This book is designed as an advanced guide for experiments in Modern Physics. It covers the Spectroscopy of Lasers: Absorption, induced and spontaneous emission, Widths and profiles of Spectral Lines, Natural Linewidth, Absorption, Doppler broadening, Homogeneous and Inhomogeneous broadening, Saturation broadening, Semiconductor diode lasers, Litrow grating mounts, Fabry Perot Interferometer, Atomic Structure of Rubidium, Hyperfine Structure of Rubidium, Cross over signals, Doppler Free Saturated Spectroscopy, Coherent States, Experimental procedure, Data analysis, Finesse of the Fabry Perot (FP) Interferometer, Doppler broadened absorption, Average velocity of Rb atoms, Doppler free saturation absorption, etc. It is included the deduction of many formulas and the statistical analysis in order to clear the concepts and to get the concordance between the theory and the experiment. It is hoped that this book fills all needs of the students to get the fundaments of advanced experiments and to achieve the interest and motivation of the students for the real nature of the Physics.