The frequency of the dark resonance in coherent population trapping experim
ents has been measured as a function of the degree of ellipticity and the i
ntensity of the probe Light. The results have been used to find the quantum
limit of sensitivity of an optical magnetometer based on the nonlinear Far
aday effect. (C) 2000 Optical Society of America OCIS codes: 270.0270, 140.
0140, 020.6580, 020.7490, 230.2240.