shift and tilt of the fiber, as well as other components in the system.

Long trace profiler (LTP) is used to measure the large radius mirror surface profile. The in-situ LTP can be used to measure the X-ray mirror of an adaptive mirror bending system inside the vacuum chamber. In this study, the in-situ LTP measure head is outside of vacuum chamber. Therefore, the vacuum chamber and window glass thermal effect can introduce errors into the measurement results. This study calculated temperature distribution and deformation using the finite element method (FEM) software and calculate incident ray through the window glass. The incident ray through window glass with thermal gradient could increase optical path difference (OPD). The calculation resulted in an evaluation of in-situ LTP measurement error by thermal deformation.

JANUS (Jupiter Amorum ac Natorum Undique Scrutator) is a high-resolution camera to be flown on board JUICE Spacecraft, devoted to investigate the atmosphere of Jupiter and the surfaces of his icy moons (Europa, Ganymede and Callisto), in the frame of ESA “Cosmic Vision” program. The scientific objectives that JANUS will reach constrained the design of JANUS Optical Head Unit (OHU), and in particular the specific measurement of Ganymede Libration, imposes highly stringent requirement on the Line of Sight (LoS) knowledge of the instrument. The differential thermal environment conditions of the mission orbits, as well as the instrument heat dissipation timelines, induce optical elements translation and rotations that correspond to a Line of Sight variation. During the mission, the LoS can be characterized with a stellar field or single star observation but none direct measurement of its variation can be retrieved during the scientific imaging sessions. To recover the LoS knowledge, a Structural Thermal Optical and Performance Analysis (STOP) is implemented. The optics and the instrument structure are Finit Element Modeled and processed (NASTRAN) imposing the temperature distributions obtained by the OHU Thermal Model (ESATAN-TMS). The obtained thermo-elastic deformations are then considered in the OHU Optical Model (ZEMAX). The resulting LoS and the dominant OHU temperature gradients are finally entangled with a proportionality relation, as well as its direction uncertainty. The indirect estimation of the LoS variation, and its uncertainty, can be establish, thanks to STOP analysis, in real time during operations as function of installed temperature sensor measurements.

The proposed Habitable Exoplanet (HabEx) astrophysics facility is one of four large such facilities being proposed to the 2020 decadal. It is a large telescope that is sensitive to ultraviolet, optical, and near-infrared photons. The proposed design’s overall length is on the order of 17.2 m and its maximum cross section is on the order of 5.25 × 5.25 m. The primary mirror is 4 m in diameter. A transient dynamic analysis was performed to estimate the order of magnitude of ring down time after moving the telescope and pointing at a new target for science planning purposes. Without uncertainty factors, results from a simple re-pointing maneuver indicate that primary to secondary mirror LOS errors are on the order of 10^-4 pico-m after 5 minutes. Also, a frequency response analysis was performed to predict the impact of planned micro-thruster vibrations on required stability. Based on provided noise level associated with the micro-thrusters and loading assumptions and without uncertainty factors, the assessed vibrations do not impact predicted performance requirements.

In this paper, we study the thermo-optical impact of a window on the imaging performance of electro-optical (EO) and infrared (IR) multi-sensors. For demonstration, high-temperature imaging experiment was performed with a broadband EO/IR window heated up to 650°C and the EO/IR multi-cameras were used to capture a blackbody target radiation through the heated window. Both EO/IR cameras captured out-of-focus images as the window was heated at 400°C or above. Image processing technique was performed to extract the thermo-optical spread patterns from impacted images. Results show the significant impact on EO/IR imaging performance as compared to the original unaffected images.

We introduce the concept of stray-light entrance pupil (SL-EP), which can be seen as the analogous regarding stray-light of the entrance pupil in optical design. It takes the shape of the rays footprint which by entering the system undergo stray-light effects before ultimately reaching the detector, modulated by the relative area importance. This paper discusses the properties of the SL-EP and the way it can be computed. The SL-EP can be used to improve stray-light simulations, as it allows to improve significantly the simulation accuracy while limiting the ray-tracing time. For that, sources can be defined based on the SL-EP and non-uniform ray densities can even be used to reach even higher performances. Time reduction factor up to more than 20 times can be demonstrated on the design of the 3MI earth observation instrument. Furthermore, the SL-EP concept can be used to facilitate experimental characterization of spatial point source transmittance maps.

Knowledge of bidirectional reflectance angular distribution of low reflectivity surfaces is important for predicting stray light in optical systems. We have performed bidirectional reflectance distribution measurements at wavelengths of 633 nm and 850 nm for surfaces coated with Z302, a commercial optical coating material widely used in optical instruments. The bidirectional reflectance properties of these surfaces depend on surface topology characterized by thickness and roughness parameters. To explain our results, we have employed directional scatter analysis of the experimental data that can be represented by a linear combination of diffuse, glossy, and specular, components. Modeling the bidirectional reflectance distribution in the context of a ray-trace can provide important information of stray light, coated surfaces, and their impact on performance of optical instruments.

We investigated several curved vane baffle designs coated with specular black paint as an inexpensive manufacturing alternative to traditional diffuse baffles vanes to reduce stray light and heat due to absorption in a standard Cassegrain telescope design configuration. The heat absorption is a very large problem in infrared systems and the specular designs solve this part of the problem. Our study involved simulating two different types of baffle systems, diffuse and specular painted vanes on the main barrel baffle. The first type of baffle consisted of evenly and non-evenly spaced diffusely black coated straight planar vanes on the main barrel baffle and a second type using specular black paint on curved vanes. TracePro, a stray light simulation software from Lambda Research Corporation, was used to simulate and compare each of the nine baffle systems for stray light rejection. The diffuse black painted straight vane baffle design was used as the baseline design to compare results to the other eight designs. In all designs except for the baseline design, TracePro's local downhill simplex optimization method was used to optimize each vane curvature and spacing in the main barrel baffle to reject incoming stray light. These curved vanes were designed to reject stray light back out of the main barrel baffle rather than to be absorbed by diffuse black paint.

Stray light mitigation is a critical aspect in many optical systems and can be approached in many ways. One method to reduce stray light is to incorporate baffle vanes into a system. To go from an ideal baffle vane to a manufacturable baffle adds complexity in how to mechanically reduce stray light. This paper discusses the manufacturable aspect of baffle vane fabrication in determining the optimal orientation a bevel should have on a straight-vane baffle. Various configurations for bevel orientation were analyzed to provide a deeper understanding of past research.

Optical waveguide devices typically have dimensions transverse to the main propagation direction on the order of a fraction of a millimeter and therefore, cannot be modeled by ray or beam tracing techniques. In this small domain, numerical solutions of the fundamental field equations are usually employed. Two such implicit FD-BPM (Finite- Difference Beam Propagation Method) techniques have been integrated into a general optical engineering code: a full vector paraxial and scalar non-paraxial. Along with a more rigorous FDTD (Finite-Difference Time-Domain) calculation, their relative accuracies and efficiencies are compared on the practical 3D problem of coupling an optical system's focused spot into a single-mode fiber using a tapered mode converter. In all but the most extreme cases, the agreement between all three is better than expected especially considering that the runtimes vary drastically on a manycore desktop, with and without the help of a modern number-crunching GPU (Graphics Processing Unit).

The growing importance of diffractive and meta-lenses in modern optical systems makes it vital to investigate and understand their capabilities. They play an important role in various applications like imaging systems, laser-beam shaping, bio/medical-optics, etc. We propose methods for the modeling of diffractive and meta-lenses based on the concept of the fast-physical-optics approach. A diffractive or meta-lens can be modeled as a series of structures functioning locally (e.g. local gratings) on a base interface. Each local structure introduces a certain local phase modulation, and by putting all of them together, the lens functionality can be achieved. In our approach, the rigorous Fourier modal method (FMM), also known as the rigorous coupled wave analysis (RCWA), is applied for the analysis of the local micro-/nanostructures, with all vectorial effects and possible higher-order effects taken into consideration; then the phase modulations can be collected for the lens function modeling. In this manner, a multi-scale simulation of optical systems with diffractive/meta-lenses becomes feasible and efficient in practice.

The conventional finite-difference time-domain (FDTD) algorithm, based on 2nd-order finite difference (FD) approximations to the derivatives in Maxwell’s equations, is a simple and flexible methodology that can be used to solve a wide class of problems, but its accuracy is low unless a very fine grid is used. For grid spacing h=Δx=Δy=Δz the error is (epsilon) ~ (h/λ)⁴ where λ is the wavelength. Putting h → h/2, reduces the error by a factor of 16 but the computation cost rises 16-fold (in three dimensions) because the time step must scale with h to maintain numerical stability. In principle, higher-order FD approximations would improve the accuracy, but they not only complicate the algorithm, but can also render it numerically unstable. We introduced an 8th-order accurate FDTD algorithm with respect to basis function solutions of Maxwell's equations by superposing 2nd-order FDs. This methodology, originally applied to monochromatic propagation, is extended to broadband computations. We validate our methodology on a problem with a known solution.

Silicon photonics offers the ability to fabricate and integrate photonic and electronic components using existing integrated circuit fabrication infrastructure. Recent work seeks to understand the impact of IC process variations on performance of photonic components. In particular, methods for analysis that identify sensitivity of photonic components to process variations are crucial to enable viable design and manufacturing of silicon photonic systems. We present two different and complementary methods for understanding the impact of geometric process variations on photonics components: ensemble statistical virtual fabrication simulations, and adjoint methods. These are utilized to identify the most sensitive regions of a Y-splitter photonic component to line edge roughness (LER) due to inherent lithography and etch process variations. In the ensemble approach, we simulate multiple instantiations with random LER applied to specific sections of the Y-splitter. This enables localization and quantification of LER impact on transmission, phase imbalance, and excess losses. These evaluations, however, come at the cost of many simulations. In adjoint sensitivity evaluation, only one or two simulations can identify regions most sensitive to LER. While first-order linear sensitivity is extracted, the adjoint has challenges in quantifying mean variation impacts. Both methods reveal that the Y-splitter is most sensitive to LER in the input taper, accounting for over 95% of the imbalance transmission. These two methods can be combined to quantify mean, variance, and sensitivity of photonic device components in the face of statistical variations. Incorporated into future photonic process design kits (PDKs), these analysis methods will help designers predict and optimize photonic component performance and yield.

Integrated silicon photonics offers great potential for monolithic integrated photonic and electronic components using existing integrated circuit fabrication infrastructure. However, understanding of the impact of IC process variations on performance of photonic components remains limited. Methods for analysis that identify sensitivity of photonic components to the variety of process variations encountered during fabrication are crucial to enable viable design and manufacturing of silicon photonic systems. We present the application of the adjoint method to predict the impact of different types of particle defects on silicon photonic circuits. The adjoint method is applied for both component and circuit level analysis to reduce computational cost, and shows good consistency with direct simulations. The results for complicated device components and small circuits are shown and discussed. The model and results can be used to help generate layout design rules and critical area extraction methods, and to assist silicon photonics designers in predicting and optimizing yield of complex silicon photonics devices and circuits.

For off-axis and wide angle systems, the calculation, calibration and removal of distortion effects from the images are often challenging tasks. Specific procedures have been implemented to assess and remove the distortion from the images acquired by the OSIRIS imaging instrument on-board the Rosetta ESA mission. OSIRIS consisted in a narrow and a wide angle camera. The Wide Angle Camera (WAC) is an off-axis, unobstructed and wide FoV (i.e. about 12°x12°) optical system. It has a peculiar optical configuration, and due to the off-axis design the camera presents a high level of intrinsic distortion, with the major component being anamorphism. The distortion has been estimated theoretically via raytracing during the design phase, then measured on-ground and inflight during the calibration campaigns. To obtain correct undistorted images, a distortion removal procedure has been implemented. The first step of the process has been to remove from the images the theoretical distortion. Then the distortion correction procedure has been refined using on-ground and in-flight calibration measurements. This work describes in detail the development of the procedure adopted to define, calculate and remove the distortion from the WAC images.

Gratings in optical fibers have been increasingly used in a variety of applications such as sensors and Telecomm. Depending on perturbation separation, they are classified as: fiber Bragg gratings (FBG), and long period gratings (LPG), whose each spectral output offer advantages for certain applications. Nowadays there is a great interest in the study of arrays formed by the combination of long period gratings and Bragg gratings in cascade (CLBG), where the propagation modes of the core and the cladding propagate in the Bragg grating after they propagate in the LPG. In this work, analysis and modeling of Cascaded Long Bragg Gratings using the Transfer Matrix method was performed for the case of two gratings in series along one fiber. We analyzed the variation of the FWHM of the reflectance and transmittance spectra for different values of the difference of the refractive indexes of the core and the perturbation of the grating, using the typical core refractive index of an SMF-28 as reference value. For smaller index difference a narrow intensity peak was observed. After the number of perturbations was varied, when there is a greater number of perturbations in the grating, there is greater intensity in reflectance. However, as our results show, this dependence is not a linear function. The results were obtained under the maximum-reflectivity condition (tuned) for each single grating. The development of the mathematical model, the results of the simulation and the analysis of results are part of the development of the present work.

Chromatic focal shift is used to characterize the variation of focal wavelength of optical system with wavelength. It is an important tool and index to analyze the color difference of optical system. In this paper, the function relationship between focal length and wavelength of optical system is analyzed by using chromatic focal shift. According to the dispersion characteristics of the optical system material, the Conrady formula which fits the refractive index curve well with less data is selected. Simulations show that the chromatic focal shift of most of monochromatic wavelength systems in the 400-1000 nm wavelength range is monotonically increasing and can be expressed by the Conrady dispersion formula. The chromatic focal shift of achromatic systems which consists of a variety of glass materials usually has an inflection point, and the shapes of most of the achromatic chromatic focal shift are the same, so the formula for the curve should be consistent.The calculation results show that the Conrady formula can also solve the curve of the achromatic system effectively in a relatively short band, such as 400nm~700nm. In fact, the chromatic aberration correction of most achromatic systems is limited, and the effective working band is short. Therefore, the Conrady formula is a very good expression for both monochrome and achromatic systems. By studying the chromatic focal shift analytical equation of the optical system, it provides a reference for the theoretical calculation and detection of focal length at a specific wavelength.