Laser Emission Spatial FTIR Spectrometer
The thermal infrared surface contamination sensor (TIRSCS) described below is a new type of laser emission, spatial FTIR spectrometer that has no moving parts. It captures a complete interferogram instantaneously and then Fourier transforms the interferogram into a spectrum. The CAD model shows its internal components. The TIRSCS uses a 9.6 micron laser to vaporize surface contaminants. The contaminants are transparent in the visible but opaque at 9.6 microns. The vaporized contaminants generate a hot plume that can be detected using an infrared detector array. We used a 7 - 11 micron mercury cadmium telluride (MCT) detector with 7 x 576 pixels, developed by AIM. The MCT array was cooled to 60K using a stirling cycle cryo-cooler to eliminate dark current.
A second cryo-cooler was used to cool the spectrometer optics to 200K to eliminate thermal background radiation, and thereby increase the detection sensitivity of the system. Cooling the optics required us to mount the optical components in a dewar along with the detector array. This was very challenging because the optics needed to be isolated from the warm dewar walls, and it also needed to stay in alignment after cool-down. The most challenging optics to maintain in alignment were the beam shearing prisms shown below.
The operation of the TIRSCS is shown below on the left where the laser is firing at themal paper mounted on teflon. The teflon spectrum is shown below the plume photo. The very first interferogram and spectrum captured by the TIRSCS are shown on the right. The spectrum shows a wavy long pass filter.
