Fiber Bragg Grid

The spectral selectivity of reflection from fiber Bragg gratings is caused by a periodically varying refractive index of the light-bearing core and is due to diffraction by these periodic optical inhomogeneities.

The spatial period of heterogeneities${\displaystyle \mathrm{<span\; class="mwe-math-element"><span\; class="mwe-math-mathml-inline\; mwe-math-mathml-a11y"\; style="display:\; none;">\Lambda </span></span>}}$ {\ displaystyle \ Lambda}   selected in such a way that light waves with the desired wavelength are reflected in it. If the length-averaged refractive index of the structure${\displaystyle \mathrm{<span\; class="mwe-math-element"><span\; class="mwe-math-mathml-inline\; mwe-math-mathml-a11y"\; style="display:\; none;">n</span></span>}}$ {\ displaystyle n}   , then reflection will be observed at wavelengths:

${\displaystyle {\mathrm{<span\; class="mwe-math-element"><span\; class="mwe-math-mathml-inline\; mwe-math-mathml-a11y"\; style="display:\; none;">\lambda </span></span>}}_{\mathrm{<span\; class="mwe-math-element"><span\; class="mwe-math-mathml-inline\; mwe-math-mathml-a11y"\; style="display:\; none;">B</span></span>}}<span\; class="mwe-math-element"><span\; class="mwe-math-mathml-inline\; mwe-math-mathml-a11y"\; style="display:\; none;">=</span></span>\mathrm{<span\; class="mwe-math-element"><span\; class="mwe-math-mathml-inline\; mwe-math-mathml-a11y"\; style="display:\; none;">2</span></span>}\mathrm{<span\; class="mwe-math-element"><span\; class="mwe-math-mathml-inline\; mwe-math-mathml-a11y"\; style="display:\; none;">n</span></span>}\mathrm{<span\; class="mwe-math-element"><span\; class="mwe-math-mathml-inline\; mwe-math-mathml-a11y"\; style="display:\; none;">\Lambda </span></span>}}${\ displaystyle \ lambda _ {B} = 2n \ Lambda}  

For example, at a structure period of 530 nm , reflection at a wavelength of about 1540 nm is observed. For comparison, the period of a long-period fiber lattice is 100 μm or more. ^[one]

The characteristic length of the periodic FBG structure is from 1 mm to several cm, that is, the number of inhomogeneities from a thousand to tens of thousands. The relative change in the refractive index from the average order${\displaystyle {\mathrm{<span\; class="mwe-math-element"><span\; class="mwe-math-mathml-inline\; mwe-math-mathml-a11y"\; style="display:\; none;">ten</span></span>}}^{<span\; class="mwe-math-element"><span\; class="mwe-math-mathml-inline\; mwe-math-mathml-a11y"\; style="display:\; none;">-</span></span>\mathrm{<span\; class="mwe-math-element"><span\; class="mwe-math-mathml-inline\; mwe-math-mathml-a11y"\; style="display:\; none;">four</span></span>}}}$ {\ displaystyle 10 ^ {- 4}}   . A large number of strokes with a small change in the refractive index leads to a very narrow reflection spectrum - the width of the reflection spectrum is usually a fraction of nanometers.

The period of the structure and, accordingly, the wavelength of the reflected light changes upon mechanical compression or tension of the fiber. This phenomenon is used in fiber-optic sensors, for example, in tensometric measurements and for tuning within narrow limits of the laser wavelength. A change in temperature leads to a thermal change in the length of the structure and also shifts the reflection spectrum, which can be used in thermometers .

Fused silica doped with germanium oxide (the core material of an optical fiber) or doped with compounds of other chemical elements has the property of changing the refractive index of a material under the influence of ultraviolet radiation (UV). The periodic spatial structure of UV radiation ( interference bands ) to create a Bragg grating in the fiber is formed by the interference of two beams of UV radiation focused by a cylindrical lens into the core region in the direction transverse to the axis of the optical fiber. To do this, the UV laser beam is divided into 2 parts.

Various methods of creating FBGs are used: directly using a phase mask , dividing the beam using a phase mask or dividing plate and mixing using additional mirrors, as well as using the Lloyd interferometer . To do this, a polymer coating that absorbs UV radiation is removed from the fiber section in which the FBG is created.

Various sources of UV radiation (usually lasers) can be used to create FBGs: with the generation of the second harmonic of a cw argon ion laser, excimer KrF and ArF UV lasers, with generation of the fourth harmonic of an Nd: YAG laser .

In addition to the interference method of creating FBGs, the formation of inhomogeneities by individual points is applied, and each heterogeneity is formed sequentially by a sharply focused radiation beam.

It is shown that in addition to ultraviolet lasers, femtosecond lasers (harmonics of infrared lasers ) can be used.

• Long period fiber grating
• Fiber laser

1. Vasiliev S. A., Medvedkov O. I., Korolev I. G. et al. Fiber gratings of the refractive index and their applications. Quantum. electronics, 2005, 35 (12), 1085-1103 http://www.quantum-electron.ru/php/paper_rus.phtml?journal_id=qe&paper_id=13041
2. Varzhel S.V., Kulikov A.V., Brunov V.S. Method for lowering the reflection coefficient of fiber Bragg gratings using the photochromism effect - Article. - Scientific and Technical Journal ITMO - January-February 2012 - UDC 681.7.063
3. ↑ The effect of wrapping the fiber Bragg grating on the temperature stability of a broadband radiation source (neopr.) . cyberleninka.ru. Date of treatment January 21, 2019.