Laser linewidth is the spectral linewidth of a laser beam.
Two of the most distinctive characteristics of laser emission are spatial coherence and spectral coherence. While spatial coherence is related to the beam divergence of the laser, spectral coherence is evaluated by measuring the laser linewidth of the radiation.
In short, Linewidth is nothing but the accuracy or the bandwidth of the laser it emits. Let us suppose you buy a laser of 1550nm with linewidth of 10MHz, then it will generate 1550nm +- 10MHz bandwidth.
In order to understand the concept better watch videos of laser fundamentals given at this link
This will explain the concept of linewidth in a better way.
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