Fiber Bragg grating

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A fiber Bragg grating is a type of distributed Bragg reflector constructed in a short segment of optical fiber that reflects particular wavelengths of light and transmits all others. This is achieved by adding a periodic variation to the refractive index of the fiber core. A fiber Bragg grating can therefore be used as an inline optical filter to block certain wavelengths, or as a wavelength-specific reflector.

1 History
2 Manufacture
3 Theory
4 Types of gratings
- 4.1 Chirped fiber Bragg gratings
5 Applications
6 See also
7 References
8 External links

[edit] History

The first in-fiber Bragg grating was demonstrated by Hill in 1978.^[1] Initially, the gratings were fabricated using a visible laser propagating along the fiber core. In 1989, Meltz and colleagues demonstrated the modern transverse holographic technique from the side of the fiber utilizing the interference pattern of ultraviolet light.^[2]

[edit] Manufacture

Fiber Bragg gratings are created by "inscribing" or "writing" the periodic variation of refractive index into the core of an optical fiber using an intense ultraviolet (UV) source such as a UV laser. Two main processes are used: interference and masking. Which is best depends on the type of grating to be manufactured. A special germanium-doped silica fiber is used in the manufacture of fiber Bragg gratings. The germanium-doped fiber is photosensitive: the refractive index of the core changes with exposure to UV light, with the amount of change depending on the exposure intensity and duration.

[edit] Interference

The first manufacturing method, specifically used for uniform gratings, is the use of two-beam interference. Here the UV laser is split into two beams which interfere with each other creating a periodic intensity distribution along the interference pattern. The refractive index of the photosensitive fiber changes according to the intensity of light that it is exposed to. This method allows for quick and easy changes to the Bragg wavelength, which is a directly related to the interference period, which is a function of the incident angle of the laser light.

[edit] Photomask

A photomask having the intended grating features may also be used in the manufacture of fiber Bragg gratings. The photomask is placed between the UV light source and the photosensitive fiber. The shadow of the photomask then determines the grating structure based on the transmitted intensity of light striking the fiber. Photomasks are specifically used in the manufacture of chirped Fiber Bragg gratings, which cannot be manufactured using an interference pattern.

[edit] Point-by-point

A single UV laser beam may also be used to 'write' the grating into the fiber point-by-point. Here, the laser has a narrow beam that is equal to the grating period. This method is specifically applicable to the fabrication of long period gratings. Point-by-point is also used in the fabrication of tilted gratings.

[edit] Production

Originally, the manufacture of the photosensitive optical fiber and the 'writing' of the fiber Bragg grating were done separately. Today, production lines typically draw the fiber from the preform and 'write' the grating, all in a single stage. As well as reducing associated costs and time, this also enables the mass production of fiber Bragg gratings. Mass production is in particular facilitating applications in smart structures utilizing large numbers (3000) of embedded fiber Bragg gratings along a single length of fiber.

[edit] Theory

The grating will typically have a sinusoidal refractive index variation over a defined length. The reflected wavelength ( $λ B$ ), called the Bragg wavelength, is defined by the relationship,

$\lambda_B= 2 n \Lambda\,$ ,

where $n$ is the average refractive index of the grating and $Λ$ is the grating period.

The wavelength spacing between the first minimums (nulls), or the bandwidth ( $Δλ$ ), is given by,

$\Delta \lambda=\left[\frac{2 \delta n_0 \eta}{\pi}\right]\lambda_B$ ,

where $δ n 0$ is the variation in the refractive index and $η$ is the fraction of power in the core.

The peak reflection ( $P B (λ B)$ ) is approximately given by,

$P_B(\lambda_B) \approx tanh^2 \left[\frac{N \eta (V) \delta n_0}{n}\right]$ ,

The full equation for the reflected power ( $P B (λ)$ ), is given by,

$P_B(\lambda) = \frac{sinh^2\left[ N \eta (V) \delta n_0 \sqrt(1-\Gamma^2) N \Lambda / \lambda\right]}{cosh^2\left[\eta (V) \delta n_0 \sqrt(1-\Gamma^2) N \Lambda / \lambda\right]-\Gamma^2}$ ,

where,

$\Gamma (\lambda)=\frac{\pi}{\eta (V) \delta n_0}\left[\frac{\lambda}{\lambda_B}-1\right]$ ,

and $N$ is the number of periodic variations.

[edit] Types of gratings

There are six common types of Fiber Bragg Grating;^[3]

uniform positive-only index change,
Gaussian apodized,
raised-cosine apodized,
chirped,
discrete phase shift, and
superstructure.

[edit] Chirped fiber Bragg gratings

The pattern may be modified to add other features such as a linear variation in the grating period (called a chirp) to broaden the reflected spectrum. A grating possessing a chirp has the property of adding dispersion - namely, different wavelengths reflected from the grating will be subject to different delays. This property has been used in the development of phased-array antenna systems.

[edit] Applications

The primary application of fiber Bragg gratings is in optical communications systems. They are specifically used as notch filters. They are also used in demultiplexers with an optical circulator, and in tunable demultiplexers. In a tunable demultiplexer the Bragg wavelength of the fiber Bragg grating can be tuned by strain applied by a piezoelectric transducer.

As well as being sensitive to strain, the Bragg wavelength is also sensitive to temperature. This means that fiber Bragg gratings can be used as sensing elements in optical fiber sensors. The sensitivity of the Bragg wavelength to an applied strain ( $S$ ) and a change in temperature ( $Δ T$ ) is approximately given by,

$\left[\frac{\Delta \lambda_B}{\lambda}\right]= C_SS + C_T\Delta T$ ,

where $C$ _S is the coefficient of strain, and $C$ _T is the coefficient of temperature.

Fiber Bragg gratings can then be used as direct sensing elements for strain and temperature. They can also be used as transduction elements, converting the output of another sensor, which generates a strain or temperature change from the measurand, for example fiber Bragg grating gas sensors use an absorbent coating, which in the presence of a gas expands generating a strain, which is measurable by the grating. Technically, the absorbent material is the sensing element, converting the amount of gas to a strain. The Bragg grating then transduces the strain to the change in wavelength.

Specifically, fiber Bragg gratings are finding uses in instrumentation applications such as seismology and as downhole sensors in oil and gas wells for measurement of the effects of external pressure, temperature, seismic vibrations and inline flow measurement. As such they offer a significant advantage over traditional electronic gauges used for these applications in that they are less sensitive to vibration or heat and consequently are far more reliable.

[edit] See also

Bragg's law
Bragg diffraction
Diffraction
- Diffraction grating
Dielectric mirror
Hydrogen microsensor
Long-period fiber grating
Photonic crystal fiber
Distributed temperature sensing by fiber optics

[edit] References

^ Hill, K.O. (1978). "Photosensitivity in optical fiber waveguides: application to reflection fiber fabrication". Appl. Phys. Lett. 32: 647.
^ Meltz, G.; et al. (1989). "Formation of Bragg gratings in optical fibers by a transverse holographic method". Opt. Lett. 14: 823.
^ Erdogan, Turan (August 1997). "Fiber Grating Spectra". Journal of Lightwave Technology 15 (8): 1277 – 1294. DOI:10.1109/50.618322.