Laser Operation Fundamentals: Gain and Cavity Dynamics

Amplification of Light in Lasers

All lasers contain a substance that can increase the intensity of light passing through it.

The Active Medium

This substance is called the active medium and may be a solid, liquid, or gas. The mechanism by which the active medium increases the intensity will be explained later. For the moment, assume that light amplification is possible.

For example, in a YAG laser (Nd:YAG), the active medium is a Yttrium Aluminum Garnet (YAG) bar containing neodymium ions.

Understanding Gain

The factor by which the light intensity increases in the active medium is known as gain.

The gain is not constant for a particular type of medium; it depends on:

• The wavelength of light.
• The length of the active medium.
• The extent to which the active medium has been energized.

The Laser Cavity and Its Functions

The space between the mirrors is known as the laser cavity. The beam passes through the cavity via multiple reflections between the mirrors and is amplified every time it traverses the active medium.

Cavity Mirror Configuration

The cavity uses two mirrors:

1. One mirror reflects almost all light that falls on it (the total reflector).
2. The other mirror reflects between 20% and 98% of incident light, depending on the type of laser. The light that is not reflected is transmitted through the mirror, and this transmitted light constitutes the laser output beam.

Key Functions of the Laser Cavity

The laser cavity serves several important functions:

1. Initiating Oscillation

After pumping, spontaneous emission of light from excited atoms in the active medium initiates low-intensity light emission within the laser cavity. This light intensity is increased each time it passes through the active medium, quickly turning into an intense beam. In the absence of mirrors, this process of self-initiation or oscillation would not occur.

2. Controlling Beam Divergence

The cavity helps ensure that beam divergence is small. Only light traveling in a direction nearly parallel to the axis of the cavity can undergo multiple reflections and pass several times through the active medium. More divergent rays follow a zigzag path in the cavity and exit quickly.

3. Improving Spectral Purity

The laser cavity also improves the spectral purity of the laser beam. Generally, the active medium amplifies light within a narrow range of wavelengths, but within this range, only the light of certain wavelengths can pass through multiple reflections inside the cavity.

Absorption and Emission

Transition and Quantum Leap

An electron absorbs a photon and undergoes a transition. It then returns to its ground state in a tiny fraction of a second, emitting a photon (spontaneous emission).