Intracavity line selection

There are some cases, where one operates a CO-laser without intracavity line selection. Although this mode of laser operation is called "all line", the spectral analysis of the output shows a wavelength distribution, that is... 

If some of the simultaneously oscillating laser transitions share a common upper or lower level, such as the lines 1, 2, and 3 in Fig. 5.27c and Fig. 5.29a, gain competition diminishes the output of each line. In this case, it is advantageous to use intracavity line selection in order to suppress all but one of the competing transitions. Sometimes, however, the laser may oscillate on cascade transitions (Fig. 5.29b). In such a case, the laser transition 1 -> 2 increases the population of level 2 and therefore enhances the gain for the transition 2- 3... 

Fig. 3.5 Experimental arrangement for intracavity Raman spectroscopy with an argon laser CM, multiple reflection four-mirror system for efficient collection of scattered light LM, laser-resonator mirror DP, Dove prism, which turns the image of the horizontal interaction plane by 90° in order to match it to the vertical entrance slit S of the spectrograph FPE, Fabry-Perot etalon to enforce single-mode operation of the argon laser LP, Littrow prism for line selection [315]...

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