Tapered grating eﬀects on static properties of a bistable QWS-DFB semiconductor laser ampliﬁer Mahmoud Aleshams a , M.K. Moravvej-Farshi a, * , M.H. Sheikhi b a Department of Electrical and Computer Engineering, Tarbiat Modares University, P.O. Box 144115-143, Tehran 1411713116, Iran b Department of Electrical Engineering, Shiraz University, Shiraz, Iran Received 22 April 2007; received in revised form 1 July 2007; accepted 3 July 2007 Available online 14 August 2007 The review of this paper was arranged by Prof. A. Zaslavsky Abstract Using the steady-state model based on the coupled-mode and carrier rate equations, in this paper, we analyze the eﬀects of trapezoidal tapered gratings on the dispersive bistable characteristics of a quarter wavelength phase-shifted distributed feedback semiconductor laser ampliﬁer (QWS-DFB-SLA). We also compare the results with those of a purely QWS-DFB-SLA. For laser ampliﬁers driven to yield the same unsaturated peak ampliﬁer gain, trapezoidal taper widens the spectral range of low-threshold bistable switching and increases the on–oﬀ switching contrast. The transfer-matrix method (TMM) is utilized for the numerical simulations. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Distributed feedback semiconductor laser ampliﬁer; Optical dispersive bistability; Phase shift; Trapezoidal tapering 1. Introduction When a distributed feedback semiconductor laser diode is biased below oscillation threshold, it acts as a DFB semi- conductor laser ampliﬁer (DFB-SLA). A DFB-SLA, similar to a Fabry-Perot semiconductor laser ampliﬁer (FP-SLA), shows a dispersive optical bistability (OB) behavior. Researchers have utilized optical bistability in SLA’s for optical logic and optical signal processing [1–4]. This is due to the fact that SLA’s exhibit exceptional bistability char- acteristics such as low switching powers (lW), fast switching speeds (ns), inherent optical gain, and wave- length compatibility with optical communication systems [5]. Meanwhile, a bistable DFB-SLA in comparison to a bistable FP-SLA, oﬀers the advantage of controlling the input/output characteristics by tailoring the DFB transmis- sion through some non-uniformities of its grating [6]. It should be pointed out that when a bistable laser works above its threshold, it may run in the ‘‘oﬀ’’ state, and the ‘‘free-running mode’’ results i.e., the laser lases in a wave- length diﬀerent from that of the injecting light. In practical applications, only the optical power in the locked fre- quency is required and the undesirable free-running mode has to be optically ﬁltered out. Meanwhile above threshold the OB loop width decreases with the bias level [7]. Hence, the behavior of the optically bistabe lasers are, often, stud- ied below threshold. The ﬁrst team to study of bistability in DFB-SLA’s was Kawaguchi et al., followed by Adams et al. [8–10]. In all these works, the DFB laser ampliﬁer had a uniform ideal grating structure and two important results were obtained. The ﬁrst was that optical bistability was observed at input power levels of a few lW, and the second was that, as a consequence of the spectral asymmetry of the bistable DFB-SLA, the hysteresis loops on either side of the stop band exhibited diﬀerent shapes. In 1990, it was demon- strated that the nonlinearity diﬀerence of the bistability loops in the two pass bands of a uniform DFB-SLA could 0038-1101/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.sse.2007.07.003 * Corresponding author. Tel.: +98 21 88332900x3367; fax: +98 21 88005040. E-mail address: farshi_k@modares.ac.ir (M.K. Moravvej-Farshi). www.elsevier.com/locate/sse Available online at www.sciencedirect.com Solid-State Electronics 52 (2008) 156–163