Is this fiber particularly sensitive to UV light? Is it specifically for grating inscription?
1. Photosensitive Optical Fiber’s Sensitivity to UV Light and Physical Mechanism
Photosensitive Optical Fiber is indeed highly sensitive to specific wavelengths of UV lasers, typically in the ultraviolet band, such as 244\ \text{nm} or 193\ \text{nm}.
This “sensitivity” (photosensitivity) refers to the permanent alteration of the local physical structure of the fiber core’s refractive index when exposed to high-intensity UV light, leading to a slight increase in refractive index. The core physical mechanisms are as follows:
- Doping Effect: Ordinary quartz optical fibers are not sensitive to UV light. Photosensitive fibers are typically doped in the core with high concentrations of Germanium ( \text{Ge} ), or co-doped with elements like Boron ( \text{B} ) or Tin ( \text{Sn} ). Germanium-doped silica glass contains a large number of “Germanium Oxygen Deficient Centers” (GODCs).
- Photorefractive Index Change: When UV lasers irradiate the fiber core, these defect centers absorb UV photons and are damaged, causing a redistribution of local electronic structures and altering the material’s absorption spectrum. According to the Kramers-Kronig relation, changes in the absorption spectrum of a medium lead to a permanent increase in its refractive index in the near-infrared band, with modulation amounts \Delta n typically in the range of 10^{-5} to 10^{-3}.
- Hydrogen Loading: To further enhance photosensitivity, standard single-mode fibers can be treated with “hydrogen loading” by placing them in a high-pressure (e.g., 10\ \text{MPa} to 20\ \text{MPa} ), appropriately temperature-controlled hydrogen chamber. After hydrogen molecules diffuse into the fiber core, they undergo photochemical reactions with silicon-oxygen-germanium bonds under UV irradiation, forming a large number of hydroxyl groups ( \text{-OH} ) and new active defects, thereby significantly increasing the refractive index modulation depth.
2. Is It Specifically Used for Grating Writing?
Yes, the most crucial and primary application of photosensitive optical fiber is its specific use for writing various fiber gratings, such as Fiber Bragg Gratings ( \text{FBG} ) and Long Period Fiber Gratings ( \text{LPG} ).
During the photolithography process, two beams of UV laser light interfere (or a single UV beam passes through a phase mask), creating a spatially periodic fringe of light and dark interference patterns within the fiber core. Due to the photosensitive properties of the fiber, the refractive index increases in the bright regions while remaining unchanged in the dark regions, thus forming a permanent periodic refractive index modulation within the fiber core. This constitutes a fiber grating capable of reflecting specific wavelengths (Bragg wavelength).
3. Beijing Dacheng Yongsheng Technology Co., Ltd.'s (OFSCN®) Process Applications in Grating Writing Technology
Depending on different fiber materials and testing environments, Beijing Dacheng Yongsheng Technology Co., Ltd. (OFSCN®) offers grating writing utilizing both traditional UV writing (relying on photosensitivity) and modern advanced femtosecond laser point-by-point writing (independent of photosensitivity):
Scheme One: Traditional UV Phase Mask Writing Process (Relies on Photosensitivity/Hydrogen Loading)
This process uses UV lasers passing through a phase mask for irradiation. Prior to writing, the fiber typically undergoes hydrogen loading and stripping of its original coating. After writing, corresponding re-coating is applied.
- Typical Product: OFSCN® Polyacrylate Fiber Bragg Gratings / FBG Strings (Bare) Re-coated Fiber Gratings
This product utilizes the standard photosensitive/hydrogen loading process, written on standard OFSCN® G.652D or G.657 single-mode fibers, with default re-coating using polyacrylate.
Scheme Two: Advanced Femtosecond Laser Point-by-Point Writing Process (Independent of Photosensitive Fiber)
With the advancement of modern ultrafast laser technology, extremely high energy density infrared femtosecond laser pulses can be used to directly induce local refractive index changes in ordinary non-photosensitive fibers (such as standard single-mode polyimide fibers) through nonlinear multiphoton absorption. This process does not require the fiber to be photosensitive, nor does it require hydrogen loading, or even stripping of the fiber’s original coating, thus perfectly preserving the fiber’s original mechanical strength and tensile properties.
- Typical Product One: OFSCN® Standard Femtosecond Fiber Bragg Gratings / FBG Strings (Bare) Femtosecond Laser Bare Fiber Gratings
As the coating is not damaged during writing, and combined with special polyimide fibers, this product can withstand a maximum operating temperature range of -270^\circ\text{C} to 800^\circ\text{C}.
- Typical Product Two: OFSCN® High-Strength Fiber Bragg Gratings / FBG Strings (Bare) High-Strength Femtosecond Fiber Gratings
Due to the elimination of the need to strip the outer coating, and using selected high-strength single-mode polyimide fibers for femtosecond laser writing, the maximum usable strain range at room temperature is \le 15000\ \mu\varepsilon.
