Optical switching architecture based on fast tunable laser and arrayed waveguide grating is an ideal choice to realize optical switches with large capacity. However, the fast tuning requirement of the fast tunable laser makes it necessary for the fast tunable laser to keep seeding light even under idle state without data, and the optical light from the idle fast tunable laser may introduce significant interference to a valid optical signal if they are routed to the same receiver. In order to solve the influence of the fast tunable laser constraint, a method is proposed to use the invalid demands to fill idle time slots so that the valid demands and invalid demands have different output ports and no longer interfere with each other. Two different methods to generate the invalid demands are proposed, namely the random strategy and the sequential strategy, and the performances of the two strategies with different traffic loads are studied and compared through simulations. Meanwhile, in order to reduce the number of rearrangement in the scheduling algorithm, a threshold-based strategy to find the path for an invalid demand is proposed. The simulation results show that this threshold-based method can greatly reduce the number of rearrangement, and improve the computation efficiency of the scheduling algorithm.