Precision separation of thick double-sided microlens arrays via stepwise immersion ultrafast laser modification and controlled thermal stress

• The permissible refractive index matching deviation for fully modified surfaces was determined. • Complete modified surfaces were established inside thick microlens arrays using stepwise layer processing. • The proposed method for correcting Bessel beam verticality ensures smooth alignment of multi-modified surfaces. • Burst and power are the key laser parameters affecting bonding strength and surface roughness, respectively. • The heat bath simultaneously causes crack propagation from edges of the modified surface for complete separation. Double-sided microlens arrays (DSMLAs) are integral to large field of view imaging and precise beam shaping in compact optical systems. However, separating thick DSMLAs with precision and efficiency remains challenging, particularly due to laser aberration, insufficient working distance, and surface damage. This study introduces an advanced separation technique termed immersion stepwise layer cutting (ISLC), which combines immersion-based ultrafast laser multilayer modification with thermal stress-driven separation, resulting in precise and rapid separation of 7.95 mm-thick DSMLAs. The separated surfaces exhibited a low surface roughness of 0.62 μm at a cutting speed of 100 mm/s. Additionally, the heat bath method enabled batch separation, enhancing efficiency and applicability. The permissible refractive index deviation range for this DSMLA was determined through integrated optical simulations and experiments. A generalized observation method for Bessel beam verticality was proposed, including a mathematical definition for filament angular deviation. Laser parameters in the ISLC process were optimized via orthogonal experiments. The temperature and thermal strain evolution of the modified surface during the heat bath were analyzed to determine the critical duration required for crack propagation at a given bath temperature. The ISLC technique greatly enhances the versatility of ultrafast laser separation in processing brittle materials.