Collagen fibers exhibit natural autofluorescence and are commonly imaged using label-free multiphoton and nonlinear optical microscopy techniques.
The retrieved papers repeatedly mention the label-free imaging of collagen using multiphoton microscopy, combining second harmonic generation (SHG) and autofluorescence or two-photon excited fluorescence (TPEF) to visualize collagen structures naturally. Therefore, the claim that collagen fibers are naturally autofluorescent is well-supported by the literature.