Metrological features of diffractive high-efficiency objectives for laser interferometry

It has been shown that high precision diffractive objectives are an alternative to their refractive counterparts for application in interferometers. A design for an all-diffractive, double-sided objective that fulfills the Abbe sine condition has been proposed. It allows eliminating aberrations due to a finite field angle. Ray tracing simulations show that field angles up to 0.1degrees cause aberrations less than lambda/20 PV in single pass. A single-sided prototype (diameter -80 mm; N.A. -0.158; designed wavelength -632.8 nm) has been fabricated by direct laser writing on photoresist. It was written on a polar coordinate laser system CLWS-300 that is capable of writing high precision DOEs up to a diameter of 300 mm. The blazed diffractive structures were written directly into a photoresist layer that was spinned on a high-precision substrate. Recalibration of the radial coordinate by reading marker positions on the substrate was used to eliminate machine drifts. The writing parameters were optimized by modelling the writing process in order to maximize. the diffraction efficiency. The fabricated objective has a rms wavefront error of less than lambda/20 in single pass. The residual errors are predictable using manufacturing data that is recorded during the writing process for each element. This permits to supply calibration data for each element. Measurements of the fabricated-DOE show excellent agreement between the predicted and measured wavefront quality.