Abstract
During grating inscription in photonic crystal fibers (PCFs) the intensity of the inscribing laser beam is non-uniformly distributed over the core region due to the interaction with the air holes in the fiber's microstructure. In this paper we model and study the non-uniformity of the index modification and its influence on the grating reflection spectra, taking into account the non-linear nature of the index change. For femtosecond laser inscription pulses at 800 nm, we show that the intensity redistribution in the PCF core region can result in Type II index changes even if the peak intensity of the incident beam is well below the corresponding threshold. Our coupled mode analysis reveals that the non-uniform nature of the index change can seriously affect the reflectivity of the grating due to a limited overlap of the guided mode with the transverse index modulation profile for almost all angular orientations of the PCFs with respect to the inscription beam. We also evaluate the influence of PCF tapering and we found that for the considered PCF a significant increase in the induced index change and reflectivity is observed only for taper diameters below 40 μm.
Highlights
The development of photonic crystal fiber (PCF) technology can be considered as a major achievement in the field of photonics
Influence of PCF tapering on the resulting grating efficiency we study the influence of fiber tapering on the induced index change and resulting reflectivity of the grating
In this report we proposed, for the first time to our knowledge, a consistent methodology for modeling the refractive index modification profile and the resulting reflectivity of a grating inscribed in the core region of a PCF with femtosecond laser pulses at 800 nm and taking into account the non-linear nature of the refractive index change
Summary
The development of photonic crystal fiber (PCF) technology can be considered as a major achievement in the field of photonics. We have used such an approach to study the influence of lattice parameters and of the angular orientation of a hexagonal lattice PCFs with respect to the inscribing beam on the transverse coupling efficiency [18,19,27,28] Such simulations can only give partial insights into the influence of the microstructure because they do not account for the effect of the irregularities of the induced index changes on the reflection strength of the grating. We present an extension to our previous approach and we propose a new methodology to study the influence of air holes in PCF cladding region on the resulting grating efficiency for femtosecond laser based inscription methods at 800 nm. To do so we model the intensity distribution, we estimate the resulting induced index change in the PCF core taking the non-linear nature of light-matter interaction into account and we calculate the reflection strength of the inscribed FBG. We note upfront that exact values of the index change will depend on the details of the actual inscription setup, such as the focusing geometry, laser parameters, illumination time, etc., and on the exact composition of the fiber glass, our approach should offer reasonable understanding of the influence of the PCF’s microstructure on the induced index change distribution
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