End-to-end precision characterisation of broadband anti-reflection-coated mm-wave components

Future sub-millimetre optical systems require low-loss, low-reflectance, broadband anti-reflection (AR) coatings, for which interspersed polymer layers with matched refractive indices are a proven option. However, hot-press bonding can change refractive index and deform the optic substrate, degrading lens performance. We address this by annealing the substrate before machining, relieving internal stresses and stabilizing optical properties so the AR-coating process does not distort the lens. We investigate cast polypropylene (CPP) and porous PTFE (pPTFE) as coating materials for UHMWPE optics and identify an optimal annealing cycle enabling reliable fabrication and characterization. Using spectroscopy and an MCMC-based transfer-matrix model, we extract refractive index and loss from 60–900 GHz. These measurements inform optimized 3- and 5-layer AR-coating designs validated on 70 mm pucks and demonstrated on a full-size two-lens system operating over 166–448 GHz.