Wide-range nano-displacement measurement via dual-wavelength interferometry with an ultra-broadband OAM converter

In this study, we propose a source-to-application framework addressing critical challenges in ultra-broadband ±2 nd order orbital angular momentum (OAM) generation and wide-range nano-displacement measurement. What we believe to be a novel all-fiber mode converter was developed using a chiral long-period fiber grating (CLPG)—fabricated by twisting a four-mode fiber (4MF)—to harness dual resonance near the dispersion turning point (DTP). Conventional chirping limitations are overcome via raised-cosine optimization of the CLPG profile, guided by coupling characteristic analyses. This design enables robust ±2 nd order OAM generation with a 37 dB maximum extinction ratio over a 146 nm flat-top bandwidth (20 dB suppression), resolving the broadband ±2 nd order OAM bottleneck. Based on this OAM converter, a dual-wavelength conjugate vortex interferometric system is constructed for wide-range nano-displacement measurement. Ultra-broadband 2 nd order OAM beams eliminate range ambiguity in traditional interferometers, achieving an 80 μm measurement range (90-fold expansion versus conventional methods) with a 0.04% average error with a displacement step size of 10 nm via centroid demodulation. This work establishes an end-to-end framework bridging advanced OAM generation and metrological applications, advancing broadband, high-precision, wide-range nano-displacement measurement. It lays a foundation for applications in micro/nano manufacturing and precision metrology.