Geometric calibration is critical to the measurement accuracy of monocular phase measuring deflectometry (PMD). The traditional three-mirror method is attractive because of its simple operation and low cost, but its calibration accuracy and robustness remain limited. To address these limitations, this paper proposes an improved three-mirror calibration method without introducing additional hardware. First, a hybrid feature constraint mechanism is constructed to establish flexible feature correspondences across different mirror positions. Building on these correspondences, a graph-constrained position selection strategy determines the optimal three-position combination from multiple mirror positions. The selected combination is then used for nonlinear optimization with phase-modulation-based weighting to reduce the influence of low-quality phase observations and obtain the final calibration parameters. Simulation results verify the effectiveness of the proposed method. Physical experiments show that, compared with the traditional three-mirror method, the proposed method reduces the reconstruction root mean square (RMS) residual from 213.85 nm to 75.96 nm for a plane mirror and from 440.67 nm to 205.08 nm for a spherical mirror. These results demonstrate that the proposed method improves calibration accuracy and robustness while maintaining a low-cost configuration.
Moiré fringe-based alignment methods are widely used in lithography, where fringe contrast directly determines measurement accuracy. Conventional off-axis illumination schemes typically rely on a fixed incident angle, which fundamentally fails to accommodate the wavelength-dependent angular requirements imposed by different process layers and grating structures. To overcome this limitation, we propose a wavelength-selective off-axis illumination method that, for the first time, decouples the incident angle from the illumination wavelength through the coordinated control of two galvanometers. This design enables arbitrary and precise tuning of the off-axis angle from 15.68° to 18.55° at the alignment mark, allowing each wavelength to satisfy its optimal Littrow condition. As a result, the proposed method not only maintains consistently high fringe contrast across different wavebands but also significantly outperforms conventional fixed-angle schemes in terms of signal-to-noise ratio and imaging robustness. Experimental results demonstrate that our approach achieves superior Moiré fringe quality with enhanced contrast and measurement repeatability.
To overcome the weak intrinsic birefringence and limited polarization-splitting capability of conventional buried silica waveguides, we propose and fabricate a polarization beam splitter (PBS) based on a Mach–Zehnder interferometer and enabled by stress-induced birefringence on a silica planar lightwave circuit (PLC) platform. The device consists of two 3-dB directional couplers, a reference arm, and a stress-modulation arm. Stress-control trenches are etched on both sides of the modulation arm to break the stress symmetry in the waveguide core, thereby inducing different phase accumulations for the transverse-electric (TE) and transverse-magnetic (TM) modes and routing them to separate output ports. The stress-control trenches are optimized using the finite-element method, while the directional couplers and the complete PBS are optimized using the beam propagation method. Experimental results show that, over the C band, the insertion losses of the target TE and TM channels are below 0.91 and 1.55 dB, respectively. Within the wavelength range of 1545–1560 nm, the polarization extinction ratios of the two channels are higher than 20 and 25 dB, respectively. These results demonstrate that the proposed structure enables on-chip polarization splitting compatible with standard silica PLC fabrication while preserving the low-loss characteristics of the platform.
