When the reflection coefficient of one of the facets of fabry- perot structure is lower than 100%, a portion of the light will come out, while the rest will continue the back and forth oscillation. This device is well known as the Fabry-Perot (FP) type laser.
Since, the fundamental structure in a LASER diode is indeed a p-n junction; there is a linear relationship between the injected current and the optical power output as in case of a LED.
The only difference is the higher value of efficiency that accompanies this emission.
And the reason for this sudden increase in efficiency is the feed-back mechanism which increases the gain of device and the coherency in the optical output and as whole the total efficiency of the device increases.
At low input drive currents, the slope of the power vs current curve is very low indicating the low efficiency of the basic diode. This curve is called as “P-I”.
As the input drive current increases, a point arrives when the slope abruptly increases and the device efficiency increases to a very high value.
This particular value of current at which the slope of the curve increases abruptly is called the threshold current $(I_th)$ of the device.
The threshold current indicates the start of the stimulated emission process which begins to dominate and nullify the losses in the device due to the material properties and other factors by amplifying the output optical power at a much higher rate than the basic diode.
The optical radiation spectrum of FP lasers contains several distinct wavelength peaks placed under the spectral envelope.
These pulses are known as longitudinal or spectral modes, and they are typically for any structure based on the FP resonator.