Nano-displacement sensing by phase-diversity optical digital coherent detection utilizing alternating quadrature phase-modulated reference light.

We have introduced a nanometer-scale non-contact displacement sensing method that relies on phase-diversity optical digital coherent detection. In our prior work, we used a conventional setup involving a 90°optical hybrid, two balanced amplified photodetectors (BAPs), and a narrow-linewidth (NLW) laser, which is complex and costly. However, in this paper, we have streamlined the system configuration by employing alternating quadrature phase modulation (AQPM) reference light, implemented using a phase modulator and a BAP. Moreover, we've employed an economical distributed feedback (DFB) laser, enabling us to achieve displacement sensing at 1.6 nm with a resolution of 0.6 nm. It is notable that there is some degradation in the performance due to the phase noise compared to the NLW laser, which achieves a displacement sensing down to 0.6 nm with a 0.2 nm resolution. Nevertheless, the DFB-AQPM system holds a significant potential for cost-effective, high-resolution nanometer-scale sensing applications.