Packet contention is a major issue in optical packet switching networks. Optical buffer is proposed to resolve contention, witch is implemented by fiber delay lines, slow-light substances, or fiber loop. As many previous works focus on fiber-delay-line-based buffer, we focus on fiber-loop-based buffer in this paper which offers variable delay. In slotted optical packet switching networks, several packets may arrive at the switching node with the same delay time requirements. They will contend for the buffer element which offers the same delay time. This phenomena is called buffer conflicts. First of all, we present a theoretical model to calculate the buffer conflicts probability distribution. Then packet loss rate is worked out based on this probability distribution. Finally, numerical simulations are conducted to verify the accuracy of our model..