Rami Arieli: "The Laser Adventure" Chapter 3.3 page 1
3.3 Feedback Mechanism
The feedback mechanism returns part of the coherent laser radiation which was created inside the active medium, back.

Usually the feedback is done by using mirrors at both ends of the active medium. These mirrors are aligned so that the radiation is moving back and forth between them. In this way an optical cavity is created (as explained in details in chapter 4).

Usually one mirror is 100% reflecting, so all the radiation coming toward the mirror is reflected back to the active medium. The other mirror is partially reflecting (10%-99%), according to the laser type.

The part of the radiation which is not reflected back into the optical cavity, is transmitted out, and it is the laser output.

The feedback allows each photon to pass many times through the active medium, so enough amplification will result.

Because of the feedback mechanism, only photons which move between the mirrors remain in the active medium, which give the directionality of the output beam.