Femtosecond Sagnac interferometer for phase spectroscopy.

We developed a novel femtosecond Sagnac interferometer that enables us to take single-shot measurements of both the difference transmission and the difference phase spectra. Owing to the excellent stability of the Sagnac interferometer, a difference phase spectrum was obtained with a high phase change sensitivity of ~lambda/100 after a 1000-shot accumulation. The spectral range, wavelength, and time resolution were 530-650 nm, 1 nm, and 200 fs, respectively.