Abstract
The symmetry of cladding modes excited in microbend and arcinduced long-period fiber gratings is investigated. An optimization technique is developed to determine the fiber parameters and to associate grating resonances with cladding modes of a particular symmetry. Using this optimization procedure, we show that the gratings induced in a standard fiber by arc discharges and microbends couple light to the antisymmetric cladding modes. In the case of a boron-germanium codoped fiber, the cladding modes excited by arc-induced gratings are found to be symmetric. Measurements of the near-field intensity distribution of cladding modes confirm the mode symmetry ascertained by the optimization technique.
Highlights
A long-period fiber grating (LPFG) is a fiber structure with a periodic perturbation of its refractive index (RI), which couples the core mode and copropagating cladding modes
It is unclear if symmetric or antisymmetric perturbation is induced by the arc discharge and, correspondingly, if symmetric or antisymmetric cladding modes are involved in the coupling in these gratings
The shift of resonance wavelength of a standard LPFG as a result of a 2% change in the difference between the core and cladding RIs is of the order of 30 nm. Some parameters such as core radius or dopant concentration are frequently unknown or unspecified. Such fiber parameters as refractive index, core and cladding radii can be changed as a result of grating formation in the fiber: refractive index is changed in the photoinduced gratings and, in addition, fiber dimensions are changed in the arc-induced gratings
Summary
A long-period fiber grating (LPFG) is a fiber structure with a periodic perturbation of its refractive index (RI), which couples the core mode and copropagating cladding modes. But no predominant mechanism was found, which indicates that more than one may contribute to the grating formation It is unclear if symmetric or antisymmetric perturbation is induced by the arc discharge and, correspondingly, if symmetric or antisymmetric cladding modes are involved in the coupling in these gratings. The symmetry of cladding modes can be determined by comparison of the experimental spectrum of an LPFG and the spectrum obtained by simulation for modes of a particular symmetry Such a comparison is impossible because usually the accuracy of fiber parameters given in the specification sheet is insufficient for accurate reproduction of LPFG spectra and for discrimination between symmetric and antisymmetric modes. We employ an optimization technique to fit experimental and theoretical dependences of resonance positions on the grating period, which allows us to determine the fiber parameters. The results obtained by the optimization technique are supported by measurements of the near-field intensity distributions of the cladding modes
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
