ICTON 2012 Mo.B4.5 978-1-4673-2229-4/12/$31.00 ©2012 IEEE 1 Short Pulse Transmission Characteristics in Multi-Contact SOA Kevin Carney, Robert Lennox*, Regan Watts, Severine Philippe, Louise Bradley*, Pascal Landais, Senior Member, IEEE The Rince Institute, School of Electronic Engineering, Dublin City University, Dublin 9, Ireland * School of Physics, Trinity College Dublin, Dublin 2, Ireland Tel: (+353) 01 7005884, e-mail: kevin.carney2@mail.dcu.ie ABSTRACT An experimental characterisation of a multi-contact semiconductor optical amplifier using ultrashort optical pulses is presented. The SOA in question allows the injection of bias current through multiple independent electrical contacts, allowing the direct control of the carrier density. Picosecond-scale optical pulses are transmitted through the SOA. The non-linear effects on the pulse shape and spectrum after transmission are determined. It is found that the bias current distribution in the SOA is a significant factor in determining the extent of the non-linearities affecting the pulses. Additionally, amplification of negatively chirped pulses in the saturated SOA is found to reduce the spectral width of the pulses. Keywords: Semiconductor optical amplifier, carrier density, ultrashort pulses, non-linearities. 1. INTRODUCTION The use of ultrashort optical pulses is a critical aspect of optical communications schemes as bit rates reach 40 Gb/s and beyond. At speeds over 100 Gb/s, pulse widths on the order of a few picoseconds are necessary in order to avoid inter-symbol interference. The narrow pulse widths attainable through the use of mode locked laser diodes makes this possible [1] and allows efficient utilization of the available bandwidth. Semiconductor optical amplifiers (SOAs) are suitable devices for use in pulse amplification schemes due to their wide amplification bandwidth (> 5 THz). This allows them to amplify pulses on the order of a few picoseconds in duration without distortion due to gain dispersion and other effects [2]. The penalizing effect of dispersion that affects ultrashort pulses in long stretches of fibre does not have a significant effect in SOAs due to the short length of the devices, however non-linear phenomena such as gain saturation and self phase modulation can cause pulse distortions, depending on the pulse energy injected into the SOA. An optical pulse with energy below the saturation energy of the SOA is generally amplified with few distortions, but non-linear effects come into play once the pulse energy reaches this limit. The SOA that is presented in this work is designed to either reduce noise figure or increase saturation power, depending on the distribution of carrier density within the device. This device has been previously characterised [3]. Control over this distribution is accomplished through the use of multiple electrical contacts for the injection of bias current. In this way, multiple carrier density profiles can be created depending on the required use of the SOA. The SOA presented herein has three electrical contacts, although the number of contacts is limited only by practical considerations. In order to decrease the SOA noise figure, the bias current is increased at the input of the SOA, in order to increase population inversion at the beginning of the chain and thus reduce NF for the entire device [4]. Conversely, in order to increase saturation power, the carrier density must be increased at the output of the SOA. This is due to the inverse dependence of the saturation intensity on the spontaneous carrier lifetime, s , which decreases with increasing bias current. As the signal is amplified, it encounters a steadily increasing carrier density, keeping the SOA in linear operation. 2. NON-LINEAR EFFECTS ON ULTRASHORT PULSES IN SOAS When the input power to the SOA is high enough, the gain saturates, due to the bleaching of carriers by stimulated emission. Gain saturation in SOAs can give rise to a number of non-linearities that affect both the temporal and spectral characteristics of optical pulses. These non-linearities have the ability to significantly impede data transmission. 2.1 Temporal effects The instantaneous gain G of an SOA in response to an optical pulse is shown in equation (1). () ( ) () ( ) 0 0 0 1 exp sat G G G G E E τ τ = - - - (1) where G 0 is the unsaturated SOA gain, E( ) is the energy of the pulse integrated to time , and E sat is the pulse energy at which the SOA saturates. When a pulse propagates in an SOA, the leading edge of the pulse will experience the maximum unsaturated gain G 0 . The corollary of this result is that the trailing edge of the pulse experiences the least amount of amplification, due to the saturation of the gain by the leading edge. In this case E( ) is replaced in equation (1) by E in , the total input energy of the pulse. This saturation temporally broadens the