Light has always piqued humanity's interest for many years and many scientists studied in this regard. Willebrord Snell developed the law of refraction in 1621, while Francesco Maria Grimaldi and Robert Hooke recognized the phenomena of diffraction by the mid-1600s. Sir Isaac Newton contributed significantly to optics, claiming that white light was a mixture of all colors. Thomas Young, who resurrected the wave theory at the turn of the 19th century by introducing the notion of superposition, is another key player in the history of optics. A laser is a device that uses a process called stimulated emission to produce light, which is a kind of electromagnetic radiation. Laser light is often spatially coherent, which means it is either emitted in a narrow, low-divergence beam or may be transformed into one using optical components like lenses. The emergence of semiconductor-based pump sources, including InxGa1-xN-laser diodes or frequency-doubled optically pumped semiconductor lasers with emission wavelengths in the blue, has prompted a reconsideration of the characteristics of the rare-earth ions such as Sm3+, Pr3+, Tb3+, Dy3+, Er3+ and Ho3+, as active ions in crystalline solid-state laser materials with direct emission in the visible spectral range. In terms of effectiveness, several of these pumped visible lasers now compete with Nd3+-lasers, and direct lasing at different colors ranging from cyan-blue to deep red may be achieved using relatively simple and small laser systems. This study highlights the trends in light and optics, different types of lasers and their applications, and reviews the latest progress in the field of visible rare-earth doped solid-state lasers.
