Figure 6.26: Output power from a diode laser as a function of input current.
The condition for population inversion depends on the pumping.
By increasing the current injected through the
p-n junction, we arrive at threshold
current,
which fulfills this condition.
An example of the power output from a laser diode as a function of the injected current is shown in figure 6.26.
It is easily seen that the slope of this graph in a stimulated emission
(laser) is far greater than the slope at spontaneous emission (LED).
This experiment can be easily done in the laboratory, and an example
of laboratory instructions can be found by clicking on I-V
Lab.
The threshold current for lasing
is determined by the intercept of the tangent to the graph at stimulated
emission with the current axis (this point is very close to the point of
change in the slope).
When the current threshold is low, less energy will be wasted in the form of heat, and more energy will be transmitted as laser radiation (The laser efficiency increases). Practically, the important parameter is current density, which is measured in units of Amperes per centimeter squared [A/cm2] of the cross section of the junction.