Rotational Doppler Velocimetry of a Surface at Larger Tilt Angles

Here, we experimentally demonstrate a straightforward-to-implement scheme that enables the rotating velocimetry of a noncooperative target at large deflection angles. This scheme is based upon the combination of digital modal decomposition with the rotational Doppler effect of orbital angular momentum (OAM)-carried light modes. To achieve this, we first theoretically analyzed the tilt effect of a rotating surface on the OAM complex spectra and rotational Doppler spectrum. Our findings validate that the tilted surface causes not only the broadening of OAM power and phase spectra, but also the broadening of the scattered Doppler spectrum. Furthermore, we introduce a compensation phase for tilted OAM light that effectively suppresses the sidebands of the OAM power spectrum, thereby restraining the sideband amplitudes of the Doppler spectrum. As a consequence, the rotating velocimetry can be extended to cover larger tilt angles (as large as 70 degrees) than those of existing systems. Our outcomes have the advantages of providing, in addition to profound insight into the interaction between OAM-carrying light and object motion, potential opportunities for noncontact optical metrology and the telemetry of angular speeds, particularly in meteorological applications.