Experimental Realization of Single-Mode Lasers

As pointed out in the previous section, all methods of achieving singlemode operation are based on mode suppression by increasing the losses beyond the gain for all but the wanted mode. A possible realization of this idea is illustrated in Fig.6.14, which shows longitudinal mode selection by a 

Gain profile, resonator modes, and spectral transmission of the etalon tuned for single-mode operation 

In the argon-ion laser, the width of the gain profile is about 8 GHz. With a free spectral width Av = c/(2nt) = 10 GHz of the intracavity etalon, single-mode operation can be achieved. 

The finesse F of the etalon has to be sufficiently high to introduce losses for the modes adjacent to the selected mode which overcome their gain (Fig.6.14). Fortunately in many cases this gain is already reduced by the oscillating mode due to gain competition. This allows the less stringent demand that the losses of the etalon must exceed the unsaturated gain at a distance away from the transmission peak. 

Often a Michel son interferometer is used for mode selection, coupled by a beam splitter St to the laser resonator (Fig.6.16). The free spectral range of this Fox-Smith cavity [6.13], which is AX=c/[2(L2+L )], has 

In the previous sections we have seen that without specific manipulation a laser generally oscillates in many modes, for which the gain exceeds the total losses. In order to select a single wanted mode one has to suppress all others by increasing their losses to such an amount that they do not reach oscillation threshold. The suppression of higher-order transverse modes demands other actions than the selection of a single longitudinal mode out of many other TEMqo modes. 

Many types of lasers, in particular, gaseous lasers, may reach oscillation threshold for several atomic or molecular transitions. The laser then can simultaneously oscillate on these transitions [5.35]. In order to reach singlemode operation one has at first to select a single transition. 

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Usually, mainly Doppler broadening determines the gain profile of a particular laser transition. Indeed, due to the different configurations achievable with gas lasers (namely, a large cavity length), the laser line can be narrower than the Doppler linewidth. Different experimental realizations of single-mode lasers are detailed elsewhere (Demtroder, 2(X)3). 

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