Rami Arieli: "The Laser Adventure"

Rami Arieli: "The Laser Adventure" Chapter 3.2 page 2

b) Electrical excitation of a gas:

When the active medium is in the gas state, the best excitation is by electrical discharge of the gas (see figure 3.3).

Figure 3.3: Electrical Excitation of a Gas Laser

The gas in the tube is electrically neutral, and as long as no external energy is applied, most of the molecules are in the ground state. When the high electrical voltage is applied, electrons are released from the cathode and accelerated toward the anode. On their way, these electrons collide with the gas molecules and transfer energy to them. Thus, the gas molecules are raised to excited state.

Higher voltage is required to start the electrical discharge in the tube than to keep the discharge. Thus, a preliminary high voltage pulse is applied for initial discharge, and then the voltage is lowered to its operating value.

The ballast resistor is used to limit the current in the tube after discharge is achieved.

Since the right conditions for direct excitation of the lasing gas are difficult to find, a variation of this method is used: