James D. Barrie

Root cause determination of on-orbit degradation of the VIIRS rotating telescope assembly

The Visible Infrared Imaging Radiometer Suite (VIIRS) is a sensor onboard the recently launched Suomi NPP spacecraft. Shortly after launch, VIIRS was found to exhibit a pronounced decrease in the optical throughput of several bands, with the near-infrared bands being more affected than those in the visible. The anomaly investigation team performed several experiments that concluded the primary source of degradation was throughput loss in the VIIRS rotating telescope assembly, likely caused by ultraviolet light illumination. This paper will discuss the laboratory investigation that determined the root cause of the telescope degradation to be UV photo-darkening of a tungsten oxide contaminant film that had been inadvertently deposited during the mirror manufacturing process. We will present data from experiments conducted on witness mirrors manufactured along with the telescope, as well as other mirrors of the same type that were not contaminated.

Corrosion characterization of durable silver coatings by electrochemical impedance spectroscopy and accelerated environmental testing

Highly reflective front-surface silver mirrors are needed for many optical applications. While various protective dielectric coating schemes have been developed, the long-term durability of Ag mirrors is still of great concern in the optics community for a variety of applications under harsh environments. The corrosion protection behavior of a SiNx-coated silver-mirror coating scheme was tested with electrochemical impedance spectroscopy (EIS) and accelerated environmental testing, including humidity and salt fog tests. The EIS data obtained were fitted with different equivalent circuit models. The results suggested that the 100A thick SiNx coating produced by rf magnetron sputtering was porous and acted as a leaky capacitor on the Ag film, whereas the addition of a NiCrNx interlayer as thin as 3A between SiNx and Ag films resulted in a much denser SiNx coating with a low-frequency impedance value of 2 orders of magnitude higher than that without the interlayer. Humidity and salt fog testing of different silver coatings showed similar results. The 100A SiNx/3A-NiCrNx/Ag coating exhibited excellent corrosion resistance against the corrosive environments used in this study.

Environmental durability of protected silver mirrors prepared by plasma beam sputtering

Various overcoat layers have been developed to protect silver mirrors from tarnish and corrosion. However, the mechanisms by which these protective layers improve mirror durability are not fully understood. Mixed flowing gas exposure was used to investigate the corrosion behavior of plasma beam sputtered silver mirrors with different adhesion layer materials. A small amount of nickel in the adhesion layer had a significant impact on the silver-dielectric interface. Additionally, lateral transport of silver was found to be an important factor in the corrosion process. Better stability at all layer interfaces is suggested to improve mirror durability.

Control of stress in protected silver mirrors prepared by plasma beam sputtering

Plasma beam sputtering was used to deposit dielectric-protected silver mirrors that exhibited excellent durability and controlled stress. The durability of the mirrors was strongly dependent upon the presence of a very thin chromium adhesion layer between the silver layer and the dielectric overcoat. The stress of the five-layer mirror was balanced by controlling the compressive stress in the top dielectric layer, offsetting the net tensile stress of the combination of layers below.

Optical Properties and Corrosion Resistance of Durable Silver Coatings

There is a need in the optics community for highly reflective mirrors that do not degrade in harsh environments. While silver can be a highly efficient reflector of visible light, it suffers from poor resistance to corrosive atmospheric gases. Here, we investigate a durable silver coating scheme, in which silver is sputter deposited between very thin layers of nichrome nitride. Excellent corrosion resistance is achieved with a silicon nitride overcoat. Samples were exposed to a variety of atmospheres and it was found that the nichrome nitride dramatically increased corrosion resistance when compared to bare silver or silicon nitride coated silver. Similar results were found when electrochemical impedance spectroscopy was used to evaluate samples. The enhanced corrosion resistance is achieved even when the thickness of the nichrome nilTide adhesion layer is too thin to be considered continuous.

Simulated space environmental exposure of optical coatings for spacecraft solar rejection

Dielectric multilayers composed of niobium pentoxide and silicon dioxide, designed for broadband solar rejection, were exposed to a simulated space environment of ultraviolet light and low-energy (10-20-keV) electron radiation. Samples exhibited various degrees of exposure-induced absorption extending from the ultraviolet to the infrared. Processing variations were correlated to damage susceptibility, and methods were identified that produced parts that exhibited no degradation even though the same materials and coating design were used. Coatings prepared under energetic deposition conditions that provided the densest and most moisture-stable coatings exhibited the best stability to the exposure conditions used.

Chemical Microsensors for Satellite Applications

We have been investigating several chemical microsensor technologies for the detection of chemicals found in propellant and rocket exhaust plumes and as contaminants for satellite components. In this work we have developed a catalytic metal sensor for the detection of H 2 contamination in electronic device packages. The sensor is based on the resistance of a thin film Pd/Ni alloy. Data have been obtained on its response at room temperature to various levels of H 2 as well as the effects of H 2 O and O 2 on its performance. Hydrogen levels to 10.5 ppm have been detected and the responsivity is about twice that of similar sensors. The sensor is insensitive to H 2 O but its response to H 2 is strongly inhibited by O 2 .

Observation and characterization of the Stierwalt effect in dielectric filters with model coating defects

Many applications of infrared technology require the use of narrow bandpass filters with excellent out-of-band rejection. Frequently, system designs require that the filter be placed in close proximity to a detector or focal plane array. More than twenty-five years ago Donald Stierwalt discovered that some filters that met out-of-band rejection specifications in a spectrophotometer did not meet specification when integrated into a focal plane assembly. In fact, he reported that proximal to the detector, one filter passed three orders of magnitude more out-of-band light. Since then, the Stierwalt Effect has become widely quoted and poorly studied. Many assume that it has to do with scatter in the film, but very little data has been reported. Here, we report the observation of the Stierwalt effect in an optical filter that was seeded with model defects. The seeding was done by depositing sparse 1μm polystyrene spheres upon a clean substrate before sputter depositing a simple band-stop filter. Light rejection from filters prepared in this way showed a strong dependence upon the distance between the film and the detector. Filters deposited without the spheres showed a much smaller effect.

Out of band scatter measurements from OLI optical bandpass filters

Multi-spectral Earth imaging sensors commonly use edge-bonded filter arrays (also known as “butcher blocks”) for spectral selection. These arrays are built from small filter “sticks” that are diced from coated wafers and then bonded together and placed in very close proximity to the detector array. Some filter designs are susceptible to excessive high angle scatter if the filters are constructed under less than ideal deposition conditions. This scatter can lead to optical crosstalk, which degrades system performance. Insufficient specifications and sub-optimum manufacturing practices lead to a phenomenon called angle resolved scatter (ARS), where light that should have been rejected by the filter is scattered into a very high-angle leak path, leading to optical crosstalk. The Landsat Data Continuity Mission’s (LDCM’s) operational land imager (OLI) instrument uses proximal filter arrays for spectral selection, so it is important to quantify the amount of transmitted, scattered light in wavelength ranges outside the pass band. This paper describes the scatter measurement techniques and Bi-Directional Transmission Distribution Function (BTDF) results for 3 OLI filters.

Lessons Learned from Optics Flown on the Materials International Space Station Experiment

The Materials International Space Station Experiment (MISSE) provided opportunities to study the effect of the low earth orbit space environment on materials. This paper presents observations made on optical samples flown as part of MISSE.