Long-period fiber grating

In contrast to the fiber Bragg gratings, 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).

Various LPFG-based devices have been developed: filters, sensors, fiber dispersion compensators, etc.