Freeform surfaces are a revolution in the field of spatial imaging because they allow the correction of optical aberrations in off-axis systems. Freeform surfaces are defined as non-rotationally symmetric surfaces, which also cannot be described as an off-axis part of a conicoid. The use of such surfaces can also enable to increase performances, such as the field of view, F-number or compactness of off-axis, fully reflective telescopes, and is thus interesting for nanosatellite imaging applications. In this paper, we will present a proof of concept for a fast, compact and well-corrected freeform Three Mirror Anastigmat (TMA) design suited for nanosatellite infrared thermal imaging using an uncooled micro-bolometer. The performance and tolerance analysis will be presented, along with an analysis of the mirrors’ shape and surface quality using an industrial surface characterisation tool. The consequences of the mirrors’ shape error on the optical quality will also be discussed, as well as a method to compensate for the loss in image quality induced by these shape defects.
These last years have seen a raising interest for ground to GEO satellites optical very high throughput links, i.e. GEO-feeder links, or GEO-FL. However, despite their potential, these applications have to overcome atmospheric turbulence, which requires the development of mitigation techniques, such as adaptive optics (AO). In the case of GEO-FL, AO performance is limited by the Point-Ahead Angle (PAA) induced anisoplanatism. We describe here how our feedback on our field experiments helped us to design ONERA’s AO-compensated ground station, FEELINGS, and the status of said ground station in the fall of 2022.
In optical design, the designer's experience is critical. Indeed, an experienced optical designer will often choose a better starting point for optimization than an inexperienced one. Most of the time, lens design software use a local optimization algorithm, which is why the starting point is so important to get an excellent optical system. We present here an alternative to the classical optical design method and a solution to reduce the impact of the designer's experience. Our alternative couples the Simultaneous Multiple Surfaces (SMS) method, introduced by Benítez and Miñano with optimization in Zemax OpticStudio. The SMS method is a direct construction method of optical systems without optical aberrations for as many field points as the system contains surfaces. This method can deal with both aspheric and freeform optical systems depending on the dimension of the method implemented. Our implementation of the SMS method can design optical systems with three surfaces. We use the SMS method to define a freeform system with an F-number of 0.85. Then, we use this fast freeform system as a starting point to perform further optimization in Zemax OpticStudio. Finally, we achieve to design two diffraction-limited freeform systems, one over a square field of view of ±30° and another over a rectangular field of view of ±33° × ±26°.
Infrared cameras could serve automotive applications by delivering breakthrough perception systems for both in-cabin passengers monitoring and car surrounding. However, low-cost and high-throughput manufacturing methods are essential to sustain the growth in thermal imaging markets for automotive applications, and for other close-to-consumer applications, which have a fast growth potential. With the reduction of the pixel pitch of microbolometer detectors, their cost has decreased considerably and now the optical part represents a significant part of the system cost. Fast low cost infrared lenses suitable for microbolometers are already sold by companies like Umicore, Lightpath, FLIR… Chalcogenide glasses are widely used as materials for optics because they have many cost advantages, especially due to the possibility of mass molding the optics. However, with the reduction of the pixel pitch, it is more and more difficult to design high performance lenses with a limited number of optics. The possibility of molding the optics allows us to use many highly aspherical surfaces at affordable costs. However, Chalcogenide glasses have usually a lower refractive index than other more expensive infrared materials such as Germanium. Indeed, high refractive index materials are known to be effective in attenuating the amplitude of many geometric aberrations. In this presentation, we evaluate the interest of high index Chalcogenide lenses, especially TGG and TGS, to design optical systems meeting the needs of the automobile with a limited number of optics. TGG glass has an index of refraction of 3.396 at a wavelength of 10µm, i.e. its index of refraction is close to the Silicon one and was initially studied for space applications. TGS has a lower index of refraction (3.12@10µm) but can be used in a cost effective manufacturing process by using flash spark plasma sintering (SPS) on raw powder. Demonstrators with TGG glass have been made and their performance evaluated.
Co-design methods started to incorporate neural networks a few years ago when deep learning showed promising results in computer vision. This requires the computation of the point spread function (PSF) of an optical system as well as its gradients with respect to the optical parameters so that they can be optimized using gradient descent. In previous works, several approaches have been proposed to obtain the PSF, most notably using paraxial optics, Fourier optics or differential ray tracers. All these models have limitations and strengths regarding their ability to compute a precise PSF and their computational cost. We propose to compare them in a simple co-design task to discuss their relevance. We will discuss the computational cost of these methods as well as their applicability.
Infrared cameras could serve automotive applications by delivering breakthrough perception systems for both in-cabin passengers monitoring and car surrounding. However, low-cost and high-throughput manufacturing methods are essential to sustain the growth in thermal imaging markets for automotive applications, and for other close-to-consumer applications which have a fast growth potential. Fast low cost infrared lenses suitable for microbolometers are currently already sold by companies like Umicore, Lightpath, FLIR… They are either made of a single inverse meniscus Chalcogenide glass or of two Silicon optics. In this paper, we explore hybrid systems with a large field of view around 40° combining Chalcogenide and Silicon in order to take advantage of both materials. Both are compatible with wafer-level process. Silicon optics can be manufactured by photolithography process and are expected to be more cost-effective than Chalcogenide ones. However they are constrained in shape and sag height. On the other hand, Chalcogenide optics can be collectively molded and could have more free shapes. They are thus more suitable to reach high-demanding performance. So hybrid designs could be seen as a compromise between cost and performance. In this paper, we show that fast lenses with diameter constraints to few millimeters to make affordable wafer-level process lead to small size detectors. As a consequence, the pixel pitch reduction of microbolometers is a key point to maintain a good resolution. Finally, strategies to improve the production yield of hybrid lenses are explored.
The accurate simulation of straylight is essential for the verification of the contrast requirements in optical instruments.
In a spectrometer, the scattering from reflective gratings is hardly understood and difficult to characterize while
contributing significantly to the overall system straylight and reduction of the spectrometer contrast. In this article we present an experimental setup for, and measurement results from, the characterization of the bidirectional scattering distribution function (BSDF) of a grating in the scope of the FLORIS project of the ESA FLEX Mission. The grating is an Engineering Model and will be subject to further optimization. Measurement of the BSDF showed approximately a Harvey-Shack profile parallel to the grating grooves, consistent with a dominant contribution from roughness scatter and minor distinctive features. Moreover, we observed distinct straylight peaks out of the diffraction plane, which are called “satellites”. The main challenges in the measurement of grating BSDFs arise from the near angle limit, the determination of the instrument signature and the selection of the appropriate sampling (2D or 3D). Theoretical analysis has been performed to investigate the influence of, and limitations introduced by, the measurement setup combined with the convex curvature of the grating. The next step is to introduce these measured BSDFs into straylight simulation. We have done that by fitting appropriate functions to the measured BSDF and defining them in the optical analysis software ASAP as a user-defined BSDF.
The Rapid Nano is a particle inspection system developed by TNO for the qualification of EUV reticle handling equipment. The sensitivity of this system has been improved by model based design. Our model identified two parameters that could be tuned to be able to detect smaller particles. The first step is a multi azimuth illumination mode and the second parameter is the illumination wavelength. Here we report on the results of the Rapid Nano 4, which has both of these parameters optimized to have a sub 20 nm LSE detection limit on EUV mask blanks.
The fact that every spectrometer can sort light by wavelength at the speed of light is intriguing. The field of spectrometry is a long-existing and ever-changing one. The application areas extend from optical communication to possible extraterrestrial life detection, health monitoring, environmental monitoring and quite a long list of other topics. TNO has played a role in several of these areas, always using state of the art designs and components. Some of the recent developments are described, as well as a possible path for (near) future developments. Any spectrometer consists of a telescope, slit, collimator, disperser and an imager. Each of these functions is discussed using and even pushing progress in the manufacturing and design capabilities of the industry. The progress from a two-mirror spherical telescope for a pushbroom space-based daily global coverage spectroscopy instrument OMI to a two-mirror freeform telescope for TROPOMI is described, the design and manufacturing of supergratings showing very little straylight, freeform mirrors and the use of deliberately decentered lenses is shown. A near-future small-satellite system is shown that is being built and tested as this paper was written.
Designing a novel optical system is a nested iterative process. The optimization loop, from a starting point to final system is already mostly automated. However this loop is part of a wider loop which is not. This wider loop starts with an optical specification and ends with a manufacturability assessment. When designing a new spectrometer with emphasis on weight and cost, numerous iterations between the optical- and mechanical designer are inevitable. The optical designer must then be able to reliably produce optical designs based on new input gained from multidisciplinary studies. This paper presents a procedure that can automatically generate new starting points based on any kind of input or new constraint that might arise. These starting points can then be handed over to a generic optimization routine to make the design tasks extremely efficient. The optical designer job is then not to design optical systems, but to meta-design a procedure that produces optical systems paving the way for system level optimization. We present here this procedure and its application to the design of TROPOLITE a lightweight push broom imaging spectrometer.
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