Long-period fiber grating

Long-period fiber gratings ( LPGs ) are enrolled in fiber optical notch filter. A periodical index modification of the core with a grating period of ~ 0.1-1 mm leads to a resonant coupling of the guided mode in the core ( red in the picture ) into a cladding mode of the fiber ( blue in the picture). Due to the resonance, this coupling is wavelength dependent. In the area of the resonance wavelength? D the light of the core mode can be almost completely coupled into the cladding mode, which manifests itself as a corresponding notch in the transmission spectrum of the core of the fiber. Bandwidth, shape and depth of the spectral notch may be influenced by varying the number of periods and the index modification. In LPGs have involved above-mentioned Modes have the same propagation direction, while the fiber Bragg gratings ( FBGs ), the core mode is coupled into the resonant opposite core mode. The mathematical description of both LPG and FBGs is very similar, since both are based on the theory of coupled modes.

Production

The modification of the refractive index in the core is mainly in two ways:

  • Irradiating an optical fiber with photosensitive core with UV light. By an amplitude mask through the core of the fiber is irradiated with a UV laser. A rasterized amplitude mask leads with and without irradiation to periodically alternating zones, the refractive index is increased accordingly or not.
  • By means of a carbon dioxide laser, the fiber is heated locally by the page and limited above the melting temperature. After each melting process, the carbon dioxide laser moves around the grating period along the fiber on. Caused by a change in melting, stress-induced birefringence in the core of the fiber pressure causes a modification in the refractive index.

Function

The effect of the LPG in the core of the fiber can be described as an optical grating. The LPG bends the light that propagates in the core mode in the cladding mode. In other words, the LPG causes phase matching between the core and cladding mode, and there is a transfer of the power from the core to cladding.

The modes are eigenvectors (solutions ) of the Helmholtz equation. For cylindrical index distributions as in an optical fiber is in the solutions to Bessel functions. The effective refractive indices can be understood as the eigenvalues ​​of the Helmholtz equation. Today there are ready software solutions for the determination of the Bessel functions and the corresponding effective refractive indices.

The resonant wavelength of the LPG? D results from the grating period Λ, as well as from the effective refractive indices of each of the core mode and cladding mode:

The strength of the coupling over between core and cladding mode is expressed by the coupling coefficient κ [1 / m]. It results from the overlap integral of the core and cladding mode as well as the strength of the actual index modification. The cross-coupling of the core to the cladding mode is optimal if:

This corresponds to the length of the LPG and the number of grating periods.

Applications

  • Long-period grating as a notch filter is the complement to the fiber Bragg gratings, which have characteristics of a spectral band pass filter. In this manner, an optical spectrum are specifically influenced by attenuating specific wavelengths by means of the LPG's ..
  • Changes in the effective refractive indices of core and cladding mode in the range of 10-5 making their presence felt through a well measurable shift of the resonance wavelength. LPGs are therefore often used in sensor.

Credentials

  • Optical component
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