Direct Laser Conversion of Transparent Polymers into Laser-Induced-Graphene for Realizing Ultra-Thin Optics

Optical systems include refractive, reflective, and diffractive optical elements. Modern industries have mostly adopted refractive lenses out of these. Fundamentally, the optical lens has evolved from glasses for correcting the human eye's imaging capability, to miniaturized cameras inside the mobile phones for taking pictures or face-recognition-based unlocking, also to LIDAR sensors for image processing in autonomous vehicles. Although such refractive lens systems are widespread, they cannot be reduced in terms of the weight and size due to the required geometrical thickness of the refractive material and the volume for performance. Here, we suggest a direct-laser-patterning of diffractive optical elements by converting transparent polymers into laser-induced-graphene (LIG) to realize ultra-thin and light-weight optical system. Colorless polyimide (CPI) was used as the transparent polymer, which is one of the well-known materials for its strong chemical and abrasion resistance. A femtosecond pulse laser was utilized to pattern a diffractive lens to enable non-thermal laser patterning with higher patterning resolution and minimal material damage. CPI is transparent in the visible wavelength region, so it is difficult to process the CPI using a visible laser system (e.g., green laser at 532 nm). Therefore, we imposed an ultra-violet laser (at 343 nm center wavelength) in this fabrication process through the third harmonic generation of a 1030-nm ytterbium-doped fiber femtosecond laser. The transparent CPI film has relatively a high absorption coefficient in ultraviolet region, so it is possible to convert CPI to LIG efficiently even at low laser intensities. An ultra-thin Fresnel zone plate was patterned successfully with a 5-μm linewidth. The resulting diffractive optics have focal lengths ranging from a few millimeters to hundreds of millimeters.<br/>Two key applications of the realized ultra-thin diffractive optics will be introduced in the presentation; the first one is the micro-endoscopic optical coherence tomography and the other one is the micro-satellites optical communication. Firstly, a more compact micro-optical coherence tomography (OCT) was realized by replacing the existing graded-index (GRIN) lens to an ultra-thin diffractive LIG lens with the astigmatism correction capability. Secondly, we applied ultra-thin LIG optics to satellite applications where weight and volume reduction is crucial. Solar sensors for satellite position control were realized with LIG lens with a 10-μm thickness and 10-g weight while providing a higher attitude control resolution.