\(dx\) and \(dy\) are 1 um, and the diameter of the laser spot is 2 mm. The relationship between the signal to noise ratio (SNR) of \(S(t)\) and the roughness Ra can be simulated, as shown in Figure S1. The SNR of the LDV is considerably diminished for common surface roughness (> 5 um). The average SNR is roughly 1.4 dB, which is very low considering only phase noise is counted. Shot noises from both the reference and the signal will further reduce the SNR of LDV.