Waveguide-array-based integrated photonic spectrometers are widely researched in astronomy for their high performance and compact structure. However， conventional star-coupler-based splitting architectures are restricted by fabrication constraints and film residual stress in thick-film silicon nitride platforms， which limits spectral resolution. High-performance optical power splitter structure， specifically ultra-broadband splitters with low insertion loss and high-precision power controllability， are essential for advanced spectroscopic survey facilities.A curved-inverse-taper-based arbitrary-ratio splitter was designed on a 300-nanometer-thick silicon nitride platform. Subwavelength grating structures were integrated into the coupling region to mitigate excess scattering losses associated with minimum feature size limits in traditional Y-branches. The subwavelength grating was treated as an effective medium with a tunable refractive index governed by the duty cycle. An gradient duty cycle design was applied to transition the effective index linearly， which ensured smooth modal evolution and suppressed back-reflections. The effective refractive index was calculated based on the square root of the weighted average of the dielectric constants of silicon nitride and silicon dioxide. Arbitrary power splitting was achieved by introducing asymmetry in the widths of the curved output branches. Modal propagation constants were tuned by varying the branch width difference， which enabled continuous adjustment of the power splitting ratio from 50：50 to 90：10. To maintain single-mode transmission， waveguide dimensions were constrained to a width of 1 micrometer and a height of 0.3 micrometer. The gap between the two output branches was set to 200 nanometers to comply with standard fabrication limits.Three-dimensional finite-difference time-domain simulations were performed to optimize structural parameters and analyze modal propagation. The subwavelength grating period， the uniform duty cycle， and the coupling length were systematically scanned to identify the minimum loss regime. The optimized configuration utilized a 400-nanometer subwavelength grating period and an gradient duty cycle transitioning from 0.5 to 0.3. A coupling length of 5 micrometers was selected to ensure adiabatic transition. The wavelength range from 1400 to 1800 nanometers was evaluated for ultra-broadband performance. Simulation results show that insertion loss remains below 0.28 decibels across the 400-nanometer bandwidth. The power splitting ratio variation is maintained under 1.7 percent for all investigated ratios， including 50：50， 60：40， 70：30， 80：20， and 90：10. The simulation data confirm that the curved-inverse-taper structure provides superior spectral flatness and wavelength insensitivity compared to conventional multimode interference couplers or directional couplers.Devices were fabricated using electron-beam lithography and inductively-coupled plasma etching. A 300-nanometer-thick silicon nitride film was deposited on a 3-micrometer buried oxide layer. Scanning electron microscopy confirmed the fidelity of the waveguide sidewalls and the high precision of the subwavelength grating patterns. Optical characterization was carried out in the 1525 to 1565 nanometer wavelength range using a tunable laser and an infrared charge-coupled device camera. End-face coupling was facilitated by a lensed fiber. To ensure measurement accuracy， background noise was removed by recording images without laser input. Each output channel was sampled using 15 pixels with a size of 15 micrometers to capture the full mode energy. Experimental results demonstrate an insertion loss under 0.23 decibels and power splitting ratio fluctuation under 2.5 percent. These measurements are consistent with theoretical predictions. Fabrication tolerance was analyzed by simulating the effects of sidewall roughness and width biases on the power splitting ratio of a 90：10 splitter. Discrepancies between simulation and experiment were attributed to a 3-nanometer sidewall roughness and plus or minus 30-nanometer width deviations resulting from the etching process.The curved-inverse-taper arbitrary-ratio splitter combines ultra-broad bandwidth， low loss， and high stability. By cascading this structure to replace the large-area free propagation regions in traditional arrayed waveguide gratings， this architecture provides a potential solution to the film stress issues that typically plague large-scale thick-film devices. The splitter performance remains stable even under significant fabrication biases， suggesting high yield for commercial CMOS foundry processes. This device provides essential technical support for high-resolution integrated spectrometers in next-generation large-scale astronomical spectroscopic surveys.