Rigid and high-numerical-aperture two-photon fluorescence endoscope

We present a rigid miniaturized optical system block fiber-optic two-photon endoscope based on a compact two-axis piezo scanner system and a miniature high (0.65) NA GRIN lens objective. The optical system is scanned as a whole by a piezo scanner allowing always an on-axis beam irradiation of the optical system. A photonic crystal fiber is used for excitation and ultrashort laser pulses can be delivered with typical power up to 100 mW at 800 run. Two-photon fluorescence signal is collected by the use of a multimode fiber. Lateral resolution values for the system were experimentally measured to be 0.67 mu m vertically and 1.08 mu m horizontally. Axial resolution was found to be 5.8 mu m. The endoscope is highly flexible and controllable in terms of time acquisition, resolution, and magnification. Fluorescence images were acquired over a 420 mu m x 420 mu m field of view. Results presented here demonstrate the ability of the system to resolve subcellular details and the potential of the technology for in vivo applications.