Measurement of the velocity difference by photon correlation spectroscopy: An improved scheme

Homodyne photon correlation spectroscopy is used to measure the velocity difference δv(l) over varying distance l. Different length scales are probed when the magnification factor M and the width S of a slit in the collecting optics are varied. The measured intensity autocorrelation function is found to be of scaling form for different values of M, provided S is kept at a value below the critical width S_c. A new convenient collecting optics is devised to expand the variable range of l up to 2 decades, over which δv(l) can be accurately measured. The new scheme is useful for the study of turbulent and other self-similar flows.