Design Requirement of LWIR Optical Filter for LAPAN-A4 Satellite

Abstract

A filter is a device or instrument which is able to pass a specific signal and also remove other signals. There are several types of optical filters: bandpass filter, long-pass filter, and short-pass filter. The next microsatellite developed by Satellite Technology Center is LAPAN-A4 Satellite. This satellite brings Medium Resolution Multispectral Imager (MRI) using SLIM4 and also Experiment LAPAN Line Imager Space Application (ELLISA). Moreover, this satellite also brings thermal imager which is called microbolometer, and experiment Short Wave Infrared (SWIR) camera. Especially for spectral band Long Wave Infrared (LWIR) cameras in LAPAN-A4 satellite will be narrowed down to $10.4\mu \mathrm{m}- 12.5\mu \mathrm{m}$, therefore a band filter is needed for blocking and passing these wavelengths. The method used to design the optical bandpass filter LWIR microbolometer camera is by determining the required spectral response. The response spectral required refers to the LANDSAT 8 Thermal Infrared Sensors (TIRS) bands 10 and 11 with spectral response $10.60 - 12.51\mu \mathrm{m}$, Centre Wavelength (CWL) of band 10 and 11 are $10.9\mu \mathrm{m}$ and $12.0\ \mu \mathrm{m}$, and Full-Width Half-Maximum (FWHM) of both band are 0.6 $\mu \mathrm{m}$ and $1\mu \mathrm{m})$), and also determine the substrate filter material to be used. The design results show that, the LWIR bandpass filter has a spectral response of 10.4- $12.5\mu \mathrm{m}$, CWL $11.45\mu \mathrm{m}$, and FWHM $2.1\mu \mathrm{m},\ \max$ peak transmission is 80.21%, and capable of blocking at wavelengths of $7-10.4\ \mu \mathrm{m}$ and $13-16.5 \mu \mathrm{m}$, the material used in this design uses Germanium (Ge) because it has a wavelength of $2-16\ \mu \mathrm{m}$ which can work on spectral IR transmission.