Visible solid-state lasers based on Pr3+ and Tb3+

Visible lasers are sought for in a variety of applications. They are required in fields as diverse as medicine, materials processing, display and entertainment technology and many others. Moreover, in contrast to infrared lasers, they enable very simple and efficient access to the UV spectral range by a single frequency doubling step. Currently, the choice of direct visibly emitting lasers is limited: The ‘green gap’ prohibits the development of semiconductor lasers with emission in the green and yellow spectral range and only few laser active ions allow for efficient visible lasing. In particular trivalent praseodymium (Pr3+) and terbium (Tb3+) ions have been shown to be the most successful candidates for efficient high power visible solid-state lasers. Compared to semiconductor lasers, solid-state lasers also provide other advantages, e.g., in terms of energy storage in Q-switched operation as well as beam quality at high output power. In recent years, visibly emitting solid-state lasers have seen a revival enabled by the increasing commercial availability of GaN-based blue emitting pump diodes and an ever-increasing number of publications evidences the vivid research activities in this field. Still, due to the relatively short history of diode-pumped visible solid-state lasers, these are still in an early stage of their development and up to now only few direct visibly emitting solid-state lasers with comparably low output power are commercially available. However, we are convinced that visibly emitting solid-state lasers based on Pr3+ and Tb3+ have the potential for 100-W-class continuous wave output power levels as well as sub-ns pulse durations in Q-switched and sub-ps-pulse durations in mode-locked operation, which would qualify them to fulfil the requirements of most of the applications named above. In this work, we review the state of the art of continuous wave and pulsed visibly emitting solid-state lasers and amplifiers based on Pr3+ and Tb3+ as the active ion. After an introduction, we briefly review the spectroscopic properties of these two ions and their particularities for laser operation as well as the requirements for suitable host materials. In the third chapter, we present the state of the art in the field of continuous wave Pr3+-lasers emitting in the cyan-blue, green, orange, red, and deep-red spectral range based on fluoride, glass, and oxide host materials and discuss prospects for further power scaling. The fourth chapter is devoted to the current state of Tb3+-based continuous wave green and yellow emitting solid-state lasers. In the fifth and sixth chapter we give an overview over existing pulsed visibly emitting solid-state lasers in Q-switched and mode-locked operation mode, respectively. Finally, the seventh chapter is devoted to pulse amplifiers for ultrafast visible lasers before this review closes with a short conclusion.