Intelligence, surveillance, and reconnaissance (ISR) sensor technologies have evolved as mission requirements become much more rigorous. As this technology has evolved, the demands have increased for the highest levels of performance and reliability in high-precision windows. Custom windows used in mission-critical applications require careful engineering to ensure optimal performance under a wide variety of operating conditions, which necessitates in-house expertise in both design and manufacturing processes. So what sort of criteria should you evaluate before working with a supplier?
The material should be selected based on the project requirements and other trade-offs such as price, schedule or availability. A supplier should make objective recommendations, coming to your project with an open mind and selecting the right material for your application, rather than the one that they prefer.
First, let’s take a look at material selection.
Material choice – and indeed material thickness – directly influences among other things the transmission characteristics and mechanical strength of your ISR optic. Understanding the trade-offs when assessing materials, for example while also balancing the requirements for robustness against optical performance is important. Equally critical, is a deep knowledge of the manufacturing limits of different material suppliers and different fabrication technologies.
Once the optimal material for your application has been chosen, what else needs to be considered when specifying your window? Is your window supplier strictly a “build-to-print” or do they have the expertise to guide you to a window design that will work for your application?
Typically, the most critical requirement for a window is the Transmitted Wavefront Error (TWE) which quantifies the deviation of a wavefront when transmitted through a window. Then there is the parallelism between optical surfaces, which is effectively an assessment of the maximum tolerance away from “perfectly parallel” one surface can be from a second surface.
It is important to consider the window geometry around its “usable area” also known as the clear aperture. The clear aperture defines the area of the optic surface over which the optical requirements are met, and its design can have a dramatic influence on costs and lead times. For example, if the mounting for the window compromises the clear aperture area then this can obviously have dramatic time and cost implications.
You must also consider whether your window is parallel or wedged. If its wedged, angle, direction, and tolerance must be specified. To measure highly parallel windows, at ZYGO we use a multiple surface test (MST) interferometer. One measurement through one click of a button reports key metrics from various individual surfaces, such as transmitted wave front, wedge, and even material homogeneity. ZYGO has the ability to make them “extremely” parallel, to < 1 arc second and in most case we are only limited by the variation in refractive index of the raw material.
In-House Optical Coating Experience Matters
In addition, coating requirements must be considered at the design stage as well. Most fundamentally of all, the question needs to be asked does your material choice support your coating requirements. There are trade-offs, for example, between spectral performance, environmental, and durability requirements.
You will likely want to look for a supplier that designs coatings that are qualified to meet RTCA/DO-160D or military specifications.
Optical Metrology Proficiency
Measurement is also key when producing ISR windows, and your supplier will need extensive experience producing customized metrology solutions for the verification of optical components. ZYGO can measure TWE over large areas, and we can test assembled windows at use angle, different polarizations, and in their mounting configurations.
Choosing the right partner matters.
When you choose the right partner, you can significantly de-risk your program. Customers who elect to work with a supplier who has industry leading experts in metrology, optical design, optical assembly, plano, asphere, spherical, and freeform polishing as well as thin film coating will be able to maximize product quality and long-term reliability.