Optical range finder using semiconductor laser frequency noise

Semiconductor laser range-finder systems use so-called "time-of-flight" methods that require us to modulate semiconductor lasers' intensity and frequency, and detect those of reflected lights, in order to compare optical paths to the reference and the target. But, accurate measurement requires both high-speed modulation and detection systems. By taking advantage of semiconductor lasers' broad-spectrum frequency noise, which has a range of up to a few GHz, and converting it to intensity noise, we were able to generate a set of high-speed physical random numbers that we used to precisely measure the distance. We tuned the semiconductor lasers' oscillation frequencies loosely to the Rb absorption line and converted their frequency noise to intensity noise, in the light transmitted. Observed through a frequency discriminator, beams traveling along two different paths will always share intensity noise patterns, but there is a time lag. We calculate the cross-correlation of the two signals by sweeping their time lags. The one with the highest degree of correlation was that corresponding to the difference in the length of the two optical paths. Through our experiments, we confirmed that the system was accurate up to a distance of 50 m, at a resolution of 0.03 m, when the sampling rate was adjusted to 0.2 ns.