# Gain Curve for a Diode laser

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?

Last edited by a moderator:

Claude Bile
A gain curve depicts how much gain a particular frequency will have per round trip in the laser cavity. Actual gain will depend on mirror reflectivities. Loop gain is a good way to depict the gain in a particular active medium without having to rely on parameters of the laser cavity.

The gain curve gives a general idea of how many longitudinal modes will appear and their relative intensities. For example, if the longitudinal mode separation was 1 THz and the gain curve had a width of 5 THz above threshold, you can expect about 5 longitudinal modes to appear, with the relative intensities of each tapering off as one moves from the centre of the gain curve.

The Gain curve is not the same as the natural linewidth of a laser.

Claude.

Ok, so would you say that the natural linewidth is the width of the mode that the laser is currently lasing at?

Or is the natural linewidth [as the term 'natural' suggests] the linewidth associated with a single photon as emited from an atom?

Claude Bile
The natural linewidth is the linewidth due to natural broadening. Natural Broadening is due to the uncertainty of the width of the energy gap, which arises because of the finite lifetime of atoms in the upper state.

Typically the natural linewidth is about 8 MHz. The natural linewidth is dependant on the upper state lifetime.