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

1. .

. 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.

), especially close to threshold. Under this assumption the threshold condition can be rearranged as

1. .

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 equation above then simplifies to

1. .

In most cases the pumping. (This analysis is equally applicable to considering the threshold energy instead of the threshold power. This is more relevant for pulsed lasers). The equation can be rewritten:

1. ,

to be determined experimentally.

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 can then be made.

requires extrapolation.

A good empirical discussion of laser loss quantification is given in the book by W. Koechner.

References