Cavity Fourier-transformed cross correlation measures the wavelength dispersion from the cross-correlation signal taken on spontaneous emission emerging from a cavity under test. This method provides a universal and quick way to detect femtosecond group delay. However, test cavity-length fluctuations have been hindering its application to femtosecond laser cavities. We introduce a dual-interferometer scheme in which one of the interferometers is dedicated to detecting length change in the test cavity, thereby enabling the influence of fluctuations to be nullified. This method is demonstrated on a Ti:Al2O3 laser as well as on a NaCl color-center laser.
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