Quasi-Three-Level Lasers

Abstract

One of the key developments in the renaissance in solid-state laser technology has been the rapid improvement of diode laser pump sources.^1–3 Some of the advantages of diode-pumping relative to lamp-pumping of solid-state lasers include higher overall efficiency, reduced thermal loading of the gain medium, higher reliability, and reduced size. In addition, there are other important differences between these two types of pump sources. For example, diode lasers have higher spectral and spatial brightness, in other words, the diode laser output is narrowband spectrally, and it is directional. The consequence is that much higher volumetric pumping density can be achieved using diode-laser pumps even with low power single-stripe diode lasers relative to high-power lamps. The high pump densities has led to the demonstration of good laser performance at room temperature of several transitions that performed only poorly or not at all at room temperature under lamp pumping. These transitions in rare-earth ions include the ⁴F_3/2- ⁴I_9/2 near 0.94 µm in Nd³⁺,^4–6 the ⁵I₇ - ⁵I₈ near 2.1 µm in Ho³⁺,^7–10 the ³F₄ -³H₆ near 2.0 µm in Tm³+,^11–13 the ⁴I_13/2 - ⁴I_15/2 near 1.5 µm in Er³⁺,^13,14 and the ²F_5/2 - ²F_7/2 near 1.0 µm in Yb³⁺ (refs. 15, 16) as shown in Fig. 1. The common element in these laser transitions is that the lower laser levels are in the ground-state multiplet which means that the lower laser levels are only a few hundred cm-¹ above the ground state. Thus the lower levels have significant population in thermal equilibrium at room temperature since kT is 207 cm^-1 at 300 K. This is in contrast to the common four-level Nd³⁺ transition near 1.06 µm which has a lower level about 10kT above the ground-state at 300 K thus can be considered unpopulated in thermal equilibruim, or ruby in which the lower laser level is the ground-state. With lamp-pumping, these lasers with lower levels in the ground-state manifold were typically operated at cryogenic temperatures to reduce the lower-level population in thermal equilibruim; at sufficiently low temperature these lasers become four-level lasers and efficient, low threshold performance was obtained.