optical adhesives (original) (raw)

Definition: specialty adhesives for use in optical systems, usually with high transparency for light

Alternative terms: optical cements, optical glues

Categories: general optics, optical materials, methods

• optical materials
  • dielectric materials
  • optical crystals
  • semiconductors
  • scintillator materials
  • mirror substrates
  • ceramics
  • metals
  • fused silica
  • nonlinear crystal materials
  • photochromic materials
  • index-matching fluids
  • optical adhesives
  • photonic metamaterials
  • (more topics)

Related: optical contact bondingindex-matching fluids

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DOI: 10.61835/4xw Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

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Contents

The Function of Optical Adhesives

Typical Applications

Curing of Adhesives

Fundamental Qualities of Optical Adhesives

Varieties and Composition

Alternatives to Using Adhesives

Frequently Asked Questions

Summary: This article provides a detailed overview of optical adhesives, also known as optical cements or glues. It explains their function in bonding optical components and lists typical applications in optics fabrication, fiber optics, and display technology. Key properties like transparency, refractive index matching, low shrinkage, and long-term stability are discussed in detail, along with different curing mechanisms such as UV curing and chemical hardening.

The article also compares common types of adhesives, including epoxies, acrylates, and silicones, and briefly touches upon alternatives like optical contact bonding.

The Function of Optical 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 high transparency.

Typical Applications

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%.
  • Outgassing 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 neighboring 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 to 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 outgassing, 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.

Frequently Asked Questions

What is an optical adhesive?

An optical adhesive, also called optical cement or glue, is a special adhesive for use in optical systems. It is typically designed to be highly transparent and have a specific refractive index to minimize its impact on light propagation when bonding optical components.

What are typical applications of optical adhesives?

They are used for fabricating optical components like achromat doublets, mounting optics in instruments such as microscopes and lasers, assembling fiber optic components, and bonding layers in display technologies like LCDs and touchscreens.

How are optical adhesives cured?

Curing, the process of hardening the adhesive, is typically achieved either by exposure to ultraviolet light (UV curing) or by mixing the adhesive with a hardener to initiate a chemical reaction. Heat may be used to accelerate the latter process.

Why is the refractive index of an optical adhesive so important?

The refractive index should closely match that of the bonded optical elements to minimize unwanted surface parasitic reflections and to prevent distortions of the optical wavefronts passing through the adhesive layer.

What are the consequences of adhesive shrinkage during curing?

Shrinkage during curing can induce mechanical stress in the bonded optical components, potentially leading to misalignment, performance degradation, or even damage. Adhesives with low shrinkage (e.g., 0.1%) are therefore highly desirable.

What are common types of optical adhesives?

Common types include epoxies (known for strong adhesion and heat resistance), acrylates (often UV-curable for rapid processing), silicones (noted for flexibility and resistance to temperature extremes), and urethanes (offering a balance of flexibility and strength).

Are there alternatives to using optical adhesives for bonding optics?

Yes, alternatives include optical contact bonding, which creates a direct bond without adhesive but requires highly precise surfaces, and mechanical mounting using clamps or holders, which allows for disassembly but can be bulkier and less stable.

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