Non-contact sub-nanometer displacement sensing based on optical digital coherent detection using frequency-domain filters

Abstract

We have introduced a sub-nanometer non-contact displacement sensing method using phase-diversity optical digital coherent detection. The initial work achieved 0.62nm displacement sensing with a 0.21nm resolution using a temporal domain filter. However, optimizing the filter for a sharp transition band and low order posed challenges, limiting noise filtering. To overcome this, frequency-domain filter is proposed and investigated in this work. Employing a frequency-domain bandpass filter improved resolution to 0.07nm at 0.62nm displacement, resulting in an 18.5dB signal-to-noise ratio--8dB higher than temporal domain filtering. This advancement enhances precision for applications in precision instruments without any additional cost.