Ge immersed grating manufacturing and optical verification for the METIS high-resolution spectrograph

Abstract

We present the manufacturing and optical verification of the germanium immersed grating for the L/M band high resolution spectrograph (LMS). The LMS is one of the science subsystems of the Mid-infrared ELT Imager and Spectrograph, METIS. The immersed grating has very demanding requirement specifications: <100 nm RMS transmitted Wave Front Error (WFE) after double pass, and >70% peak throughput in all orders within the 2.9-5.3 μm wavelength range over the pupil. The grating has a period of 18.2 μm, a sawtooth groove profile with 89.6 degrees apex angle and a grating area of 150mm x 60mm. The germanium immersed grating was produced by Canon's high precision mechanical cutting technology. We present the interferometric tests that were performed in order to verify WFE and two different measurements (based on cascade laser and Fourier Transform Spectrometer, respectively) for throughput verification.