Effect of Wet Expansion Behavior on Polyimide Membrane Diffractive Lens

Abstract

Optical polyimide (PI) membrane is a potential substrate to fabricate a diffractive primary lens in large aperture space-borne telescope. In this paper, we investigated the wet expansion behavior of PI via a strain gauge method and measured the coefficient of wet expansion (CWE) of PI membrane supported by different fixtures. The CWE of silica-fixed, ring-fixed, and free state PI membrane were near zero, 5 ppm/%, and 10 ppm/% respectively. It is found that Fresnel zone lens (FZL), based on PI membrane with lower CWE, achieve a better imaging quality with smaller wave-front errors. The underlying mechanisms and methods to improve imaging quality of ring-fixed FZL are discussed. This work provides direction in fabricating a flexible membrane diffractive lens with applicable imaging quality.

The Tailored Material Removal Distribution on Polyimide Membrane Can Be Obtained by Introducing Additional Electrodes

“…These homemade PI membranes were synthesized by 1,2,4,5-Benzenetetracarboxylic anhydride and 2,2′-Bistrifluoromethylbenzidine, and formed homogeneous film by multiple spin coats and imidization. Young’s modulus is about 7 GPa, and the transmittance is more than 80% at 632.8 nm [ 9 ]. In order to keep the PI membrane dimensionally stable and minimize the distortion of the etched pattern, the PI membrane substrates were fixed on a special circular tooling.…”

“…In order to facilitate the measurement of etch depth, a diffractive micro-structure mask pattern using a photoresist (AZ1500) was exposed on the PI membrane substrate before RIE. The photolithography process mainly includes spin coating photoresist on the PI substrate, exposure and development, hard bake, dry etching and cleaning the PI substrate [ 9 ]. During the etching process, an additional electrode made of aluminum alloy (2A12) was placed under the PI membrane substrate.…”

“…G.H. Gao studied the wet expansion behaviors of PI membranes, supported by different fixtures, and found that the Fresnel zone lens, based on a silica-fixed PI membrane, achieves near zero coefficient of wet expansion, and good imaging quality [ 9 ]. Adopting a polyimide composite membrane with SiO 2 antireflective membrane on both sides provided a solution to improve optical homogeneity and achieve high transmittance [ 10 ].…”

Investigation of Photoelastic Property and Stress Analysis for Optical Polyimide Membrane through Stress Birefringence Method

“…The optical PI membrane is a promising isotropic material for large-aperture-membrane diffractive lenses applied in next-generation space telescopes [11]. Recent research on transmissive diffractive membrane optics for large lightweight optical telescopes showed that the PI membrane easily deformed in-plane, resulting in failure of accurate light focusing [12,13]. The deformation of the PI membrane is accompanied by stress distribution variation, but it is difficult to quantitatively investigate how stress distribution changes during this process without inflicting damage [14][15][16].…”