An approach to improve the coupling efficiency of bidirectional optical subassembly (BOSA) modules is proposed and experimentally demonstrated. We analyzed the wavefront aberration coefficients of a typical BOSA. It was found that the wavelength filter induces coma and astigmatism, and then it further deteriorates the laser diode to fiber coupling. We measured the BOSA efficiencies based on a series of different filters. For a typical filter, 25% coupling efficiency improvement was achieved by optimizing the filter parameters.