Ultrafast and precise distance measurement via real-time chirped pulse interferometry

Laser frequency combs, which are composed of a series of equally spaced coherent frequency components, have triggered revolutionary progress in precision spectroscopy and optical metrology. Length/distance is of fundamental importance in both science and technology. We describe a ranging scheme based on chirped pulse interferometry. In contrast to the traditional spectral interferometry, the local oscillator is strongly chirped which is able to meet the measurement pulses at arbitrary distances, and therefore, the dead zones can be removed. The distances can be precisely determined via two measurement steps based on the time-of-flight method and synthetic wavelength interferometry, respectively. To overcome the speed limitation of the optical spectrum analyzer, the spectrograms are stretched and detected by a fast photodetector and oscilloscope and consequently mapped into the time domain in real time. The experimental results indicate that the measurement uncertainty can be well within ±2 μm, compared with the reference distance meter. The Allan deviation can reach 0.4 μm at 4 ns averaging time and 25 nm at 1 μs and can achieve 2 nm at 100 μs averaging time. We also measured a spinning disk with grooves of different depths to verify the measurement speed, and the results show that the grooves with about 150 m/s line speed can be clearly captured. Our method provides a unique combination of non-dead zones, ultrafast measurement speed, high precision and accuracy, large ambiguity range, and only one single comb source. This system could offer a powerful solution for field measurements in practical applications in the future.