All-fiber two-photon endomicroscopic probe based on dual-metalens telescope structure for excitation modulation and fluorescence collection

Metalenses have emerged as highly promising compact optical components in fiber-optic two-photon endomicroscopy systems due to their exceptional light control capabilities. However, the limited core diameter of single-mode fiber (SMF) constrains the numerical aperture (NA) and wavefront control of the metalens. Although wavefront modulation can be improved by introducing a beam expansion structure, the probe size and transmission loss increase accordingly, which affects the fluorescence collection efficiency. Herein, an all-fiber two-photon endomicroscopic probe based on a dual-metalens telescope structure is demonstrated, which achieves precise control of beam propagation through the coordinated optimization design of metalenses and a coreless glass section. Based on independently optimized phase profiles, the metalens 1 with a short focal length of 6.75 µm and the metalens 2 with an NA of 0.89 are theoretically designed using the particle swarm optimization (PSO) algorithm to achieve dual-wavelength achromatic design at 1060 nm and 548 nm. To achieve adequate wavefront control, the excitation beam was diverged by the metalens 1 located at the end of SMF, and then collimated and refocused by the metalens 2 located at the end of the coreless glass section. The emitted fluorescence was collected by the fiber core via the metalens 2, the coreless glass section and the metalens 1 successively. By leveraging dual-metalens telescope structure, the length of the coreless glass section is shortened, which can enhance the fluorescence collection efficiency to 5.50%. Therefore, the dual-metalens telescope structure based all-fiber two-photon endomicroscopic probe offers significant potential as an ultra-compact and efficient solution for high-resolution minimally invasive two-photon imaging in deep tissues.