Optical sensing has been subject to a great interest for the moderate intrusiveness of its operation. The introduction of random lasers in ’90s has opened the door for developing a new kind of optical sensors. In such a source, disorder is introduced within an inverted medium, increasing the lifetime of the radiation without the presence of an optical cavity. The striking point is that the spectral characteristics of the output emission are strongly dependent on the scattering properties of the medium, suggesting new methods to investigate disordered materials. Recently, a novel concept for optical sensing based on the physics of random laser has been reported,<sup>1</sup> overcoming the limits due to the alteration of the investigated sample by injecting an active material. Here we present a characterization of such a kind of sensor, suggesting non-invasive and also <i>in-vivo</i> applications.
We present an experimental realization of slow and fast light schemes for a few ns long optical pulses that makes use of incoherent interactions in an atomic medium. The combination of such different schemes allows us to demonstrate that the propagation delay acquired in the slow light stage, can be completely recovered in a fast light one. The use of an incoherent interactions scheme makes the control of the propagation dynamics of light pulses easer to realize. Delays up to 13 ns, in slow light regime, and advances up to 500 ps, in fast light regime, are reported when the stages work individually for a 3 ns long pulse. When both stages are switched-on the fast light stage is able to recover a previously induced delay and even to produce an extra advance, with an overall advance up to 1 ns. Since every optical transmission line needs an amplification system to overcome the unavoidable losses, the results suggest the opportunity and perspective of a proper tailoring of the amplification stage for data timing purposes.