Optical needle with narrow lateral focal width and extended longitudinal focal depth enabled by multi-level...

In flat lenses, focusing typically arises from the interference of first-order diffracted light after the incident optical field passes through the lens structure. However, the focused field exhibits a fundamental trade-off between achieving a narrow lateral focal width and maintaining an extended longitudinal focal depth. This limitation poses a significant challenge for simultaneously realizing high lateral resolution and large imaging depth in practical imaging systems. To overcome this constraint, we propose a multi-level diffractive lens (MDL) architecture in which focusing is produced by the constructive interference of multiple diffraction orders. Based on this principle, broadband light in the wavelength range of 800-900 nm is focused into an optical needle with an average lateral focal width of 2.4 μm and a longitudinal focal depth of 2640 μm, corresponding to an exceptional depth-to-width ratio of 1100:1. The fabricated MDL is integrated into a spectral-domain optical coherence tomography (SD-OCT) system, yielding nearly a 9-fold improvement in imaging depth compared with conventional objective lens-based OCT systems over comparable lateral resolution. These results demonstrate the strong potential of the proposed MDL for high-resolution imaging applications such as clinical diagnostics and defect inspection.