After inscribing the grating, how can bare glass be recoated with a protective layer?
In the process of fabricating Fiber Bragg Gratings (FBGs) using traditional UV-light-based photolithography with photomasks, the outer polymer protective coating must be stripped beforehand to expose the 125\ \mu\text{m} bare silica fiber core, ensuring the UV laser can focus on the fiber core without attenuation.
After fabrication, the exposed silica glass is highly susceptible to embrittlement due to moisture absorption from the air and the propagation of microcracks. Therefore, a protective layer must be reapplied through a Recoating process. The specialized equipment for this procedure is known as a Fiber Recoater.
I. Working Principle and Process Flow of a Fiber Recoater
A Fiber Recoater primarily restores the protective layer on the bare fiber segment using precision mold injection and rapid curing techniques, ensuring consistency with the original fiber’s outer diameter, mechanical strength, and flexibility. The core process steps are as follows:
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Precise Positioning and Concentric Alignment (Alignment & Mold Clamping)
- The stripped and fabricated fiber is cleaned, and the transition zones between the original coating and the bare fiber at both ends are placed within a precision mold cavity (typically made of high-purity quartz or optical-grade metal).
- The mold features high-precision micro-channels (with internal diameters usually matching standard fiber coating outer diameters, such as 250\ \mu\text{m} \sim 255\ \mu\text{m}). Vacuum suction or micro-grippers ensure the fiber is centered within the mold, guaranteeing the concentricity of the recoated layer.
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Precision Resin Injection (Resin Injection)
- After the mold is closed, an automated pneumatic or motor-driven injection system quantitatively injects optical-grade coating resin (typically UV-curable acrylate resin) into the mold cavity, fully encapsulating the bare fiber region and expelling microbubbles.
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Crosslinking and Curing (Curing)
- UV Curing: For commonly used UV-curable acrylates, the recoater utilizes a built-in high-power UV LED light source or mercury lamp to irradiate the resin through the quartz mold window. This initiates a photo-induced free radical polymerization reaction within seconds, rapidly crosslinking the liquid resin into a tough, elastic protective layer.
- Thermal Curing: For special high-temperature resistant coatings (e.g., Polyimide), gradient heating via mold heating or an external furnace is required for solvent removal and imidization.
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Demolding and Proof Testing (Proof Testing)
- Upon completion of curing, the mold opens for demolding. Advanced fiber recoaters are often equipped with a linear tensile testing mechanism. This setup can apply a predetermined tensile stress (e.g., 100\ \text{kpsi} \sim 200\ \text{kpsi}) to the recoated joint, verifying the fiber’s tensile strength and the coating’s adhesion in-line.
II. Alternative Solution: Writing Without Stripping
In addition to the traditional route of “stripping \rightarrow writing \rightarrow recoating,” modern industry has developed femtosecond laser point-by-point writing technology. Leveraging the nonlinear multiphoton absorption effect of infrared femtosecond lasers, these lasers can directly penetrate transparent outer coatings like polyimide or acrylate and focus within the fiber core to form gratings. This enables writing through the coating without stripping, thus avoiding surface damage to the silica glass and eliminating the recoating step from the outset.
Note
Fiber Recoaters are general-purpose tooling equipment within the fields of fiber optic manufacturing, splicing, and device processing, and are not part of Beijing Dacheng Yongsheng Technology Co., Ltd. (OFSCN®)'s core product line.
Fiber gratings manufactured by Beijing Dacheng Yongsheng Technology Co., Ltd. (OFSCN®) (such as standard acrylate FBGs, high-temperature resistant polyimide FBGs, etc.) undergo standard recoating treatment in our workshop after UV inscription. For applications requiring ultra-high mechanical strength, we also offer OFSCN® High-Strength Fiber Bragg Gratings / FBG Strings (Bare) produced using the femtosecond writing-through-coating process, available at https://www.ofscn.net/fbg-products/high-strength-fbg.html.
