Can someone please explain what exactly the gain curve of a laser is? Is it exactly the same thing as the natural linewidth of the laser [a graph of intensity vs frequency] or am I off track? I found this graphhttp://www.phys.ksu.edu/perg/vqm/laserweb/Ch-5/C5s1p5.htm [Broken] that tries to explain the gain curve but it has the y-axis labelled as GL, the loop gain. What exactly is this? Is it a measure of intensity/laser power?