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Quantitative analysis of slow light experiments utilising coherent population oscillation (CPO) in a range of saturably absorbing media, including ruby and alexandrite, Er 3+ :Y 2 SiO 5 , bacteriorhodopsin, semi-conductor quantum devices and erbium doped optical fibres, shows that the observations may be more simply interpreted as saturable absorption phenomena. A basic two-level model of a saturable absorber displays all the effects normally associated with slow light, namely phase shift and modulation gain of the transmitted signal, hole burning in the modulation frequency spectrum and power broadening of the spectral hole, arising from the finite response time of the non-linear absorption. Only where hole-burning in the optical spectrum is observed (using independent pump and probe beams), or pulse delays exceeding the limits set by saturable absorption are obtained, can reasonable confidence be placed in the observation of slow light in such experiments. Superluminal ('fast light') phenomena in media with reverse saturable absorption (RSA) may be similarly explained. PACS Nos. 42.50.Gy, 42.65.−k

The distortion of an intense light pulse by a saturable absorber is analysed on the basis of the dynamic transmission equation. The conclusions are in accord with an earlier numerical analysis for a particular pulse function, but are here proved for arbitrary pulse shape. Bounds are derived for maximum transmittance and energy loss, and the existence of stable periodic solutions is demonstrated. The region in which saturable absorption theory is valid is discussed and compared with that for self-induced transparency. The two phenomena are examples of completely incoherent and completely coherent absorption, respectively.