Optical bistability and switching performance in QWS distributed coupling coefficient DFB SLA’s

Abstract

Optical bistability (OB) behavior in quarter-wave-shift (QWS) distributed coupling coefficient (DCC) distributed feedback (DFB) semiconductor laser amplifiers (SLA's) is investigated. The simulations are based on a modified transfer-matrix method, which considers grating non-uniformities along the structure as well as gain saturation effect. Switching characteristics, including structural dependence of OB in QWS DCC DFB SLA's is demonstrated in the steady state regime. Time-dependent analysis of OB is performed as well, and the dynamics of optical switching is demonstrated. Keywords: distributed coupling coefficient (DCC), distributed feedback (DFB), dynamical analysis, optical bistability, optical switching, semiconductor laser amplifier (SLA). INTRODUCTIONDistributed feedback (DFB) structures are considered as one of the key elements for optical communication systems. They are already in use as single mode laser sources and have the potential to serve as the basis of alloptical processing components. In DFB semiconductor laser amplifiers (SLA's) the inherent optical gain is accompanied by an internal filtering effect and a strong nonlinear phenomenon, which makes them act as functional devices for applications in optical switching and photonic logic [1]- [4]. The nonlinear effect in SLA's arises from mutual changes in the gain and refractive index of the optically active medium, caused by carrier density dependence [5], and results in regions of dispersive bistability. Optical bistability (OB) in DFB SLA's involves a feedback procedure including the gain-dependent refractive index, a Bragg resonance, and the internal optical intensity. When an optical signal enters the SLA the amplifier internal power increases and, in appropriate conditions, the optical gain saturates, which makes the Bragg resonance shift towards the input signal wavelength. As a result, the internal power increases even more and the refractive index continues to increase as well [6]. Such positive feedback loop results in a sudden increase in the internal power and consequently, an upward jump in the output power from the SLA, which causes the switching performance. It is quite common to represent the dispersive OB behavior with the aid of an input-output intensity hysteresis curve.The switching action is the functional basis of several required applications in the field of all-optical signal processing. Since each application has unique requirements, it is desirable to express switching characteristics in terms of amplifier's bistability hysteresis, and also to have a means of tailoring the bistability behavior to fit the application. The object of this paper is to represent the OB switching performance of quarter-wave-shift (QWS) distributed coupling coefficient (DCC) structures. For the first time, the structural dependence of switching characteristics of optical bistability in DCC DFB SLA's, and the optical switching dynamics is investigated.