LINEARIZATION OF NONLINEAR BEAT FREQUENCY IN FMCW INTERFEROMETRY THROUGH WAVEFORM MODIFYING TECHNIQUE

Frequency-modulated continuous-wave (FMCW) interferometry has become a more popular technique in recent years and is being widely used in optical ranging measurements. In an ideal system, the optical frequency sweep of the laser occurs linearly and periodically in time resulting in a constant beat frequency in time, in which the information regarding the distance can be extracted after FFT analysis. However, practically, linearity in the optical frequency sweep is difficult to obtain, and this leads to variation of the beat frequency in time. Nonlinear optical frequency sweep has become a challenging issue in the research topic to be solved recently. In this report, we proposed a linearization method by modifying the frequency modulation (FM) waveform through the external sampling technique to reduce the effect of nonlinear optical frequency sweep. In this technique, triangle FM sweep is generated and transmitted through the system and re-sampled by the Analog Digital Converter at the external sampling rate (temporal sampling). As temporal sampling changed with time, this resulted in a tiny deterioration in the FM waveform at the beginning of each ramp. Thus, a pre-distorted FM waveform was obtained. One period interval of that distorted FM waveform was extracted and used to reconstruct a new FM waveform signal. This new constructed signal was later retransmitted to the system as a new FM sweep signal. The process was repeated until the stability of the beat frequency was noticeably improved. The proposed linearization method has been worked out through experimentation, and after the 2 nd iteration the result showed that this method effectively reduced the issue of nonlinear optical frequency sweep.