Signal diversity for the reduction of signal dropouts and speckle noise in a laser-Doppler extensometer

A recently demonstrated laser-Doppler extensometer is affected by signal dropouts caused by speckle noise, whereby the carrier-to-noise ratio (C/N) falls below the so-called frequency-modulation threshold. In this article, we present a new optical setup with polarization signal diversity for reducing speckle-induced dropouts dramatically. The probability of a signal dropout can be substantially reduced using weighted combination signals from different photodetectors collecting the scattered light at two orthogonal polarizations. Therefore, we prove in this paper that signal diversity enables a laser-Doppler extensometer for industrial applications. Furthermore, a theoretical model based on the speckle statistics for estimating the probability of signal dropout is introduced. The relationship between the occurrence of signal dropouts and the demodulation bandwidth is studied. The experimental results of the occurrence of dropouts highly match the theoretical estimated probability distribution. Finally, strain measurement demonstrates a significant reduction of speckle noise with the implemented technique