How does a CO2 laser work?

in simplistic terms, a laser cavity consists of a gain medium between two mirrors, and a means of “pumping”, or supplying energy to the gain medium.

In our CO2 lasers the gain medium is a gas mixture which occupies the space between two planar metal electrodes.   These electrodes are water-cooled, and RF energy is used to excite a gas discharge between them.   Nitrogen molecules are excited into a vibrational level, and subsequently transfer their excitation energy to the CO2 molecules.

The lasing transition then occurs in the excited CO2 molecules.   As the excited electrons return to their ground state, photons are emitted.   These photons are all of the same wavelength (defined by the energy difference between the excited and ground states) and are coherent – these properties distinguish laser light from “normal” light.   As the photons travel back and forth through the gain medium, reflected by the cavity mirrors at each end, they stimulate other excited electrons to return to the ground state, releasing more photons with the same wavelength and phase – this process is known as stimulated emission.

A rapid cumulative effect occurs, and soon there are many photons of the same wavelength and phase travelling in the same direction within the cavity – this is the laser beam.   One of the cavity mirrors is partially transmitting, to allow a portion of the beam to exit the cavity, while the remainder continues to propagate between the resonator mirrors, maintaining the stimulated emission process.

Our CO2 lasers produce laser light at wavelengths between 9 and 11µm.   The most prominent wavelength is 10.6µm, although other spectral lines are present; we can use different gas isotopes and/or mirror coatings to encourage these to propagate and suppress the 10.6µm emissions.   This is how we produce lasers at alternative wavelengths, namely 10.2µm and 9.3µm.

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