Summary: This article explains in detail
- what optical adhesives are used for,
- which curing processes are available and what are their implications,
- what are fundamental qualities and how they are relevant for applications,
- which types of optical adhesive exist,
- and what are possibly alternatives to using adhesives.
Optical adhesives, also referred to as optical cements or optical glues, are specialty adhesives developed for use in optical systems. In a narrower sense, optical adhesives are those which transmit light in an application – a classical example would be adhesives used for bonding prism parts together. In numerous instances, such adhesives play a pivotal role in bonding optical components together, and are developed to achieve minimal impact on the propagation and characteristics of light passing through them. In a wider sense, optical adhesives are also used at locations where they are not directly exposed to light, but may fulfill specific requirements of optical systems. There are also cases where strong light absorption by an adhesive layer is desired. This article, however, focuses on adhesives in the explained narrower sense, with good transparency.
Optical adhesives have a broad range of applications, often as an integral part of the device's functionality. Some examples of typical applications are given in the following:
- Fabrication of optical components: Adhesives are instrumental in bonding certain types of optical elements, such as prisms and lenses (e.g. achromat doublets).
- Mounting optical components: This includes the assembly of intricate systems like microscopes, telescopes, camera objectives, and even complex optoelectronic devices. For example, compact and rugged laser setups may be made by gluing a laser crystal, some laser resonator mirrors and other components to a baseplate.
- Fiber optics: Particularly in optical fiber communications, optical adhesives serve in the assembly and packaging of components. They may be used to bond fibers into ferrules for use in fiber connectors, to package photonic integrated circuits, or to secure mechanical components.
- Display technology: Optical adhesives are used in various display technologies. They may bond layers in liquid crystal displays (LCDs), OLED displays, or touchscreens, often serving to enhance image quality as well as durability by attachment of protecting optical windows.
- Medical devices: Optical adhesives are also used in the manufacture of medical devices such as endoscopes (e.g. containing fiber bundles) and medical imaging equipment.
Curing of Adhesives
A fundamental principle of adhesives is that they are applied in a state of low or moderate viscosity, and are then cured, i.e., transformed into a rigid state where they provide a stable mechanical connection of parts. This is generally some mechanism of polymerization, i.e., chemical linking together of long molecules. For optical adhesives, curing is usually achieved with one of the following techniques:
- UV curing: One can use irradiation with ultraviolet light which acts on some kind of photoinitiator in the adhesive formulation. UV curing might be accomplished within only a couple of seconds and typically is done at room temperature. Special UV curing lamps are available for that purpose, but special precautions may need to be applied, e.g. to achieve sufficiently uniform irradiation.
- Using a hardener: The other option is using a spontaneously running chemical reaction which is initiated by adding some hardener to the main component just before applying it. That process may also be accelerated by applying heat, but still usually takes minutes or longer. Accurate mixing of such two-part formulations (with proper mix ratio and high homogeneity) may be vital.
Depending on the application, different curing mechanisms may have disadvantages, such as being inconvenient to apply reliably, or taking too much time. For fabricating large parts, it can be relevant that heat curing tends to be substantially more energy-intensive than UV curing.
Fundamental Qualities of Optical Adhesives
The efficacy of an optical adhesive (in the narrower sense explained above) depends on its ability to meet certain key criteria:
- Transparency: In most applications, optical adhesives are required to be highly transparent within specific wavelength ranges, which may span from ultraviolet (UV) to near-infrared (NIR) wavelengths. Inadequate transparency could not only lead to the loss of light energy, but possibly also to thermal effects (in applications with high light intensities), eventually also to degradation of the adhesive e.g. by overheating.
Refractive index: Optical adhesives often need to have a refractive index closely matched to that of the bonded optical elements (assuming that one bonds two pieces with the same refractive index). There are essentially two reasons for that:
- the minimization of parasitic reflections
- the minimization of wavefront distortions resulting from irregularities of the interface
There is a range of products with different refractive indices for diverse requirements, but most common are refractive indices close to those of typical optical materials, i.e., in the range from 1.4 to 1.7 (and often around 1.5). (See also the article on index matching fluids.)
- Homogeneity: While some change in optical phase of transmitted light may often be acceptable, non-uniform phase changes lead to wavefront distortions, which can – depending on the application – have various detrimental effects. For achieving sufficiently high homogeneity, one requires both a uniform thickness of an adhesive layer and a uniform refractive index. Further, as mentioned above, it helps if the refractive index difference between adhesive and bonded materials is small because that reduces the importance of a uniform layer thickness.
- Initial viscosity: It may be vital to initially have a rather low viscosity, e.g. allowing the adhesive to fully fill the void between two optical parts, i.e., not leaving air bubbles which would greatly degrade the optical properties due to the high refractive index contrast. In other cases, a higher viscosity is preferred, such that there is some initial mechanical stability even before curing, but that usually implies a poorer void filling capability.
- Curing properties: It is crucial that the curing process does neither adversely affect the optical properties of the adhesive nor generate disturbing side effects:
- The material should stay optically homogeneous during the whole curing process. This may be more challenging for adhesives requiring a hardener because homogeneity of the prepared mixture may be difficult to achieve reliably – or may at least require a well-defined procedure and possibly some auxiliary means. However, UV curing may also not be ideal in cases where the refractive index somewhat changes with irradiation, and the applied UV dose may not be fully uniform.
- It is desirable to have low shrinkage during the curing, as this could induce mechanical stress or misalignment in the optical components. While some adhesives exhibit shrinkage of several percent, others reach the order of only 0.1%.
- Outgasing of organic substances during curing or at later stages may be quite problematic, particularly in applications involving ultraviolet light because that light may chemically decompose such substances, leading to the deposition of black soot on optical components.
- Of course, UV curing can work only if the neighbored optical parts are sufficiently transparent to UV light and are not degraded by it.
- Mechanical strength: Depending on the application, a reliably high mechanical strength of the created bonds may be required. In certain cases, however, it is better to keep the option to remove bonds with a defined procedure, possibly applying heat or a solvent. The obtained mechanical strength can depend on which materials are bonded and what surface preparation procedures are applied. Note also that issues with mechanical stability may not only arise in the form of broken bonds, but also as creeping under sustained loads.
- Long-term stability: It is essential that optical adhesives maintain their physical and optical properties across a range of operational temperatures and resist mechanical stresses without delaminating or cracking. Resistance to certain chemicals or antimicrobial properties are important in some cases. Also, optical adhesives should not gradually lose their optical transparency. (Some adhesives tend to develop increasing absorption in the blue spectral region over time, thus developing a yellowish appearance, based on chemical changes induced by absorbed short-wavelength light.)
There may be further relevant properties, e.g. the laser-induced damage threshold, the cost of required substances and procedures, and safety risks created by toxic substances. For example, some adhesives are detrimental for skin, and some of them outgas poisonous substances; such properties can also affect regulatory compliance with REACH and RoHS, for example. Chemical compatibility with the bonded materials can also be an issue; some adhesives are corrosive. In some cases, thermal and electrical conductivity may also be relevant.
Varieties and Composition
Optical adhesives can be based on various chemistries, including epoxies, acrylates, silicones, and urethanes. Each has distinct characteristics making them suitable for different applications:
- Epoxy resins are known for their strong adhesion, excellent chemical and heat resistance, and impressive dimensional stability, and are thus widely used in optical applications. However, they often require heat curing (possibly making the process inconvenient, particularly time-consuming), exhibit substantial shrinkage, and may turn yellow over time, especially under UV exposure. Also, they may cause some outgasing, particularly before being fully cured.
- Acrylate adhesives are usually UV-curable, making them well-suited for rapid production processes. They can offer excellent transparency and good adhesion to a wide range of substrates.
- Silicones (silicone-based optical adhesives) are noted for their high flexibility, low shrinkage upon curing, and high resistance to temperature extremes and UV radiation. Their flexibility can make them useful in applications involving thermal cycling or where stress relief is needed. However, their mechanical strength and chemical resistance are limited.
- Urethanes offer a good balance of flexibility and adhesion strength, are suitable for a wide variety of plastics, can (after curing) even be used underwater, and can have good optical properties. However, their UV resistance may be lower than silicones or epoxies.
Due to the possibly complex requirements as explained above, the selection of a well-suited optical adhesive can be a non-trivial task and may have wide-reaching consequences for a production process. In some cases, even a custom adhesive formulation may need to be developed.
Alternatives to Using Adhesives
In some cases, it is preferable to avoid the use of adhesives due to their intrinsic disadvantages, e.g. concerning resistance to high |optical intensities (→ laser-induced damage threshold) and long-term stability. Some options are briefly explained in the following:
- Optical contact bonding is a method of obtaining high-quality bonds without using any adhesives. However, this requires particularly accurate surface preparation, the details of which strongly depend on the used optical materials.
- Mechanical contacts may be obtained by using additional mechanical means, such as clamps. This may also give one the valuable option of easily disassembling parts later on, e.g. during optics repair. However, that approach may be substantially more costly in mass fabrication and may increase the weight while decreasing mechanical stability.
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