Fibre Bragg gratings (FBG) are periodic changes of the refractive index in the core of optical fibres and are written to filter specific wavelengths from a broadband spectrum to be re-routed to another area of an optical network. The filtered wavelengths, or channels, act as a means for carrying differentiated information down the optical fibre. To maximise the amount of information that can be transmitted through the optical fibre, channels may be as close as 100 GHz, or 0.2 nm at l = 1550 nm. However, temperature fluctuations experienced by FBGs cause the mean filtered wavelength to shift as much as 1.5 nm. Either an increase in channel spacing is required, which will reduce the network capacity, or the temperature-dependent shift in Bragg wavelength must be eliminated. Compressing the grating as the temperature increases can reduce the shift in Bragg wavelength. This can be done by mounting the FBG on a platform with the appropriate negative thermal expansion. This research presents a device to reduce the temperature-dependent shift. The dimensional changes of the device were studied both theoretically and experimentally with predictions giving good agreement with experimental results. It is also shown that the tolerances are improved six-fold over current devices. It gives the smallest temperature-dependent shift in Bragg wavelength over the required temperature range that has been reported. These experiments show that the temperature-dependent shift in Bragg wavelength is non-linear and to further reduce the shift requires a platform with a non-linear coefficient of thermal expansion. No such device currently exists. It is shown that the structure presented can be modified to give the required non-liner strain. A theoretical analysis describes the effects of temperature on the dimensional changes and shows it can obtain the required non-linearity with the required overall thermal expansion with suitable modifications. Experimental results show the structure non-linearly compensates for the temperature-dependent shift in Bragg wavelength. However, further work is required to achieve manageable tolerances to completely thermally compensate a FBG.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:604676 |
| Date | January 2004 |
| Creators | Howlett, A. L. |
| Publisher | University of Cambridge |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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