OSA Open Access Publishing AgreementCifuentes, AngelPikálek, TomášOndráčková, PetraAmezcua-Correa, RodrigoAntonio-Lopez, José EnriqueČižmár, TomášTrägårdh, Johanna2022-03-312022-03-312021https://oa.tib.eu/renate/handle/123456789/8485https://doi.org/10.34657/7523Multimode fiber-based endoscopes have recently emerged as a tool for minimally invasive endoscopy in tissue, at depths well beyond the reach of multiphoton imaging. Here, we demonstrate label-free second-harmonic generation (SHG) microscopy through such a fiber endoscope. We simultaneously fully control the excitation polarization state and the spatial distribution of the light at the fiber tip, and we use this to implement polarization-resolved SHG imaging, which allows imaging and identification of structural proteins such as collagen and myosin. We image mouse tail tendon and heart tissue, employing the endoscope at depths up to 1 mm, demonstrating that we can differentiate these structural proteins. This method has the potential for enabling instant and in situ diagnosis of tumors and fibrotic conditions in sensitive tissue with minimal damage.enghttps://www.osapublishing.org/library/license_v1.cfm530620EndoscopyExcited statesFibersMultimode fibersNonlinear opticsPolarizationProteinsTissueHeart tissuesMinimal damageMinimally invasiveMultiphoton imagingPolarization stateSecond harmonic generation microscopies (SHG)Second harmonicsStructural proteinsHarmonic generationArticle