OUTPUT  CHARACTERISTICS (CO₂ LASER)

          Most CO₂ lasers are used for their high output powers in materials-processing applications where beam quality is not important. In applications such as ceramics cutting, longitudinal modes or the presence of multiple output wavelengths in the output beam is hardly a concern. A carbon dioxide laser operating with standard, broadband optics, for example, has output on many lines between 9 and 11 μm. Where single-frequency operation is required (e.g., in spectroscopy applications), a wavelength selector such as a diffraction grating may be included to tune a single line.

       In most lasers, single-line operation usually implies that multiple longitudinal modes will oscillate, but in most CO₂ lasers, only a single longitudinal mode will usually oscillate. Consider that a 1-m laser cavity has a free spectral range (FSR) of c/2L or 150 MHz, so that longitudinal modes are spaced that far apart. Now the natural linewidth of this laser, like that of most gas lasers, is broadened primarily by Doppler shifting, allows us to calculate the linewidth of this laser at around 60 MHz (the atomic mass of the CO₂ molecule is 44 atomic mass units or 7.3 × 10^-26 kg). This means that the FSR of the cavity is larger than the linewidth of a single emission line, so only a single longitudinal mode will oscillate an etalon is not required. Exacting wavelength control and stabilization can be done by adjusting the cavity length in minute amounts using piezoelectric positioners. This method of active cavity-length control is especially important in longer-wavelength lasers operating in far-IR regions.