A fiber Bragg grating (FBG) is a type of optical filter that is inscribed or "written" into the core of an optical fiber. It consists of a periodic modulation of the refractive index along the length of the fiber. This modulation forms a grating structure that reflects a specific wavelength of light while transmitting others.
Key features of FBGs include:
Periodic modulation: The refractive index of the optical fiber's core is periodically modulated, creating a series of alternating high and low index regions along the length of the fiber.
Wavelength selectivity: The spacing between the index modulations determines the wavelength of light that is selectively reflected. This wavelength is known as the Bragg wavelength.
Reflective element: FBGs act as reflective elements for a specific wavelength of light. When light with the Bragg wavelength propagates through the fiber, it is reflected back due to constructive interference resulting from the periodic index modulation.
Applications:
Sensing: FBGs are widely used in fiber-optic sensors for measuring various physical parameters such as temperature, strain, pressure, or acceleration. Changes in these parameters alter the Bragg wavelength, allowing for precise measurements.
Telecommunications: In wavelength-division multiplexing (WDM) systems, FBGs are used as wavelength-specific filters to selectively reflect or transmit signals in optical communication networks.
Laser technology: FBGs can be incorporated into laser systems as wavelength-selective elements or as feedback elements in distributed feedback (DFB) lasers.
Fabrication: FBGs are typically created using ultraviolet (UV) light to induce a photosensitive change in the refractive index of the optical fiber. The grating structure is "written" into the fiber by exposing it to an interference pattern produced by two UV beams.
Temperature stability: FBGs exhibit temperature stability, making them suitable for sensing applications in environments with varying temperatures.
Fiber Bragg gratings provide a versatile and robust means of manipulating and sensing light within optical fibers. Their wavelength-selective characteristics make them valuable components in various optical systems and applications.