Large-Scale Dynamic Laser Frequency Modulation and Central Wavelength Stabilization in a Fiber-Optic Microprobe Interferometer

Abstract

This paper presents a fiber-optic microprobe interferometer tailored for ultraprecise measurements in confined spaces, such as lithography and precision manufacturing. Through nonlinear error suppression analyses, we demonstrate that achieving long-scale displacement measurements strictly requires large-range dynamic adjustment of the laser frequency modulation amplitude. This approach simultaneously yields a two-order-of-magnitude improvement in accuracy over conventional methods. Because the laser wavelength serves as a measurement standard, its accuracy and stability directly determine the measurement precision. Further analysis reveals that the accompanied optical intensity modulation arising from large-scale amplitude modulation causes drift between the locking and reference points, highlighting the need for improved stabilization and control of the laser wavelength. To enhance the accuracy of the measurement standard, we propose a method that detects peaks of the frequency discrimination curve and eliminates the frequency locking-point drift. Experimental results demonstrate a more than 3-fold reduction in the frequency shift induced by large-scale dynamic modulation, thereby improving the displacement-measurement accuracy by several-fold. To improve the stability of the measurement standard, a joint current–temperature fast frequency-control method is proposed, enabling real-time correction prior to frequency unlocking. Allan deviation analysis over various averaging times yielded a short-term wavelength stability of 1.57 × 10–10 (τ = 1 s). Moreover, robust long-term performance is confirmed by a relative wavelength stability of 1.2 × 10–8 (k = 3) over 250 min, representing an improvement of approximately 2 orders of magnitude over the conventional method.