Signal Process And Research On FMCW Laser Rader

Frequency Modulated Continuous Wave (FMCW) laser rader has the advantage ofhigh precision, high efficiency, non-contact without target, large size without blinddistrict, etc., can work in a variety of occasions, is one of the important method thatmeasure large size of the target. Through the study of key technical points of FMCWlaser rader, a linear frequency modulated continuous wave laser radar ranging system isestablished, to improve the accuracy of the absolute distance measurement the systemsignal is analyzed and processed, according to the system requirements, the key devicewere selected to build an experimental platform.FMCW absolute distance measurement is dependent on the laser chirp, but actuallylaser frequency tuning is difficult to be in a strict linear, which will cause interferencebeat frequency broadening, thereby introducing large measurement error. According tothe basic FMCW distance measurement technology principle, the frequency samplemethod to correct the nonlinearity of laser tuning based on Mach-Zehnderinterferometer which used as the calibration clock is study, the external clock signal isused to sample the original measurement signal. Since the measurement signal and thecorrection signal contain the same non-linear factor, the simultaneous samplingeliminates the nonlinear components. Because the system uses all-fiber device design,the optical path of different laser frequencies through the optical fiber is not the same,the change of the optical path in the free space is negligible, so the frequency dispersionbetween the measurement path and the correction path can not be counteracted, whichcause the dispersion error, the paper discuss the dispersion problem in the system, andbuild a model to simulate the dispersion errors. The essence of FMCW technology is theinterference measurement, actually bending of the fiber due to stress or environmentalchanges are likely to affect the polarization state of two light beams involved in themeasuring arm and the reference interferometer arm, which cause fading of theamplitude of the interference signal, even decline to zero that interference phenomenoncompletely disappeared, the paper discuss the impact of the polarization fading ofFMCW. In the external sampling clock, delay variation due to the change of interval offrequency or the sample delay of the AD sampling or the conversion process ofcorrection signal to the clock signal will lead to clock error and the sampling error. Inthis paper, sampling error and the dispersion of sampling error were discussed.Based on the FMCW theory, the1550nm band continuously tunable external cavitylaser is chose as the light source, using all-fiber optical device and balanced detector,the final signal is acquired by a high speed data acquisition card. Labview measurementsoftware is written, the ranging experiments of the fiber end and the block of free space is set, using high-precision guide rail and dual laser interferometer to calibrate theranging results, experimental results show that the frequency sample method correct thenonlinearity well with high ranging stability, but the fiber dispersion and sampling errorbring a greater impact on ranging accuracy, and only correct the dispersion and thesampling errors with optical path delay compensation or algorithm can lead to higheraccuracy.