Off-axis freeform surface reflective collimator for pumping in miniaturized NMR co-magnetometers

•An off-axis freeform surface design integrates beam expansion, collimation, and reflection.•Collimator structure with a volume under 0.5 cm3 optimizes space for atomic polarization and probing.•Reflective design achieves over 98% linear polarization.•High collimation with only 1.5 milliradians of divergence.•Effective and feasible for polarizing atomic ensembles in NMR co-magnetometers. The design of the optical paths for pumping and probing plays an essential role in miniaturized nuclear magnetic resonance (NMR) co-magnetometers by constraining sensor size limitations. Traditional bulky optical systems for expanding, collimating, and reflecting laser beams hinder the achievement of compact designs. This study introduces an optical path design using an off-axis freeform surface that combines beam expansion, collimation, and reflection into one. For a vertical cavity surface-emitting laser (VCSEL) with a divergence angle of 15°, the off-axis collimation spot has a diameter of 2.3 mm, a divergence angle of 1.5 milliradians, and a degree of linear polarization of reflection more than 98 % at an optical path of 8.7 mm. The overall reflective collimator structure is less than 0.5 cm3, simplifying the optical path significantly. The device is purposed for spin polarizing electrons and nucleons, demonstrating the feasibility and effectiveness of the design to achieve atomic polarization and meets the requirements for atomic pumping and probing. This approach holds promise for broad applications and introduces a novel method to miniaturize compact atomic sensors like gyroscopes and magnetometers.