When procuring optics for use in critical applications such as aerospace, defense, semiconductor, research, and life/health sciences, risk mitigation is vital. Risks are events affecting timeline, cost, and technical performance. This article outlines four key capabilities to look for in optical coating suppliers in order to minimize program risks.
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The fabrication, testing, coating, and assembly of optics by a single manufacturer has a number of advantages. An overall reduction in project completion time is the most obvious advantage.
By coordinating all activities, a single supplier takes full responsibility for the completion of the project. This eliminates the requirement to transport optics to a coating supplier, then back to the originator for final acceptance testing and perhaps a third supplier for assembly, saving both expense and time.
After receiving the optics from Supplier A, ultimately you are responsible for ensuring damage does not happen when sending the optics on to Supplier B. In these cases, it is a prudent choice to insure high quality optics.
Most optics represent a significant financial expenditure, so any delays in reimbursement by the insurance company will only lengthen the project delays, completely derailing a program schedule.
When working with multiple suppliers, establishing who is at fault when an optic fails to meet the specifications is a risk which is not immediately apparent. For instance, a high aspect ratio optic with a demanding surface figure. Did coating stress distort the optic or did the fabricator fail to meet specifications?
If the root cause of the failure is ambiguous or unknown, assigning responsibility and taking corrective action is impossible. The substrate will need grinding, polishing, and recoating services from additional suppliers even if one supplier is willing to take responsibility.
A single source supplier of coated optics and optical assemblies takes full ownership of all manufacturing processes. Image Credit: Zygo Corporation
For over 45 years, ZYGO has been fabricating cutting-edge optics. Before developing their own coating capabilities in the s, they had to learn this painful lesson first hand. It was those situations that inspired them to develop the broad range of coating capabilities they now possess.
ZYGO’s customers know that they can rely on a single supplier who has the technical expertise to choose the most appropriate substrate material and then fabricate, coat, assemble, and carry out metrology on their unique optics.
If applied to a substrate that is not properly cleaned, then a durable, high quality coating design is subject to failure. When looking at potential coating suppliers, it is vital to enquire about their cleaning processes, facilities, and packaging capabilities.
ZYGO’s facilities include Class and Class 100 cleanrooms which are set up to process optics sensitive to Airborne Molecular Contaminants. Cleaning standards are certified by the National Ignition Facility (NIF) and per MIL-STD-. They can ship completed assemblies and optics in vacuum or nitrogen purged packaging.
It is standard practice to model initial designs using a number of software applications when designing sophisticated coatings. Yet, when real-world variables clash with textbook theories, will these coatings perform as expected?
If a high performance coating is required for your application, it is crucial to ask prospective suppliers about their prior experience in applying similar coatings to similar substrates.
For example, when coating very large optics, it is vital to understand the supplier’s level of experience in handling, cleaning, and controlling coating uniformity. Understanding the variables involved is crucial to ensuring the finished product meets or exceeds specifications. Decades of experience enable ZYGO’s design team to use out-of-the-box thinking to solve complex, unique problems.
In addition to designing coatings, ZYGO’s in-house research and development engineers possess the skill set to design and fabricate proprietary fabrication equipment and fixtures for optics. Their production staff has techniques in place to control post-coating figure, surface quality, and uniformity — even with unusually shaped or large optics.
Coating stress simulated in FEA. Image Credit: Zygo Corporation
Coating stress from left used to predict figure change. Image Credit: Zygo Corporation
A custom coating design could be needed in order to meet the durability and environmental specifications for an application. Usually, the end-users of coated optics are not interested in the type of deposition technology employed to apply a coating onto a substrate.
Even so, a strong argument can be made that top tier coating suppliers must have a number of deposition technologies at their disposal to balance the requirements of a coating’s spectral performance and durability with schedule and cost.
ZYGO uses a number of deposition technologies which enable them to optimize their manufacturing processes. They use enabling technologies like resistive sputtering and ion-assisted deposition, in addition to more conventional processes.
It is crucial to use a coating supplier that has in-house testing capabilities for your durability and environmental requirements. Coating designs are tested by utilizing witness samples throughout the development phase and during production for performance verification.
High reflective, antireflective and specialty coatings are applied in 72-inch coating chambers. Image Credit: Zygo Corporation
Outsourcing these tests both increases costs and slows down the coating design and production processes significantly. Typical requests include ISO and military specifications for temperature cycling, salt fog, humidity, adhesion, and abrasion.
ZYGO maintains the capability to carry out all of the testing listed here, so they can develop advanced coatings in an efficient and cost-effective manner.
When certain types of coatings are applied an optic is at risk for deformation. This risk is dependent on the coating materials, deposition technique used, layer count, substrate material, and aspect ratio.
The stress that is applied can distort an otherwise perfect substrate so that it will no longer adhere to transmitted wavefront or flatness requirements. The proprietary coating design is almost never shared with the fabricator when choosing different suppliers for fabricating and coating an optic.
The fabricator is unable to anticipate and compensate for the stress induced on the optic by the coating without that information. Choosing a single supplier, like ZYGO, to fabricate and coat an optic enables the engineering and manufacturing teams to share coating design information freely to create an optic that meets specifications both before and after the coating is applied.
Always be sure to ask suppliers if they have established methods for coating stress compensation and if they will verify specifications both before and after coating is applied.
ZYGO utilizes various techniques to prevent substrate deformation due to tensile or compressive coating stress. Image Credit: Zygo Corporation
It is crucial to understand the metrology capabilities of potential coating suppliers. A coating supplier must possess a spectrophotometer that can measure reflectance and transmission at various angles of incidence and polarizations.
The chosen supplier must have interferometric testing equipment on hand to ensure an optic meets all specifications before and after coating. It is particularly desirable that the interferometer is large enough to test the optic at full aperture. In order to ensure that accurate results are acquired, ask suppliers if their equipment is in a climate-controlled environment.
ZYGO’s metrology equipment and expertise are recognized around the globe. Their equipment includes interferometers up to 32 inches (813 mm) diameter, enabling them to make full aperture measurements on large optics without the requirement to use stitching.
For more information, please visit Custom Thin-film Coatings.
Large aperture interferometry ensures optics meet specifications before and after coating. Image Credit: Zygo Corporation
Program risks do not end with the delivery of a new assembly or optic. A premier coating supplier must be able to support their optics after they leave a facility and face the wear and tear of the real world, whether that is after extended use in an ultra-high power laser system or from the battlefield.
By definition, high-durability coatings cannot simply be chemically stripped and reapplied. Grinding the substrate to remove the damaged coating and repolishing to meet specifications prior to recoating is necessary for rework. Make sure that the chosen coating supplier has the in-house capabilities to carry out all of the required refurbishment steps.
Typically, extensive delays happen when the substrate requires transportation to different facilities to complete the rework process when working with a coating supplier that does not possess ZYGO’s full in-house capability to manufacture substrates. Choosing the right coating supplier ensures support is available over the life of a program.
Refurbishing highly durable coated optics requires in-house grinding and polishing capabilities. Image Credit: Zygo Corporation
Produced from materials originally authored by Michael Albrecht from Zygo Corporation.
This information has been sourced, reviewed and adapted from materials provided by Zygo Corporation.
We continue our discussion through the basic concepts of light and optics by making a detour away from the fundamentals and instead focusing on an often undiscussed aspect of our field – how to request an optic. This article is directed to the procurement specialist, college student and any individual who needs an optical component but has very little experience in contacting an optical supplier for a specific need. Perhaps you were tasked with finding a window or directed to purchase a lens with a 100mm focal length and simply don’t know where to begin. Our goal is to offer a reference to assist you in finding the most appropriate optic for your application in as little as time as possible.
Often times at Esco we receive inquiries that generate more questions than answers. In fact, this is a near daily occurrence. Through repetition, we’ve found that in order to understand a customer’s requirements, we generally ask many of the same questions and, by simply offering a few additional details up front, a solution is almost always readily available. For example, as noted above, it is not uncommon for us to receive a request for a lens with a defined focal length but with no further information. While this is a good starting point, it presents a vast array of choices that may be fulfilled from one of our stock product lines (or perhaps move into the realm of a custom optic) but we simply don’t know since we have no idea how the optic is being used or how it is installed in your system. By providing details that address the optic’s dimensions, spectrum of use, application and environment, as we as, expected performance including the possible addition of thin film coatings, you can avoid the back and forth dialogue that may take days to resolve, delaying your procurement and schedule.
Dimensions
Surprising to us is the number of inquiries we receive that lack information regarding the size of a customer’s required optic. Whether a diameter or rectangle, without the basics of size and tolerances, determining any further details, let alone cost, is impossible. If the optic in questions is a lens, information on any restrictions in center or edge thickness is important to begin the process of finding the appropriate match. If unknown, refer back to how the optic is mounted in the system to better understand the required final configuration.
Material & Spectrum
We tie these two aspects of optics together because not all materials are created equal, both in performance and cost. If you are unfamiliar with the electromagnetic spectrum, we encourage you to read our article Essential Concepts in Light and Optics for a better understanding of how light works. That said, if you’re pressed for time, we can break down the spectrum into three simple optical regions and letting us know your areas of interest will help narrow down your choices of material considerably. Your regions may overlap somewhat, but don’t worry, a lot of materials do as well so we can recommend what may work best for you. What follows are generalities and not considered true scientific definitions – consider them a guideline.
Ultraviolet: 250nm – 400nm
These are wavelength below the visible spectrum where fused silica reigns supreme. If you are working below 250nm, still considered ultraviolet, we will need to know more about your application (see section below) as this region moves into some highly specific optical materials requiring special considerations.
Visible: 400nm – 750nm
There are literally hundreds of materials to choose from in this range, however, there is also a reason why Esco’s stock product line of windows is kept simple with the choice of standard optical glass (BK7), fused silica/quartz, low expansion (Borofloat) and sapphire. These four material variations cover the vast majority of applications and optical environments that may be encountered when working in the visible region. Usually it simply comes down to the consideration of cost in relation to performance and durability.
Near Infrared / Infrared: >750nm
This is the region above the visible spectrum most commonly associated with the detection and imaging of heat signatures. It is important to specify because many of the materials that work in the visible and ultraviolet regions no longer transmit light above a certain defined wavelength. They essentially “shut off” and offer no functionality.
Application & Environment
This section could literally become a full article in its own right, but our goal is to keep things simple. In short, when requesting an optic, offering a few details on how it will be used will provide us almost all the information we need to present a solution. Is it an imaging application and, if so, what are you imaging? Is it a basic viewing window that only requires transparency? Will you be directing a laser through the optic (see section on lasers below)? Having covered hundreds of different applications over Esco’s 60+ years in business, we’ve probably come across a similar, if not exact request for optics used in your setup. Let our expertise be your guide by letting us know the basics of how it’s used.
Environment is also important because not all materials withstand the rigors of exposure to harsh conditions. Are there temperature considerations? Will the optic be exposed to moisture, salt, caustic chemicals or excessive handling? Perhaps the optic is used in environments where extremely low or high pressure is a concern. Again, providing a few details on these type of factors will help us narrow down the available choices of materials and specifications.
Optical Specifications & Performance
Esco’s commercial optics are specified to approximately 5 waves in surface form and for most basic applications, this is sufficient for functionality. For more complex systems, determining functionality without the assistance of an optical designer or engineer is difficult and we always prefer a drawing of the optic to assist in providing a quotation, especially if it is a custom configuration. Unfortunately, optical specifications can be quite complex and if you are uncertain of how to specify your optic we recommend reading our articles on how optical surfaces are characterized (Lenses Part 2 & Plano Optics). If you still are unsure how to proceed, an Esco sales representative can assist in determining how precise your optics may need to be by understanding the details of dimensions, application/environment and the spectrum in which it will be used. Please contact us directly for further assistance and we can take you through the steps of characterizing your optic.
Lasers
Lasers deserve their own consideration because not all optics are made to withstand the level of power density some lasers can generate. You need not understand how lasers work but there is no guarantee of functionality or longevity without the following details:
Type/Wavelength: What kind of laser are you using and what portion of the spectrum does it operate? For example, a common laser type is the Nd/YAG (Neodymium-doped / Yttrium Aluminum Garnet) which produces a beam of light at nm. Understanding the type of laser is very important in determining the appropriate material. Also of interest is the power rating of the laser, specifically whether the laser is pulsed or continuous.
Pulsed Lasers: These laser can generate an incredible amount of power and cycle billions, if not more times per second. The output power is specified in joules or millijoules per square centimeter, written mJ/cm^2 or J/cm^2 and this information is important when requesting an optic for use in the lasers path. The second characteristic is the physical spot size or, more simply, the actual diameter of the laser beam.
Continuous Lasers: Continuous lasers, as the name implies, are laser that have a continuous output of power. Their power rating is specified in watts or milliwatts per square centimeter, written mW/cm^2 or W/cm^2. Please specify the power rating when using a continuous laser.
Coatings
Optical thin film coatings, like application and environment, could warrant its own article, but for the novice, we can breakdown the requirements one simple parameter – spectrum. Coatings are designed to function over specific regions of the electromagnetic spectrum and we need to know where to optimize the design. Coatings used with lasers are also specially designed to withstand the intense power in order not to fail in the system. By specifying application, environment and the possible use of lasers, you’ve already covered a large portion of the information we need to determine what type of coating works best.
Summation
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