Observation of the rotational Doppler shift of a white-light, orbital-angular-momentum-carrying beam backscattered from a rotating body

We observe the rotational Doppler shift of an orbital angular momentum (OAM)-carrying white-light beam after it is backscattered from a rotating object. Unlike the well known linear shift, this rotational shift is independent of the optical frequency, and hence each spectral component of the scattered light is shifted by the same value. Consequently, even a white-light source can give rise to a single-valued frequency shift. We show that the size of this shift is proportional to the OAM of the light and that superpositions of different OAM states give rise to multiple frequency sidebands. The observability of this rotational shift for white-light illumination highlights the potential for the rotational Doppler effect to form the basis of a rotational sensor for the remote detection of spinning objects. (C) 2014 Optical Society of America