The optical community frequently relies on optical adhesives to join multi-component optical assemblies. However, options for bonding infrared materials are limited, since most optical adhesives are organic which means poor index matching to high index materials, and the introduction of their own IR absorption bands. Contact bonding is a potential solution, as it provides adhesion through intermolecular forces, capillary forces, and covalent bonds between bonding surfaces. However, to be successful, a reliable non-destructive characterization technique must be designed to evaluate bond quality. Many optical applications require low temperature processing to avoid degradation of any optical coatings. At room temperature, trapped water in the interface is a dominant contributor to the strength of contact bonded assemblies. Therefore, evaluating the interface is a crucial portion of determining bond quality. This report will analyze the potential of acoustic microscopy as a non-destructive technique to probe trapped water in the bonded interface. Several experiments were conducted to ascertain the sensitivity of acoustic microscopy to trapped water: (i) FTIR spectroscopy was used to provide a benchmark, (ii) bulk silicon samples were bonded in ambient atmosphere and aged over several weeks, (iii) and various annealing procedures were also performed to demonstrate water content evolution. A spring model of the bonded interface is used to quantify the amount of water in these studies through interfacial stiffness. Through these experiments, acoustic microscopy is shown to be an effective non-destructive technique to observe changes in interfacial water and may be used alongside other methods to screen high-value optical assemblies.
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.
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.
Infrared optical systems require a range of exotic materials, many with large indices of refraction. The mismatch in the
index between the optical element and the surrounding medium can result in reflection losses that approach 50%. Antireflection
(AR) coatings are applied to these elements in order to minimize "ghost" reflections and improve the optical
transmission through a system. The coatings are designed to be highly transparent; however, significant infrared
absorption has been observed in some AR coatings. Likely candidates for the loss mechanism are water trapped into the
optical coatings during deposition and water being incorporated into coating voids or grain boundaries during exposure
to ambient humidity. Five different AR coatings have been procured from four manufacturers to study the cause of the
observed losses. Upon receipt of the coated samples, infrared transmission measurements were made which showed the
presence of incorporated water/hydroxyl, as evidenced by reduced transmission around 2.9 μm. Four of the five sample
types placed in laboratory air for two months continued to absorb water, whereas those placed in flowing dry nitrogen
showed no change. Samples placed in a humid environment for one, three and ten days also showed additional water
incorporation with the magnitude of the change in transmission on the order of that observed with the two-month air
exposure.
The exterior optical surfaces of satellites are directly exposed to the harsh space environment. Here, a multilayer dielectric solar rejection filter was deposited on a silicon substrate and then subjected to electron and proton irradiation, simulating an orbital environment. Following the exposure, damage was observed that was attributed to dielectric breakdown. Optical and scanning electron microscopy revealed extensive pitting as a result of this exposure. The typical size of dischrage pits was 50 - 100 microns at the surface, extending to the substrate material, where a 10 micron diameter melt region was found. Pit damage occurred at pre-existing coating defects and was accelerated by pre-exposure to proton radiation. Pitting was not observed on similar samples that had also been overcoated with a conductive thin-film.
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.
By definition Nanosatellites are space systems that can weigh 1010 kg and can perform unique missions (e.g. global cloud cover monitoring, store-and-forward communications) acting either in constellation of distributed sensor-nodes or in a many-satellite platoon that flies in formation. The Aerospace Corporation has been exploring the application of microelectronics fabrication and advanced packaging technology to the development of a mass-producible nanosatellite. Particular attention is being directed at M3 (Micromachining/MEMS/Microsystems) technology which appears to be important in the integration and manufacturing of these satellites. Laser direct-write processing techniques are being applied for rapid prototyping and to specific 3D fabrication steps where conventional microelectronics fabrication techniques fall short. In particular, a laser based technique has been developed that combines the rapid prototyping aspects of direct-write and the low cost/process uniformity aspects of batch processing. This technique has been used to develop various fluidic components and a microthruster subsystem in a photostructurable glass/ceramic material.
A pulsed UV laser based technique has been developed which permits the transfer, by direct-write exposure, of 3D image into a photosensitive glass/ceramic material. The exposed latent image volume is developed via temperature programmed bake process and then etched away using HF in solution. The height of the 3D microstructures is controlled by the initial laser wavelength used during the exposure and the time duration of the etching cycle. Using this technique we have fabricated large arrays of microstructures which have applications to microfluidics, microelectromechanical systems and optoelectronics. The resulting master copy can be used either as is or by use standard injection modeling techniques converted into a metallic or plastic copies. We present these results and others which have specific applications to miniature 1Kg class satellites - nanosatellites.
A number of different organic molecules have been used as optical probes of the sol-gel process. There is relatively little information, however, as to whether these molecules remain well isolated within the sol-gel structure or if they tend to form dimers or higher aggregates within the network. This issue is particularly important for doped sol-gel optical materials as dimer formation can exert a significant influence on the optical properties of dyes. The present paper uses the optical absorption characteristics of copper phthalocyanine tetrasulfonate (CuPcts) to determine how the state of the dye is affected by the chemical changes during the sol-gel process. The absorption spectra of CuPcts indicate that the dye molecules are dimerized in acid-catalyzed silica xerogels prepared from TMOS. The dimerization is largely controlled by the chemical environment inside the pores. By using appropriate reference solutions, we are able to identify the factors which cause dimerization and the stages of the sol-gel-xerogel transformation when the dimers form. These factors include the quantity of solvent remaining in the pores, the alcohol/water content of the solvent and its acidity. It is shown that by modifying the sol-gel processing conditions and the solvent chemistry within the pores, it is possible to reduce significantly dimer formation in silica xerogels.
We demonstrate optical limiting in phthalocyanine doped solid host materials such as poly- (methylmethacrylate) and organically modified sol-gels. It is shown that the nanosecond nonlinear absorptive properties and the excited-state properties, such as the triplet-triplet absorption spectrum and the triplet quantum yields, of the metallophthalocyanines in these solid hosts are very similar to those in solutions. The key figures-of-merit for these materials suggest potential for the realization of high-performance solid-state optical limiters based on phthalocyanines.
Optical limiters are devices that have the ability to limit the intensity of light that passes through them. They function like 'optical surge protectors,' blocking a greater fraction of photons when the incident light intensity is very high than when the incident intensity is low. This might allow an electronic sensor or a human eye to function under ambient lighting conditions and yet be protected from being blinded by direct laser illumination. Phthalocyanine (Pc) dyes which exhibit nonlinear optical behavior known as reverse saturable absorption (RSA) have been demonstrated to be effective in solution-based optical limiters. There is considerable interest in the possibility of fabricating optical limiters based on solid-state materials containing phthalocyanine dyes. Sol-gel processing is a solution based technique for preparing porous metal oxide monoliths at low temperatures. By adding an organic dye to the precursor solution, one can trap these organic molecules in the pores of a silica host. These potentially thermally stable materials are of interest for the fabrication of monolithic lens/limiter optical components. Our study of sol-gel synthetic approaches to phthalocyanine doped glassy materials show that phthalocyanine dyes in silica hosts are stable and exhibit optical nonlinearities comparable to those of the corresponding dye solutions.
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