The present invention is directed to a method of applying radiation through an optical fiber for detecting disease within a plant or animal or other penetrable tissue, or imaging a particular tissue of a plant or animal. In addition, fluorescence and nonlinear scattering signals can be detected and localized within a subject by such application of radiation through an optical fiber. The radiation is effective to promote simultaneous multiphoton excitation. The optical fibers are used alone to examine internal regions of tissue, in conjunction with an optical biopsy needle to evaluate sub-surface tissue, or with an endoscope to evaluate tissue within body cavities. The present invention also relates to a device for coupling in radiation from an ultrashort mode-locked laser into the beam path of a microscope.