Doppler-free frequency-modulation spectroscopy of atomic erbium in a hollow-cathode discharge cell

The erbium atomic system is a promising candidate for an atomic Bose-Einstein condensate of atoms with a non-vanishing orbital angular momentum (L not equal 0) of the electronic ground state. In this paper we report on the frequency stabilization of a blue external cavity diode laser system on the 400.91 nm laser cooling transition of atomic erbium. Doppler-free saturation spectroscopy is applied within a hollow-cathode discharge tube to the corresponding electronic transition of several of the erbium isotopes. Using the technique of frequency-modulation spectroscopy, a zero-crossing error signal is produced to lock the diode laser frequency on the atomic erbium resonance. The latter is taken as a reference laser to which a second main laser system, used for laser cooling of atomic erbium, is frequency stabilized.