Balanced optical cross-correlators (BOCs) enable sub-femtosecond pulse timing interval measurements and are widely utilized in ultrafast laser diagnostics and synchronization control. Conventional free-space BOCs depend on bulk nonlinear crystals, which demonstrate low second-harmonic generation (SHG) conversion efficiency (0.4%, corresponding to a normalized efficiency of 0.0065%·W^-1·cm^-2), thus necessitating relatively high pump power. Thin-film lithium niobate (TFLN) photonic platforms provide exceptional second-order nonlinearity (d₃₃=25 pm/V, d₃₁=4.6 pm/V), electric-field-induced domain inversion capability, a broad transparency window (400 nm to 5 μm), and strong optical confinement (with a refractive index contrast of ~0.7 relative to silicon dioxide), presenting a promising approach toward fully integrated on-chip BOCs. In this study, we demonstrate a polarization-independent reflector on TFLN photonic platforms. A polarization- rotating Bragg grating (PRBG) structure is implemented by introducing bidirectional asymmetry to suppress the polarization dependence of both the TE₀ and TM₀ modes. We designed and fabricated a 210 μm-long asymmetric Bragg grating. Experimental results show that within the 1548.3?1556.8 nm wavelength range, the transmission spectra of the TE₀ and TM₀ modes are nearly identical, with a 3 dB bandwidth of approximately 8.5 nm and a polarization extinction ratio exceeding 20 dB, confirming the strong polarization-independent performance of the structure. These results provide a key technological foundation for the realization of fully integrated on-chip BOCs.

This study employs the finite element method (FEM) to simulate the wavelength-dependent effective refractive indices of the TE₀ and TM₀ modes in a z-cut TFLN waveguide with a width of W=0.9 μm. Considering the fabrication constraint, a sidewall tilt angle of 67° was incorporated into the design. The performance of the PRBG reflector was analyzed using the eigenmode expansion method (EME), through which the effects of variations in waveguide width, unetched thickness, sidewall angle, duty cycle, and grating period were systematically investigated. Based on design results, the device was patterned using 100 kV electro-beam lithography with ZEP520A positive-tone resist, followed by pattern transfer to the TFLN layer through an optimized argon ion beam milling process. After cleavage to expose the waveguide facet, the device performance was characterized using a fiber-to-chip coupling system.

To analyze the experimental results, the transmission spectra under TE₀ and TM₀ modes incidence are initially simulated using EME method, as shown in Fig. 5(b) and Fig. 5(c). Considering fabrication-induced deviations, the grating period in the simulation is adjusted from the designed value of 420 nm to 419 nm to better match the actual structure. The results demonstrate that the TE₀ mode is effectively reflected in the wavelength ranges of 1509.6?1521.1 nm and 1549.2?1557.1 nm, with the TM₀ mode being the dominant transmitted component. Conversely, the TM₀ mode is effectively reflected within 1549.2?1557.1 nm, with TE₀ as the primary transmitted mode. By superimposing and normalizing the transmission spectra, the overall simulated transmission spectrum is obtained, as shown by the black curves in Fig. 5(d) and Fig. 5(e). From experimental measurements, we have recorded the normalized transmission spectra, as shown in the red curves in Fig. 5(d) and Fig. 5(e). The reflector exhibits excellent reflection characteristics for both TE₀ and TM₀ modes within the 1548.3?1556.8 nm wavelength range, with a central transmission wavelength of approximately 1552.6 nm, a 3 dB bandwidth of ~8.5 nm, and a polarization extinction ratio exceeding 20 dB—indicating strong polarization-independent reflection performance. Furthermore, the experimental results demonstrate high consistency with the simulated spectra in this range, validating the accuracy and reliability of the device design parameters. Additionally, when the input is TE₀, strong reflection is observed in the 1511.4?1519.0 nm band, which aligns well with the expected design.

This research presents the pioneering implementation of a polarization-independent photonic reflector utilizing the TFLN photonic platform. The design incorporates a PRBG structure, developed through a systematic parameter optimization methodology. A comprehensive analysis examined the effects of critical structural parameters, including waveguide width, unetched thickness, sidewall angle, duty cycle, grating period, and number of periods, on device performance. The reflector fabrication is accomplished through a single-step electron-beam lithography and dry etching process. Experimental measurements demonstrate that both TE₀ and TM₀ modes achieve high reflectivity within the wavelength range of 1548.3?1556.8 nm, featuring a 3 dB bandwidth of approximately 8.5 nm and a polarization extinction ratio exceeding 20 dB. The measured transmission spectra demonstrate excellent agreement with simulation results, confirming the validity of the design methodology. When combined with existing z-cut TFLN periodic poling techniques, this polarization-independent reflector demonstrates significant potential for monolithic integration with type-II QPM PPLN waveguides, advancing the development of fully integrated on-chip BOC devices and enabling ultrafast optical signal processing.