He-4 optical pumping with frequency modulated light

Recently a distributed Bragg reflection (DBR) laser diode was commercialised by SDL (USA) for helium optical pumping experiments. An interesting feature of the new device is the possibility of frequency modulation of the diode output by modulating the current intensity, leading to a new pumping scheme called optical pumping with frequency modulated light. We develop this method to induce spin precession and obtain new resonances in He-4 atomic gas, simply by modulating the frequency of the pumping light at the Larmor frequency of atomic states in a static magnetic field. These resonances are analysed with a simple model taking into account velocity changing collisions.