Rami Arieli: "The Laser Adventure" Chapter 4 Applet Cavity - Rays
Applet: Rays Trajectories in an Optical Cavity

A visual mean to see if a cavity is stable, is to send laser beam from certain point inside the cavity, to specific directions.
First the cavity parameters must be defined (Mirror radius and cavity length).
For each mirror, a small triangle on the cavity axis marks the center of curvature of that mirror.
It is possible to choose a plane mirror, by clicking on the sign of infinity, which mark an infinite radius of curvature for that mirror.

To se how the beam advances, the starting point and the direction of the beam must be defined.
The starting point of the beam is marked by clicking the left button of the mouse.
The starting direction of the beam is marked by clicking the right button of the mouse.
The computer calculates the beam path, and it is immediately seen if the beam remains inside the cavity, or hit the walls, and vanish (absorbed by the cavity walls).

To check the output beam from the laser, it is possible to change the reflection coefficient of one of the mirrors (output coupler).
It is possible to determine the number of reflection of the beam from the mirrors (in case it is not exiting first), by choosing a number between 1 and 400.


  1. Find conditions of stable cavity, and show that the beam is confined within the cavity.
  2. Determine the conditions that cause minimal divergence to the output beam from the laser. Explain.