Hi Hamdi,
For low-power laser, linewidths up to a few kiloHertz can also be found according to wikipedia. However, the ampunt I mentioned for the linewidth was used in the paper “Transmission simulation of coherent optical OFDM signals in WDM systems” by Bao as a conventional value for commercially available lasers, and the references are given in the paper (even though they are not datasheet). I hope this helps.
Regards