Many aspects about Optical Packet-Switching (OPS) networks have been studied in the last few years and it has been shown that their performance depends on several determinant factors, such as the transmission mode and the optical packet format A synchronous slotted mode is more adapted to MAN network specifications, since it produces higher throughput comparing to the asynchronous mode. In this paper, we propose an optimized mechanism for creation of fixed-size optical payloads filled with variable size electronic client packets. Our algorithm is called GPFO for graduated packet filling optimization. It avoids the need for complex segmentation methods. It also provides high network throughput and good performance results in terms of packet loss ratio and mean access delay. We investigated the performance of our algorithm throw simulation works. Using the ns simulator, our experiments are performed on an optical MAN network with a ring topology. Numerical results have shown that, compared to existing solutions, the GPFO enhances the network throughput and optimizes the global network performance in terms of queuing delay and bit loss ratio