The ESA mission Euclid is designed to explore the dark side of the Universe and to understand the nature of the dark energy responsible of the accelerating expansion of the Universe. One of the two probes carried by this mission is based on the Baryonic Acoustic Oscillation (BAO) method that requires the redshift measurements of millions of galaxies. In the Euclid design the spectroscopic channel uses slitless low resolution grisms. Classical grisms, manufactured by replication of a ruled master on the hypotenuse of a prism, are extremely difficult to make for Euclid due to the combination of low groove density and small blaze angle. Two years ago we started an RandD program to develop grisms by the photolithography process that is well adapted to coarse gratings and allows introducing aberration correction by ruling curved and non parallel grooves. During the Euclid Phase A, we developed several prototypes made by photolithography and we present in this paper the test results done in the specific environment of the Euclid mission.
The optical damage behaviour of different LiNbO3 optical waveguides has been experimentally studied by measuring the intensity output of a single beam as a function of the intensity input. Parallel measurements of photovoltaic currents have been carried out as a function of the input intensity and they have been correlated with the optical damage data. The following LiNbO3 guides have been studied and compared: proton exchanged (PE) belonging to the phases alpha, beta1, beta2 and reverse proton exchanged (RPE), and Zn in-diffused waveguides. The greatest intensity thresholds for optical damage, about 2x103 times greater than that of the substrate, have been obtained in RPE guides (they support ordinary polarization and have similar nonlinear optic activity as the substrate) and beta2 guides which support extraordinary polarization (they have no nonlinear optic activity). On the other hand, the lowest photovoltaic currents have been measured in beta1,2-phases. As a function of the light intensity, the photovoltaic current exhibits a superlinear behaviour, strong in alpha-phase and weaker in Zn in-diffused and RPE guides. The results for optical damage are discussed in connection with those of photovoltaic currents, paying particular attention to the main mechanisms involved.