What is "dehydrogenation" treatment?

OFSCN Global Technical Center OFSCN Technical Knowledge Base
1

After etching the grating, why is it necessary to remove hydrogen by high temperature heating?

2

In the field of Fiber Bragg Grating (FBG) manufacturing and sensing, “Hydrogen Degassing” treatment (often combined with the “High-Temperature Annealing” process) after grating inscription is a crucial core process.

To understand why hydrogen needs to be driven out at high temperatures, we must first understand the “Hydrogen Loading” process and then examine the physical and optical hazards that residual hydrogen poses to FBG performance.

1. What is “Hydrogen Loading”?

The core of standard silica ( \text{SiO}_2 ) fiber (typically Germanium-doped) has inherently low photosensitivity. To facilitate the inscription of high-reflectivity gratings in the core using conventional UV lasers (such as 248\ \text{nm} excimer lasers), the fiber is usually placed in a high-pressure (e.g., 10\ \text{MPa} to 20\ \text{MPa} ) and moderately heated (e.g., 60^\circ\text{C} to 80^\circ\text{C} ) hydrogen ( \text{H}_2 ) environment. This allows hydrogen molecules to dissolve and diffuse into the fiber core. This process is known as Hydrogen Loading.

H$_{2}$ loading significantly enhances the photosensitivity of the fiber core, leading to a substantial increase in the refractive index modulation depth (refractive index change) when exposed to UV light.

2. Why is High-Temperature “Hydrogen Degassing” Necessary?

After grating inscription, a large amount of unreacted free hydrogen molecules remain trapped within the fiber. If this residual hydrogen is not removed through high-temperature “degassing,” it can lead to the following critical issues for FBG sensors:

A. Preventing Wavelength Drift Over Time (Enhancing Wavelength Stability)

According to the Bragg reflection principle of fiber gratings, their center reflection wavelength is determined by the following physical formula:

\lambda_B = 2 n_{\text{eff}} \Lambda

Where \lambda_B is the Bragg wavelength, n_{\text{eff}} is the effective refractive index of the fiber core, and \Lambda is the grating period.

Free hydrogen molecules ( \text{H}_2 ) dissolved in the fiber core cause the effective refractive index n_{\text{eff}} to increase. If the sensor is used at room temperature immediately after inscription, these unreacted hydrogen molecules will slowly diffuse outwards and escape the fiber over weeks or months. As the hydrogen concentration decreases, n_{\text{eff}} gradually reduces, causing the grating’s center wavelength \lambda_B to drift slowly towards shorter wavelengths. For FBG sensors requiring high-precision wavelength modulation, this uncertain wavelength drift will directly compromise their measurement accuracy.

B. Stabilizing the Refractive Index Modulation Structure (Eliminating Unstable Defects)

During UV exposure, the refractive index modulation formed in the fiber core consists of two parts: one is a stable structure modification with high activation energy, and the other is composed of unstable defect centers with low activation energy.

These unstable structures are prone to spontaneous decay in subsequent environmental temperature fluctuations, leading to a gradual decrease in the grating’s reflectivity.

By performing high-temperature annealing and degassing in a constant temperature environment above the sensor’s maximum operating temperature (typically 100^\circ\text{C} to 150^\circ\text{C} for 12 to 24 hours or longer), the release of free hydrogen molecules is accelerated, and unstable, decay-prone refractive index modulations are eliminated in advance, retaining only permanent refractive index changes with high activation energy. This ensures that the spectral profile and reflectivity of the FBG sensor remain constant throughout its operational life.

C. Reducing Optical Loss and Absorption

Catalyzed by light and heat, residual free hydrogen molecules can react with silica defects in the fiber network, forming hydroxyl groups ( \text{-OH} ) or creating color center defects. This leads to significant infrared absorption peaks at 1.24\ \mu\text{m} , 1.38\ \mu\text{m} , and near the C-band ( ext{around } 1.55\ \mu\text{m} ) used for communication. Thoroughly removing hydrogen through the degassing process prevents the subsequent formation of hydroxyl groups, ensuring the fiber’s ultra-low optical loss.

3. Femtosecond Laser Processes Requiring No Degassing

It is important to note that not all FBGs require hydrogen loading and degassing. For instance, gratings inscribed using Femtosecond Lasers utilize nonlinear multiphoton absorption generated by ultra-strong, ultra-fast pulses to directly modify the refractive index in localized regions of the quartz material.

These processes do not require hydrogen loading prior to inscription, and therefore, do not need subsequent degassing. Their thermal stability and high-temperature resistance (often up to 800^\circ\text{C} ) are generally superior.

4. Related OFSCN® Official Products Involving This Process

In Beijing Dacheng Yongsheng Technology Co., Ltd.'s (OFSCN®) product line, bare FBGs inscribed via traditional UV exposure undergo extremely rigorous high-temperature hydrogen degassing and aging annealing processes before recoating to eliminate hydrogen-induced wavelength instability:

Through these precise scientific annealing and degassing treatments, the FBG sensors leaving the factory possess excellent fatigue resistance, unimpeded temperature/strain transfer, and a center wavelength that remains drift-free for decades.