The lasing threshold is the lowest excitation level at which a laser's output is dominated by stimulated emission rather than by spontaneous emission.
Below the threshold, the laser's output power rises slowly with increasing excitation.
Above threshold, the slope of power vs. excitation is orders of magnitude greater.
The linewidth of the laser's emission also becomes orders of magnitude smaller above the threshold than it is below.
The term "lasing" is a back formation from "laser," which is an acronym, not an agent noun.
The lasing threshold is reached when the optical gain of the laser medium is exactly balanced by the sum of all the losses experienced by light in one round trip of the laser's optical cavity.
is the round-trip threshold power gain, and
is the round trip power loss.
The experimenter typically has little control over the distributed losses.
The optical loss is nearly constant for any particular laser (
Under this assumption the threshold condition can be rearranged as[1] Since
requires low distributed losses and high reflectivity mirrors.
in the denominator suggests that the required threshold gain would be decreased by lengthening the gain medium, but this is not generally the case.
The problem is that the laser output power varies by orders of magnitude depending on whether the laser is above or below threshold.
When very close to threshold, the smallest perturbation is able to cause huge swings in the output laser power.
The formalism can, however, be used to obtain good measurements of the internal losses of the laser as follows:[2] Most types of laser use one mirror that is highly reflecting, and another (called the output coupler) that is partially reflective.
Reflectivities greater than 99.5% are routinely achieved in dielectric mirrors.
The reflectivity of the output coupler can then be denoted
The equation above then simplifies to In most cases the pumping power required to achieve lasing threshold will be proportional to the left side of the equation, that is
In order to use this expression, a series of slope efficiencies have to be obtained from a laser, with each slope obtained using a different output coupler reflectivity.
The power threshold in each case is given by the intercept of the slope with the x-axis.
The resulting power thresholds are then plotted versus
The theory above suggests that this graph is a straight line.
At this point the x value is equal to the round trip loss
This allows for measurements with low random error, however it does mean that each estimate of
A good empirical discussion of laser loss quantification is given in the book by W.