Intracavity characterization of a three-level system in atomic samarium

The fact that the two counterpropagating pulses in a modelocked dye ring laser can be completely decoupled in frequency makes such a laser a sensitive probe of phase differences between the counterpropagating pulses. The difference in center frequency between the two counterpropagating pulses can be resolved down to the noise limit of 100 Hz. This limit corresponds to a phase difference between the counterpropagating pulses of 10-6, or an effective cavity length difference of 10-12 m. When combined with fluorescence measurements and the intensities of the counterpropagating fields, the steady-state elements of the density matrix describing the system can be inferred. In this study, a ring dye laser is modified to create a linear 'tail' where a heated cell (5-cm long) containing atomic samarium is inserted.