Characteristic analysis of tapered lens fibers for light focusing and butt-coupling to a silicon rib waveguide

The characteristics of tapered lens fibers (TLFs) are analyzed by using a finite-difference time-domain (FDTD) method. Two types of TLFs are considered, namely, a tapered-cladding TLF and a tapered-core TLF. The radial FDTD method is first used to simulate light focusing for these two types of TLFs. It is shown that the tapered-core TLF has a smaller focus spot size and a larger transmission loss than the tapered-cladding TLF. The butt-coupling between a TLF of either type and a Si rib waveguide is then simulated by a three-dimensional FDTD method. The tapered-core TLF has a smaller coupling loss to the Si rib waveguide but a similar total loss (the sum of the coupling loss and the transmission loss), as compared with the tapered-cladding TLF.