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This paper investigates and analyzes the in-band crosstalk generated in the photonic packet synchronizer that consists of a sequence of optical fiber delay lines and 2 × 2 optical switches. This paper shows that the second-order in-band crosstalk is dominant and its contribution is proportional to the square of the number of delay lines. It is found that coherent, partially coherent, and incoherent in-band crosstalk may coexist at the output of the synchronizer, causing the power penalty to vary with the delay time of the main signal. A close-form formula is derived for the mean and variance of the beating noise between the main signal and a crosstalk element. The power penalty caused by the in-band crosstalk is further numerically studied and a guideline for the requirement of the optical switches used in the synchronizer is provided.