Frequency upconversion detection of rotational Doppler effect

We demonstrated an efficient scheme of measuring the angular velocity of a rotating object with the detection light working at the infrared regime.Our method benefits from the combination of second-harmonic generation(SHG)and rotational Doppler effect,i.e.,frequency upconversion detection of rotational Doppler effect.In our experiment,we use one infrared light as the fundamental wave(FW)to probe the rotating objects while preparing the other FW to carry the desired superpositions of orbital angular momentum.Then these two FWs are mixed collinearly in a potassium titanyl phosphate crystal via type Ⅱ phase matching,which produces the visible second-harmonic light wave.The experimental results show that both the angular velocity and geometric symmetry of rotating objects can be identified from the detected frequency-shift signals at the photon-count level.Our scheme will find potential applications in infrared monitoring.