DOUBLE-CLAD FIBER COUPLERS
Double clad fiber couplers (DCFC) combine a double clad fiber with a multimode fiber in order to efficiently transfer light from and to the inner cladding of the double-clad fiber.
In all DCFC types, light in the single-mode core of the double-clad fiber travels unperturbed through the coupler, in either direction. This can be used to send and collect single mode light.
Coupling of light within DCFC's occurs in the inner cladding of the double-clad fiber. This coupling, depending on the coupler type, may be from the inner cladding to a multimode fiber OR from a multimode fiber to the inner cladding
In collection-type DCFC, multimode light collected by the inner cladding of the double-clad fiber is transferred to a multimode branch of the coupler for detection. We design DCFCs for various ratios of diameters between the inner cladding and core. What we call large collection DCFCs have an inner cladding diameter typically larger than 7x that of its core. These are used for incoherent detection, for example, when combining optical coherence tomography (OCT) through the core and fluorescence detection through the inner cladding. On the contrary, when the inner cladding diameter is less than 7x larger, we call them small collection DCFCs. These are used for partially coherent detection, in confocal microscopy, for example.
Injection-type DCFC are designed to transfer light into the inner cladding of the DCFC.
Large inner cladding
Collection (large) DCFCs are designed to transfer light from the double-clad fiber inner cladding region into a separate multimode branch of the coupler.
Small inner cladding
Collection (small) DCFCs function as their large counter part, but with a small diameter inner cladding, for applications requiring partial coherence.
Injection DCFCs allow transfering light from a multimode fiber into the inner cladding of a double-clad fiber. Applications include monitoring laser therapy using imaging.