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

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.