NPRO laser feedback interferometer with improved sensitivity

A non-planar ring oscillator (NPRO) laser feedback interferometer (NPRO-LFI), with frequency-shifted self-mixing frequency (SMF), has demonstrated the capability to detect vibration signals with amplitudes as low as sub-picometer level. However, external magnetic noise has remained a major limitation for frequency stability, restricting its application in high-precision measurements. To address this issue, we introduce an active magnetic stabilization technique based on an electromagnet, achieving a noise suppression ratio of 10⁵ at a 1 mHz Fourier frequency for an SMF of 614 kHz. Under closed-loop control, the stabilized SMF enables vibration detection within the 4 Hz-100 kHz range with a maximum deviation of only 0.17 Hz. Furthermore, the system achieves a weak-light detection limit of 12.2 fW, allowing reliable vibration sensing under extremely low optical feedback conditions. Rotational speed measurements within the range of 25-56 rad/s exhibit a maximum error of 1.53%. Such an NPRO-LFI with improved sensitivity will enable high-precision, long-range interferometric sensing under weak-feedback and noisy environments.