We report the observation of a coherent multimode instability in quantum cascade lasers (QCLs), which is driven by the same fundamental mechanism of Rabi oscillations as the elusive RiskenNummedal-Graham-Haken (RNGH) instability predicted 40 years ago for ring lasers. The threshold of the observed instability is significantly lower than in the original RNGH instability, which we attribute to saturable-absorption nonlinearity in the laser. Coherent effects, which cannot be reproduced by standard laser rate equations, can play therefore a key role in the multimode dynamics of QCLs, and in lasers with fast gain recovery in general.PACS numbers: 42.55. Px, 42.60.Mi, 42.65.Sf The fundamental coherent mechanism that can destabilize a single-mode laser was predicted in the early 60s [1] and was later extended to multi-mode lasers [2,3] where it became known as the RNGH instability. These instabilities became classic landmarks for the general field of nonlinear dynamics [4,5] because they emerge in conceptually the simplest laser model, which in the single-mode case was shown to be equivalent to the Lorentz model of deterministic chaos [6]. Another feature that makes these instabilities so interesting and unique is their coherent nature that involves the polarization of the medium as a nontrivial dynamical variable. Most other physical mechanisms that can drive a laser from a singlemode to a multi-mode regime, such as spatial and spectral hole burning, Q-switching, and saturable absorption [7,8], can be adequately described within the standard rate equation formalism, in which the polarization of the active medium is adiabatically eliminated. The RNGH instability and its single-mode twin cannot be explained by the rate equations. Such coherent effects can be only observed when the polarization is driven faster than or comparable to the dephasing time T 2 [9].The origin of both phenomena is the oscillation of the population inversion at the Rabi frequency Ω Rabi that takes place when the intracavity laser intensity becomes large. This results in a modification of the gain spectrum and the emergence of sidebands separated from the maximum of the gain curve by an amount corresponding to the Rabi frequency. These sidebands can be regarded as a manifestation of parametric gain. The instability sets in when the intracavity power is sufficiently large: the Rabi angular frequency Ω Rabi has to be greater than the relaxation timescales of the gain medium (more precisely, Ω Rabi is sufficiently greater than (T 1 T 2 ) −1/2 , where T 1 is the gain relaxation time). The instability threshold is often called the second laser threshold due to its universal nature.Pioneering theoretical works stimulated extensive experimental studies that finally resulted in the observation of the Lorenz-type chaos in a far-infrared singlemode laser [10]. However, despite almost 40 years of efforts, the experimental demonstration of the multimode RNGH instability has remained somewhat controversial [11,12,13,14,15,16].In lasers with long gain recov...