By using an optical heterodyne technique, we have demonstrated the detection of ballistic photons traveling through a suspension. We measured the propagation time of light in a sample with a variable optical delay line in the reference arm of an interferometer, using a superluminescent diode as a light source. The resolution and accuracy in propagation time measurement were 400 and 3 fs, respectively. The minimum detectable rotation angle of 1/10,000 deg was achieved with an integration time of 5 s. This system can measure the refractive index and optical rotatory power of the suspension in a solvent without disturbing heavy scattering from particles in the suspension, because ballistic photons are detected.
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