A CO2 laser may oscillate at any one of many possible lines. As the cavity length of such a laser is varied over a distance of λ/2, a large number of lines oscillate one at a time in succession. The listing of the identities of the sequences of lines constitutes a signature of the laser and is a matter of practical importance in the operation of these lasers in remote controlled applications without the use of mode-selecting elements in the cavity of the laser. We report here a rather remarkable result, namely, that the lines P(20) and P(16) of the 10.4-µm band compete so effectively relative to all the other modes that they can be relied on to oscillate over quite a wide gain curve and for a wide range of operating conditions.
P. C. Claspy and Yoh-Han Pao, "CO2 Laser Signature Problem," Appl. Opt. 10, 136-136 (1971)