Optical Design for Manufacturability

This is my first blog post and to be honest, writing is not my forté. Additionally, that will probably be the last time I'll use the word forté.   

Today, I'm going to describe Optical Design for Manufacturability, but before I can dive into what that means I must profess, I am not an optical engineer. After serving 4 years in the Army with Bravo Troop 9^th Cavalry, as a 19 Delta Cav Scout, I returned home to Rochester, NY to start my career
and attend college. I was able to get started in the industry with Sydor Optics and attend night school at Monroe Community College in the evenings. My introduction to optical manufacturing began on double-sided machines, cutting machines, and creating precision bevels. Just like being in the Army, when you immerse yourself with a specific focus, you can learn a lot and very quickly. After 5 years in the shop, I transitioned from shop floor into the sales department. My knowledge of hands-on optical fabrication now translated into putting those skills onto paper and working with clients. Knowing how to make a piece of machined glass and how it will perform is essential in working with the end-user. Although, I've been with Esco Optics for only just over a year, I have found that all of the engineers and optical designers that I interface with daily have a combined their knowledge that dwarfs my 20 years of experience. 

Ok, with the intro over I'm ready to get into “Optical Design for Manufacturability” and WTF does that mean? Well, in short, a manufacturer's role is to fabricate something based on drawing or print from a customer. With precision optics, there are a ton of variables and final specifications that greatly impact manufacturing costs. In my experience, material selection, finished surface, and whether the component requires a coating are the major factors in determining fabrication costs.  

Let's talk about  material  selection for a moment. Optical glass, crystals, and specialty glasses all perform differently depending on the application. Many customers aren't always aware of raw material costs and the potentially drastic difference in pricing. For example, a customer might be purchasing 100 machined windows 5" x 5" out of UV fused silica for say $275ea. That same customer might contact me on another project for 10" square windows, but the raw material might only come from the supplier at the largest an 8" block. I've had to go to my glass suppliers for a custom melt to get a blank large enough to then begin fabrication which can add a great amount of time and money to the project. Not all glass is created equal and helping customers choose the right material is an important part of the process.  

Surface specifications  are extremely important for an optic. Whether the final result is a lens for a high-powered laser or a window for deep-space operation applications, one thing that I've found is that there’s always conversations around the difference between reflective versus transmissive specifications for Plano-optics, and how often they are confused. If there is a need for a reflective component, which would be a mirror or something you need light to reflect off of, the manufacturing term for this application is flatness or “Reflected Wavefront Error”. That specification determines how well the light, image or laser beam will bounce off the surface, and how much it distorted upon return, defined by how “flat” the surface is to be manufactured to.

Transmitted Wavefront Error (TWE), which is also known as Transmitted Wavefront Distortion (TWD), is when an image or laser-beam goes through the optical window, and how well it comes out the other side without distortion. The reason these two distinctions are so important is due to the manufacturing methods and the impact on cost. When we’re talking 1-2 waves flat, no big deal either way. But when you get into fractional wavelengths, ¼ wave to 1/20th wave, it makes a big difference.  

• Double sided  polishing machines polish both surfaces at the same time, making the surfaces very parallel, hence you get a high precision Transmitted Wavefront, just by the nature of the process and the machine. You also get a high degree of flatness, but not a 1/20th wave.  

• Single sided  “Pitch Polishing” is a slower, costlier polishing process as you remove very small amounts of material, hence longer, more expensive processing.  This is where you go after high precision “Reflected Wavefront” (1/4 – 1/20th wave flatness) which is normally needed for high precision laser mirrors, reference/test flats, or extremely high precision windows that need 1/10^th -1/20th wave TWE. 

Next up is  optical coatings. I tend to think of coatings like the secret sauce. It pulls everything together in a recipe. You have your manufactured optic made from the finest material, polished to precision surface specifications, and now you need a coating to ensure it performs perfectly. The coating is a complex piece of the puzzle and once again can greatly impact the price. For example, a single layer AR coating such as Magnesium Flouride (MgF2) is a relatively low-cost coating because the coating process is priced by how many layers are deposited, which is relative to how long it is in the coating chamber. Coatings are defined spectrally by each layer and the tighter the performance of the coating will influence costs.  

Overall, what I have found helpful for customers to understand the impacts of all-optical manufacturing is to allow your manufacturer into the process early. Esco has been around for 65 years and we have a wealth of knowledge to help customers find the optimal solution. I have one customer in particular that brought us into the conversation early in their R & D process. Their comments were, "you've created these parts for us in the past and for our next iteration of this design we want the performance to do 2X better." We took that information and looked at how we produced the components previously and came back with two options. The material they had specified was a premium glass and not necessary for the UV spectrum, so we proposed an alternate glass which in turn was a lower-cost material. Additionally, when manufacturing for production, scaling efficiency is a key component. For this particular customer, they would order sporadically throughout the year. Our conversation changed to be more of a partner, where we approached them and said, "if we were able to forecast or commit to a larger quantity for a year then the price drops significantly." This was due to a few things as we could prepare optical blanks first for the entirety of the job. That means we don't have to process rough optical stock each time an order would come in. Additionally, we worked with them to optimize what quantities were needed based on what capacity could fit into a coating chamber. That minor change increased our coating efficiencies allowing the max output each time we produced finished optics.  

Day-in and day-out, Esco is here to help our customers navigate the manufacturing process. Paperwork, material certifications, ultra-sonic cleaning and thorough metrology are some of the additional ways we set ourselves apart from our competition. 

If you would like to discuss this topic further I'd be happy to start a conversation. Please email me at Mike@EscoOptics.com. To read Optical Coatings and Cost from Mike click here.