Objective Single-layer PB phase metasurfaces, while compact and broadband, face inherent limitations in complex wavefront shaping and arbitrary polarization control. Specifically, they cannot achieve beam translation, which requires spatially separated elements, nor can they independently manipulate the diagonal terms of the Jones matrix, limiting their ability to modulate linear polarization. Multilayer cascaded devices address these issues but often introduce fabrication complexities such as interlayer Fresnel reflections, reduced refractive index contrast, or demanding alignment processes. This study aims to overcome these limitations by leveraging the true three-dimensional direct writing capability of femtosecond laser processing to fabricate a compact, cascaded PB phase beam splitter within a single fused silica substrate. The objective is to realize a device that simultaneously achieves beam splitting, wavefront collimation, and linear-to-linear polarization conversion, all while maintaining high efficiency and robustness for high-power applications.

Methods The proposed device is a three-stage cascaded structure integrated monolithically in fused silica. It consists of two identical PB phase polarization gratings and a quarter-wave plate (QWP). The first PG acts as a beam splitter, diffracting the orthogonal circular polarization components of an incident linear polarization into ±1 orders. The second PG, placed at a distance, acts as a collimator, redirecting these divergent beams back to their original propagation direction. The final QWP converts the circularly polarized outputs back to linear polarization.

A rigorous vector diffraction analysis was first conducted to theoretically evaluate the impact of fabrication errors—specifically, non-uniform retardance and nonlinear azimuthal angle variations—on the diffraction efficiency and polarization state of the ±1, 0, and higher orders. This analysis established strict requirements for the linearity of the azimuthal angle and the uniformity of the retardance across the grating to achieve ideal 50:50 beam splitting with high polarization purity.

To meet these stringent requirements, a dynamic pulse line density compensation strategy was developed to address the polarization-dependent fabrication characteristics inherent in femtosecond laser writing. This method compensates for variations in birefringence strength induced by different laser polarization states during scanning, which arise from spatio-temporal asymmetries like pulse front tilt and angular dispersion introduced by the focusing objective. Instead of modifying the laser’s spatio-temporal profile, this approach dynamically adjusts the pulse line density for each scanning line based on its polarization orientation. This allows for uniform birefringence retardance across the entire device, regardless of the slow-axis orientation, while maintaining low propagation loss. The three-layer structure was fabricated in Corning 7980 fused silica using a 1030 nm femtosecond laser and a low numerical aperture objective. The final device was characterized at a 510 nm operating wavelength.

Results and Discussions The theoretical analysis confirmed that deviations from ideal conditions—a uniform π retardance and a strictly linear azimuthal angle variation across the grating period—lead to severe performance degradation. Such errors cause energy coupling between the ±1 and 0 orders, introduce higher-order diffractions, and degrade the polarization ellipticity of the output beams. The analysis quantitatively showed that azimuthal angle nonlinearity is a primary source of efficiency loss, while non-uniform retardance causes unbalanced energy distribution between the two output beams.

The experimental investigation of the fabrication process revealed a significant polarization-dependent writing effect. Birefringent structures written with laser polarization at 90° (slow axis horizontal) exhibited a lower modification threshold, resulting in wider, longer, and higher-retardance structures compared to those written with 0° polarization. This "aggressive" growth of structures in the preferred polarization direction distorted the designed linear azimuthal angle variation, effectively pulling the orientation of neighboring structures towards the preferred angle. This phenomenon was found to be objective-dependent, with different objectives (5×, 20×, 40×) showing reversed trends, confirming the role of the optical system’s spatio-temporal aberrations.

The dynamic pulse line density compensation was highly effective in mitigating this polarization-dependent non-uniformity. Compared to a non-compensated grating, the compensated grating showed a reduction in normalized retardance deviation from 30%–40% to just 12%. This significantly improved the linearity of the azimuthal angle profile. The fabricated polarization grating, when tested alone, demonstrated remarkable input polarization robustness. For any linear input polarization, the power splitting ratio between the ±1 orders remained above 90.69%, with the device operating at its design state (90° linear input) achieving a 97.01% splitting ratio and a total diffraction efficiency of 94.8%, far exceeding the ~81% of conventional binary optics.

The complete three-stage collimated beam splitter exhibited excellent performance. The output beams showed a measured separation angle of 0.16 mrad, closely matching the theoretical design, indicating successful collimation. The final output beams were linear with a high splitting ratio of 94.01% and a polarization angle difference of only 3.3°. The device also demonstrated outstanding power handling capability, maintaining a stable ~83% efficiency under 3.6 W of continuous 510 nm laser irradiation over 2400 seconds, with a calculated power tolerance exceeding 52.67 W/cm².

Conclusions This work successfully demonstrates a new paradigm for fabricating multifunctional PB phase devices by leveraging the true three-dimensional capability of femtosecond laser direct writing. A cascaded PB phase collimated beam splitter was integrated into a single fused silica substrate, overcoming the wavefront shaping and polarization modulation limitations of single-layer devices. The critical challenge of polarization-dependent writing non-uniformity was effectively addressed by a novel dynamic pulse line density compensation method, which is more practical than complex optical path compensation. This approach enabled precise control over the retardance and azimuthal angle distributions, yielding a high-performance device with a splitting ratio of 94.01%, an inter-beam angle of 0.16 mrad, a polarization angle difference of 3.3°, and high-power tolerance. This work paves a robust and scalable way for creating compact, high-performance, and damage-resistant integrated photonic elements for applications such as high-power laser parallel processing, atomic electric field sensing, and advanced beam manipulation systems.