Ion lasers operate by using an ionized gas as their lasing medium within a resonant cavity. Here's a breakdown of the process:
1. The Laser Medium: Ionized Gas
- Unlike helium-neon lasers which use neutral gas atoms, ion lasers employ ionized gas, meaning the gas atoms have lost one or more electrons, becoming positively charged ions. Common gases used include argon, krypton, and xenon. The specific gas determines the laser's output wavelength and color.
2. The Resonant Cavity
- Like all lasers, ion lasers utilize a resonant cavity, typically a Fabry-Pérot resonator. This cavity consists of two mirrors: one highly reflective (typically >99%) and one partially reflective (output coupler). The mirrors are precisely aligned to reflect light back and forth through the gain medium (the ionized gas).
3. Excitation (Pumping)
- The gas is ionized and excited within the laser tube. This is commonly achieved using a high-current electrical discharge.
- A strong electrical current is passed through the gas, colliding with the gas atoms and stripping away electrons, creating ions.
- These collisions also elevate the ions to higher energy levels.
4. Population Inversion
- The key to laser action is creating a population inversion, where more ions are in an excited energy state than in a lower energy state. This is achieved by carefully controlling the excitation process.
5. Spontaneous and Stimulated Emission
- Ions in the excited state spontaneously decay back to a lower energy state, emitting photons (light particles) in random directions. This is spontaneous emission.
- When a photon encounters another excited ion, it can trigger stimulated emission, where the excited ion decays and emits a photon identical to the triggering photon (same wavelength, phase, and direction). This amplifies the light.
6. Amplification and Laser Output
- The photons travel back and forth between the mirrors, passing through the ionized gas medium. Each pass stimulates more emission, amplifying the light.
- The partially reflective output coupler allows a fraction of the light to escape the cavity, forming the laser beam.
- The laser beam produced is highly coherent, monochromatic (single wavelength), and collimated (narrow, parallel beam).
7. Wavelength Selection
- Ion lasers can emit at multiple wavelengths. Prisms or other wavelength-selective elements within the cavity are often used to select a specific wavelength of operation.
Example: Argon Ion Laser
An Argon ion laser uses argon gas. When a high current is passed through the argon gas, argon atoms lose electrons and become Argon ions. These ions are excited to high energy levels. As these ions decay back to lower energy levels, they emit photons at various wavelengths, primarily in the blue and green regions of the visible spectrum. Different energy level transitions within the argon ion produce different wavelengths.
In summary, ion lasers work by ionizing a gas, creating a population inversion of ions in excited states, and then using a resonant cavity to amplify light emitted by these ions, resulting in a coherent, monochromatic laser beam.
