Lamb Dip Stabilization of Gas Lasers

If the center frequency o)q of the molecular absorption line is different from the center frequency of the gain profile G(o) - o) ) of the amplifying laser medium, the absorbing sample inside the laser resonator causes a Lamb peak at o)q in the laser output 1, which lies at the slo e of the gain profile (see Fig.10.27). This not only causes a slight shift of the Lamb peak 

In such cases it is often advantageous to use the third derivative technique which is based on the following considerations. The frequency dependence 1 (0)) of the laser intensity is determined by the superposition of the 

In a small interval around u)q we may approximate the Doppler profiles of G((d - o)j) and olqM by a quadratic function and obtain for (10.26) the approximation 

These derivatives are shown in Fig.10.27 which demonstrates that the Doppler-broadened background disappears for the higher derivatives. It is therefore advantageous to stabilize the laser frequency a)j to the zero point wq of the third derivative. This can be performed as follows. 

If the laser frequency wq is modulated at a frequency the laser intensity 

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