An output coupler (OC) is the component of an optical resonator that allows the extraction of a portion of the light from the laser's intracavity beam. An output coupler most often consists of a partially reflective mirror, allowing a certain portion of the intracavity beam to transmit through. Other methods include the use of almost-totally reflective mirrors at each end of the cavity, emitting the beam either by focusing it into a small hole drilled in the center of one mirror, or by redirecting through the use of rotating mirrors, prisms, or other optical devices, causing the beam to bypass one of the end mirrors at a given time.
In its most common form, an output coupler consists of a partially reflective mirror, sometimes called a beamsplitter. The reflectance and transmittance of the mirror is usually determined by the gain of the laser medium. In some lasers the gain is very low, so the beam must make hundreds of passes through the medium for sufficient gain. In this case the output coupler may be as high as 99% reflective, transmitting only 1% of the cavity's beam to be used. A dye laser has very high gain compared to most solid-state lasers, so the beam needs to make just a few passes through the liquid to reach its optimum gain, thus the output coupler is typically around 80% reflective. In others, such as an excimer laser, the 4% reflectivity of uncoated glass provides enough of a mirror, transmitting nearly 96% of the intracavity beam.
Lasers operate by reflecting light between two or more mirrors that have an active laser medium between them. The medium amplifies the light by stimulated emission. For lasing to occur, the gain of the active medium must be larger than the total loss, which includes both unwanted effects such as absorption, emission in directions other than the beam path, and the intentional release of energy through the output coupler. In other words, the laser must attain threshold.
There are three important properties of the output coupler:
A cavity dumper is an output coupler that performs the function of a Q-switch. It allows the energy to build up in the optical cavity and then releases it at a specifically timed interval. This allows the beam to build up to high levels and then be released in a very short time; often within the time it takes a light wave to complete one round-trip through the cavity, hence the name. After building in intensity the cavity suddenly "dumps" its energy. Cavity dumpers usually use a high-reflective mirror on each end of the cavity, allowing the beam to receive full gain from the medium. At a specific interval, the beam is redirected, using a device such as a Pockels cell, an acousto-optic modulator, or a fast-rotating prism or mirror. This redirected beam bypasses the end mirror, allowing a very powerful pulse to be emitted. Cavity dumpers can be used for continuous-wave operation, but their most common use is with mode-locked lasers, to extract a very short pulse at its peak intensity.