Abstract
The fabrication of Bragg reflectors in hydrogenated, all-silica, fluorine cladding depressed and microstructured optical fibers using 248 nm, 5 ps laser radiation, is investigated here. Comparative Bragg grating recordings are performed in both optical fibers, for investigating effects related to the scattering induced by the capillary micro-structure, to the photosensitivity and index engineering yield. Further, finite difference time domain method is employed for simulating the scattering from the above capillary structure and the nominal intensity reaching the fiber core for side-illumination. The maximum modulated refractive index changes inscribed in the standard, step-index fiber were of the order of 8.3x10-5, while the maximum refractive index changes inscribed in one of the microstructured optical fibers was 32% lower and 5.7x10-5, for nominal pulse intensities of 20 GW/cm2 and modest accumulated energy densities.
Highlights
The straightforward and high yield inscription of strong, photosensitive Bragg reflectors and long-period periodicities in all-silica microstructured optical fibers (MOFs) [1], is a significant step towards the development of functional photonic devices based on these unique optical platforms
Focusing on the case of Bragg reflectors, there are two fundamental parameters that dominate the yield of the inscription process: the low photosensitivity of the pristine silicate glass matrix, and the scattering effects induced by the guiding capillary microstructure
That anomaly may indicate the occurrence of a manifold underlying photosensitivity mechanism, which possibly may include some contribution from stresses and singularities that preferentially take place at the fluorinated silica core-cladding interface [26]
Summary
The straightforward and high yield inscription of strong, photosensitive Bragg reflectors and long-period periodicities in all-silica microstructured optical fibers (MOFs) [1], is a significant step towards the development of functional photonic devices based on these unique optical platforms. Fused silica is an optical material of 9.3 eV bandgap, and low intrinsic defect concentration, while exhibiting a high resistance to radiation induced damage These unique material characteristics prompt the use of high intensity and photon energy lasers as a promising route for the inscription of exploitable refractive index changes in that glass matrix. Our approach refers to idealized cases, where the fiber exhibits known orientation with respect to the laser beam This analysis will be used for obtaining some characteristic figures related to the amount of light that reaches the core of the fibers examined here; which in turn can be used as a rough guide for justifying/understanding the experimental results. The 248 nm photon highly overlaps with the oxygen deficiency and hydrogen based defect centers absorbing at 5 eV vicinity, increasing the possibility of single photon related excitations, which in turn may contribute to the overall photosensitivity yield [16, 17]
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