Exposed core microstructured optical fiber Bragg gratings: refractive index sensing.

Area of Science:

• Photonics and Optical Engineering
• Materials Science

Background:

• Microstructured optical fibers (MOFs) offer unique light-guiding properties.
• Bragg gratings are crucial components in optical sensing and filtering.

Purpose of the Study:

• To demonstrate the first-time fabrication of Bragg gratings in exposed-core MOFs using a femtosecond laser.
• To investigate the sensing capabilities of these gratings for refractive index and temperature measurements.

Main Methods:

• Femtosecond laser ablation was employed to inscribe second and third-order Bragg gratings into exposed-core MOFs.
• The fabricated gratings were characterized for their reflectivity, bandwidth, and response to external refractive index changes.
• High-temperature sensing performance was evaluated up to 800°C.

Main Results:

• Strong reflectivity Bragg gratings with narrow bandwidths (as low as 60 pm) were successfully inscribed at 1550 nm.
• The Bragg gratings exhibited sensitivity to external refractive index due to field penetration outside the fiber.
• Temperature-compensated refractive index measurements were achieved by exploiting differential mode sensitivities.
• The devices demonstrated robust performance for high-temperature sensing up to 800°C.

Conclusions:

• Femtosecond laser inscription is a viable method for creating Bragg gratings in exposed-core MOFs.
• These novel fiber Bragg gratings are suitable for advanced optical sensing, including simultaneous temperature-compensated refractive index and high-temperature measurements.
• Splicing to single-mode fiber facilitates integration with standard optical interrogation systems.