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Significance Statement
Gratings in optical fibers have been identified as periodic modulations of the fiber structure that couple energy between different modes. They have been classified in two categories, which are Bragg gratings with periodicity matching half a guided wavelength, and long period gratings with a period longer than the wavelength. Bragg gratings couple energy at selected wavelengths between counter propagating modes and to radiation modes. Long period gratings couple energy between co-propagating modes.
The bandwidth of the Bragg gratings is in the range of a few pm to hundreds of pm. On the other hand, long period gratings have broader spectra due to the fact that the number of periods of the gratings is smaller. Their bandwidth ranges between a few nanometers to hundreds of nanometers. To reduce the rejection bandwidth below 1nm, association of gratings in coaxial interferometric configuration is needed. However, individual gratings possess a larger bandwidth while the spectral sub-bands cannot be designed independently.
The need for designing long period fiber gratings with narrow rejection bands for core—cladding conversion is still outstanding. These grating can be implemented in signal processing, rejection filters for telecommunications, and DWDM components. Therefore, researchers led by Dr. José Luis Cruz at the University of Valencia in Spain demonstrated a long period grating written in an optical fiber with 3dB bandwidth below the frontier of 1nm at 1527 nm and more than 100 nm of free spectral range. In their work, they discussed the fabrication procedure, grating attributes and sensitivity to mechanical and thermal perturbations. Their work is published in Optics Letters.
In their experiments the authors used a fiber with NA of 0.29. The selected fiber had a modal field diameter of 4.2µm and a cutoff wavelength of 1387nm. They wrote a Bragg grating in order to get the information about the indices of the cladding modes. They then inscribed the Bragg grating over the fiber through a phase mask. A laser beam at 244nm was focused with lens, and the flux of energy in the fiber approximated as 160J/mm2. A number of couplings to cladding modes appeared at short wavelengths. The main series of resonances corresponded to HE1m and EH1m cladding modes. A secondary series of notches appeared between notches of the main series. This series corresponded to modes TE0m, TM0m and HE2m. These resonances have been observed to appear in strong gratings owing to fiber imperfections as well as fabrication insufficiencies.
The research team demonstrated the narrowband long period gratings fabrication implementing a high NA fiber with core and cladding modes having differences between indices larger than those of low NA fibers. The period of the gratings was short in this fiber, therefore, the number of periods for a selected length was higher, and as a result, the bandwidth was reduced.
Bandwidths of 0.83nm were realized in 15cm long gratings and were observed to have excellent spectral quality. The bandwidth was reduced to 0.68nm in 20cm long gratings, but the spectral quality was a little bit poor. The spectrum had a 125nm free spectral range. The resonance that corresponded to the HE1,18 mode exhibited superior temperature and strain sensitivities. The stability of the UV beam and room temperature in the course of fabrication forms the basis for further improvement of the main limitations of the study.
Reference
L. Poveda-Wong, J. L. Cruz, M. Delgado-Pinar, X. Roselló-Mechó, A. Díez, And M. V. Andrés. Fabrication of long period fiber gratings of subnanometric bandwidth. Vol. 42, No. 7 / April 1 2017 / Optics Letters.
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