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
An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparent fiber made by drawing glass (silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a means to ...
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
A fiber Bragg grating (FBG) 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 creating a periodic variation in th ...
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
Transmission may refer to:
Medicine, science and technology
* Power transmission
** Electric power transmission
** Propulsion transmission, technology allowing controlled application of power
*** Automatic transmission
*** Manual transmission
*** ...
has dips at the wavelengths corresponding to resonances with various cladding modes (in a single-mode fiber).
Depending on the
symmetry
Symmetry (from grc, συμμετρία "agreement in dimensions, due proportion, arrangement") in everyday language refers to a sense of harmonious and beautiful proportion and balance. In mathematics, "symmetry" has a more precise definit ...
of the
perturbation
Perturbation or perturb may refer to:
* Perturbation theory, mathematical methods that give approximate solutions to problems that cannot be solved exactly
* Perturbation (geology), changes in the nature of alluvial deposits over time
* Perturbat ...
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 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 ...
, which are
antisymmetric 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.
References
*S.W. James, R.P. Tatam, "Optical fibre long-period grating sensors: characteristics and application," Meas. Sci. Technol. 14, R49–R61 (2003).
*O.V. Ivanov, S.A. Nikitov, Yu.V. Gulyaev, "Cladding modes of optical fibers: properties and applications," Physics-Uspekhi 49, 175-202 (2006).
*T. Erdogan, "Cladding-mode resonances in short- and long-period fiber grating filters", J. Opt. Soc. Am. A, 14, pp. 1760–1773, 1997.
External links
Theory of LPFGs (Fiber Optic Research Center)
Fiber optics
Diffraction