Dear Muhammad,
when you directly modulate a laser with sinusoidal signal, you should expect to get sinusoidal signal at the output of the laser as long as you choose the bias and amplitude of the signal correctly, as well as the LI characteristic is linear. This implies that you should get single tone. Else, you may have many tones as the output signal is no more sinusoidal. I am attaching an image to show you the results when modulating the laser with correct settings.
Regards,
Ahmad
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