This book explores the modern problems of quantum optics, and shows that, in simple optical systems, it is possible to obtain quantum states that are interesting from the point of view of modern quantum physics and quantum optics. In particular, the quantum behavior of the second and third harmonics and subharmonics generation processes is investigated, highlighting that, in subharmonic processes, it is possible to obtain Schrödinger's cat-type states of light, which are one of the main problems of quantum physics. The book uses few formulas and mathematical conclusions, opting instead for a large amount of graphic material, in order to make the concepts explored easier to understand. It will be of interest to scientists working in quantum optics, as well as teachers and students of physics.