An optical module with a laser diode (LD) without any temperature control and an optical fiber that is coupled with the LD through the two lens system is disclosed. The two lens system first converts laser beam into collimated beam and second concentrates the collimated beam onto the optical fiber. A beam splitter is disposed between the lenses and splits the collimated beam toward a photodiode (PD). The PD, which receives the split collimated beam in a back surface thereof, provides an anti-reflection film in the back surface. The anti-reflection film eliminates multi reflections occurred within the PD.