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Single-mode sapphire fiber Bragg grating

Mohan Wang, Patrick S. Salter, Frank P. Payne, Adrian Shipley, Stephen M. Morris, Martin J. Booth, and Julian A. J. Fells

Open Access Open Access

Abstract

Sapphire optical fiber has the ability to withstand ultrahigh temperatures and high radiation, but it is multimoded which prevents its use in many sensing applications. Problematically, Bragg gratings in such fiber exhibit multiple reflection peaks with a fluctuating power distribution. In this work, we write single-mode waveguides with Bragg gratings in sapphire using a novel multi-layer depressed cladding design in the 1550 nm telecommunications waveband. The Bragg gratings have a narrow bandwidth (<0.5 nm) and have survived annealing at 1000°C. The structures are inscribed with femtosecond laser direct writing, using adaptive beam shaping with a non-immersion objective. A single-mode sapphire fiber Bragg grating is created by writing a waveguide with a Bragg grating within a 425 µm diameter sapphire optical fiber, providing significant potential for accurate remote sensing in ultra-extreme environments.

Published by Optica Publishing Group under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

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Supplementary Material (12)

NameDescription
Data File 1       "Single-mode sapphire fiber Bragg grating"Data File 1 (Underlying data for Figure 2(c)Experimental measured dimensions of the laser-induced single-tracks using different pulse energies and repetition rates
Data File 2       "Single-mode sapphire fiber Bragg grating"Data File 1 (Underlying data for Figure 4(a)Measured guided mode profile of a sapphire depressed cladding waveguide for TE mode
Data File 3       "Single-mode sapphire fiber Bragg grating"Data File 1 (Underlying data for Figure 4(b)Measured guided mode profile of a sapphire depressed cladding waveguide for TM mode
Data File 4       Single-mode sapphire fiber Bragg grating Data File 1 (Underlying data for Figure 7(c)Measured guided mode profile of a sapphire waveguide Bragg grating
Data File 5       "Single-mode sapphire fiber Bragg grating"Data File 1 (Underlying data for Figure 7(d)Measured reflection spectrum of a sapphire waveguide Bragg grating
Data File 6       "Single-mode sapphire fiber Bragg grating"Data File 1 (Underlying data for Figure 8)Measured reflection spectrums of second-order sapphire waveguide Bragg grating before and after annealing at 1000 degreeC
Data File 7       "Single-mode sapphire fiber Bragg grating"Data File 1 (Underlying data for Figure 9(d)Measured guided mode profile of a sapphire fiber Bragg grating
Data File 8       "Single-mode sapphire fiber Bragg grating"Data File 1 (Underlying data for Figure 10)Measured reflection spectrum of a sapphire fiber Bragg grating
Visualization 1       Video of femtosecond laser fabrication in sapphire. Fabrication of waeguide with Bragg grating.
Visualization 2       Video of femtosecond laser fabrication in sapphire. Fabrication of waeguide with Bragg grating.
Visualization 3       Video of femtosecond laser fabrication in sapphire. Fabrication of waeguide with Bragg grating.
Visualization 4       Video of femtosecond laser fabrication in sapphire. Fabrication of waeguide with Bragg grating.

Data availability

Data underlying the results presented in this paper are available in Data File 1, Data File 2, Data File 3, Data File 4, Data File 5, Data File 6, Data File 7 and Data File 8 of the supplementary information.

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Figures (10)
Fig. 1.
Fig. 1. The femtosecond laser fabrication system.

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Fig. 2.
Fig. 2. Top-view (a) and cross-sectional view (b) of the laser-induced single-tracks using a repetition rate of 1 MHz and a scan speed of 11 mm/s, at increasing laser pulse energies, measured using a microscope; the red bar indicates a length of 20 µm; (c) graph of width (triangle) and height (square) of the femtosecond laser-written single-tracks for different pulse energies, at repetition rates between 10 kHz and 1 MHz. See Data File 1 for underlying data.

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Fig. 3.
Fig. 3. (a) the DCW design; (b) microscope image of the fabricated DCW from the side facet; and (c) the measured mode profile at 1550 nm with the waveguide design superimposed on top.

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Fig. 4.
Fig. 4. The experimentally measured guided mode profiles of a DCW at 1550 nm for (a) TE mode (See Data File 2 for underlying data) and (b) TM mode (see Data File 3 for underlying data). Adjacent are their respective mode fields along the axis (blue straight line) together with a Gaussian fit (orange dashed line).

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Fig. 5.
Fig. 5. (a) Experimentally measured mode intensity profile of the DCW fitted to the analytic solution to the wave equation; (b) mode loss as a function of cladding to core ratio, calculated from an analytic expression in Ref. [25].

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Fig. 6.
Fig. 6. (a) cross-sectional views for Step 1(a1), Step 2 (a2), and Step 3 (a3) during the three-step fabrication process, (b) diagram of the three-step multi-layer WBG fabrication process, and (c) the top-view of a fabricated multi-layer second-order WBG (Step 3). The red bars indicate a length of 20 µm. See Visualization 1, Visualization 2, Visualization 3, and Visualization 4 for video clips of the laser writing process.

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Fig. 7.
Fig. 7. (a) The design schematic of the multi-layer WBG, the red bar indicates a length of 10 µm, (b) the cross-sectional view of the fabricated sample, measured using a transmission microscope, (c) experimentally measured guided mode at 1550 nm, with the mode profile along the horizontal and vertical central axis plotted at the top and the left axes (see Data File 4 for underlying data), and (d) the experimentally measured reflection spectrum (see Data File 5 for underlying data).

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Fig. 8.
Fig. 8. The reflection spectrum of a WBG before and after annealing. See Data File 6 for the underlying data.

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Fig. 9.
Fig. 9. (a) Top-view microscope image of a sapphire fiber with an FBG inscribed in the center, (b) a magnified view of (a), (c) microscope image of the end-facet, with a three-layer FBG (Inset: magnified view with the orange bar indicating a length of 20 µm); and (d) the measured transmitted mode profile (See Data File 7 for the underlying data).

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Fig. 10.
Fig. 10. The experimentally measured reflection spectrum of the single-mode sapphire FBG. See Data File 8 for the underlying data.

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