Fiber Bragg gratings for low-temperature measurement

Area of Science:

• Optics and Photonics
• Materials Science
• Sensor Technology

Background:

• Fiber Bragg gratings (FBGs) are widely used optical sensors.
• Accurate temperature sensing at cryogenic temperatures (e.g., liquid nitrogen) is crucial for various scientific and industrial applications.
• Existing FBG temperature sensing methods may face limitations at low temperatures.

Purpose of the Study:

• To demonstrate the efficacy of FBGs as monolithic temperature sensors across a wide temperature range, including cryogenic conditions.
• To investigate and quantify the nonlinear thermal sensitivity of FBGs in different fiber types.
• To provide a consistent model for FBG behavior at low temperatures.

Main Methods:

• Fabrication of Bragg gratings in three distinct fiber types (pure-silica-core, boron-doped, germanium-doped).
• Characterization of FBG performance with high-density point measurements from ambient to 77 K (liquid nitrogen temperature).
• Utilizing a conventional interrogation scheme to determine temperature resolution.

Main Results:

• FBGs function as monolithic temperature sensors without auxiliary structures, from ambient down to liquid nitrogen temperatures.
• Nonlinear thermal sensitivity was confirmed across all tested fiber types, attributed to thermo-optic and thermal expansion nonlinearities.
• Achieved temperature resolutions of 0.5 K for pure-silica-core FBGs and 0.25 K for doped fibers at 77 K.

Conclusions:

• FBGs are effective monolithic temperature sensors for cryogenic applications.
• The nonlinear thermal sensitivity of FBGs at low temperatures is quantitatively explained by the nonlinear thermo-optic and thermal expansion coefficients.
• A consistent modeling approach for FBGs at low temperatures is now achievable.