A laser light source outputs a virtually linearly-polarized light beam that has plural mode components arranged at an interval of an almost equal angular frequency. A polarization dispersion device is connected to the laser light source in such a way that a polarization axis of the polarization dispersion device is at 45° with respect to a polarization axis of the light beam output from the laser light source. This polarization dispersion device effects on the light beam output from the laser light source a polarization mode dispersion that is not in the vicinity of (2π/Δω), where Δω is a mode interval angular frequency.