Advances in multimodal imaging using double-clad fiber couplers

Double-clad fibers (DCFs) are versatile waveguides comprising a single-mode core surrounded by a multimode inner cladding. DCFs are increasingly used for multimodal biomedical applications, such as imaging or therapy, for which the core is typically used for coherent illumination and the inner cladding to support a concurrent modality. Double-clad fiber couplers (DCFCs) were developed to allow the separation or combination of these signals in a robust and efficient fashion. In this paper, we review the operation and types of DCFCs in addition to current biomedical applications.