Laser frequency offset locking via tripod-type electromagnetically induced transparency

We have demonstrated laser frequency offset locking via the Rb-87 tripod-type double-dark resonances electromagnetically induced transparency (EIT) system. The influence of coupling fields' power and detuning on the tripod-type EIT profile is studied in detail. In a wide coupling field's detuning range, the narrower EIT dip has an ultranarrow linewidth of similar to 590 kHz, which is about one order narrower than the natural linewidth of Rb-87. Without the additional frequency stabilization of the coupling lasers, we achieve the relative frequency fluctuation of 60 kHz in a long time of similar to 2000 s, which is narrower than the short-time linewidth of each individual laser. (C) 2014 Optical Society of Americ