Curved imaging sensors bring significant size, weight and cost reduction to imaging systems while mitigating off-axis optical aberrations, as opposed to current flat sensors. Unlocking these key features has captured the interest of major players over the last two decades. SILINA has been developing a CMOS Image Sensor (CIS) curving process, which adapts to various sensor characteristics. This enables the design of image focal planes to various shapes to specifically maximize the optical performance of every single imaging system. We demonstrated the manufacturing of spherical and aspherical CIS in 2021, opening a new area of compact, fast, wide-angle and high-resolution optical system solutions. From concave to convex, spherical, aspherical, cylindrical, toroidal, or even freeform shapes can be reached. These new degrees of freedom offered to CIS can significantly simplify optical systems through a reduction of the number of elements and improve optical performance in many different ways. The field of view, the contrast, the F-number can be increased while optical aberrations and distortion can be minimized. At the end, the different costs related to manufacturing, metrology, integration, and alignment are reduced. This is of great importance for applications requiring compact payload, high resolution, high transmission factor and shorter integration time. In this paper we address the challenges caused by the adoption of curved image sensors, notably the different approaches for the adaptation of existing designs with a highlight on the optimization and tolerancing methodology. We describe our approach to enhance existing designs by including a curved focal plane, from the first analysis to the last optimization run, in order to improve key performance criteria while maintaining first order parameters. We discuss the improvement expected for space observation through a focus on a two-mirror telescope design based on the Hubble Space Telescope characteristics.

Curved imaging sensors bring significant size, weight and cost reduction to imaging systems while mitigating off-axis optical aberrations, as opposed to current flat sensors. Unlocking these key features has captured the interest of major players over the last two decades. SILINA has been developing a CMOS image sensor curving process which adapts to various sensor characteristics. This process enables the deformation of image sensors to various shapes to specifically maximize the performance of every single imaging system. Indeed, curved CMOS image sensors (CIS) help to create compact optical instruments, notably imagers, telescopes and spectrographs. Simplifying optical systems enables to release opto-mechanical constraints from the design to the integration phase. Nowadays, freeform optical elements participate in the development of solutions that meet the common needs of compact, fast, wide-angle and high-resolution systems. Nevertheless, freeform surfaces remain extremely expensive in terms of manufacturing and metrology. Moreover, field curvature aberration is still difficult to correct and curved CIS bring a suitable solution for that. Early 2021, SILINA has demonstrated the manufacturing of spherical and aspherical CIS, opening a new area to optical system design. Optical designers can now consider various sensor shapes to optimize their systems by considering spherical, aspherical or more complex focal surfaces. In 2022, SILINA is generating a data set of electro-optical performances of curved CIS. EMVA1288 compliant CMOS image sensor characterization test beds are being developed in order to fulfill this challenge. Experimental data of dark current, photo-response non-uniformity, fixed pattern noise, quantum efficiency and more can be measured. Furthermore, characterizing a large number of curved CIS will enable to assess the influence of SILINA’s curving process on key performance criteria and extract statistic results. In parallel to the electro-optical characterization campaign, reliability tests such as thermal tests, thermo-vacuum cycle and aging tests, following the ECSS norms, will be performed in order to qualify the robustness of the curving process . This paper gives an overview of SILINA‘s activities. The benefits of curved CIS are illustrated through on-going projects, the main features of the curving process and the electro-optical characterization are introduced. A focus is done on the improvement of shape accuracy and on the performances of visible curved CIS. Finally, the results of reliability tests are presented.

Curved imaging sensors bring significant size, weight and cost reduction to imaging systems while mitigating off-axis optical aberrations, as opposed to current flat sensors. Unlocking these key features has captured the interest of major players over the last two decades. SILINA has been developing a CMOS Image Sensor (CIS) curving process, which adapts to various sensor characteristics. This enables to maximize the optical performance of every single imaging system. We have demonstrated the manufacturing of curved CMOS Front-Side Illuminated (FSI) and Back Side Illuminated (BSI), opening a new area of compact, fast, wide-angle and high-resolution optical lenses. An EMVA1288 compliant CMOS image sensor characterization testbeds has been developed in order to measure the electro-optical performances of curved CIS. Experimental data of dark current, photo-response non-uniformity, fixed pattern noise, quantum efficiency and more can be measured. In parallel to the electro-optical characterization campaign, reliability tests such as thermal tests (ageing, cycling) have been performed. This paper gives an overview of SILINA`s main technological developments and electro-optical performance of CMOS Back Side Illuminated (BSI) curved image sensors is presented.

This study proposes a new way to manufacture large lightweight aspherics for space telescopes using stress mirror polishing (SMP). This technique is well known to allow reaching high quality optical surfaces in a minimum time period, thanks to a spherical full-size polishing tool. To obtain the correct surface's aspheric shape, it is necessary to define precisely the thickness distribution of the mirror to be deformed, according to the manufacturing parameters. We first introduce active optics and stress mirror polishing techniques, and then we describe the process to obtain the appropriate thickness mirror distribution, allowing to generate the required aspheric shape during polishing phase. Shape optimization procedure using PYTHON programing and NASTRAN optimization solver using finite element model (FEM) is developed and discussed in order to assist this process. The main result of this paper is the ability of the shape optimization process to support SMP technique to generate a peculiar aspherical shape from a spherical optical surface thanks to a thickness distribution reshaping. This paper is primarily focused on a theoretical framework with numerical simulations as the first step before the manufacturing of a demonstrator. This two-step approach was successfully used for previous projects.

This work explores the variable curvature mirror's (VCM) elastic bending rules through modeling it as a thin elastic plate with an exponential thickness distribution actuated with a uniform pressure under simply supported boundary conditions. By using the small-parameter method, the general analytical expression of a plate's deflection is worked out. The results calculated by the analytical solution are compared to the finite element analysis of a VCM model with the same specific parameters. We demonstrate that the two have a good correlation with the each other. This analytical solution is an effective way to predict a VCM's deflection. (C) 2019 Optical Society of America

Curved sensors are a suitable technological solution to enhance the vast majority of optical systems. In this work, we show the entire process to create curved sensor-based optical systems and the possibilities they offer. This paper defines the boundaries of the reachable curvatures for a full range of monolithic sensors. We discuss how the curved focal plane shape is related to the imaged scenes and optical parameters. Two camera prototypes are designed, realized, and tested, demonstrating a new compact optical architecture for a 40 deg compact objective as well as a wide-field fisheye zoom objective using a convex sensor to image a 180 deg field of view.

Exoplanet imaging requires super polished off-axis parabolas (OAP) with the utmost surface quality. In this paper we describe an innovative manufacturing process combining 3D printing and stress polishing, to create a warping harness capable of producing any off axis parabola profile with a single actuator. The warping harness is manufactured by 3D printing. This method will be applied to the production of the WFIRST coronagraph's off axis parabolas. The evolution of the warping harness design is presented, starting from a ring warping harness generating astigmatism, to an innovative thickness distribution harness optimised to generate an off axis parabola shape. Several design options are available for the prototyping phase, with their advantages and disadvantages which will be discussed.

3D printing, also called additive manufacturing, offers a new vision for optical fabrication in term of achievable optical quality and reduction of weight and cost. In this paper we describe two different ways to use this technique in the fabrication process. The first method makes use of 3D printing in the fabrication of warping harnesses for stress polishing, and we apply that to the fabrication of the WFIRST coronagraph off axis parabolas. The second method considers a proof of concept for 3D printing of lightweight X-Ray mirrors, targeting the next generation of X-rays telescopes. Stress polishing is well suited for the fabrication of the high quality off axis parabolas required by the coronagraph to image exoplanets.. Here we describe a new design of warping harness which can generate astigmatism and coma with only one actuator. The idea is to incorporate 3D printing in the manufacturing of the warping harness. The method depicted in this paper demonstrates that we reach the tight precision required at the mirrors surface. Moreover the error introduced by the warping harness fabricated by 3D printing does not impact the final error budget. Concerning the proof of concept project, we investigate 3D printing towards lightweight X-ray mirrors. We present the surface metrology of test samples fabricated by stereo lithography (SLA) and Selective Laser Sintering (SLS) with different materials. The lightweighting of the samples is composed of a series of arches. By complementing 3D printing with finite element analysis topology optimization we can simulate a specific optimum shape for the given input parameters and external boundary conditions. The next set of prototypes is designed taking to account the calculation of topology optimisation.

This paper describes the development of a new manufacturing process to directly obtain off-axis parabolas (OAP) by combining 3D printing and stress polishing. 3D printing opens up a new vision for optics fabrication by providing innovative lightweight structures which are not fabricable with traditional mechanical manufacturing. It also provides a new range of materials covering plastic up to metal through composite materials and ceramics. The direct imaging of exoplanets using coronagraphic instruments provides a good example of an astronomical application that can greatly benefit from such developments. Exoplanets imaging is very demanding in terms of optical surface quality, however, the majority of coronagraphic instruments use off axis optics, which manufacturing of such optics could present some drawbacks: either the optics are cut out of a parent large mirror, resulting in a material loss, or the surfaces are machined with sub-aperture tools, resulting in high spatial frequency ripples which must be avoided for this application. Thanks to 3D printing and topology optimisation we created an innovative warping harness design which can generate any off axis parabola shapes with only one actuator. We optimised the harness thickness distribution in order to reach non symmetrical deformation composed of astigmatism and coma. The warping is applied by micrometric screws and the high transmission factor of the system allows to keep stable the final error budget despite the error introduced by the warping harness fabricated by 3D printing. Several warping harness designs and materials were explored for the prototyping phase. This study is part of WFIRST satellite which will be launch in 2024 by NASA to observe galaxies via a wide field instrument and also perform exoplanet direct imaging via coronagraph. In the case of the WFIRST coronagraphic instrument, eight off axis parabolas are used to relay the beam from one pupil to another. We present the first prototyping results dedicated to the WFIRST off axis parabolas. Deformation surface results are performed by interferometric measurements and compared to Finite Element Analysis predictions.

FAME is a four-year project and part of the OPTICON/FP7 program that is aimed at providing a breakthrough component for future compact, wide field, high resolution imagers or spectrographs, based on both Freeform technology, and the flexibility and versatility of active systems. Due to the opening of a new parameter space in optical design, Freeform Optics are a revolution in imaging systems for a broad range of applications from high tech cameras to astronomy, via earth observation systems, drones and defense. Freeform mirrors are defined by a non-rotational symmetry of the surface shape, and the fact that the surface shape cannot be simply described by conicoids extensions, or off-axis conicoids. An extreme freeform surface is a significantly challenging optical surface, especially for UV/VIS/NIR diffraction limited instruments. The aim of the FAME effort is to use an extreme freeform mirror with standard optics in order to propose an integrated system solution for use in future instruments. The work done so far concentrated on identification of compact, fast, widefield optical designs working in the visible, with diffraction limited performance; optimization of the number of required actuators and their layout; the design of an active array to manipulate the face sheet, as well as the actuator design. In this paper we present the status of the demonstrator development, with focus on the different building blocks: an extreme freeform thin face sheet, the active array, a highly controllable thermal actuator array, and the metrology and control system.

We present two ways to generate or compensate for first order optical aberrations using smart warping harnesses. In these cases, we used the same methodology leading to replace a previous actuation system currently on-sky and to get a freeform mirror intended to a demonstrator. Starting from specifications, a warping harness is designed, followed by a meshing model in the finite elements software. For the two projects, two different ways of astigmatism generation are presented. The first one, on the VLT-SPHERE instrument, with a single actuator, is able to generate a nearly pure astigmatism via a rotating motorization. Two actuators are sufficient to produce the same aberration for the active freeform mirror, main part of the OPTICON-FAME project, in order to use stress-polishing method.

We present the conception of an anamorphic and telecentric scale changer with no distortion, able to provide magnifications in the range of 2 to 30 without any interchangeable optics, dedicated to ground or space applications. Several optical designs are investigated and the final configuration is based on off-axis five mirrors system with no moving elements. Four active mirrors are adapted to four different zoom configurations. A specific mechanical profile with variable thickness distribution is simulated and optimized on each mirror to allow using a minimal number of actuators. An opto-mechanical design will be presented, showing the implementation of actuators on the system. This work is done in the frame of the ANR project OASIX and will produce a lab prototype in 2015.