In laser science, 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 the 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. Above the threshold, the laser is said to be lasing. The term "lasing" is a back formation from "laser," which is an acronym, not an agent noun.
Theory 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. This can be expressed, assuming steady-state operation, as
R 1 R 2 exp ( 2 g threshold l ) exp ( − 2 α l ) = 1 {\displaystyle R_{1}R_{2}\exp(2g_{\text{threshold}}\,l)\exp(-2\alpha l)=1} . Here R 1 {\displaystyle R_{1}} and R 2 {\displaystyle R_{2}} are the mirror (power) reflectivities, l {\displaystyle l} is the length of the gain medium, exp ( 2 g threshold l ) {\displaystyle \exp(2g_{\text{threshold}}\,l)} is the round-trip threshold power gain, and exp ( − 2 α l ) {\displaystyle \exp(-2\alpha l)} is the round trip power loss. Note that α > 0 {\displaystyle \alpha >0} . This equation separates the losses in a laser into localised losses due to the mirrors, over which the experimenter has control, and distributed losses such as absorption and scattering. The experimenter typically has little control over the distributed losses. The optical loss is nearly constant for any particular laser ( α = α 0 {\displaystyle \alpha =\alpha _{0}} ), especially close to threshold. Under this assumption the threshold condition can be rearranged as
g threshold = α 0 − 1 2 l ln ( R 1 R 2 ) {\displaystyle g_{\text{threshold}}=\alpha _{0}-{\frac {1}{2l}}\ln(R_{1}R_{2})} . Since R 1 R 2 < 1 {\displaystyle R_{1}R_{2}<1} , both terms on the right side are positive, hence both terms increase the required threshold gain parameter. This means that minimising the gain parameter g threshold {\displaystyle g_{\text{threshold}}} requires low distributed losses and high reflectivity mirrors. The appearance of l {\displaystyle l} 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 dependence on l {\displaystyle l} is more complicated because α 0 {\displaystyle \alpha _{0}} generally increases with l {\displaystyle l} due to diffraction losses.
Measuring the internal losses The analysis above is predicated on the laser operating in a steady-state at the laser threshold. However, this is not an assumption which can ever be fully satisfied. 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: 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 analysis can be simplified by taking R 1 = 1 {\displaystyle R_{1}=1} . The reflectivity of the output coupler can then be denoted R OC {\displaystyle R_{\text{OC}}} . The equation above then simplifies to
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