When we talk about risk mitigation related to optics, a “risk” is anything in the product development process that can have an impact on cost, schedule, and, of course, technical performance. One such risk is coating and a good place to begin your evaluation of the risk on performance is by vetting the coating supplier. Here are some useful questions to ask. 


1. What is your approach to vertical integration?
When considering coating, vertical integration negates the time and expense of transporting optics to a separate coating supplier and back again. It also counteracts the risk of trying to apportion blame if an optic fails. Was it poorly fabricated, or did it fail because of distortion due to coating stress? You need to know you can rely on a single supplier that possesses the technical expertise to select the most appropriate substrate material. The supplier should be able to then fabricate, coat, assemble, and perform testing and verification via sophisticated metrology tools on their unique optics. Facilities should include Class 1000 and Class 100 cleanrooms to ensure a clean substrate, and a supplier’s packaging capabilities should be suitable for protection of delicate coated optics. 

2. How do you handle environmental and durability requirements?
Ideally your coating suppliers should have multiple deposition technologies available to balance the needs of a coating’s spectral performance and durability with cost and schedule. This allows the manufacturing processes to be optimized. Suppliers should also offer enabling technologies such as ion-assisted deposition and resistive sputtering. In-house testing capabilities for environmental and durability requirements are also important, allowing for the development of advanced coatings in an efficient and cost-effective manner. 

3. What are your processes for managing coating stress?
An optic is at risk for deformation when certain types of coatings are applied. Selecting a single supplier to fabricate and coat an optic allows the engineering and manufacturing teams to share coating design information freely. This produces an optic that meets specifications both before and after coating is applied. Does your chosen supplier have established techniques for coating stress compensation and will they verify specifications both before and after coating is applied? 

4. Do you have testing equipment on site? If so, what can it handle?
Coating suppliers must have spectrophotometers available to measure reflectance and transmission at various angles of incidence and polarizations. In order to ensure an optic meets all specifications before and after coating, suppliers must also have interferometric testing equipment on hand, preferably large enough to test the optic at full aperture. Customers should ask suppliers if their equipment is in a climate- controlled environment, and whether they have the capability to design appropriate holding fixtures and provide an error budget based on specific testing requirements.

5. What’s your approach to refurbishment? 
High-durability coatings cannot simply be chemically stripped and reapplied. Ensuring that your chosen coating supplier has the in-house capabilities to perform all of the required refurbishment steps is extremely important. Without this, extensive delays can occur when the substrate requires transportation to different facilities to complete the rework process. 

Good questions drive good practices
While evaluating coating suppliers don't forget to ask questions to engineers, end users, and stakeholders. All should have good answers if they are playing their roles properly. Your supplier’s team need to be able to think outside the box and draw on real-world know-how. Working with a supplier with in-house coating capabilities and in-depth coating experience, such as Zygo is extremely important to optimize your outcomes. Selecting a single supplier to fabricate and coat an optic allows the engineering and manufacturing teams to share coating design information freely—producing an optic that meets specifications both before and after coating is applied.