High-Temperature fiber Bragg Gratings

Considering the structure of FBGs, specialists use metal fiber coatings for FBG sensors to withstand the highest temperatures. In addition, the metal coating may prevent mechanical fatigue and chemical corrosion. Fibers coated with copper, aluminum, gold, and steel can survive in a wide temperature range.

Gold-coated FBG sensors are sure to be the most effective in extreme environments. They provide mechanical protection, hermetic sealing, high corrosion resistance, and high sensitivity.

There are several types of high-temperature fiber Bragg gratings:

• Type-I FBGs are the least temperature-resistant, operating from -20 to +80°C, but they are well-suited for telecommunications.
• To operate at higher temperatures, fiber Bragg gratings have to be regenerated. Type-I FBGs are regenerated with various H2-free silicate fibers. Thermal regeneration is achieved by prolonged irradiation using CW or UV light. The fiber formation consists in relaxing a partial internal core due to the UV inscription. First-order FBGs reached temperatures about 500°C, but higher intensity exposures made it possible to get up to 700-800°C. Their key advantage is that they can be produced in a single step, avoiding H2 loading or post-thermal processing.

Silica regeneration allows FBGs to operate above 800°C. They are produced through annealing a seed structure enabling them to operate at as high as 1295°C and higher. Elevated temperatures cause stress relaxation in the fiber core and cladding. In spite of FBGs strong stability rates, they become fragile because of removing compressive stress between cladding and core.

The other two types can operate at higher temperatures:

• Femtosecond FBGs are fabricated using ultrafast lasers either point-by-point or phase-masked. Both categories of femtosecond fiber Bragg gratings demonstrate high thermal stability that can reach up to 1000 °C.
  The first category of femtosecond fiber Bragg gratings is generated by laser pulses with energy below the glass damage threshold. The localized heating and cooling of the glass changes the index and leads to localized densification.
  The second category is created at energy above the glass damage threshold. In this case, the glass surface is ionized, resulting in the changed refractive index and structural changes.
• Sapphire FBGs are another type of the strongest FBGs. They can operate at the highest temperatures, up to 1900°C. Such FBGs are inscribed into a sapphire optical fiber or alumina with a femtosecond laser. However, these gratings contain a single sapphire fiber without conventional cladding.

Despite the laser type, the wavelength of FBGs can shift upon the first exposure, and this occurs because the structure needs to fit into the environment. High-temperature FBGs can be pre-calibrated or calibrated in the field, depending on customers’ requirements.