Rami Arieli: "The Laser Adventure"

Rami Arieli: "The Laser Adventure" Chapter 8 Section 2, page 12

8.2.3 Choosing specific Longitudinal Mode with an Etalon

Usually under the gain curve there are several longitudinal modes that have enough gain to appear in the laser output (see figure 5.4).

The longitudinal mode near the center of the gain curve will be amplify more than the others.

Choosing a specific longitudinal mode can be done by an optical element that cause high losses for the other longitudinal modes inside the cavity.

The frequency difference between two adjacent longitudinal modes is determined by the length of the cavity. This is the condition for creating standing waves inside the cavity, as explained in chapter 4.

When the cavity is long, the frequency difference between two adjacent longitudinal modes is smaller. Thus, more longitudinal modes will be under the gain curve.

Reducing the length of the optical cavity will cause an increase in the frequency difference between adjacent longitudinal modes, thus reducing the number of modes under the gain curve.

By controlling the length of the optical cavity, a control is achieved over the number of longitudinal modes emitted from the laser.