Three-Dimensional Object Profiling Using Highly Accurate FMCW Optical Ranging System

Abstract

We have developed three-dimensional object profiling system using a highly accurate frequency-modulated continuous-wave optical ranging system. A DFB laser and a vertical-cavity surface-emitting laser (VCSEL) are used as the laser source and the optical frequency is swept by the injection current modulation with a symmetric triangular wave. The influence of nonlinearity in the optical frequency sweep is canceled by utilizing $k$ -sampling technique. Since the optical frequency sweep range of a VCSEL is larger than that of a DFB laser, high spatial resolution and high ranging accuracy are achieved by using a VCSEL. The spatial resolution evaluated by the full-width at half-maximum (FWHM) is 460 $\mu$ m and the standard deviation of the ranging accuracy is $\sigma = \text{2.7} \, \mu$ m when a VCSEL is used as the frequency-swept laser source, and the FWHM is 2.3 mm and $\sigma = 14.8 \, \mu$ m when a DFB laser is used as the frequency-swept laser source. Accordingly, fine and clear object profiling of a coin and a printed circuit board is achieved by using a VCSEL as the frequency-swept laser source. Finally, we speed up the measurement time by increasing the repetition frequency of the injection current modulation and optimizing timing of the data acquisition, the galvano mirror scan and the fast Fourier transform analysis by taking account of the transient response of the galvano mirror, and the fastest measurement time of 22.6 s is achieved for $201 \times 201$ measurement points, which is four times faster than our previous results. We also discuss the effect of moving average filtering and median filtering to improve profiling quality.