Optical Packet Switching (OPS) has emerged as a promising candidate for the future all-optical core network. A crucial issue in such networks is how the packet length distribution (PLD) influences the performance under different traffic scenarios. In this paper we investigate how a deterministic, empirical and exponential PLD influences the Packet Loss Rate (PLR) in an optical packet switch in the case of a Poisson arrival process and a bursty Hyperexponential arrival process. We consider optical packet switches with and without Fibre Delay Line (FDL) contention buffers. Our results show that in the case of OPS without FDL buffering and with a Hyperexponential arrival process, the empirical PLD yields the lowest PLR. However, in the case of OPS with FDL buffering, the deterministic PLD yields the lowest PLR.