  Quantitative analysis of slow light experiments utilising coherent population
oscillation (CPO) in a range of saturably absorbing media, including ruby and
alexandrite, Er3+:Y2SiO5, 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.
