Experimental demonstration of non-contact and quasi OPD-independent nanoscale-displacement measurement by phase-diversity optical digital coherent detection and comb filtering.

We experimentally demonstrated and quantitatively evaluated a non-contact nanometer-displacement measurement using phase-diversity optical digital coherent detection implemented by a 90 ° optical hybrid and a narrow-linewidth probe laser without fine-tuning of optical path length difference (OPD). Combined with a comb filter, the system exhibits 99.99% linearity detection with a scale and resolution of approximately 7 nm and 2 nm respectively, as well as a wideband vibration of 5.85 MHz. We also experimentally analyzed the effect of noise arising from the OPD and demonstrated the detection of displacement down to 85 nm with a resolution of 24 nm at an OPD of 342 cm.