We describe theoretically the form of non-linear absorption resonance used
for the frequency stabilization of laser standards, when the absorber is pl
aced in an external Fabry-Perot (FP) resonator. We calculate the frequency
shifts induced by diaphragms located outside the FP resonator. The resultin
g shifts can be of the order of magnitude 10(-3)-10(-2)gamma, where gamma i
s the width of non-linear resonance. They strongly depend on the transit ti
me phenomenon, which even changes the sign of the shift in the transit time
limit. (C) 2000 Elsevier Science B.V. All rights reserved.