Orbital angular momentum-based dual-comb interferometer for ranging and rotation sensing.

We present a dual-comb interferometer capable of measuring both the range to a target as well as the target's transverse rotation rate. Measurement of the transverse rotation of the target is achieved by preparing the probe comb with orbital angular momentum and measuring the resultant phase shift between interferograms, which arises from the rotational Doppler shift. The distance to the target is measured simultaneously by measuring the time-of-flight delay between the target and reference interferogram centerbursts. With 40 ms of averaging, we measure rotation rates up to 313 Hz with a precision reaching 1 Hz. Distances are measured with an ambiguity range of 75 cm and with a precision of 5.9 µm for rotating targets and 400 nm for a static target. This is the first dual-comb ranging system capable of measuring transverse rotation of a target. This technique has many potential terrestrial and space-based applications for lidar and remote sensing systems.

Observation of the rotational Doppler shift with spatially incoherent light.

The rotational Doppler shift (RDS) is typically measured by illuminating a rotating target with a laser prepared in a simple, known orbital angular momentum (OAM) superposition. We establish theoretically and experimentally that detecting the rotational Doppler shift does not require the incident light to have a well-defined OAM spectrum but instead requires well-defined correlations within the OAM spectrum. We demonstrate measurement of the rotational Doppler shift using spatially incoherent light.

Centering a beam of light to the axis of rotation of a planar object.

We detail an experimentally simple approach for centering a beam of light to the axis of a rotating surface. This technique can be understood as a rotating analog to knife-edge profilometry, a common experimental technique wherein the intensity (or power) of various masked portions of a beam is used to ascertain the transverse intensity profile of the beam. Instead of collecting the light transmitted through a mask, we give the surface a variable reflectivity (such as with a strip of retro-reflective tape) and sample the light scattered from the surface as it rotates. We co-align the transverse position (not the tilt) of the axis of rotation and the beam centroid by minimizing the modulation amplitude of this scattered light. In a controlled experiment, we compare the centroid found using this approach to the centroid found using the canonical knife-edge approach in two directions. We find our results to be accurate to within the uncertainty of the benchmark measurement, ±0.03 mm (±2.9% of the beam waist). Using simulations that mimic the experiments, we estimate that the uncertainty of the technique is much smaller than that of the benchmark measurement, ±0.01 mm (±1% of the beam waist), limited here by the size of the components used in these experiments. We expect this centering technique to find applications in experimental and industrial fabrication and processing settings where alignment involving rotating surfaces is critical.

Detection technique effect on rotational Doppler measurements.

There are two established methods for measuring rotational Doppler shift: (1) heterodyne and (2) fringe. We identify a key distinction, that only the heterodyne method is sensitive to the rotating object's phase, which results in significant differences in the signal-to-noise ratio (SNR) when measuring multiple rotating particles. When used to measure randomly distributed rotating particles, the fringe method produces its strongest SNR when a single particle is present and its SNR tends to zero as the number of particles increases, whereas the heterodyne method's SNR increases proportionally to the number of particles in the beam.