Spectral narrowing of magnetic resonance in transverse optical pumping by recurrently using free decay

We propose a new type of spectroscopic method to reduce spectral width by recurrently using free decay (FD) in magnetic resonance of transverse optical pumping. Simultaneously with the excitation of the atomic system by a pump beam, the induced FD signal was recorded by a digital waveform memory. Then, in order to induce the next FD, we excited the atomic system by a pump beam, the intensity of which was modulated by the recorded FD, and recorded the FD induced by this pump beam. Recurrent use of this procedure reduced the spectral width of the atomic resonance and increased the signal-to-noise ratio. The narrowing procedure was demonstrated by using a randomly modulated stationary pump beam. Theoretical and experimental studies are presented. Our method serves to increase the precision of the measurement of the centre frequency of the atomic resonance.