Phase-shifted fiber Bragg grating filters based on counter-propagating cladding modes coupling.

Area of Science:

• Photonics and Optical Engineering
• Fiber Optic Sensing
• Nanophotonics

Background:

• Conventional fiber Bragg gratings (FBGs) have limitations in achieving ultra-narrow bandwidths.
• Cladding mode coupling in FBGs is typically avoided, but offers unique spectral control opportunities.

Purpose of the Study:

• To analyze counter-propagating cladding mode assisted phase-shifted FBGs.
• To propose and investigate an ultra-narrow bandwidth laser line filter based on these gratings.
• To explore their potential in high-accuracy sensing and optical communication systems.

Main Methods:

• Full vector modal analysis to understand mode guiding and coupling.
• Incorporation of single or multiple phase shifts to tailor transmission spectra.
• Investigation of grating parameters (length, strength, phase shift location, apodization) effects on linewidth.

Main Results:

• Phase shifts create tunable narrowband transmission windows within the FBG stopband.
• The proposed grating offers bandwidths two orders of magnitude smaller than conventional LPGs.
• Refractive index-based tuning of resonance wavelength is achievable via evanescent field access.

Conclusions:

• Phase-shifted FBGs utilizing counter-propagating cladding modes enable ultra-narrow bandwidth filtering.
• These gratings are suitable for high-accuracy sensors and all-fiber demultiplexers.
• The study provides a physical explanation for cladding mode coupling in these novel gratings.