Frequency Spectrum of Multimode Lasers

The He-Ne laser is probably the most thoroughly investigated gas laser [6.3]. From the level scheme in Fig.6.6 which uses the Paschen notation [6.3a], we see that the transitions at X = 3.39 ym and = 0.6328 ym share a common up-per level. Suppression of the 3.390 ym line therefore enhances the output power at 0.6328 ym. The 1.15 ym and the 0.6328 ym lines, on the other hand, share a common lower level, and they also compete for gain, since both laser transitions increase the lower-level population and therefore decrease the inversion. If the 3.3903 ym transition is suppressed, e.g., by placing an absorbing CH cell inside the resonator, the population of the upper 3 2 level increases, and a new line at X = 3.3913 ym reaches threshold. 

Level diagram of the He-Ne system in Paschen notation with the main laser transitions 

This laser transition populates the 3P level, and produces gain for another line at X = 2.3951 ym. This last line only oscillates together with the 3.3913 ym, which acts as pumping source. This is an example of cascade transitions in laser media [6.4] as depicted in Fig.6.5b. 

and the 465.8 nm share the same lower level, suppression of the competing lines will enhance the inversion and the output power of the selected line. The mutual interaction of the various laser transitions has been therefore extensively studied [6.6,7] in order to optimize the output power. Line selection is generally achieved with an internal Brewster prism (Fig.6.3). The homogeneous width Av is mainly caused by collision broadening due to electron-ion collisions. Additional broadening and shifts 

A section of the level diagram is illustrated by Fig.6.8. The vibrational levels (VpV jV ) are characterized by the number of quanta in the three normal vibrational modes. The upper index of the degenerate vibration V2 gives the quantum number of the corresponding angular momentum 1 [6.9]. 

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The different gain saturation of homogeneous and inhomogeneous transitions strongly affects the frequency spectrum of multimode lasers, as can be understood from the following arguments ... 

If such a multimode laser is used for spectroscopy and is scanned - for instance, with a grating or prism inside the laser resonator (Sect.5.5) -through the spectral range of interest, this nonuniform spectral structure Il(0) may cause artificial structures in the measured spectrum. In order to avoid this problem and to obtain a smooth intensity profile Il(i ), the length d of the laser resonator can be wobbled at the frequency f > 1/r which... 

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