Multimodal nonlinear endomicroscopic imaging probe using a double-core double-clad fiber and focus-combining micro-optical concept

Multimodal non-linear microscopy combining coherent anti-Stokes Raman scattering,second harmonic generation,and two-photon excited fluorescence has proved to be a versatile and powerful tool enabling the label-free investigation of tissue structure,molecular composition,and correlation with function and disease status.For a routine medical application,the implementation of this approach into an in vivo imaging endoscope is required.However,this is a difficult task due to the requirements of a multicolour ultrashort laser delivery from a compact and robust laser source through a fiber with low losses and temporal synchronization,the efficient signal collection in epi-direction,the need for small-diameter but highly corrected endomicroobjectives of high numerical aperture and compact scanners.Here,we introduce an ultra-compact fiber-scanning endoscope platform for multimodal non-linear endomicroscopy in combination with a compact four-wave mixing based fiber laser.The heart of this fiber-scanning endoscope is an in-house custom-designed,single mode,double clad,double core pure silica fiber in combination with a 2.4 mm diameter NIR-dual-waveband corrected endomicroscopic objective of 0.55 numerical aperture and 180µm field of view for non-linear imaging,allowing a background free,low-loss,high peak power laser delivery,and an efficient signal collection in backward direction.A linear diffractive optical grating overlays pump and Stokes laser foci across the full field of view,such that diffraction-limited performance is demonstrated for tissue imaging at one frame per second with sub-micron spatial resolution and at a high transmission of 65%from the laser to the specimen using a distal resonant fiber scanner.