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In recent years, many researches have been focused on the flexible bandwidth optical networks in which the spectrum can be flexibly assigned in an efficient way. Thanks to the finer granularity in optical layer, flexible bandwidth optical networks achieve higher spectrum efficiency than wavelength routed optical networks. This benefit offers a good choice to provide connections between IP routers which the converged IP traffic flow is variable and time-dependent. In most cases, network operators often over-allocate network capacity to time-dependent traffic. However, it causes wasted of resource when the traffic volume becomes low at some time periods. To utilise the spectral resource effectively, the authors propose a novel time-dependent spectrum resource sharing (T-SRS) model in flexible bandwidth optical networks, in which the idle spectrum can be reused by other traffics. An interference factor (IF) is also introduced to evaluate the performance of T-SRS model. The authors formulate the problem using the integer linear programming (ILP) formulations to minimise IF optimally. Two efficient heuristic algorithms are proposed based on the T-SRS model and compared with ILP results. The numerical simulations show that more idle spectrum can be reused by reducing the IF.