Material and process innovations for high-energy laser coatings on compliant polymer substrates are needed to improve the adhesion and survivability of space-based optical membranes. These reflective membranes consist of a dielectric coating stack of neobium pentoxide (Nb2O5) and silicon dioxide (SiO2 ) laminated to a fluorinated polyimide membrane. Two prevalent limitations of these coated membrane structures are the lack of adhesion of the dielectric coatings to the polyimide film and the space survivability. Adhesion loss can be caused by a mismatch of the coefficient of thermal expansion (CTE) of the dielectric coating and the polyimide or the inability of the polyimide to withstand oxygen plasmas. MicroChem Corp. is proposing two new stable polymer membranes, which are compatible with current, dielectric coating technology and provide greater than 99.99% reflectance at a wavelength of 1.315um. The first polymer, OPI-N2005, is a new fluorinated polyimide, which is transparent at 1.315um and has a low, negative CTE, which practically eliminates the compressive stress created by dielectric coatings. The second polymer, SU-8, is an epoxy-novolak which has more than twice the resistance to degradation by radio frequency (RF) generated oxygen plasma as compared to polyimdes, which has been shown to accurately predict in-space durability.
Benefits: This technology has already been demonstrated to be a low cost alternative to wafer fabricated quartz in the production of band-pass filters for telecommunication waveguides and fiber optic connections. These dielectrically coated polyimide filters have survived stress tests which simulate the rigors of buried underground cables. Both the high-energy COIL [laser] and fiber optic telecommunications operate at the same wavelength of 1.315 um, which means that the same dielectric stack and production techniques can be applied. Reflective optics for space-based and tactical applications are commonly used in both Newtonian style telescope designs and for collimating coherent light sources. Lightweight and compliant, deployable space optics are often limited to coherent light sources because their structural support does not produce the needed wave front accuracy. However, it has been shown that membrane optics are capable of producing near-diffraction limited images. This means that compliant optics can penetrate both commercial and military markets that were previously reliant upon heavy, costly and slow-to-produce monolithic designs. This could allow compliant optics to be used for satellite imagery and other space-based communications applications. Many of the polymers to be tested in this Phase I research proposal have been originally designed for use in the telecommunications and electronics industries. It is therefore, reasonable to assume that improvements in the materials and processes for space-based applications can also be used in the same applications. Many of the polyimides and other polymers in these applications are not photoactive or photo-imageable, i.e., the coated optical membrane must be mechanically separated into many small elements for use. Converting these polymers into photo-crosslinakable polymers, like SU-8 would allow the use of light to pattern the filters and obviate the need for mechanical separation, such as dicing or other forms of cutting. Therefore, it is easy to re-incorporate the findings of this Phase I research directly back into the products for commercialization in their current applications.
Keywords: Fluorinated Polyimide, Epoxidized Bisphenol A Novolak, Coefficient of Thermal Expansion, Oxygen Plasma, Neobium Pentoxide, Silicon Dioxide, Nano particulate fillers
