characterizing the ring down either using a temporary interferometer to measure the sine and cosine beams separately or by carefully fitting the summed signal with a bi exponential if the difference between decay times is expected to change slowly then an averaging loop in the analysis can be used to permit that difference to vary slowly over time thus partially limiting the degrees of freedom of the fit and reducing its shot to shot variability in the short term fig1 shows an embodiment where an optical circulator is used to facilitate separation of the backward mode signal from the incident source radiation here an optical circulator 1002 is present in the system which has the characteristic behavior that light incident on port 1 is emitted from port 2 and light incident on port 2 is emitted from port 3 thus light from laser source 110 is incident on mirror 102 of the resonant cavity and drives the forward propagating mode within the cavity heavy solid arrows forward propagating light emitted from the cavity is received at ringdown detector 1006 an optional fold mirror 1004 is shown in this example but has no significant effect on operation within the cavity formed by mirrors 102 104 and 106 scattering or other processes can lead to the propagation of a backward mode light dotted arrows such backward propagating light will be incident on port 2 of the circulator when it couples out of the resonant cavity and will therefore be emitted from port 3 of the circular to be received by ringdown detector 1008 thus circulator 1002 separates the source radiation from the backward mode signal.