What Is Optical Bonding? | IRD Glass

Optical bonding is a process used when two or more optic components must be joined or fused. The ability to perform optical bonding is a must for any modern company in the optics industry. Historically, optical bonding has been used in several breakthrough designs, including apochromatic, achromatic, and aplanatic lenses. Optical bonding uses optical-grade adhesive to join the components, so compliance with FDA regulations is imperative for optics vendors. There are several types of optical bonding to be familiar with.

What Are the Different Types of Optical Bonding?

The types of optical bonding are mainly differentiated by the adhesive used to complete the process. The most common types of optical bonding are silicone cured with UV, epoxy, and polyurethane. However, polyurethane has somewhat fallen out of more common usage because the adhesive turns yellowish over time when exposed to light. 

• Silicone bonding cured with UV. Silicone has more flexibility during the actual bonding process, making it a widely used choice for many applications. Because of its flexibility, you can also use it to rework damaged existing bonds. Silicone has several features that make it a popular choice: 
  • Repels water from watertight seals 
  • Low conductivity
  • Low chemical reactivity
  • Thermal resistance 

However, a drawback with silicone bonding is that debris can often form around bond edges if they are exposed during handling. The edges of the optical components can be covered to thwart this. 

• Epoxy. Epoxy is another adhesive used in the optical bonding process. Epoxy forms a tighter bond than silicone, which may be more suitable for certain applications. Because of its strength, epoxy does not leave room for debris to form on the bond’s edges. However, epoxy cannot be reworked like silicone can, so it depends on the particular application whether this is a good fit or not. 
• Polyurethane. Polyurethane is the less commonly used of the three adhesives because its bonds can yellow over time. However, it is still used in polar and avionic applications.

Additionally, new innovations in optical bonding methods include diffusion bonding, thermal annealing, and frit bonding.

Other Optical Bonding Methods

There are other optical bonding methods that differ from the list above. They are as follows: 

Optical Contact Bonding 

Optical contact bonding is a method performed without any adhesive. In optical bonding, the deviation between the components must be smaller than one nanometer. Provided this is met, the components can be joined together using intermolecular and physical adhesion. In addition, components used for optical contact bonding must have a high degree of surface quality, using the polishing and surface quality standards found here. Advantages of optical contact bonding include that it reduces thickness, allows no air, dust, or steam, and reduces reflections. A disadvantage is that if an optical contact bonding component is broken, it is very expensive to replace. Optical contact bonding is one of our capabilities, as we can produce the optical components necessary for this level of high surface quality. 

Anodic Bonding

Anodic bonding is an optical contact bonding that uses hermetic sealing between glass and silicone but without a middle layer. The process of anodic bonding requires the application of heat and an electrical field to the materials. Advantages of anodic bonding include the strength of the bond, hermetic seal, and low pressure. Drawbacks include the temperatures and high voltage requirements in order to perform anodic bonding.