Why does hydrogen need to be released after etching the grating? What happens if it isn’t?
During the fabrication of Fiber Bragg Gratings (FBGs), to enhance the photosensitivity of silica fibers (especially germanium-doped fibers), the fiber is typically placed in a hydrogen chamber under high pressure and a certain temperature for Hydrogen Loading. This process allows free hydrogen molecules (H_2) to diffuse into the fiber core.
After inscribing the grating with a UV laser, a “Hydrogen Unloading / Outgassing” process is mandatory. This usually involves high-temperature thermal annealing to forcibly expel any remaining unreacted free hydrogen molecules. If the hydrogen is not removed, it will have severe negative impacts on the long-term stability and optical performance of the grating.
I. Why Remove Hydrogen? (What happens if it’s not removed?)
If artificial hydrogen unloading is not performed after grating inscription, the residual free hydrogen molecules will cause the following three major physical and engineering hazards:
1. Long-term drift of the Bragg center wavelength (\lambda_B) towards shorter wavelengths
When free hydrogen molecules enter the fiber core, they increase the effective refractive index (n_{eff}) of the fiber core. In a room-temperature environment, these hydrogen molecules, which are not chemically bonded to the silica network, will diffuse out of the fiber very slowly and spontaneously. This natural diffusion process can take weeks or even months at room temperature.
According to the Bragg formula for fiber gratings:
\lambda_B = 2 n_{eff} \Lambda
(where \lambda_B is the reflection center wavelength, n_{eff} is the effective refractive index of the core, and \Lambda is the grating period)
As hydrogen spontaneously diffuses out over weeks or months, the core’s n_{eff} continuously decreases, causing the grating’s center wavelength \lambda_B to undergo a continuous and unpredictable drift towards shorter wavelengths. This renders the sensor unusable for accurate temperature or strain measurements in practical applications, as it loses its reference point.
2. Hydroxyl ( -OH ) absorption loss, increasing optical signal attenuation
Under elevated ambient temperatures, UV exposure, or high-intensity light within the fiber, residual free H_2 molecules can react with defects or silica bonds in the fiber glass network, forming hydroxyl ( -OH ).
Hydroxyl groups have extremely strong absorption peaks in the near-infrared spectrum, particularly at strong absorption bands around 1.38\ \mu\text{m} and 1.24\ \mu\text{m}. The edges of these absorption spectra (absorption tails) extend into the 1550\ \text{nm} wavelength (C-band), which is commonly used for fiber optic communications and grating sensing. This leads to a sharp increase in fiber transmission loss and severe signal attenuation, making it impossible for demodulators to detect a high signal-to-noise ratio grating reflection spectrum.
3. Poor thermal stability
During use, if exposed to high-temperature environments, the diffusion and reaction rates of residual hydrogen will accelerate instantaneously, causing abrupt shifts in the grating wavelength or rapid degradation at high temperatures.
II. Common Engineering Methods for Hydrogen Unloading
To avoid the hazards mentioned above, Thermal Annealing stabilization treatment is typically performed immediately after fiber Bragg grating inscription:
- The inscribed gratings are placed in a constant temperature oven (e.g., baked at 80^\circ\text{C} to 120^\circ\text{C} for tens of hours).
- High temperatures allow unreacted free hydrogen molecules to diffuse out of the fiber quickly and completely.
- Simultaneously, thermal annealing can also pre-emptively eliminate unstable refractive index changes caused by UV exposure, resulting in high-quality fiber Bragg gratings with extremely stable long-term spectral characteristics, center wavelength, and reflectivity.
III. Beijing Dacheng Yongsheng Technology Co., Ltd. (OFSCN®) Related Grating Products Technology
Regarding grating inscription and annealing processes, Beijing Dacheng Yongsheng Technology Co., Ltd. offers two types of bare grating products with different physical manufacturing processes:
1. Traditional UV Laser Inscribed Gratings (Requires Hydrogen Loading and Unloading)
These gratings are inscribed using traditional UV light mask projection. Before inscription, the fiber undergoes high-pressure hydrogen loading. After inscription, hydrogen unloading and thermal annealing stabilization treatment are mandatory to ensure wavelength stability during long-term operation. Finally, they are recoated with materials such as polyimide.
OFSCN® Polyimide Fiber Bragg Gratings / FBG Strings (Bare)
2. Femtosecond Laser Direct-Writing Gratings (No Hydrogen Loading or Unloading Required)
If you wish to completely avoid hydrogen loading and unloading processes, as well as the mechanical strength damage associated with stripping the coating, you can opt for femtosecond laser direct-writing gratings. Femtosecond lasers utilize extremely high instantaneous power to directly alter the material’s refractive index through multi-photon nonlinear absorption. No hydrogen loading is required before inscription, thus eliminating the issues of residual hydrogen outgassing and hydroxyl absorption losses at the physical source. This provides inherent long-term wavelength stability and extremely high temperature resistance.
OFSCN® Standard Femtosecond Fiber Bragg Gratings / FBG Strings (Bare)
