DESIGN AND SIMULATION OF A DIFFRACTIVE OPTICAL ELEMENT–BASED SPECTRAL SPLITTING SYSTEM FOR EFFICIENT SEPARATION OF VISIBLE AND INFRARED SOLAR RADIATION

Abstract

Efficient utilization of the solar spectrum remains a central challenge in improving the performance of hybrid photovoltaic and thermal energy systems. Since solar radiation spans a broad wavelength range from ultraviolet to infrared, conventional photovoltaic devices cannot effectively convert the entire spectrum into useful energy. This study presents the design, fabrication, simulation, and experimental characterization of a spectral splitting system based on a multi-level diffractive optical element (DOE) integrated with a structured light-guide. The DOE was fabricated on a fused silica substrate using 8-level electron-beam lithography and reactive-ion etching, achieving a measured fundamental step height of 1.154 μm a deviation of 4.8% from the designed value of 1.204 μm, within the ±5% specification. Surface roughness was confirmed by atomic force microscopy at Ra = 1.7 nm. Numerical simulations were conducted in LightTools over the wavelength range 380–2100 nm, and experimental validation was performed using a 150 W xenon arc lamp with calibrated silicon and InGaAs photodetectors. The measured visible-band diffraction efficiency reached 83.2 ± 2.5%, in close agreement with the simulated value of 85.1% (deviation < 2.3%). The spectral separation ratio between visible and infrared output ports was measured at 18.4 ± 0.6 dB, exceeding the 15 dB design target, with insertion losses below 1.2 dB at both ports. Sensitivity analysis over four perturbation parameters incident angle (±5°), step-height fabrication error (±10%), design wavelength shift (±20 nm), and grating period error (±5%) confirmed that efficiency remains above 80% and separation ratio above 17 dB throughout all tested ranges. Two configurations were demonstrated: a two-port design for direct visible/infrared separation, and a three-port design for flexible multi-channel energy distribution. Compared with prism-based systems (70–80% efficiency) and dichroic mirror systems (≈90% efficiency, complex fabrication), the proposed DOE-based system offers a competitive balance of performance, broadband spectral coverage, compactness, and manufacturability.