Abstract
A new measurement technique for characterizing the magnitude of power
coupling from the fundamental mode to higher order core- and cladding-bounded
modes occurring in a fiber Bragg grating (FBG) inscribed in a large-mode-area
(LMA) fiber is demonstrated and studied. The method is based on inducing mode
selective fiber bending losses on the modes propagating in the core and monitoring
the power guided by the cladding of the LMA fiber. Besides transmitted, also
reflected distributions of modes can be resolved in terms of the relative
powers carried by them and thus the fraction of higher order modes (HOMs)
can be quantified. Additionally, the method can distinguish the mode content
spectrally with high resolution. Sample FBGs having a chirped index profile
are characterized using the method. The effect of direction reversal of light
propagation on mode coupling in these gratings is also studied. Distinct wavelength
regions corresponding to the fundamental mode coupling to the reflected fundamental
mode, core HOM and cladding modes can be identified. The measurement method
also reveals a wavelength dependent fine structure of the modal coupling,
i.e., at certain wavelengths there can be coupling to all of the above mentioned
modes simultaneously with varying amounts. These effects can be partly attributed
to the phase matching between different modes being dependent on the spatial
location in chirped gratings. It is shown that the method can yield information
that is useful for better design and optimization of fiber optic devices utilizing
FBGs in LMA fibers, such as fiber lasers.
© 2011 IEEE
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