This book describes two categories of work, i.e., the finite-difference full-vectorial beam propagation method (FD-FV-BPM) development and microlens design for single-mode fiber (SMF) to laser diode (LD) coupling. Chapter 2 gives a review of the general formula to solve the FV-BPM equation of electric formula by using the FD method. The FD-FV-BPM based eigenmode solver is then successfully employed to investigate the effective index of the fundamental modes in an air-filled silica index-guiding photonic crystal fiber. Chapter 3 presents an improved FD-FV-BPM for arbitrary optical waveguides...