An optical device comprises a substrate, an optical waveguide, a laser diode, photodiodes, and a filter. The substrate is provided with a flat reference surface and a V-groove recessed from the reference surface. A pin is fitted in the V-groove of the device and a V-groove of a connector. The optical waveguide is formed on the reference surface. The optical waveguide is provided with a core and a cladding. The core extends along the reference surface. The laser diode emits a signal light beam with a first wavelength toward the core and the photodiode, individually. The filter transmits the signal light beam with the first wavelength and reflects a signal light beam with a second wavelength. The signal light beam reflected by the filter is received by a photodiode.