An optical device comprises a PD array and a submount. Anode electrodes and cathode electrodes are disposed as a gold electrode pattern on a surface of the PD array facing the submount and a common cathode electrode and anode electrodes associated with respective channels are disposed as a gold electrode pattern on a mounting surface of the submount. The gold electrode pattern on the PD array and the gold electrode pattern on the submount are electrically connected to each other by a conductive layer. A gap between the PD array and the mounting surface of the submount includes a first gap corresponding to active layers and a second gap around the first gap, the first and second gaps comprising air.