Source: http://aoot.osa.org/oe/abstract.cfm?uri=oe-25-26-32879
Timestamp: 2019-04-19 03:11:39+00:00

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Using point-by-point infrared femtosecond laser inscription, a fibre Bragg grating with a resonance wavelength of 1550.28±0.02 nm, 25 dB extinction ratio, and 1.10±0.02 nm bandwidth (measured at first minima) was inscribed in a high concentration (40 mol%) germania-doped silica fibre. At this wavelength, the high concentration germania doped silica fibre had a normalized frequency of V ≃ 3.06 permitting higher order mode propagation. Subsequently, two additional Bragg resonances were recorded at 1534.40 ± 0.02 and 1535.78 ± 0.02 nm, corresponding to the coupling of the forward propagating fundamental mode and counter propagating HE21 and TM01/TE01, respectively. Thermal tests revealed the grating was stable up to 800 °C for 30 minutes. Analysis determined the grating had first and second order coefficients dλB/dT = 11.1 pm/°C and d2λB/dT2 = 8.37 × 10−3 pm2/°C2 respectively.
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Fig. 1 Refractive index profile of the NA~0.41 germania-doped silica fibre showing a slight graded index distribution between cladding and core.
Fig. 2 Schematic of the femtosecond inscription set-up. O-Objective, D-dichroic mirror, F-fibre, DS-diffraction slit, (C)-synchronisation controller, CCD-CCD camera, OSA-optical spectrum analyser.
Fig. 3 (a) Transmission spectra of the FBG of resonance wavelength λB = 1550.28 nm. Additional resonances at λB = 1534.40 nm and λB = 1535.78 nm were also observed, attributable to the forward propagating HE11 mode coupling to the counter propagating higher order modes. (b) Simulated electric field (x component) of the HE11, TE01, TM01 and HE21 modes. The HE11 and HE21 had effective modal indices of neff = 1.4850 and neff = 1.4578, respectively. The TE01 and TM01 modes were degenerate with an effective modal index of neff = 1.4583.
Fig. 4 (a) Transmission spectra of the FBG during thermal testing. The spectra only shows the shift of the fundamental Bragg resonance of λB = 1550.28 nm (at room temperature) from 50 to 750 °C. All other spectral features were observed to shift uniformly with the main Bragg resonance. (b) Thermal sensitivity plot of the high concentration germania fibre (solid). Regression analysis determined a quadratic correlation between the Bragg wavelength shift and temperature. Dataset 1 is available at .
Table 1 – Calculated effective modal indices and measured wavelengths of Bragg resonances. Below, a comparison of the simulated and theoretical results using Eq. (1) and (2). The simulated and theoretical results are in agreement to 1%.
– Calculated effective modal indices and measured wavelengths of Bragg resonances. Below, a comparison of the simulated and theoretical results using Eq. (1) and (2). The simulated and theoretical results are in agreement to 1%.

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