Optical design of two-photon endoscopy objective with high collection efficiency

Endoscopic objectives arc key components of two-photon imaging used in medical endoscopy imaging. The imaging quality and signal light collection efficiency arc crucial to the final image quality. An objective lens with high collection efficiency, 400 μim field of view, and compact structure is designed for the optical fiber scanning two-photon imaging system. In this system, a 920 nm femtosecond laser emerges from a double cladding fiber and excites the sample after the objective lens. The excited 460 nm second harmonic signal and 510 nm fluorescence arc collected into the fiber cladding through the objective lens. In order to improve the collection efficiency during this process, we design the axial chromatic aberration of the objective so that more signal light enters the cladding. The design of the system is optimized by Zcmax software. The results show that the modulation transfer function in the whole field of the objective at 920 nm is larger than 0. 25 at 700 lp/mm, which meet the requirements of the system. Meanwhile, the signal light collection efficiency of the objective lens designed in the non-sequential mode and the traditional gradient index lens is compared. The simulation results show that the signal light collection efficiency of this objective lens is 54 %, which is two times higher than that of the traditional gradient index lens.