The present disclosure generally relates to optical modules, and in particular, to an optical module comprising a printed circuit board for reducing crosstalk between differential signal lines. In one implementation, the printed circuit board comprises a top layer, a first intermediate signal transmission layer, a second intermediate signal transmission layer, a bottom layer and multiple ground layers between signal transmission layers. Each signal transmission layer comprises one or more differential signal line pairs. The top layer and the bottom layer each comprises an edge connector, and the top layer further comprises a laser driver chip. The signal transmission layers are connected to the edge connectors and laser driver chips via a combination of blind and through connection holes such that the interference between the differential signal line pairs of various signal transmission layers are reduced.