Characteristics of LPFGs Written by a CO₂-Laser Glass Processing System

Switchable Ytterbium Fiber Laser Based on a Symmetrical Long-Period Fiber Grating

“…Then, due to the micro-tapering of the cladding, the evanescent portion of the electromagnetic field is spreading out to interact with the environment close to the surface of the fiber, enhancing the sensitivity. Finally, at the end of each expanded core region, the fiber returns to its original MFD, exciting the high-order cladding modes LP0m, as is observed in [60], [65]. These modes have axial symmetry, and therefore, the coupling of the fundamental core mode and the cladding modes in this symmetrical LPFG allows to create a wavelength selective fiber [66].…”

“…Further explanation about the fabrication method of the symmetrical LPFG can be found in [60]. At each tapered waist, the cladding diameter is reduced whereas the core is expanded, due to the diffusion of dopants in the core fiber during the CO2 laser heating, obtaining a LPFG geometry not only longitudinally but also axially symmetric, as it is depicted in Fig.…”

“…3. The length of the LPFG is = Λ, where is the number of tapers and Λ is the period of the perturbation [60]. Furthermore, due to the thermal expansion of the core, the mode field diameter (MFD) is also locally expanded at each taper location [62].…”

“…The combination of the geometrical change induced by the tapering and core expansion produces changes in the effective refractive index of the fiber, mainly due to the frozen-in viscoelasticity and mechanical stress. Where the core refractive index decreases, the cladding refractive index increases [60]. This axisymmetric periodic perturbation of the fiber has the advantage of achieving symmetric coupling of energy from the core mode towards cladding modes [5].…”

“…In the case of LPFGs fabricated with CO2 laser, the birefringence increases mainly by azimuthal asymmetry of the laser exposure, since the fabrication typically adopts the surface deformation [58]. However, LPFGs fabricated with a CO2-laser LZM-100 glassprocessing system, which provides uniform heat deposition, allows the construction of axially symmetric LPFGs [59], [60]. It has been demonstrated that the polarization-dependent loss (PDL) of the axially symmetric LPFGs can be reduced to a level comparable to that of the SMF fiber [58].…”