Losses measurement in quasioptical resonator with small unstability

We investigate experimentally the model Q-factor of an asymmetric unstable resonant cavity as a function of a parameter which is equivalent to the Presnel number for each mirror. Different values of this parameter can be obtained by varying the mirror separation, this is not the case in the method of diaphragming the beam at the exit mirror which has been applied in the past. Our results show that in certain limits one can obtain the required characteristics (size of losses and mode operation) of the resonance system by simply changing its length.