Femtosecond laser direct writing of a gradient index Fresnel lens in a heavy oxide glass

An important challenge in infrared imaging today consists in addressing the SWaP problem (Size, Weight and Power), for example by simplifying as much as possible the optical system before the sensor. The work presented in this paper takes advantage of recent techniques in femtosecond laser direct writing to imprint optical systems. We want to simplify an infrared multispectral imaging system, which combines a lens array and a filter array. This work aims at merging a lens array with a filter array by writing gradient index lenses with a femtosecond laser inside a dedicated glass substrate. A classical gradient index requires a huge refractive index variation, which cannot be reached today with femtosecond laser processing (Delta n(max) similar to 0.05). So, we decided to turn towards writing a gradient index Fresnel lens. A first-order Fresnel lens was designed with a Delta n(max) < 0.05 discretized into 8 index levels to guarantee a diffraction efficiency of 85% on the overall spectral bandwidth of the filters. The multispectral design is made of an array of 2x2 Fresnel lenses in a landscape lens configuration. For a horizontal field of view of 40 degrees, each GRIN Fresnel lens has 82 rings inside a total diameter of 3.8 mm. After characterizing the photosensitive response of the material to fs-laser writing, we started writing the first prototypes of graded index Fresnel lenses. For a first approach, a discretization on 4 index levels was chosen. A focal length measurement has been performed in order to compare it with our model.