Mr. Pierre Delullier Profile

Pierre Delullier

at ONERA

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Proceedings Article | 3 October 2022 Paper

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

Pierre Delullier, Guillaume Druart, Florence de la Barrière, Matthieu Lancry

Proceedings Volume 12216, 1221602 (2022) https://doi.org/10.1117/12.2632081

KEYWORDS: Fresnel lenses, GRIN lenses, Glasses, Diffraction, Femtosecond phenomena, Cameras, Refractive index, Modulation transfer functions, Gradient-index optics

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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 (Δ𝑛𝑚𝑎𝑥 ~ 0.05). So, we decided to turn towards writing a gradient index Fresnel lens. A first-order Fresnel lens was designed with a Δ𝑛𝑚𝑎𝑥 < 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°, 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.

Proceedings Article | 28 October 2021 Presentation + Paper

Femtosecond laser direct writing of a Fresnel zone plate in glasses for mid-infrared imaging applications

Pierre Delullier, Guillaume Druart, Florence de la Barrière, Laurent Calvez, Matthieu Lancry

Proceedings Volume 11889, 1188912 (2021) https://doi.org/10.1117/12.2600695

KEYWORDS: GRIN lenses, Fresnel lenses, Femtosecond phenomena, Gradient-index optics, Cameras, Mid-IR, Glasses, Sensors

Read Abstract +

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. 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 lens requires a huge refractive index variation, which cannot be reached today with femtosecond laser processing (Δ𝑛_𝑚𝑎𝑥 ~ 0,05). So, we decided to turn towards writing a gradient index Fresnel lens. A first-order Fresnel lens was designed with a Δ𝑛_𝑚𝑎𝑥 < 5.10⁻²discretised into 8 index levels to guarantee a diffraction efficiency of 85% on the overall spectral bandwidth of the filters. We studied a chalcogenide glass because of his good properties in mid-IR optics. We compared the different laser responses of the material: threshold of permanent changes, quantitative phase and index variation. Finally, we showed that reaching a phase variation of π radians with femtosecond laser writing is possible even for a huge scanning speed (0,01 mm/s up to 25 mm/s). By overlapping the written lines, we were able to implement a maximum phase variation of 8 radians (with only one written layer), demonstrating the possibility to write quickly a GRIN Fresnel lens with a diameter of some millimeters.

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