FaserBraggGitter

FaserBraggGitter, often referred to as a fiber Bragg grating (FBG) in English, is a type of optical reflector fabricated in an optical fiber by creating a periodic modulation of the core refractive index. The resulting structure acts as a distributed Bragg reflector, reflecting light around a specific wavelength while transmitting other wavelengths.

Principle: The reflected Bragg wavelength λB is approximately 2 n_eff Λ, where n_eff is the effective refractive index of the guided mode and Λ is the grating period. Uniform FBGs provide a narrow reflection band; chirped FBGs vary Λ along the length to broaden or tailor the reflection spectrum; tilted FBGs enable coupling to cladding modes for enhanced sensing.

Fabrication: FBGs are inscribed in photosensitive silica fibers by ultraviolet exposure. The common phase-mask technique uses a UV laser and a phase mask to create the periodic index modulation, while point-by-point inscription with femtosecond lasers can write custom geometries. Gratings typically range from a few millimeters to several centimeters in length, with Λ on the order of hundreds of nanometers, yielding Bragg wavelengths in the 1260–1625 nm range for standard telecom fibers. Reflectivity can reach high values, up to nearly 100% for well-made gratings.

Applications: In telecommunications, FBGs serve as narrowband reflectors and filters in fiber lasers and in wavelength-division multiplexing systems. In sensing, they provide measurements of strain, temperature, force, pressure, or chemical changes through shifts in λB, enabling distributed sensing in aerospace, civil engineering, oil and gas, and structural health monitoring.

Advantages and limitations: FaserBraggGitter are compact, passive, immune to electromagnetic interference, and compatible with existing fiber systems, but their response can be temperature- and strain-cross sensitive and long-term stability depends on the environment.

Historically: The technology was demonstrated in the late 1970s by researchers such as Hill and Meltz at Bell Labs.

---