INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS J. Phys. D: Appl. Phys. 36 (2003) 1473–1476 PII: S0022-3727(03)52033-2 Semiconductor optical amplifier length effects on gain dynamics F Ginovart and J C Simon Groupement d’Int´ erˆ et Scientifique ‘FOTON’, Laboratoire d’Optronique/UMR CNRS 6082, ENSSAT–Universit´ e de Rennes1, 6, rue de K´ erampont, 22305 Lannion Cedex, France Received 6 August 2002, in final form 23 December 2002 Published 18 June 2003 Online at stacks.iop.org/JPhysD/36/1473 Abstract We study theoretically semiconductor optical amplifier (SOA) length effects on gain dynamics when a short saturating pulse (pump) is either co- or counter-propagative with the probe. More specifically, we focus our attention on SOA recovery dynamics and gain overshoot. Analysis is also given when the input probe power and input pump energy are increased for different SOA lengths. 1. Introduction and model During the last decade, gain saturation in semiconductor optical amplifiers (SOAs) has been thoroughly used for applications in all-optical signal processing, such as wavelength conversion or gating components in optical regeneration devices [1, 3]. Concerning gain dynamics studies including ASE, pump and CW probe propagation, few models have been proposed [4–7]. Here, we use a direct temporal analysis of a wavelength converter, based on a previous work [8]. Our model [9] includes propagation of pump, probe and forward and backward amplified spontaneous emission. The band model is the four-band Kane model and the calculations use the Ga 0.40 In 0.60 As 0.85 P 0.15 parameters from Pearsall [10] for the undoped core. The amplifier structure is a buried ridge type waveguide. The substrate is a n-type InP substrate, with the core stripe lying on it. The core is surrounded by an InP cladding material, grown up to a thickness of 2 µm above the core top interface. The cladding material is undoped except for a narrow stripe region (Zn-doped, p-type) located just above the core for carrier conduction. Usual ohmic contact and Ti–Pt–Au layers complete the structure. We did not consider intraband relaxation effects which can be not relevant, for instance for experiments using long pulse durations in bulk SOA. Here, we assume that SOA facets reflectivity is negligible and that all the injected current contributes completely to the population inversion in the active zone. We also assume that transverse effects are negligible since carrier scattering length is much longer than waveguide size, and thus carrier density is uniform in transverse cross-sections. We give in table 1 the constant modelling parameters which have been used. Table 1. Parameters used in the model. SOA length: L = parameter Confinement factor: Ŵ = 0.5 rectangular guide: 0.5 × 0.4 µm 2 Injected current density: Peak gain wavelength value: I = 0.238 A cm −1 1.54 µm Probe wavelength: 1530 nm Auger recombination coefficient: C = 4 × 10 −29 cm 6 s −1 Pump wavelength: 1540 nm Spontaneous recombination Pump pulse duration: 2.1 ps coefficient: B = 1.2 × 10 −9 cm 3 s −1 2. Results and discussion 2.1. SOA length effects on gain dynamics We plot in figures 1–3 the time evolution of the gain dynamics of three SOA, whose lengths are 630, 1170 and 1710 µm. In order to compare the effect of device length, bias current values are adjusted in order to get the same current density. Also, injected pump pulse energy has been adjusted in order to obtain the same gain compression (about 10 dB) in all cases. Injected pump energy values are given in table 2. Note that figure 1 corresponds to a weak value of the input probe power, 10 −6 µW, in order to leave undisturbed the SOA gain. Since it is a small-signal regime, we have plotted only the co-propagative configuration. Figures 2 and 3 correspond to a moderately high input probe power, 60 µW, respectively, in co- and counter-propagative configurations. We can clearly see on all curves a fast gain compression followed by a gain recovery which is more or less fast depending on probe power and device length. 0022-3727/03/131473+04$30.00 © 2003 IOP Publishing Ltd Printed in the UK 1473