The linewidth of a mode-locked semiconductor laser caused by spontaneous emission: Experimental comparison to single-mode operation

We experimentally compare the linewidth of the individual modes of an extended cavity semiconductor laser when it operates mode locked and when it operates single mode. We find that the linewidths under these two operating conditions have the same inverse dependence on the average power. Therefore, the coherence length of the mode-locked source is the same as that of the single-mode source despite lower power per mode, much broader total bandwidth, and much higher spontaneous emission noise level in the mode-locked source. It can be inferred from our data that the electric fields of over 1000 consecutive mode-locked pulses are correlated.