Fiber Bragg grating

Fiber Bragg gratings are inscribed in optical waveguides, optical interference filter. Wavelengths, which are within the filter bandwidth to? B are reflected.

Construction

The individual layers are registered by means of UV - light (e.g. an excimer laser with λ = 248 nm) into the glass fibers. In the fiber core produces a periodic modulation of the refractive index, with high and low refractive index regions, which reflects the light of a specific wavelength back ( bandstop filter ). The center wavelength of the filter bandwidth in single-mode fibers is calculated by the Bragg condition:

Is the effective refractive index n eff, and Λ is the grating period. The effective refractive index neff depends on the geometry ( the core and cladding diameter ) of the waveguide, the refractive indices n1, n2, n3 and the wave modes. ? B thereby is the wavelength in vacuum, and λ the (effective) wavelength in the waveguide. The spectral width of the strip depends on the length of the fiber Bragg grating and the magnitude of the change in refractive index between the adjacent refractive index areas.

Function

The wave propagates in both the core and in the cladding of the waveguide, similar to an electromagnetic wave (photons) in the finite potential well. Therefore, the effective refractive index of the three indices of refraction depends. In the mantle, the intensity distribution of the wave decreases with the exponential function.

The core of the fiber is composed of successive sections of the length λ / 2 = Λ ( in the medium). The length Λ consists of two λ / 4 pieces that differ in refractive index. At each interface, a part of the inputted amplitude ( (normal incidence ) Fresnel formulas ) is reflected by the Fresnel reflection. The periodic change of the refractive index or impedance of the shaft causes the reflected wave experiences a phase shift of either 0 ° or 180 ° at the end of each λ / 4 track. Due to multiple reflection occurs in the reflected wave to constructive interference (physics). This sequence of λ/2-Schichten corresponds approximately to the equivalent of an anti-reflective coating, wherein the two times of passage through λ/4-Schicht ( with the optical path difference λ / 2) leads to destructive interference.

The difference in the refractive indices n2 and n3 is manufacturing reasons not very large, therefore succeed no complete amplitude cancellation in to a few successive layers.

The electrical analog of a fiber Bragg grating at significantly higher wavelengths would be a succession of different wave λ/2-Leitungsstücken impedance. The mismatch at each connection point, a part of the energy is reflected.

Applications

• In the optical communication equipment as a filter for separating different channels in wavelength division multiplexed (WDM )
• Wavelength-selective reflector fiber in fiber- coupled diode lasers
• An optical element in the resonator of a fiber laser
• Sensors for temperature and strain based on the reflected wavelength changing:

The wavelength shifts by the temperature T and the relative elongation of the fiber:

With:

The elongation of the fiber is composed of the portion of the externally applied elongation, and thermal expansion together: and is obtained for the temperature dependence of:

With:

And the strain dependence:

And at the same time modified temperature and mechanical stress:

Fiber Bragg grating solve compressive forces of several bar and temperature changes from 100 K to quite good. A typical grating which is tuned to 1500 nm, shifted by 0.1 nm at a temperature change of 10 K, as in a change in length of 10-4.