Propagation delay of femtosecond pulses in an optical amplifier

The recent realization of tunable propagation delay of optical pulses in solid-state and semiconductor optical media1,2 has attracted great attention as such a functionality enables a whole new class of optical components in optical communications systems and signal processing3. The reported results show a large reduction in group velocity but this was achieved at the cost of a small bandwidth (e.g. 37 Hz in the case of Bigelow et al.1) of the probe signal. In this paper, we report measurements of slowing down and speeding up of the propagation of 150 fs pulses, having a very large bandwidth of 2.6 THz, through a quantum-dot (QD) semiconductor amplifier (SOA) at room temperature. This extremely large bandwidth, on the other hand, is at the cost of a rather small group index change of ?ng=4*10-3. We have performed two types of femtosecond pulse slow-down and advancement experiments. In the first experiment, we prepare a narrow peak or dip in the SOA gain spectrum by injection of a strong pump pulse4. The resulting dispersion feature is then probed by a weak pulse. In the second experiment, we measure self-slowdown or advancement as pulse energy isincreased5. In both cases, we perform measurements as function of injected bias current. Good agreement is found with simple models of the real and imaginary parts of the active material's susceptibility. 1 M.S. Bigelow, N.N. Lepeshkin, and R. Boyd, Phys. Rev. Lett. 90, 113903-1—4 (2003) 2 P.-C. Ku et al., Opt. Lett. 19, 2291—2293 (2004) 3 C.J. Chang-Hasnain, P.-C. Ku, J. Kim, S.-L. Chuang, Proc. IEEE 91, 1884—1897 (2003) 4 M. van der Poel, J. Mørk, and J.M. Hvam, paper JWB96, CLEO/QELS, Baltimore, USA, May 2005 5 M. van der Poel, J. Mørk, and J.M. Hvam, accepted for oral presentation, 13th. International symposium on nanostructures, St. Petersburg, June 2005