We continue to develop our diffraction grating fabrication technology to achieve new performance levels and extended wavelength ranges.
The Moon Mineralogy Mapper (M3) spectrometer was successfully launched aboard India’s Chandrayaan-1 on October 22, 2008, and is in orbit around the Moon. M3 is operational and is transmitting spectra of the lunar surface.
A low-polarization triple-blaze convex grating was fabricated and tested. The grating was designed to have tailored spectral efficiency from 400 to 2500 nm. Through groove shape optimization and specialized in-house coating, we demonstrated less than 2% polarization sensitivity over the critical 400–900-nm spectral region. Low-polarization sensitivity is critical for Earth-observing imaging spectrometers because our atmosphere polarizes the downwelling solar radiation to an unknown degree.
A long-wave infrared (LWIR, 8 to 12 µm) concave grating on a ZnSe diamond-turned substrate was fabricated. The grating enabled the successful demonstration of a LWIR Dyson imaging spectrometer. Dyson spectrometers are particularly well suited for LWIR operation due to their extremely compact size, allowing the entire spectrometer to be cooled to minimize the infrared background signal. In addition, Dyson spectrometers exhibit the same low image distortion as Offner spectrometers, but offer increased light throughput and reduced diffraction spread, which are especially important at long wavelengths.
We successfully developed a new process and fabricated a 2-inch-diameter convex grating for the 1.8 to 2.9 µm-wavelength band. We built a test setup and measured peak efficiency greater than 95% (>70% across the band) and ghosts less than 2x10-4 relative to the main diffraction order.