Large-range displacement measurement in narrow space scenarios: fiber microprobe sensor with subnanometer accuracy

The embedded ultra-precision displacement measurement is of great interest in developing high-end equipment as well as precision metrology. However, conventional interferometers only focus on measurement accuracy neglecting the sensor volume and requirement of embedded measurement, thus hindering their broad applications. Here we present a new sensing method for realizing large-range displacement measurement in narrow space scenarios based on the combination of a fiber microprobe interference-sensing model and precision phase-generated carrier. This is achieved by microprobe tilted-axis Gaussian optical field diffraction and high-order carrier demodulation to realize large-range displacement sensing. It is uncovered that the microprobe element misalignment and phase demodulation means play pivotal roles in the interference signal and the accuracy of large-range displacement sensing. The analysis shows that the proposed interference-sensing method can effectively reduce the nonlinearities. Experimental results illustrate that the measurement range extends from 0 to 700 mm. Furthermore, the maximum nonlinear error is reduced from tens of nanometers to 0.82 nm over the full range, allowing subnanometer accuracy for embedded measurements in the hundreds of millimeters range.