Recently, the Joint Laboratory of High Power Laser Physics, Shanghai Institute of Optics and Precision Machinery (SIPM), Chinese Academy of Sciences (CAS), has proposed a single-exposure interferometric calibration method for large aperture diffractive lenses, which provides a strong support for the engineering application of large aperture diffractive lenses. The results were published in Optics Letters as "Absolute measurement of focusing properties of a large-aperture diffractive lens". Optics Letters.
Compared with the refractive focusing lens, the diffractive optical element is flexible in design, large in aperture, light in weight, suitable for various wavelength bands, and capable of realizing complex optical functions, and the photon sieve and the ribbon sheet are the typical representatives of the diffractive lens. Considering that diffractive elements consist of a large number of microstructures, deviations are inevitable during processing, so the performance needs to be calibrated before use.
Figure 1. Measurement optical path of a large-diameter diffractive lens
In this study, the researchers used the natural background light of the large-diameter diffractive lens as a reference, which entered the shear interferometric system together with the focused beam. Based on the interferograms recorded by the detector, Fourier analysis was used to obtain the wavefront gradient of the focused beam with respect to the background light, then the mode method was used to recover the transmitted wavefront, and finally, the focal length and focal spot shape of the diffractive lens were numerically calculated.The experimental results of the 210-mm aperture diffractive lens are in accordance with theoretical expectations. Ultra-large aperture waveband slices and photon sieves can be used in space interferometric telescopes. Beam-splitting photon sieves with self-supporting structures are suitable for focused imaging of EUV and soft x-rays.
Figure 2. Self-developed shear interferometer with measured experimental results
