In optical communications, at a fiber optic module foundry, a fiber optic cable—thinner than a human hair—must be precisely aligned with a laser chip the size of a grain of rice, with an allowable error of no more than 0.5 micrometers—roughly the width of a single pollen grain.
What can be used to securely fasten them so that they do not shift?
The answer is: special-purpose resin.
This material, which is rarely mentioned in everyday conversation, actually accounts for half of the entire manufacturing landscape of optical communication devices. From the thin coating layer within the optical cables beneath your feet to the precision optical modules in 5G base stations, it is indispensable in every application.
chapter one
What specific roles do specialty resins play in optical communications?
Specialized resins are not a single type of material, but rather a class of chemical systems specifically engineered for optical communications. They address three core challenges: adhesion, fixation, and protection—while operating beneath optical signals without introducing any interference.
🏗 The “outer sheath” of the optical fiber cable
The optical cables you’ve seen actually don’t contain exposed glass fibers; instead, they are encased in multiple layers of “sheaths.”
At the innermost layer is a glass fiber core, surrounded by a soft inner coating (buffer), followed by a rigid outer coating, and possibly an additional colored coating and another buffer layer on the exterior. Together, these four to five layers are composed entirely of specialized resins.
Why is such precision required? The glass core of the fiber has a diameter of only 9 microns (single-mode fiber), making it as brittle as glass fibers. Without a coating, it would break at the slightest bend. Beyond providing mechanical protection, the coating serves a critical function: reducing micro-bend loss. When optical cables are laid underground, they are subject to temperature and pressure variations; even a slight bend in the fiber can cause the optical signal to leak out, leading to attenuation. The coating material, through its elastic support, minimizes this micro-bend effect to the lowest possible level.
💡 What is the amount of fiber coating resin used? Globally, over 200 million core-kilometers of optical fiber are deployed each year, and nearly every single core-kilometer relies on resin coating. In short: resin coating is a standard component of the foundational infrastructure of global optical fiber networks.
The evolutionary path of coating resins has been well-defined: from nylon-based systems to UV-curable acrylate-based systems. The advantage of UV curing is its “instant-drying” property, which has enabled production line speeds to increase from the initial few dozen meters per minute to over 2,000 meters per minute today, significantly boosting production capacity.
🔌 The precision “glue” in optical modules
If fiber optic coatings are mass-produced, standardized products, then the resin adhesive used in optical modules represents a domain of precision customization.
Inside an optical module, the laser chip, lens array, and fiber array are secured using resin. The most critical component in this process is the coupling adhesive—ensuring that optical signals travel from the chip, pass through the lens, and couple into the fiber, with no “light leakage” occurring at any stage of the process.
How demanding are the requirements for coupling adhesive?
(1) Transmittance: When light at wavelengths of 1310 nm and 1550 nm passes through the optical adhesive layer, the transmittance must exceed 99%/mm. Any point absorption constitutes direct loss. For a transmission rate of 112 Gbps, an additional loss of 0.1 dB may represent the critical threshold between acceptable bit error rates and unacceptable performance.
(2) Alignment precision: The spacing and angular alignment between the lens array and the fiber array must be maintained by leveraging the rheological properties of the adhesive. After the adhesive is injected but before it fully cures, the components undergo slight drift. If the adhesive is too fluid, it may displace the components; if it is too viscous, the dispensing may become uneven.
(3) Curing shrinkage rate: The resin undergoes shrinkage during the curing process; if this shrinkage is uneven, the component may be “distorted.” For high-end coupling adhesives, the shrinkage rate must be <3%, and the shrinkage direction must be highly controllable.
⚡ The “waveguide core” in a PLC optical splitter
There is another type of resin whose function is not “adhesion,” but rather to directly participate in optical transmission—the polymer waveguide core layer of a PLC optical splitter.
PLC stands for Planar Lightwave Circuit. Its fundamental principle involves depositing a polymer film onto a silicon substrate, etching optical paths into the film, and enabling light to propagate through this film. The material used for this film is a specialized polymer resin.
What is the purpose of a PLC splitter? In Fiber-to-the-Home (FTTH) networks, this device enables the splitting of 32 or 64 user access channels from the backbone fiber optic cable. In a residential area, dozens or even hundreds of PLC devices may be operating simultaneously—yet you may not even notice their presence; the light within the fiber optic waveguide quietly splits into multiple paths, supporting your home broadband connection.
▲ Figure Caption | Optical fiber cross-sectional structure: resin coating provides layered protection for the glass core
chapter two
Where is the resin located? See a clear diagram below.
Within optical modules, every square inch of space is invaluable, and specialized resins can be found far more extensively than one might imagine.
🏗 Optical fiber coating
The outermost protective sheath of the optical fiber, present on every kilometer of optical cable.
🔌 Coupling adhesive fixation
Precision bonding of lens arrays and fiber arrays
⚡ PLC waveguide core
Optical transmission medium for FTTH splitters
🔬 Coloration and Curing
Rapid UV-curable layer for fiber identification coloration
▲ Figure Caption | Cross-section of an optical module: optical components secured with precision resin adhesive
chapter three
Who is supplying the goods? An overview of the global landscape
Specialized resins for optical communications are highly concentrated among a handful of companies. This mirrors the landscape of the entire optical communication device supply chain—appearing fragmented, yet in reality relying on only a few core technological assets.
| manufacturer | core product | Application Phase | area |
| DSM | Fiber Optic Coating Resin Series | Optical fiber coating | ����Holland |
| Henkel | Lucifer Series Coupling Adhesive | Optical module coupling and fixation | ����Germany |
| DELO | UV adhesive for optical components | Precision bonding of optical modules | ����Germany |
| Mitsui Chemicals | Polymer waveguide material | PLC waveguide core layer | ����Japan |
| JOME | UV-curable adhesive | Optical module coupling | ����China |
| JOME | Optical fiber coating material | Optical fiber coating | ����China |
💡 A noteworthy trend: In the fiber optic coating sector, DSM holds a dominant position, whereas the optical module coupling adhesive market remains relatively fragmented, with Henkel and DELO as the two key players. In recent years, Chinese manufacturers have made significant progress in the coupling adhesive field, and companies such as Shanghai Feikai have already been able to replace imported products for certain specifications. PLC waveguide core materials, however, remain dominated by Japanese companies, with Mitsui Chemicals holding an unequivocal presence among global PLC device manufacturers.
Several noteworthy developments in recent times
New requirements in the 800G/1.6T era: As optical modules evolve from 400G to 800G, optical power density increases significantly, and the challenge facing resins has shifted from “adequate light transmission” to “resistance to aging, yellowing, and degradation under high-power optical illumination.” Several Japanese material companies have already been developing high-stability coupling gels for CPO applications, aiming to maintain additional loss below 0.2 dB over a service life exceeding 10 years.
Widespread adoption of UV-LED curing: Traditional UV curing systems utilize mercury lamps, which offer rapid curing speeds but suffer from high energy consumption, significant heat generation, and the presence of mercury. UV-LED curing light sources are now gradually gaining popularity, reducing energy consumption by over 70%, although the wavelength spectrum of LED light sources is concentrated at fixed peaks of 365 nm, 385 nm, and 405 nm, which imposes new formulation requirements on resin photoinitiator systems. Currently, both Henkel and DELO have launched specialized UV-LED models.
chapter four
How far has domestic production progressed?
Within the optical communication industry chain, the domestic production of specialty resins has only begun to receive genuine attention in the past two years. Compared to the design and manufacturing of optical modules and optical chips, the domestic production of materials has started later; however, the urgency of this endeavor is by no means lower.
China’s landscape in optical communication materials exhibits two distinctive characteristics:
First, domestic production has already achieved breakthroughs in mid-to-low-end segments, while the high-end segment remains a target for catch-up efforts. Regarding optical fiber coating resins, domestic enterprises such as Realma Chemical and Guda Optoelectronics are now capable of supplying products whose quality approaches international standards. In the field of UV-curable adhesives, companies including Shanghai Feikai, Qiangli New Materials, and Jiuri New Materials have already succeeded in replacing imported products for certain specifications. These breakthroughs in these sectors are primarily driven by cost-effectiveness and the advantages of localized service capabilities.
Secondly, there remain key technical barriers for high-end PLC waveguide core materials and precision coupling adhesives. These products not only require expertise in synthetic chemistry but also involve precise formulation, stringent control of production environments (including cleanliness and dryness), and the accumulation of long-term reliability validation data.
Where exactly do the barriers lie?
The barriers to entering the specialty resin market may appear to stem from chemical formulations at first glance, but in reality, they arise from process expertise and consistent quality control.
For the same type of resin, preparing samples in a laboratory is straightforward; the challenge lies in achieving high batch-to-batch consistency during mass production. If one batch has a refractive index of 1.501 and the next has 1.503, this discrepancy can lead to uneven spectral distribution within a PLC waveguide. To address this, strict control over monomer purity at the raw material stage, precise management of synthesis processes, and ongoing calibration of detection methodologies are essential—each step must be executed flawlessly.
Another implicit barrier is the validation cycle. When an optical module manufacturer decides to replace any given material, it must conduct a full suite of reliability tests—including temperature cycling, mechanical shock, and humidity-heat aging—with a typical test duration ranging from 6 to 18 months. Given this requirement, no optical module manufacturer would readily switch to a new supplier unless it has sufficient confidence in the new supplier’s capabilities.
Where lies the opportunity for domestication?
Despite high entry barriers, an opportunity window for domestic production is now emerging.
First is the endogenous driving force for supply chain security. In the past two years, leading optical module manufacturers such as Huawei and Zhongji Xuchuang have actively promoted domestic validation of core materials, with resins being included on a key verification list. This has provided domestic resin manufacturers with a rare validation opportunity—whereas in the past, it required considerable effort to gain access to testing at major manufacturers, now these major manufacturers are proactively reaching out to them.
The second opportunity lies in the potential for formulation redesign driven by CPO and co-packaged optics. Under the CPO architecture, the optical density within optical modules increases significantly, which may necessitate a redesign of conventional coupling adhesive formulations. This presents a new starting line for all market participants, offering domestic manufacturers the chance to secure a competitive position in defining the specifications for next-generation materials.
Everything covered here draws on the UV curing adhesive range JOME manufactures in house. Engineers comparing notes with Applications of Silicone Materials in UV-Curing can move between the two guides; the JOME industrial adhesives homepage frames the range and the get input from the JOME engineering desk takes it from there.