Pulsed time-of-flight laser range finder techniques for fast, high precision measurement applications

Abstract

This thesis describes the development of high bandwidth (~1 GHz) TOF (time-of-flight) laser range finder techniques for industrial measurement applications in the measurement range of zero to a few dozen metres to diffusely reflecting targets. The main goal has been to improve single-shot precision to mm-level in order to shorten the measurement result acquisition time.

A TOF laser range finder consists of a laser transmitter, one or two receivers and timing discriminators, and a time measuring unit. In order to improve single-shot precision the slew-rate of the measurement pulse should be increased, so the optical pulse of the laser transmitter should be narrower and more powerful and the bandwidth of the receiver should be higher without increasing the noise level too much.

In the transmitter usually avalanche transistors are used for generating the short (3–10 ns) and powerful (20–100 A) current pulses for the semiconductor laser. Several avalanche transistor types were compared and the optimization of the switching circuit was studied. It was shown that as high as 130 A current pulses are achievable using commercially available surface mount avalanche transistors.

The timing discriminator was noticed to give the minimum walk error, when high slew rate measurement pulses and a high bandwidth comparator were used. A walk error of less than +/- 1 mm in an input amplitude dynamic range higher than 1:10 can be achieved with a high bandwidth receiver channel. Adding an external offset voltage between the input nodes of the comparator additionally minimized the walk error.

A prototype ~1 GHz laser range finder constructed in the thesis consists of a laser pulser and two integrated ASIC receiver channels with silicon APDs (avalanche photodiodes), crossover timing discriminators and Gilbert cell attenuators. The laser pulser utilizes an internal Q-switching mode of a commercially available SH-laser and produces optical pulses with a pulse peak power and FWHM (full-width-at-half-maximum) of 44 W and 74 ps, respectively. Using single-axis optics and 1 m long multimode fibres between the optics and receivers a total accuracy of +/-2 mm in the measurement range of 0.5–34.5 m was measured. The single-shot precision (σ-value) was 14 ps–34 ps (2–5 mm) in the measurement range. The single-shot precision agrees well with the simulations and is better with a factor of about 3-5 as compared to earlier published pulsed TOF laser radars in comparable measuring conditions.