Ultra-Low Frequency High-Precision Displacement Measurement Based on Dual-Polarization Differential Fiber Heterodyne Interferometer

Picometer displacement measurement in the ultra-low frequency band below 1 Hz is highly demanded in the fields of geophysics and space exploration for the applications of hydrophones, geophones, drag-free space controllers, and so on. In this work, high-precision displacement detection in ultra-low frequency is achieved by proposing and demonstrating a dual-polarization differential fiber heterodyne interferometer. By injecting two orthogonal polarized lights into the dual-polarization differential fiber heterodyne interferometer, two optical interferences can be co-existed in a monolithic interferometer with a fully identical optical fiber path. Through the difference of the two optical interferometric phases, the light source noise and the fiber link noise can be well suppressed. Meanwhile, a pull-push type displacement sensing module is designed to double the displacement sensitivity as well as suppress the common-mode noise derived from the environmental fluctuation. Through the calibration experiment, the displacement sensitivity is obtained as 0.01634(±0.00022) rad/nm, which is consistent with the theoretical calculation. The displacement resolutions of 1.8 pm/Hz
^1/2
at 0.1 Hz and 0.25 pm/Hz
^1/2
at 1 Hz are achieved in the experiment, which is the superior displacement measurement level for this frequency band in the existing optic displacement sensors to our knowledge. The proposed fiber optic displacement sensor with ultra-high resolution in low frequency can be of great use in scientific research and precision engineering.