To remarkably increase a threshold level of induced Brillouin scattering by suppressing intensity modulation of a light source LD generated light in the case of applying optical frequency modulation to the light source LD in order to prevent production of the induced Brillouin scattering in an optical transmission line in the configuration of the optical transmitter.
In the transmitter, an external modulator 2 modulates a light generated from a light source 1 consisting of a laser diode(LD) by using a high speed electric signal and the modulated light is amplified via an optical circuit 6 having an optical amplification function and the modulated and amplified signal is sent to an optical transmission line. In this case, a low frequency signal generating circuit 4 generating a low frequency signal and provided to the optical transmitter superimposes the low frequency signal to a bias current of the light source 1 via an adder 5 to apply optical frequency modulation to the generated light and the low frequency signal is fed to a gain control terminal of the optical circuit 6, in which intensity modulation opposite in the polarity to the intensity modulation component in the light generated from the light source 1 is conducted.
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