The distributed feedback (DFB) lasers and solar cells are devices of many potential applications. The (DFB) laser has a number of features needed by the high performance optical fiber systems eg: stable single frequency output, gigabit modulation capability, stable operation throughout lifetimes at room temperature, etc.... This book consists of two parts. In the first part of the book, a rigorous, truly, two-dimensional method for threshold analysis of (DFB) lasers based on a coupled wave transfer matrix formalism is presented. In the second part of the book, the method of the RCWA is applied to a thin textured solar cell. The analysis should help shed some light on optical modeling for both devices namely, DFB laser and Solar cell and that should be especially useful to professionals in optics and waves propagation, or anyone else who may be interested in DFB laser or light trapping techniques of thin textured solar cells.