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Diffractive Optical Elements (DOEs) are commonly used in the photonics community for several purposes, such as geometrical calibration of cameras,1 medical treatments, lithography, LIDAR applications. In the context of the optical alignment and integration of the RAman Spectrometer for MMX (RAX),2 a DOE was included in the test setup with the goal of providing a clear figure of merit to optimize the focusing of a dioptric lens objective on to the spectrometer detector. This Raman spectrometer will be integrated later this year in a small Rover on-board the Martian Moons eXploration (MMX) mission led by JAXA, and will operate on Phobos’ surface to characterize the different materials composing Phobos’ soil. To achieve this, the optical design of RAX is very challenging in terms of performance to reach in very limited volume and mass. As described in Ref. 2, the optical alignment and integration of RAX was a very challenging exercise, requiring several optical setups and methods. The usage of a DOE was introduced to solve a classical problem during the integration of a camera: how to integrate both the optical objective (lens assembly) and the detector to ensure that both the optical focal plane and the detector sensitive plane are co-planar. When illuminated by a collimated laser beam, the implemented DOE generates a regular pattern of collimated beams with well-known deviation angles from the input beam. It acts as a 2D diffraction grating, and generates a pattern field which covers the entire field of view of our camera. Thanks to this property, the Camera Interface Objective of RAX could be successfully positioned and oriented with respect to the detector mechanical interface. It was achieved by acquiring successive images of the DOE pattern with controlled defocused laser beam illuminating it. We were then able to compute the equivalent mechanical defocus needed to maximize the image quality. This maximizes the overall instrument performance and will ensure best possible scientific measurement on Phobos.
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