McCumber Theory | previous | next | feedback |
Definition: a theory applied to absorption and emission properties of laser gain media, in particular to solid-state media with Stark level splitting
Particularly in the context of solid-state lasers, one is often dealing with Stark level manifolds where degeneracies are lifted by the influences of the crystal field. As a consequence, absorption and emission transitions between Stark manifolds have a significant spectral width (bandwidth). The wavelength-dependent transition strengths are described with effective cross sections.
In the 1960s, Dean E. McCumber at Bell Laboratories worked out a detailed theory [1] – now called McCumber theory – to explore the quantitative relations between various optical properties of laser gain media such as molecular gases but also rare-earth-doped or transition-metal-doped gain media. He made use of thermodynamic principles, following earlier (and less general) theoretical investigations of Albert Einstein. The required additional assumptions – basically that within the Stark level manifolds there is a thermal equilibrium – are usually well satisfied, in particular for solid-state media due to the fast phonon-mediated transitions.
McCumber theory is particularly useful for spectroscopic evaluation of quasi-three-level gain media, e.g. of rare-earth-doped type. A result of central importance, nowadays frequently used particularly in the context of solid-state media, is the McCumber relation (or McCumber equation)
which relates the frequency-dependent effective cross sections σabs for absorption and σem for emission to each other. The quantity E0, which depends on the temperature but not on the optical frequency ν, can be calculated from the energies of the single Stark levels. Alternatively, it can be calibrated e.g. using the reciprocity method or the Fuchtbauer-Ladenburg equation. For ytterbium-doped gain media, E0 is often close to the photon energy of the zero-phonon transition, i.e., the transition between the lowest sublevels of both manifolds.
The above quoted McCumber relation is very useful e.g. to evaluate the weak absorption cross sections on the long-wavelength side of a laser transition. Calculating the absorption cross sections from the emission cross sections can be much more precise than directly measuring the weak absorption. Also, the spectral shape of the intrinsic fluorescence can be calculated from the absorption spectrum. This can be advantageous when direct fluorescence measurements would be affected by reabsorption in a highly doped sample.
Bibliography
| [1] | D. E. McCumber, "Einstein relations connecting broadband emission and absorption spectra", Phys. Rev. 136 (4A), A954 (1964) |
| [2] | J. N. Sandoe et al., "Variation of Er3+ cross section for stimulated emission with glass composition", J. Phys. D 5 (10), 1788 (1972) |
| [3] | W. J. Miniscalco et al., "General procedure for the analysis of Er3+ cross-section", Opt. Lett. 16 (4), 258 (1991) |
| [4] | R. M. Martin and R. S. Quimby, "Experimental evidence of the validity of the McCumber theory relating emission and absorption for rare-earth glasses", J. Opt. Soc. Am. B 23 (9), 1770 (2006) |
See also: gain media, four-level and three-level gain media, rare-earth-doped gain media, transition-metal-doped gain media, cross sections, fluorescence
Category: physical foundations
