Shifts of optical frequency references based on spectral-hole burning in   Eu:Y2SiO5
  Several properties of Eu:Y2SiO5 spectral holes are measured, to assess the suitability of broad-band hole-patterns for use as laser-frequency references. We measure frequency shifts due to magnetic fields, side-features of neighboring spectral holes, and changing optical probe power. A precise calibration of a temperature insensitive point is also performed, where the temperature-induced frequency shift is canceled to first order by the pressure-induced shift from the crystal's helium-gas environment. 
Michael J. Thorpe, David R. Leibrandt, and Till Rosenband
physics.atom-ph physics.optics