The emission frequency of a diode laser submitted to a frequency-dependent optoelectronic feedback is observed to have more than one stable operation point together with a stable power emission. This is, to our knowledge, the first observation of bistability exclusively in the frequency of an optical system. The experiment was carried out with a semiconductor laser coupled to the cesium $D_2$ line by an orthogonally polarized frequency-sensitive optical feedback.

• Received 9 July 2004

DOI:https://doi.org/10.1103/PhysRevLett.94.173902