The leaky-mode resonant diffraction gratings fabricated in a high-refractive index layer deposited on the facet of an optical fibre can serve as wavelength- and polarization-selective elements in fibre lasers. We predict a modal reflectance peak as high as 86% with a 3 dB spectral bandwidth of about 50 nm and a polarization extinction ratio of 17 dB using a theoretical model based on a 3D FDTD method. Gratings were fabricated by focused ion beam milling. Experimentally measured transmission spectra were compared with numerical simulations adapted to real grating profiles.
We numerically investigated the properties of diffraction gratings designated for fabrication on the facet of an optical fiber. The gratings are intended to be used in high-power fiber lasers as mirrors either with a low or high reflectivity. The modal reflectance of low reflectivity polarizing grating has a value close to 3% for TE mode while it is significantly suppressed for TM mode. Such a grating can be fabricated on laser output fiber facet. The polarizing grating with high modal reflectance is designed as a leaky-mode resonant diffraction grating. The grating can be etched in a thin layer of high index dielectric which is sputtered on fiber facet. We used refractive index of Ta2O5for such a layer. We found that modal reflectance can be close to 0.95 for TE polarization and polarization extinction ratio achieves 18 dB. Rigorous coupled wave analysis was used for fast optimization of grating parameters while aperiodic rigorous coupled wave analysis, Fourier modal method and finite difference time domain method were compared and used to compute modal reflectance of designed gratings.
We investigated behaviours of 1D binary diffraction gratings fabricated on optical fiber facets. Only sub-wavelength structures were considered to suppress higher diffraction orders. The aperiodic rigorous coupled wave analysis, the Fourier modal method and the finite-difference time-domain numerical methods were used to compute and optimize the modal reflectance of the investigated structures. Optimized gratings were milled on fiber facets by focused ion beam. One of the gratings was tested in a thulium-doped fiber laser where it acted as a low re ectivity polarizing output mirror. A slope efficiency of the laser and a beam quality parameter were conserved while lasing threshold slightly increased.
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