Conclusions

The results of DIC imaging experiments performed on various FBG samples that were presented and analysed in this chapter are summarised in Table 3-8. The refractive index structure of the type I and IIA FBGs written with a prism interferometer (samples Ip-1 and IIAp-1) were imaged. The refractive index structure of phase-mask-written FBGs were also investigated, including a type IIA FBG (sample IIA-1). Additionally, type I FBGs (samples I-1 to I-4) fabricated in standard and smaller core fibres with the standard and custom- made phase masks were investigated.

Sample Fibre Core diameter

(μm) Type Fabrication Details (mask pitch) FBG period Λ (± 0.05µm) Parallel image phase shift (rad) Talbot length ZT (± 0.5 µm) Ip-1 F-4 7.4 I prism 2.50 - - IIAp-1 F-2 8 IIA - - - IIA-1 F-2 8 IIA SM-2: 1.0668 µm 1.09 (1.02 ± 0.02) π 4.6 I-1 F-1 7.4 I 1.08 (1.0 ± 0.1) π I-2 F-3 3.6 I 1.09 (1.0 ± 0.1) π I-3 F-1 7.4 I _CM-2: 1.07 µm 1.09 (1.03 ± 0.01) π 4.6 I-4 F-3 3.6 I 1.09 (1.0 ± 0.1) π

Table 3-8. Summary of the imaging experiments and results.

The images of type I and IIA FBGs written with a prism interferometer were presented in sections 3.3.1 and 3.3.2, respectively. The expected structures for pure two-beam interference patterns could not be resolved in the images. The observed structures were complex but Talbot beating was not observed in the images. FFTs of features in the type I prism FBG images revealed much larger periods than the expected pitch of approximately 0.54 µm for two-beam interference. Three out of the six images presented for sample Ip-1 revealed grating features with a period of 2.50 ± 0.05 µm which is inconsistent with the spectral results for the sample which demonstrated strong grating reflections at approximately 1541.7 nm. According to Equation 1-3 and assuming that

from a grating period of 2.5 µm. The observed features with a period of 2.5 ± 0.05 µm are therefore likely to be an artefact. The prism-written fibre gratings are believed to possess a period closer to 0.54 µm which could not be resolved in the images due to the resolution limit of 0.58 µm of the microscope system. This limit did not prevent resolution of phase mask FBG features since the periods were around 1 µm.

The images identified as having been measured in the parallel and perpendicular planes of the type I and type IIA FBGs written with either the standard or custom-made phase masks were presented in sections 3.4 and 3.5. The images recorded in the plane parallel to the direction of the UV writing beam all revealed index perturbations which were distributed uniformly across the core as expected. The images recorded in the perpendicular plane

all revealed the existence of π phase-shifted gratings with a period consistent with the period of the phase mask used in fabrication. The measured Talbot lengths of the refractive index patterns extending across the core in the perpendicular images of the standard and custom-made phase mask FBGs were all consistent with the respective Talbot lengths expected for beating between the ± 1st _{and ± 2}nd _{orders of each mask, as detailed in Table 3-2.}

The imaged structures of the larger core FBGs written with the standard and custom phase masks, presented in sections 3.4.2 and 3.5.1, respectively, revealed full periods of the Talbot diffraction patterns in the core. However, the imaged structures of the smaller core FBGs written with the standard and custom phase masks, presented in sections 3.4.3 and 3.5.2, respectively, highlighted the size discrepancies between the core diameters and the expected Talbot lengths. The core diameter of the smaller core fibre prevents a full Talbot length from being formed across the core. This is expected to affect the spectral properties of FBGs written in this fibre, which will be investigated in Chapter 4.

Considering the differences in generated interference patterns and exposure fluences between the standard and custom made phase mask samples, it is difficult to compare the effect that each phase mask has on the images of the

resulting FBGs. Additionally, the measured intensity variations in the images were normalised for each sample and cannot be used to determine relative refractive index changes. While the intensity variations in the custom made phase mask images (samples I-3 and I-4) appear stronger than those in the standard phase mask images (samples I-1 and I-2), it is possible that this qualitative observation is due to the larger exposure fluences required to obtain high reflectances in the custom made phase mask samples. The effect of the higher order contributions of the custom made phase mask on the spectral properties of these FBGs will investigated in Chapters 4 and 5.

Chapter 4:

Spectral Characterisation

his chapter investigates the effect of the phase mask and prism interferometer techniques on the spectral properties of the different types of gratings which were discussed in Chapters 2 and 3. The Bragg wavelength and other spectral regions are investigated where reflections are expected due to the complex refractive index structure.

The techniques used for obtaining optical spectra are described and the spectra of type I and IIA FBGs are then investigated and compared. The transmission spectra measured after fabrication of FBGs written using standard and custom-made phase mask techniques are compared in different spectral regions under multimode and singlemode propagation. The spectral consequences of the π phase-shifted gratings observed in the DIC images are then discussed and, finally, the spectral properties of prism interferometer FBGs are investigated.