Axicon-hyperbolic lens for reflectivity measurements of curved surfaces

We present a quasioptical element design that transforms a diverging Gaussian beam to an approximate Bessel beam. The elements are designed to deliver millimeter waves to a curved surface in reflectivity measurements. Compared to canonical focused quasioptical designs, such as the Gaussian-beam telescope, diffraction from an axicon surface allows for significant relaxation in alignment requirements. This research is motivated by in vivo cornea measurements where achieving optimal optical alignment is difficult. Combined axicon-hyperbolic lenses were designed for 220-330 GHz and fabricated of TOPAS, a low-loss material at millimeter waves. The lens performance is evaluated with near-field measurements. The output beam is close to the Bessel beam with the radial wave number 94-96 % of the design at frequencies below 280 GHz. The match is maintained from close to the axicon to the predicted maximum range of the approximate Bessel beam. Agreement between theory and experiment is verified up to 51 mm and 87 mm for axicon external cone angles of 25 and 15 degrees, respectively. The measured on-axis power density peaks close to the theoretical location below 280 GHz. Reflection measurements from a spherical surface were obtained and the results suggest in-phase reflection back to the transceiver. Compared to the Gaussian beam, the in-range alignment requirement can be relaxed by an order of magnitude with the Bessel beam.