Why does the FBG only reflect one specific wavelength?

The periodic structure acts like a series of partially reflective mirrors. For most wavelengths, the reflections from each interface interfere destructively. Only for the Bragg wavelength (λ_B) do the reflections from all periods add in phase, resulting in strong constructive interference and high reflectivity. This is a direct consequence of the Bragg condition λ_B = 2 n_eff Λ.

How is an FBG used as a sensor?

Physical parameters like strain and temperature change either the grating period (Λ) or the effective index (n_eff). According to the Bragg condition, this shifts the reflected wavelength (λ_B). By precisely measuring this spectral shift, the FBG becomes a highly sensitive sensor for mechanical deformation, temperature, or pressure.

What does the 'grating strength' control represent?

It primarily models the depth of the refractive index modulation (Δn). A stronger modulation creates a larger impedance mismatch at each period, increasing the peak reflectivity and often broadening the spectral width. In a real FBG, strength is also influenced by the grating length.

Is the reflectivity spectrum always a perfect Lorentzian shape?

No, the Lorentzian is a useful simplification for a weak, uniform grating. Real FBGs, especially strong or long ones, exhibit a more complex spectrum described by coupled-mode theory, often with side lobes. Apodization (tapering the grating strength at the ends) is used to suppress these side lobes for practical applications.

Fiber Bragg Grating

A Fiber Bragg Grating (FBG) is a periodic modulation of the refractive index within the core of an optical fiber. This simulator models the fundamental operating principle: the FBG acts as a wavelength-selective mirror. When broadband light propagates through the fiber, the periodic structure causes constructive interference for a specific wavelength, known as the Bragg wavelength, which is reflected. All other wavelengths are transmitted with minimal loss. The core physics is captured by the Bragg condition: λ_B = 2 n_eff Λ, where λ_B is the Bragg wavelength, n_eff is the effective refractive index of the fiber mode, and Λ is the grating period. The simulator visualizes the characteristic reflectivity spectrum, typically approximated as a Lorentzian curve centered at λ_B. Its width and peak height depend on the grating's strength (modulation depth of the refractive index, Δn) and length (L). By interacting with the controls, you can explore how changing the grating period (Λ), effective index (n_eff), or grating strength alters the reflected wavelength and the shape of the reflectivity spectrum. This demonstrates the FBG's function as an optical filter or sensor, as shifts in λ_B due to strain or temperature changes form the basis for its widespread sensing applications. The model simplifies the full coupled-mode theory to a Lorentzian lineshape, neglecting side lobes and more complex apodization effects, providing a clear, intuitive understanding of the central resonance phenomenon.

Who it's for: Undergraduate and graduate students in photonics, optical engineering, and physics courses covering waveguide theory, optical filters, or fiber optic sensors.

Key terms

Bragg Wavelength
Effective Refractive Index
Grating Period
Reflectivity Spectrum
Lorentzian Lineshape
Coupled-Mode Theory
Optical Filter
Wavelength-Division Multiplexing

How it works

Narrowband reflection from a periodic index modulation in the fiber core — the telecom workhorse for filters and strain sensors.

Frequently asked questions

Why does the FBG only reflect one specific wavelength?: The periodic structure acts like a series of partially reflective mirrors. For most wavelengths, the reflections from each interface interfere destructively. Only for the Bragg wavelength (λ_B) do the reflections from all periods add in phase, resulting in strong constructive interference and high reflectivity. This is a direct consequence of the Bragg condition λ_B = 2 n_eff Λ.
How is an FBG used as a sensor?: Physical parameters like strain and temperature change either the grating period (Λ) or the effective index (n_eff). According to the Bragg condition, this shifts the reflected wavelength (λ_B). By precisely measuring this spectral shift, the FBG becomes a highly sensitive sensor for mechanical deformation, temperature, or pressure.
What does the 'grating strength' control represent?: It primarily models the depth of the refractive index modulation (Δn). A stronger modulation creates a larger impedance mismatch at each period, increasing the peak reflectivity and often broadening the spectral width. In a real FBG, strength is also influenced by the grating length.
Is the reflectivity spectrum always a perfect Lorentzian shape?: No, the Lorentzian is a useful simplification for a weak, uniform grating. Real FBGs, especially strong or long ones, exhibit a more complex spectrum described by coupled-mode theory, often with side lobes. Apodization (tapering the grating strength at the ends) is used to suppress these side lobes for practical applications.

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Frequently asked questions

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