定义：
达到激光振荡阈值时的泵浦功率。

激光器的泵浦阈值功率指的是当激光阈值满足时的泵浦功率。在这个时候，激光谐振腔内的损耗等于小信号增益。在其它的光源，如拉曼激光器和光学参量振荡器中也存在着类似的阈值功率。 

图1 光泵浦激光器中的输出与输入功率的关系曲线。泵浦阈值功率为5W，斜率效率为50％。 需要注意的是，由于放大的自发辐射的影响，低于泵浦阈值功率的曲线也会略鼓起。 
对于光泵浦激光器，阈值泵浦功率的定义可以是输入的泵浦功率也可以是被吸收的泵浦功率。对于应用来说，更关心输入的泵浦功率。但是对于判断增益介质的增益效率，被吸收的泵浦功率则更为有用。 
当谐振腔的腔内损耗较低且增益效率高时才能得到低的泵浦阈值功率。高增益效率通常利用具有高σ-τ乘积（发射截面和上能级寿命乘积）的小模场面积的增益介质获得。σ-τ乘积则受到发射带宽的限制。因此，宽带增益介质趋向于具有更高的激光阈值。 
对于一个简单的四级激光增益介质，我们可以用一个公式来计算泵浦阈值功率： 

其中，Irt是谐振腔内的损耗，hvp是泵浦源的光子能量，A是激光晶体中的光束面积，ηp是泵浦效率，τ2是上能级寿命，σem是发射截面大小。 
对于给定的泵激功率，激光输出功率的优化通常涉及到高斜率效率和低激光阈值功率的折衷。在大多数的情况下，工作状态下的泵浦功率是泵浦阈值功率的数倍。最佳泵浦阈值功率的选择是激光器设计的参数之一。 
输出功率对激光泵功率曲线并不总像图1所示的那么简单。例如，在具有高谐振腔损耗的激光器，阈值泵浦功率是通过通过外推高功率下曲线的近似线性曲线下零来定义。 
有一些特殊的激光器，如：单原子激光器，它们没有激光阈值，并因此被称为无阈值激光器。

Definition: the pump power at which the laser threshold is reached

The threshold pump power of a laser is the value of the pump power at which the laser threshold is just reached, usually assuming steady-state conditions. At this point, the small-signal gain equals the losses of the laser resonator. A similar threshold exists for some other types of light sources, such as Raman lasers and optical parametric oscillators.

Figure 1: Output versus input power for an optically pumped laser. The threshold pump power is 5 W, and the slope efficiency is 50%. The “edge” occurring at the threshold is very slightly rounded due to the influence of amplified spontaneous emission.

For an optically pumped laser, the definition of threshold pump power may be based either on the incident or absorbed pump power. For applications, the incident pump power may be more relevant, but the threshold power with respect to absorbed power can be interesting e.g. for judging the gain efficiency of the gain medium.

A low threshold power requires low resonator losses and a high gain efficiency. The latter is achieved by using, e.g., a small laser mode area in a gain medium with a high σ−τ product. The latter is fundamentally limited by the emission bandwidth. Therefore, broadband gain media tend to have higher laser thresholds.

For a simple four-level laser gain medium, we can use an equation for the gain efficiency from the corresponding article for calculating the threshold pump power:

where l_rt is the round-trip power loss of the laser resonator (taking into account the output coupler loss and parasitic losses), hν_p is the photon energy of the pump source, A is the beam area in the laser crystal, η_p is the pump efficiency, τ₂ the upper-state lifetime and σ_em the emission cross section. It is assumed that the power losses l_rt per round trip and thus the round-trip gain is small (e.g. below 20% or 1 dB). The pump efficiency contains terms for the quantum defect, for possible losses of excitation in the gain medium (e.g. by quenching processes), for pump power lost outside the region covered by the laser beam, and also (if we consider the pump threshold in terms of incident pump power) for incomplete pump absorption.

The optimization of the laser output power for a given pump power usually involves a compromise between high slope efficiency and low laser threshold power. In typical situations, the pump power used in normal operation is several times higher than the pump threshold power. The question of which value is most appropriate for the threshold pump power is one of the issues of laser design.

The dependence of output power on pump power of a laser is not always as simple as shown in Figure 1. For example, the onset of lasing may not be as well defined in some lasers with high resonator losses. The threshold pump power is then sometimes defined by extrapolating the approximately linear curve at higher powers down to zero.

There are some exotic types of lasers, e.g. single-atom lasers, which have no laser threshold, and are thus called thresholdless lasers.