Optics are applied in many diverse fields of use every day all over the planet and even in space. The fundamental purpose of optics is to control light in a manner to make it useful. Optical coatings modify the reflectance, transmittance, and absorptive properties of optics substrates to increase the efficiency and functionality of an optic.
From a manufacturer's perspective, there are several pertinent pieces of information that need to be relayed in the specification of optical coatings (essential and supplemental information). Missing information can lead to longer quoting times and may cause misconceptions. Essential information is the minimum information needed to design a coating to meet the spectral requirements. Supplemental information ensures the coating will be designed with your exact use in mind and enhance system performance.
- Glass Type
- Wavelength of Range of Wavelengths of Interest
- Transmission or Reflection Requirements
- Angle of Incidence (AOI) or AOI Range
- Laser Damage Requirements
- Environmental Durability Requirements
- Witness Piece Requirements
- Special Documentation, Marking, and Packaging Requirements
Glass Type (3:25 - 4:55)
Including the required glass type and its equivalents is necessary as coatings are designed specifically to the refractive index. This means that a set of multiple optics with a wide range of glass types may require multiple individual coatings even though the specifications may be the same. Glasses with close index can usually be coated together with a common design.
Thermal expansion coefficients are important for processing considerations. Many coating processes are high temperature processes; therefore, great care must be taken during heating and cooling of high expansion glasses to prevent thermal shock that can result in fracturing. Also, stresses can be induced which can lead to adhesion issues or form errors.
Chemical properties are important for processing considerations. A large number of optical glasses have poor chemical properties relating to humidity, water, acid, or alkali resistance. These glasses have a high potential for staining. This in turn also demands great care and special consideration in processing. How to clean surfaces before coating being the primary example. These poor chemical properties will limit the use of ultrasonic cleaning, which puts a larger demand for manual cleaning.
Wavelength or Range of Wavelengths of Interest (4:55 - 5:50)
A single wavelength or small wavelength ranges are easier than broad ranges. It is important to only specify what is necessary for your system performance. As range increases, the achievable performance diminishes and coating designs and processes become longer. Not all available coating materials transmit the same for all wavelengths.
Transmission or Reflection Requirements (5:50 - 7:18)
This is obvious but very important! There is a significant difference between 99% reflection and 99% transmission. For optics coated on one side only such as achromat elements, high transmission values cannot be measured due to uncoated surface reflection losses. These should be specified in reflection only. Very high or very low reflection values can be challenging to measure accurately. Specialized measurements such as laser ring down (LRD) may be required for demanding applications, which can add cost. Reflection cannot be measured at 0 deg due to the physical limitations of spectrophotometers.
AOI or Range of AOI (7:18 - 8:30)
As AOI is increased, optical coating performance shifts toward shorter wavelengths. Broader bandwidth designs must be used to aid in covering a large range of AOI. As previously mentioned, as range increases the achievable performance diminishes and coating designs and processes become longer. Polarization is especially important as AOI increases. Reflection of S&P polarizations separate as AOI is increased. Special design/coating material considerations can be used to combat these challenges, but there are limitations.
Clear Apertures (8:30 - 8:55)
Clear apertures that are full or near full do not allow for standard fixturing of optics for coating. Generally, 1mm per side is easily accommodated. Tighter apertures and optic configurations may need special considerations.
Laser Damage Requirements (8:55 - 9:52)
Design, material, and processing considerations are important for Laser Damage. It is important to communicate if laser damage testing is a requirement. This is a common requirement, but it comes with an additional charge. There are two common types of laser damage testing: Laser Damage Certification and Laser Damage Threshold. Laser Damage Certification qualifies a coating and tests the laser damage to a specified power level. Laser Damage Threshold quantifies the coating where power levels are increased incrementally until the damage is observed to determine the “threshold level”. As wavelength decreases, laser damage values decrease relative to material absorption and wavelength energy. To learn more about Laser Damage Testing, check out our blog on the Laser Induced Damage Threshold.
Environmental Durability Requirements (9:52 - 11:04)
Environmental durability requirements are typically specified per standard Mil specs or ISO standards. Most common are MIL-PRF-13830B for single layer MgF2 films, MIL-C-48497 or ISO 9211-3 for multilayer coatings, and Mil-M-13508C for metalized mirrors. Special or unusual environmental applications, such as space flight or underwater activities should be made known. It should also be communicated if actual testing is required for each batch. LaCroix has developed an internal testing procedure that covers all of our coatings. Testing is performed regularly to ensure our coating systems and processes are robust. Every batch is subjected to abrasion and adhesion testing as standard operating procedures. Once again, design, material, and process considerations can influence the price.
Witness Sample Requirements (11:04 - 11:46)
Witness samples from coating lots can be provided upon request at an additional cost. LaCroix maintains an extensive stock of witness samples to cover an entire range of wavelengths. Actual glass samples can also be provided upon request.
Special Documentation, Marking, and Packaging Requirements (11:46 - 12:50)
It is important to communicate if you have any special documentation, marking, or packaging requirements when sending your RFQ.
In conclusion, it is important to communicate the above essential and supplemental information with your optics manufacturer to ensure you receive the desired optical coating that enhances the performance for your optical system.