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Second harmonic generation imaging via nonlinear endomicroscopy.

A compact endomicroscope is the only solution for transferring second harmonic generation (SHG) imaging into in vivo imaging and real time monitoring the content and structure of collagen. This is important for early diagnoses of different diseases associated with collagen change. A compact nonlinear endomicroscope using a double clad fiber (DCF) is newly employed in SHG imaging. The experiment shows the core of the DCF can maintain the linear polarization of the excitation laser beam in particular directions, and the degree of polarization of the excitation laser beam directly affects signal to noise ratio of SHG imaging. The nonlinear endomicroscope can display clear three dimensional (3D) SHG images of mouse tail tendon without the aid of contrast agents, which reveals the collagen fiber structure at different depths. The high resolution of SHG imaging from the endomicroscope shows that SHG imaging can reveal additional information about the orientation and degree of organisation of proteins and collagen fibers than two-photon-excited fluorescence imaging. Therefore SHG imaging offers endomicroscopy with additional channel of imaging for understanding more about biological phenomena.