Dynamic Pump-Probe Studies of TE and TM Modes in Semiconductor Optical Amplifiers S. Philippe 1 , F. Surre 1,2 , A.L. Bradley 1 , P. Landais 2 , M. Martinez-Rosas 3 1) Trinity College Dublin, Ireland 2) Dublin City University, Ireland 3) UABC, Ensenada, Mexico ABSTRACT A contra-propagation set up is implemented and dynamic pump probe studies of a InGaAsP/InP SOA in the gain regime are undertaken using pulses of 2ps duration. The time resolved amplified probe signal is measured separately for the TE and TM modes of the semiconductor optical amplifier. Different behaviours are observed both in the gain compression and the timescales of the effect, with the TM mode displaying a faster component and a higher gain compression. Keywords: semiconductor optical amplifier, optical switching, gain, polarisation, pump-probe. 1. INTRODUCTION Semiconductor optical amplifiers (SOAs) are mainly used for optical regeneration in optical communication networks. Their main advantage over fibre amplifiers is their potential for integration in all-optical systems. As they exhibit strong non-linearities, they are not only suitable for amplification, but also for signal processing applications. Various configurations have been implemented for all-optical wavelength conversion, based on cross-gain and cross- phase modulation 1-5 . Most of the applications proposed use at least partially fiberised set-ups, where the polarisation state of the light is random or modified as it travels through the fibres. This makes it very difficult both to set the polarisation to a known state and to measure how it is modified. But it is well known that SOAs are polarisation sensitive, with different gain and refractive indices in the TE and TM modes. Although considerable efforts have been made to reduce this sensitivity, using strain and different waveguide structures, and gain anisotropy of less than 2dB has been achieved, this does not imply preservation of the state of polarisation of the input beam. In a wavelength conversion system, a high intensity laser beam (pump) modifies properties (transmission, polarisation and phase) of a low intensity laser beam (probe). Pump-probe techniques can be used to investigate the timescales over which the probe is modified by the pump, by monitoring the probe output as a function of time delay between the pump and the probe pulses. The resolution of such experiments is mainly determined by the width of the pulses. For optical telecommunications applications of polarization rotation effects it is interesting to study the dynamics on the effect in the picosecond regime. In a conventional pump-probe set-up, the pump and probe pulses travel collinearly and in the same direction through the SOA and one is delayed with respect to the other. When the pump and probe are at the same wavelength the only way to separate them is via the polarisation, the most common method being to launch cross-polarised pump and probe pulses and to separate them at the output with a polariser. In the context of a polarisation study this would obviously limit the measurements that could be undertaken. Not only do the pump and probe have to be cross-polarised but their state of polarisation must be preserved as they travel through the SOA. Practically this means they have to be polarised along the eigen modes of the device, leaving only two possible combinations, probe TE pump TM and probe TM pump TE. Although a more complicated heterodyne detection 6 as been proposed, a simpler and more flexible method to study the polarisation dependence of pump-probe measurements is to use a counter-propagation configuration where the pump and probe pulses travel in opposite directions. Then the beams can simply be separated using a beamsplitter, whatever their polarisation may be. We propose to use a contra-propagation configuration to investigate if modifications applied to the waveguide in order to achieve polarisation insensitivity can influence the dynamic behaviour of the SOA. In this paper we will present a pump-probe study of dynamic polarisation effects of SOAs in contra-propagation geometry, using picosecond Opto-Ireland 2005: Optoelectronics, Photonic Devices, and Optical Networks, edited by J. G. McInerney, G. Farrell, D. M. Denieffe, L. P. Barry, H. S. Gamble, P. J. Hughes, A. Moore, Proc. of SPIE Vol. 5825 (SPIE, Bellingham, WA, 2005) · 0277-786X/05/$15 · doi: 10.1117/12.601450 267