Enhanced measurement of deep-ultraviolet optical systems using a point diffraction heterodyne interferometer.

An interferometer is widely used for wavefront testing to evaluate optical aberration. A point diffraction interferometer (PDI) utilizes diffraction from a pinhole as a perfect reference wavefront for highly accurate detection. To further increase the resolution of manufacturing, an optical system is designed in the deep ultraviolet (DUV) region, which promotes the urgent need for a DUV interferometer. Here, we demonstrate a DUV heterodyne point diffraction interferometer (DH/PDI) to implement precise wavefront measurement. Extreme photodamage of focused DUV laser is an inevitable challenge of PDI, which is tackled by two-layer point diffraction modules in DH/PDI. By utilizing two acousto-optic modulators (AOMs) to generate a frequency difference between the test and reference beams, DH/PDI implements high-speed and anti-interference heterodyne detection for wavefront analysis. The powers of AOMs are adjustable, which enables DH/PDI to obtain high-contrast interferograms of mirrors with different reflectivity. By investigating the wavefront of a transmission relay lens, DH/PDI demonstrates a promising detection capacity of the optical system in the DUV region.