GREGOR is an assembled solar telescope with 1.5 m diameter that is designed for high-precision measurements of the magnetic field and gas motion in the solar photosphere and chromosphere with a resolution of 70 km on the sun and high-resolution stellar spectroscopy. To meet the specific requirements of measurement using the GREGOR telescope system, the surfaces of the lenses need to be modified using interference coatings. These coatings form the optical filters required to achieve the rejection bands at the specific wavelength ranges originating from fluorescence. Therefore, a multi-notch filter covering visible light with high transmittance for passbands and high optical density for rejection bands is designed and fabricated to improve the signal-to-noise ratio, where the noise signals are rejected.

In this design, two materials with high and low refractive indices are employed and the ripples in the passband regions are suppressed by modulating the layer thicknesses of the layer stacks with high order apodization functions. As the full width at half maximum (FWHM) and central wavelength of the multi-rejection bands are also specified, high-order iteration algorithms are used to insert and adjust modulating layers to achieve the desired merit functions. This realizes the global search for a multi-band notch filter with effective ripple suppression. The designed multi-notch filters are then fabricated on B270 substrates using ion-beam-assisted reactive magnetron sputtering deposition. Optical monitoring and time control methods are used to control the thickness of the non-quarter wavelength optical thickness (QWOT) layers. The trigger point of the deposition of each layer can be determined using the turning points of the QWOT if the thicknesses are larger than the QWOT, whereas the trigger point of the deposition of each layer is determined by time using the average deposition rate for thicknesses less than the QWOT. The errors in the layer thickness can be automatically compensated by the optical monitoring method. By changing the number and position of the test glass, multi-rejection notch filters with improved spectral performance are fabricated.

The measurement results of the fabricated muti-band notch filter demonstrate good agreement with the designed spectra, thus proving the feasibility of the fabrication process (Fig. 5). Considering the high number of layers in the stack for the filter, gradient temperature annealing is performed at 200 ℃. This reduces the stress from -233 MPa to -89 MPa, indicating that relaxation of the internal stress can be achieved by the annealing method. The filter deposition method where the substrates rotate at high speeds and the masks are optimized after multi-iterations enables improvements in the fabrication of filters and results in a higher thickness uniformity (Fig. 6). From the above results, we demonstrate a design for a multi-notch filter with a narrow high-rejection band, where the sidelobes can be strongly suppressed by apodization. Fabricating a multi-notch filter is always a challenge, with difficulties in the accurate control of the index of refraction and thickness of each layer during deposition. The accuracy in the layers affects the steepness at the reflecting band edges and the high transmittance in the passband, resulting in high requirements in the accuracy of the index and thickness control. The advantage of this design approach is that the layer thicknesses are all close to the half wave length at the central rejection wavelength and therefore are very suitable for accurate monitoring during deposition. These coatings are produced by ion-beam-assisted reactive magnetron sputtering deposition technology with a high deposition rate and low stress.

The apodization thickness modulated design method presented in this work generates notch filter designs with strongly suppressed ripples in the transmission regions and high optical density for the rejection bands. The total layer number of the multi-notch filter is 190 and the total filter thickness is approximately 20 μm. The good performance multi-notch filter is deposited successfully. The multi-notch filter is prepared by ion-beam-assisted reactive magnetron sputtering on B270. Optical monitoring and time control methods are utilized to monitor the non-quarter thickness. The average transmittance in the passbands of the fabricated multi-notch filter is higher than 90%, the cut-off depth for the rejection bands is better than 0.1%, the full width at half maximum of the rejection bands is less than 24 nm, and the maximum deviation of the band position is 2 nm. This indicates the fabricated multi-notch filter with excellent performance and high production yield meets the specified requirements for the GREGOR telescope application.