A long-period fiber grating couples light from a guided mode into forward propagating cladding modes where it is lost due to absorption and scattering. The coupling from the guided mode to cladding modes is wavelength dependent so we can obtain a spectrally selective loss.
It is an optical fiber

Optical fiber

An optical fiber is a flexible, transparent fiber made of a pure glass not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of...

 structure with the properties periodically varying along the fiber, such that the conditions for the interaction of several copropagating modes are satisfied. The period of such a structure is of the order of a fraction of a millimeter. In contrast to the fiber Bragg grating

Fiber Bragg grating

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, which generates a...

s, LPFGs couple copropagating modes with close propagation constants; therefore, the period of such a grating can considerably exceed the wavelength of radiation propagating in the fiber. Because the period of an LPFG is much larger than the wavelength, LPFGs are relatively simple to manufacture. Since LPFGs couple copropagating modes, their resonances can only be observed in transmission spectra. The transmission spectrum has dips at the wavelengths corresponding to resonances with various cladding modes (in a single-mode fiber).

Depending on the symmetry

Symmetry

Symmetry generally conveys two primary meanings. The first is an imprecise sense of harmonious or aesthetically pleasing proportionality and balance; such that it reflects beauty or perfection...

 of the perturbation that is used to write the LPFG, modes of different symmetries may be coupled. For instance, cylindrically symmetric gratings couple symmetric LP0m modes of the fiber. Microbend gratings

Microbend gratings

Microbend grating is a convenient method to couple light from one guided fiber mode into another. Due to its antisymmetric nature of perturbation, it can be used to couple only into antisymmetric modes. Microbend gratings are easily tunable for a wide range of wavelengths....

, which are antisymmetric

Antisymmetric

The word antisymmetric refers to a change to an opposite quantity when another quantity is symmetrically changed. This concept is related to that of Symmetry and Asymmetry. The difference between these three concepts can be simply illustrated with Latin letters. The character "A" is symmetric about...

 with respect to the fiber axis, create a resonance between the core mode and the asymmetric LP1m modes of the core and the cladding.

Long period grating has a wide variety of applications, including band-rejection filters, gain flattening filter and sensors.

Various gratings with complex structures have been
designed: gratings combining several LPFGs, LPFGs with superstructures, chirped gratings, and gratings with apodization. Various LPFG-based devices have been developed: filters, sensors, fiber dispersion compensators, etc.

Chiral long-period gratings

Chiral-fiber gratings are a variation on conventional fiber Bragg gratings. Rather that write gratings on glass, chiral gratings are mechanical gratings microformed into glass. By twisting uniformly heated standard single-mode fiber with residual nonconcentricity long-period fiber gratings can be produced. Chiral Photonics

Chiral Photonics

Chiral Photonics, Inc. is a photonics company based in Pine Brook, New Jersey, founded in 1999. The company is developing a new class of optical devices based on twisting glass optical fibers. These in-fiber devices aim to displace discrete optical elements such as lasers, filters and sensors...

, a New Jersey company, is the leading developer of this technology.

Twisting birefringent fiber with concentric noncircular cores with 180° rotation symmetry creates double-helix structures. The period then equals one half the turn, so that the pitch, P, is twice the period, a, or . Narrow dips arise in the transmission spectrum when light is resonantly coupled between the core and cladding in chiral long-period gratings (CLPGs) with a pitch much greater than the wavelength. In chiral gratings with a pitch slightly shorter than in CLPGs, the wave couples to free space and escapes the fiber. This scattering occurs at the edge of the scattering band, and is polarization selective. These chiral intermediate-period gratings (CIPGs) are the basis of linear and circular polarizers with bandwidths of over 100 nm. Twisting fiber with a nonconcentric core produces a periodicity in the refractive index equal to the pitch, , which creates single-helix structures. The periodic helical modulation of the fiber core is similar to modulation in fiber long-period gratings produced by microbending.

External links

• Theory of LPFGs (Fiber Optic Research Center)