Physics Letters A 318 (2003) 71–77 www.elsevier.com/locate/pla Amplitude modulation in a pair of time-delay coupled external-cavity semiconductor lasers Awadhesh Prasad a , Ying-Cheng Lai a,b,∗ , Athanasios Gavrielides c , Vassilios Kovanis c a Department of Mathematics and SSERC, Arizona State University, Tempe, AZ 85287, USA b Departments of Electrical Engineering and Physics, Arizona State University, Tempe, AZ 85287, USA c Nonlinear Optics Center, Air Force Research Laboratory, DELO, Kirtland AFB, NM 87117, USA Received 11 May 2002; received in revised form 4 October 2002; accepted 6 August 2003 Communicated by A.R. Bishop Abstract The phenomenon of amplitude death in coupled nonlinear oscillators has been a topic of recent interest. We demonstrate that a similar phenomenon can occur in a pair of time-delay coupled, external-cavity semiconductor lasers. In particular, with coupling chaotic oscillations of the laser field can be converted into quasiperiodic motion and low-frequency fluctuations in laser power can be suppressed.  2003 Elsevier B.V. All rights reserved. PACS: 42.65.Sf; 42.55.Px; 05.45.-a There has been an interest in the phenomenon of amplitude death [1–3] in the context of time-delay coupled limit-cycle oscillators [4,5]. The phenomenon was first observed in coupled chemical oscillators [1]. It was then established theoretically that, if the cou- pling is sufficiently strong and the spread in the nat- ural frequencies of the oscillators is sufficiently broad, the amplitudes of the oscillations can reach zero [2,3]. The issue of time-delayed coupling, which is physi- cally important, was recently addressed both theoreti- cally [4] and experimentally [5]. An interesting result is that in the presence of a time delay, amplitude death can occur even if the natural frequencies of the limit- * Corresponding author. E-mail address: yclai@chaos1.la.asu.edu (Y.-C. Lai). cycle oscillators are all identical [4], in sharp contrast to the situation of zero time delay, where a broad dis- tribution in the natural frequencies of the oscillators is necessary for the amplitude death [2,3]. Since time de- lay is present in many physical applications, amplitude death may be pervasive in coupled oscillators. The aim of this Letter is to present evidence that a similar phe- nomenon can occur in time-delay coupled, external- cavity semiconductor lasers. The observable phenom- enon is that the laser oscillation can be modulated sig- nificantly. We call the phenomenon amplitude modu- lation in coupled semiconductor lasers. In many applications of semiconductor lasers, opti- cal feedbacks are deliberately introduced to improve the performances of the laser such as the enhance- ment of the single longitudinal mode operation, spec- tral line narrowing, improved frequency stability, and 0375-9601/$ – see front matter  2003 Elsevier B.V. All rights reserved. doi:10.1016/j.physleta.2003.08.072