Impact of the spectroscopic properties of rare-earth ions on solid-state laser systems

Abstract

The electronic energy level schemes within the 4f subshells of rare-earth ions give rise to a number of fluorescence transitions ranging from the near-UV to the mid-IR spectral region. A large variety of laser lines have been demonstrated based on these fluorescence transitions. Depending on the energy level scheme of the individual rare-earth ion, the characteristics of the host material chosen, the specifications of the pump source and resonator configuration, and the resulting population mechanisms within the energy level scheme, these lasers may operate in quite different regimes. Specifically, additional absorption of pump photons at transitions originating in highly populated excited states as well as energy-transfer processes between neighboring rare-earth ions can have a significant impact on the population dynamics of a rare-earth-ion laser system.\ud Two examples shall be discussed in my presentation. First, the Nd3+ laser at 1 µm was considered as an almost ideal four-level laser system for many years. However, with increasing pump powers available, it has turned out that energy-transfer-upconversion processes from the 4F3/2 upper laser level may lead to a reduced storage time and subsequent multiphonon relaxations can generate significant extra heat dissipation in the crystal with its undesired consequences of thermal lensing and rod fracture, when the system is operated in a regime of higher excitation density such as a Q-switched laser or as an amplifier [1]. Second, the erbium 3-µm laser suffers from the long lifetime of its lower laser level. Depending on the host geometry, pump source, and dopant concentration, this laser was operated in three regimes [2] in which the problem of the depopulation of the lower laser level was solved in different ways: a) pump excited-state absorption and cascade lasing, b) energy-transfer to a co-doped rare-earth ion, and c) energy-transfer upconversion and energy recycling to the upper laser level.\ud \ud [1] M. Pollnau, P.J. Hardman, M.A. Kern, W.A. Clarkson, D.C. Hanna, Phys. Rev. B 1998, 58, 16076.\ud [2] M. Pollnau, S.D. Jackson, IEEE J. Select. Topics Quantum Electron. 2001, 7, 30