In the last 10 years aspheres have readily gone from new products and specialized components to wide acceptance in the market. Successful fabrication of aspheres requires all parts of the process chain including design, production, and measurements. Aspheres now are well-established and accepted as an equal optical element, when done properly. This segment has been the fastest growing market of all optical elements. Research and industry have now started to focus efforts to develop the next new element that propels the field forward in capability, namely the optical freeform surface. An essential factor enabling wide use of freeforms is communicating requirements. This manuscript provides an example monolithic freeform element using the recently updated relevant parts of ISO 10110. The first manufacturing of this component has been successful, and this manuscript shows the role the ISO standard has played in success. Specifically the description of the complex freeform element, as well as definitions for toleranced parameters such as surface registration (centration) and form deviation (irregularity, slope, Zernike, pv, and pvr), are indicated. The provided example also shows how to use the defined datums and definitions for metrology and data handling.
We present a novel approach to tolerancing slope errors of aspheric surfaces in relay optics of typical avionics head-up displays (HUD). In these systems, a beamlet entering the pilot eye occupies only a tiny fraction of HUD entrance pupil/eyebox with a typical diameter of 125mm. Consequently the beam footprint on any HUD optical surface is a small fraction of its clear aperture. This presents challenges to HUD tolerancing which is typically based on parallax (angular difference in line of sight between left and right eyes) analysis. Aspheric surfaces manufactured by sub-aperture grinding/polishing techniques add another source of error – surface slope error. This type of error not only degrades image quality of observed HUD symbology but also leads to its “waviness” and “floating” especially noticeable when a pilot moves his head within the HUD eyebox. The suggested approach allows aspheric surface slope error tolerancing that ensures an acceptable level of symbology “waviness”. A narrow beamlet is traced from a pilot eye position backwards through the HUD optics until it hits the light source. Due to the small beamlet size, slope error of the aspheric surface acts primarily as an overall tilt/wedge that deviates the beam and causes it to shift. The slope error is acceptable when this shift is not resolved by a pilot eye. The beamlet is scanned over entire eyebox and field of view and the slope error tolerance is established for several zones in the aspheric surface clear aperture. The procedure is then repeated for each aspheric surface.
The evolution from spherical, to aspheric, to freeform optics is quickly progressing towards more complex freeform surfaces. Freeform surfaces typically have little to no symmetry making the alignment of the surfaces difficult. The alignment of such freeform surfaces relative to the other features on the optic has been little considered. A typical alignment specification like wedge (edge thickness difference) is not well defined for freeform optics, nor is the wedge measurement. We show that by using fiducials during the manufacturing of freeform surfaces, the alignment and locating of the freeform surface can be specified and measured.
Aspheres are becoming very popular in optical design for reducing size and weight of optical systems or even allowing for novel optical performance. Innovative manufacturing enables this trend. Unfortunately, due to the type of surface form deviation introduced by CNC based grinding and polishing processes, tolerancing an optical system with aspheric surfaces becomes very complicated. Especially for serial production it is critical to know just how “good” an asphere has to be in order to guarantee the optical performance needed without overdoing it. It will be demonstrated how to overcome this limitation up to a certain level.
Setting a tolerance for the slope errors of an optical surface (e.g., surface form errors of the “mid-spatial-frequencies”) requires some knowledge of how those surface errors affect the final image of the system. While excellent tools exist for simulating those effects on a surface-by-surface basis, considerable insight may be gained by examining, for each surface, a simple sensitivity parameter that relates the slope error on the surface to the ray displacement at the final image plane. Snell’s law gives a relationship between the slope errors of a surface and the angular deviations of the rays emerging from the surface. For a singlet or thin doublet acting by itself, these angular deviations are related to ray deviations at the image plane by the focal length of the lens. However, for optical surfaces inside an optical system having a substantial axial extent, the focal length of the system is not the correct multiplier, as the sensitivity is influenced by the optical surfaces that follow. In this paper, a simple expression is derived that relates the slope errors at an arbitrary optical surface to the ray deviation at the image plane. This expression is experimentally verified by comparison to a real-ray perturbation analysis. The sensitivity parameter relates the RMS slope errors to the RMS spot radius, and also relates the peak slope error to the 100% spot radius, and may be used to create an RSS error budget for slope error. Application to various types of system are shown and discussed.
Optical systems are designed to provide a specific functionality. However, a built optical system shows some deviations from the nominal performance caused by the manufacturing process. This tutorial will demonstrate the tolerancing process using Nijboer-Zernike polynomials as an expression of wave front aberrations. Nijboer-Zernike polynomials are a special form of well-known circle polynomials developed by Zernike. They are orthogonal and have an automatic balancing of aberrations of various orders. The degradation of the Strehl Ratio can be estimated very easily for every single aberration using the value of the specific coefficient. This property is very beneficial for the definition of a tolerance budget.
The design of a 6-in, f/2.2 transmission sphere for Fizeau interferometry is presented in this paper. To predict the actual performance during design phase, we build an interferometer model combined with tolerance analysis in Zemax. Evaluating focus imaging is not enough for a double pass optical system. Thus, we study the interferometer model that includes system error, wavefronts reflected from reference surface and tested surface. Firstly, we generate a deformation map of the tested surface. Because of multiple configurations in Zemax, we can get the test wavefront and the reference wavefront reflected from the tested surface and the reference surface of transmission sphere respectively. According to the theory of interferometry, we subtract both wavefronts to acquire the phase of tested surface. Zernike polynomial is applied to transfer the map from phase to sag and to remove piston, tilt and power. The restored map is the same as original map; because of no system error exists. Secondly, perturbed tolerances including fabrication of lenses and assembly are considered. The system error occurs because the test and reference beam are no longer common path perfectly. The restored map is inaccurate while the system error is added. Although the system error can be subtracted by calibration, it should be still controlled within a small range to avoid calibration error. Generally the reference wavefront error including the system error and the irregularity of the reference surface of 6-in transmission sphere is measured within peak-to-valley (PV) 0.1 λ (λ=0.6328 um), which is not easy to approach. Consequently, it is necessary to predict the value of system error before manufacture. Finally, a prototype is developed and tested by a reference surface with PV 0.1 λ irregularity.
The Near Infrared Camera (NIRCam) instrument used to align and obtain science data for NASA’s James Webb Space Telescope (JWST) was tested at the module level at flight-like cryogenic temperature. This paper explains the background that created the innovative techniques used to measure NIRCam’s modules alignments in 6 degrees of freedom (DOF) inside a thermal vacuum chamber. All 6 DOF were measured remotely, through a single chamber window port, using only a flat reference mirror/reticle surface mounted on each module. This measured orientation was then used to determine the optical input axis and entrance pupil for each module. The accuracy achieved was on the order of 20 microns in position and 5 arc seconds in angular orientation.
The Near Infrared Camera (NIRCam) instrument used to align and obtain science data for NASA’s James Webb Space Telescope (JWST) was tested at the module level at flight-like cryogenic temperature. This paper explains the innovative techniques used to measure the precise location and orientation of the modules. A laser tracker was used to precision locate the instrument, using a flat reference mirror/reticle surface on the modules inside a chamber through its port windows. This technique established 6 degrees of freedom of position and orientation. The accuracy achieved was on the order of 20 microns in position and 5 arc-seconds in angular orientation.
Although the terms “micropositioning” and “nanopositioning” refer to different classes of positioning systems, “nanopositioning” is often used mistakenly to describe micropositioning systems. Micropositioning systems are typically motor-driven stages with travel ranges of a few millimeters up to a few hundred millimeters. Because the guiding systems in such stages — usually bearings of some kind — generate frictional forces, their resolution and repeatability are typically limited to 0.1 μm. The guiding system working principle also adds errors that are typically in the micrometer range. Nanopositioning systems are typically based on frictionless drives and guiding systems such as piezo actuators and flexures. These systems can achieve resolutions and guiding accuracies down to the sub-nanometer level. Both of these classes of precision positioning and motion systems are used extensively in precision optical and photonic systems to achieve desired performance specifications of instruments and experimental research projects. Currently, many precision positioning and motion systems have been design and implemented to cross over from the micro to the nano ranges with excellent results. This paper will describe some of the fundamental performance parameters and tolerances typical of these systems, some of the metrology used to confirm specifications and a few high end applications of general interest.
High-precision micro laser drilling with high aspect ratios requires laser imaging effects such as optical double rotation. Optical double rotation is an effect where the laser beam is guided through any optical elements with a total amount of reflections that remains uneven. Those optical elements need to be mounted in a rotary stage that spins the elements with a certain velocity. In an ideal case the optical axis is identically with the rotational axis. Few optical elements such as the Dove-prism show the effect that the beam is rotated in itself while it is moving on a helical path. That offers an independency of the beam profile. However the Dove-prism alone can not be adjusted in a way that the two axis match. This is based on geometrical errors of the Dove-prism due to manufacturing technologies. Certain deviation in length and angle lead to a helical error. Additional optical elements can compensate this effect. Alignment that only takes place in one 2D plane (e.g. the focal plane) leads most likely to a cross-over of both axes (x-alignment) in that one plane. In order to match both axes the alignment needs to be done at least in two 2D planes. That requires the opportunity to both influence the optical angle and the optical position (parallel shift) in both planes. The highly complex optical alignment method as well as the mechanical storage of the optical elements will be shown in this paper.
Threaded rings are used to fix lenses in a large portion of opto-mechanical assemblies. This is the case for the low cost drop-in approach in which the lenses are dropped into cavities cut into a barrel and clamped with threaded rings. The walls of a cavity are generally used to constrain the lateral and axial position of the lens within the cavity. In general, the drop-in approach is low cost but imposes fundamental limitations especially on the optical performances. On the other hand, active alignment methods provide a high level of centering accuracy but increase the cost of the optical assembly.
This paper first presents a review of the most common lens mounting techniques used to secure and center lenses in optical systems. Advantages and disadvantages of each mounting technique are discussed in terms of precision and cost. Then, the different contributors which affect the centering of a lens when using the drop-in approach, such as the threaded ring, friction, and manufacturing errors, are detailed. Finally, a patent pending lens mounting technique developed at INO that alleviates the drawbacks of the drop-in and the active alignment approaches is introduced. This innovative auto-centering method requires a very low assembly time, does not need tight manufacturing tolerances and offers a very high level of centering accuracy, usually less than 5 μm. Centering test results performed on real optical assemblies are also presented.
Automated alignment of optical systems saves the time and energy needed for manual alignment and is required in cases where manual intervention is impossible. This research discusses the state estimation of the misalignment of a reimaging system using a focal plane sensor. We control two moving lenses to achieve high precision alignments by feeding back state estimates calculated from images from a CCD camera. We perform a Principal Component Analysis (PCA) on a simulated data set. The monochromatic images are decoupled into Karhunen- Loève (KL) modes, which are used as the measurement in state estimation. An Extended Kalman filter (EKF) is used to estimate the misalignment of the optical components, and we describe a closed-loop control system with monochromatic beam to demonstrate the performance of the state estimation process. The state and measurement residuals converge with the Kalman observer. The automated alignment technique can be extended to reconfigurable systems with multiple lenses and other optical components.
In this study, we performed alignment state estimation simulations and compared the performance of two Computer Aided Alignment (hereafter CAA) algorithms i.e. ‘Merit Function Regression (MFR)’ and ‘Multiple Design Configuration Optimization (MDCO)’ for a TMA optical system. The former minimizes the merit function using multi-field wavefront error measurements from single configuration, while the latter minimizes the merit function using single-field measured wavefront error from multiple configurations. The optical system used is an unobscured three-mirror anastigmat (TMA) optical system of 70mm in diameter, and F/5.0. It is designed for an unmanned aerial vehicle for coastal water remote sensing. The TMA consists of two aspherical mirrors, a spherical mirror and a flat folding mirror. Based on the sensitivity analysis, we set the tilt x, y of tertiary mirror as a compensator, and not considered decenter of tertiary mirror because of its spherical characteristic. For the simulation, we introduced Gaussian distribution of initial misalignment to M3. It has the mean value of zero and standard deviation of 0.5 mrad. The initial simulation result of alignment state estimation shows that both algorithms can meet the alignment requirement, λ/10 RMS WFE at 633nm. However, when we includes measurement noise, the simulation result of MFR shows greater standard deviation in RMS WFE than that of MDCO. As for the measurement, the MDCO requires single on-axis field while the MFR requires multiple fields, we concluded that the MDCO is more practical method to align the off-axis TMA optics than MFR.
Focal plane assembly (FPA) is an important component for modern remote sensing instruments. Array or linear CCD/CMOS detectors are usually applied. Linear detectors are often used in the space project to have a wider swath. In a remote-sensing project, five spectral ranges are desired. The spectral ranges are designated to be one panchromatic (PAN) band from 450 to 700 nm and four multispectral (MS) bands from 455 to 900 nm. Pixel size of the PAN band is 10 μm, and those for MS are 20 μm. Pixel numbers are 12,000 and 6,000 for PAN and MS bands, respectively. The FPA is consisted of a filter with five stripe band-pass thin films, filter mask and a five-line detector. The arrangement of the detector is B1, B2, Pan, B3 and B4 from the top to the bottom. In order to give the alignment tolerance, an analysis of the parameters of each component in FPA has been performed. Ray tracing method has been applied to have an image projection onto the plane where thin films are located. Spread sheet computation was adopted to simulate the situations when the alignment parameters were changed. According to the analysis, some supplement wideness to the stripe thin films has to be added to have a satisfied alignment tolerance.
A Prototype of Fast-steering Secondary Mirror (FSMP) for the Giant Magellan Telescope (GMT) has been developed by the consortium consisting of institutes in Korea and the US. In 2014 the FSMP development was finalized by combining the two major sub-systems, the mirror fabricated and the mirror cell with the tip-tilt control parts. We have developed an assembly procedure in which potential difficulties, such as handling without contacting mirror surface, and optimizing bonding process, have been resolved. Supporting jigs were produced, and optimized bonding techniques have been developed. The assembled FSMP system was installed in a test tower, and stability of the system were checked. Performance of the FSMP system will be evaluated in static and dynamic environments for the validation of the FSMP system operation as the future works.