Location: Ocean Springs
Optical Bonding in Custom Precision Optics
Bill Thames and Sandesh Borse
PFG Precision Optics
733 Bienville Boulevard, Ocean Springs, MS 39564
Although technique dating back to the late 1750s optical bonding continues to present challenges in custom precision optics manufacturing. The challenges remain despite the advancements made in optical materials, designs, and manufacturing technology. There are various well known bonding techniques available today but almost all of them have certain unknowns and inherent limitations. In depth knowledge and experience is essential to choose the best method and adhesive to achieve the desired bond characteristics, performance, and accuracy for predictable high quality optical assemblies. This article explores various aspects of optical bonding with specific focus on adhesive bonding technique and alternative methods.
Adhesive bonding background
Adhesive bonding is the most popular and commonly used technique for various optical applications. It requires strict attention to the following factors for the highest level of lifetime performance and durability:
Depending on the above factors, optimal bonding techniques can be determined along with the right choice of adhesive for thickness, uniformity, and strength. Active alignment of optical elements while bonding and methodical curing of the adhesive have a critical influence on the precision, accuracy, and durability of the optical assembly and bond itself. The following sections elaborate on optical bonding of achromatic doublets and triplets, beam splitter assemblies, and special filter assemblies. Specific details are covered for the techniques, the typical failure modes that can be encountered if the proper precautions are not taken, and root cause(s) thereof.
Achromatic doublets and triplets
Typically, the achromatic doublets and triplets are comprised of crown and flint glasses with specific geometry to achieve common focus on the wavelengths of interest to minimize effects of chromatic and spherical aberrations. Depending on the material properties, physical dimensions of optical elements, coatings and the desired bond line, the mating surfaces are prefigured to achieve desired tolerances post assembly. All assembly work must be performed under a laminar flow bench equipped with HEPA or similar air filtration to avoid any contamination and of course using proper cleaning techniques.
If a UV curing adhesive is chosen for bonding, then the transmission of the materials and the coatings have to be carefully calculated to determine the best curing method and suitable UV light source settings to uniformly and fully cure the adhesive. Typical wavelengths for curing these adhesives range from 320 to 400 nm with 365nm being the most responsive wavelength to target. It is important to verify the intensity of UV light energy level before every batch being built and cured. Closely following the manufacturer’s recommended exposure for the intensity and cure time yields the optimal result. Active alignment of the doublet or triplet elements using a laser source results in the highest accuracy and precision for beam deviation while centering the achromats.
If the lenses do not transmit enough ultraviolet for a cure in a reasonable time frame, then two part epoxy adhesives should be chosen for bonding, and can be cured at room temperature or using elevated temperatures in an oven. Epoxy adhesives are also superior for bonding when high strength is required and when there is high thermal variation in the ambient environment.
It is important that test lenses are measured for any changes before and after curing for radius and surface figure to ensure the optical performance of the doublet is met. Stress testing, thermal cycling or thermal shock tests should not be performed until the adhesive has absolutely reached a fully cured state. Upon completion of curing, environmental stress testing, or thermal shock testing, the bond line should be carefully examined under an appropriate light source with magnification for defects such as feathering, delamination, or any adhesive or cohesive failure.
It is critical to have a methodical curing strategy, including pre-cure and full cure, giving adequate consideration to material properties, proper exposure to the UV light source, following the adhesive manufacturer’s instructions, and subsequent ESS testing. Bond line failures typically emanate from uneven or incomplete curing. Uneven curing can induce residual stresses in the adhesive and defects can go unnoticed for a prolonged time until stresses are relieved, resulting in delamination or voids. Other causes for the failure include, but are not limited to, the coefficient of thermal expansion difference, cleaning and contamination, mismatched elements, assembly technique, improper adhesive, inadequate testing, and the environment the achromat assembly is subjected to. If the coated surface(s) is (are) bonded, then coating adhesion failure could also result in delamination of the bond. Sometimes surface roughness of the edge or bevel at the bond line can induce failure in the form of small fractures propagating from the edge if the doublet is subjected to a higher temperature ramp rate, especially when the difference in coefficient of thermal expansion of the two elements is high.
Beam splitter assemblies
The majority of the beam splitters are two right angle prisms aligned with parallel polished entrance-exit faces and bonded on the hypotenuse. Some bonded prisms are not true cubes, such as a rhomboid and a right angle prism joined at one end of the rhomboid or a rotated porro prism set. Occasionally, waveplates, polarizers, or even lenses are bonded to the entrance-exit face of the cubes for certain applications. Most cubes are designed with the same glass type for each half and may come from the same block of glass for high precision angular deviation requirements. Generally, cube applications use UV curing adhesives. To determine the optimal curing strategy the transmission of all optical elements - including materials and coatings - need to be carefully considered.
Typically, all optical elements need to be actively aligned using autocollimators to achieve the desired beam deviation accuracy upon complete cure. Depending on the optical elements being bonded, curing times vary for a given UV light source. Any surface form changes after curing have to be taken into account prior to assembly to obtain the desired performance of the beam splitter assemblies. Again, physically checking the energy level of UV light source is highly recommended prior to using it for pre-or-full cure. Special care should be taken for evenly distributed UV light at the pre-cure and full cure stages so that the bond line is stress-free and the polished surfaces are not distorted or deformed. Cube size, angular deviation and transmitted wavefront should be measured on the first few samples prior to full curing. This will validate the process and ensure all parts are still conforming before the final cure is applied. Certain beam splitter assemblies require customized precision shaping after full cure; in which case deformation caused by shaping needs to be studied through trial parts and taken into consideration before starting the production run.
Special Filter Assemblies
Bonding two filters together may seem fairly basic, however if the surface area is relatively large compared to the thickness of the filter (high aspect ratio), then distortion of the polished surfaces becomes a major challenge. These types of assemblies require thorough study of changes in the form of filter glasses being bonded through every step of the process including grinding, polishing, coating, assembly, and shaping, as applicable. How the part is processed also has an influence on the deformation after bonding and full cure.
It is critical to exercise precaution for a very even pre-cure and full cure through energy distribution control of the UV source chamber and associated temperature changes. As an example, a long single UV bulb central to the polished face will embed a distorted trough along the bulb’s axis. Multiple bulbs with a diffuser screen to evenly spread the UV light serves the best for such applications. As mentioned in the prior sections, testing the UV intensity with a meter and following the adhesive manufacturer’s recommended energy levels goes a long way towards proper full curing. If UV curing is not an option due to transmission of the filter glass and/or coatings, then use of epoxy adhesives or adhesive free bonding, using an optical contacting technique is recommended.
Although the failure modes associated with bonding of special filter assemblies are similar to the ones discussed in the above sections (feathering, delamination, etc.) the uneven bond line thickness is more common in such assemblies compared to achromats or beam splitters because the aspect ratio of the filter is significantly higher, therefore requiring special techniques and expertise to achieve an even bond line thickness. Furthermore, this uneven bond thickness leads into uneven curing and subsequent distortion of the assembly.
Precision optical bonding of achromatic doublets, triplets, beam splitters, and special filter assemblies using UV curing adhesive can be done predictably, reliably, and productively. The most critical aspect in using UV cured adhesives when bonding, is achieving an even and complete cure deterministically using an appropriate strategy for a given application. Uneven or incomplete cure can lead to bond failure and can go undetected for a long time. Additionally, change in the form of optical surface has to be monitored carefully throughout the process to meet the specifications.
While UV curing adhesives serve well in the visible region of the spectrum for optical bonding, they have low transmission in the deep UV and far infrared regions of the spectrum. Furthermore, they have limitations on the bond strength and cannot withstand higher thermal shock. In such applications adhesive-free chemical bonding or two part epoxy can be used respectively, to reliably achieve desired bond strength and optical performance.
References and Resources
1. Eugene Hecht, Alfred Zajac, Optics: “Geometrical Optics-Paraxial Theory”, pg.142,186,187;Addison-Wesley publishing company, 1979
2. Summers Optical: “The Bonding of Optical Elements Techniques and Troubleshooting”
3. Norland Products: UV adhesive technical data sheets, and telephonic conversations with Tim Norland.
4. H. Frederick Woods: “Causes for separation in UV Adhesive Bonded Optical Assemblies”, SPIE Vol.1999 Adhesive Engineering (1993)