El proceso del pensamiento

The objective of the thesis was the fabrication, characterisation and modelling of optically active aluminium oxide thin films with the appropriate characteristics to serve as elements in integrated optoelectronics.

The main experimental aim of the project was to dope aluminium oxide thin films with erbium, and erbium and ytterbium by both plasma-enhanced chemical vapour deposition (PECVD) and ion implantation and to compare their photoluminescence characteristics with those from rare-earth doped silica thin films. The predicted signal gain from Br-doped alumina waveguides is studied and compared to the corresponding signal gain from Er- doped silica waveguides.

8.2 Summary Of Achievements

(1) Amorphous AI2O3 thin films were deposited by PECVD using both TMA and TMAA. The compatibility of TMAA with the PECVD process was demonstrated. The thin films are stoichiometric AI2O3 with low carbon contamination. The impurity content ranged from <0.2At% to 1.3At%. The refractive index of the deposited thin films ranged from 1.54 to

1.63, at 633nm, and the RMS surface roughness was as low as 4.3nm.

(2) Optically active Er^+ -doped AI2O3 thin films were grown by PECVD using an organic Er precursor; Er(thd)3. The thin films exhibited room-temperature photoluminescence at 1.53p.m. The FWHM of the Er^+ emission is as high as ~60nm; it is significantly broader than the EWHM in Si0 2 glass (circa 20nm) and in Si0 2: AI2O3 : Er3+ material (43nm). The Er^+ concentration ranged from 0.01At% to 0.2At%. The 1/e fluorescence lifetime was measured at 50mW pump laser power to be 2ms.

(3) The AI2O3 matrix was co-doped with both Er3+ and Yb^+ by ion implantation and optically active Er3+/Yb^+ co-implanted AI2O3 thin films and sapphire crystals were prepared. The implanted samples showed room-temperature photoluminescence at 1.53p.m. The FW HM of the Er^+ emission is as high as 67nm. The Er^+ and Yb^+ concentrations ranged from 0.4At% to ~ lA t% and from 2.4At% to ~8At%, respectively. The 1/e fluorescence lifetime was measured at 50mW pump laser power to be 4.2ms. High rare- earth concentrations were incorporated in the AI2O3 matrix without dramatic concentration quenching effects. The existence of an energy exchange mechanism between Er3+ and Yb^+ was observed.

Chapter 8: Conclusions

(4) The signal gain characteristics of Er^+ -doped AI2O3 and Er3+ -doped Si0 2 waveguide amplifiers were analysed and compared using the finite-element technique. The analysis showed that the AI2O3 waveguide amplifiers show improved gain performance compared to Si0 2 waveguide amplifiers mainly due to the lack of pair-induced up-conversion in the AI2O3 host. In addition, Er3+ -doped and Er3+/Yb3+ co-implanted AI2O3 thin films exhibit a very broad emission linewidth. The above characteristics make Er^+ -doped AI2O3 optical waveguides suitable as optical amplifiers at the very im portant telecom m unication wavelength 1.53|Lim, either for signal amplification or loss compensation or as elements in W DM systems. The larger optical bandwidth, compared to Er^+ -doped Si0 2 amplifiers would appear to provide a larger number of optical channels or an increase in the spacing between channels leading to reduced crosstalk.

Two 16-channel WDM transmission systems consisting of Er^+ -doped AI2O3 and Er^+ -doped Si0 2 amplifier cascades were also analysed. It was found that the transmission system using AI2O3 amplifiers showed much better performance compared to the one using Si0 2 amplifiers, leading to reduced gain peaking and thus larger maximum transmission distance. The transmission distance of 3,250km achieved for the 16-channel system using Er^+ -doped AI2O3 amplifiers makes them suitable for use in dense WDM systems for optical networks.

In general, as was discussed in chapters 4, 5 and 6 the AI2O3 thin films deposited during the present research work did not pocess good waveguiding characteristics. However, low loss material is of paramount importance in order for such waveguides to find applications as optical amplifiers at 1.5p.m. Clearly improvements must be made to both the deposition process and the experimental set-up, used for the deposition of the thin films in order to produce uniform, low loss material.

One of the main drawbacks of using Er^+ -doped AI2O3 optical waveguides as in-line optical amplifiers in optical communication systems that could compromise their use as a commercially deployable solution would be the difference in refractive index between the AI2O3 waveguides and the Si0 2 optical fibres which would result in high coupling losses. However, there are reports [4] of the planar fabrication process of a high (98%) coupling efficiency interface between optical waveguides of large index difference ( -1.8).

8.3 Future Work

The research work presented in this thesis demonstrated that high rare-earth concentrations can be incorporated in alumina by ion implantation w ithout dramatic concentration

Chapter 8: Conclusions

convenient and much cheaper fabrication technique than ion implantation. Thus, it would be interesting to compare the photoluminescence characteristics of Er/Yb co-doped alumina thin films by PECVD with those from Er/Yb co-implanted alumina thin films. A suitable Yb precursor could be Yb(thd) 3 since it is compatible with the low temperatures used in PECVD.

During the research work it was found that thermal annealing considerably enhances the photoluminescence intensity from the rare-earth doped alumina films. This is because thermal annealing eliminates both the defects in the matrix and the OH contamination and also because it re-organises the matrix, co-ordinating the Er^+ ions with oxygen ions. However, thermal energy also increases the mobility of the rare-earth ions, and thus at high temperatures (>950°C) the rare-earth ions cluster, reducing the luminescence efficiency of the samples.

A possible way to overcome this problem is maybe to use laser annealing instead of thermal annealing. Thus, it would be interesting to anneal the rare-earth doped alumina samples using pulses from an excimer laser instead of using a furnace. Experiments done on Eu- im planted sapphire crystals dem onstrated that using laser annealing enhances the photolum inescence intensity compared to the one when the samples were thermally annealed [1]. This is probably due to the fact that the laser pulses provide enough energy to anneal out the defects in the matrix but not as much to cluster the rare-earth ions. One of the possible problems associated with laser annealing is the fact that annealing is not uniform for the entire sample area.

As was shown in this thesis, Er^+ -doped AI2O3 waveguide amplifiers could find wide applications in dense WDM systems for optical networks. The higher signal gain and the relatively flat gain spectrum in the 1.54p.m - 1.55|am reduce the detrimental effect of gain peaking and thus increase the maximum transmission distance. One possibility to further reduce the negative effects of gain peaking is to use in-line gain equalising filters. For example, there are reports using two types of gain equalising filters: (1) Notch filters and (2) long-period fibre grating (LPFG) filters [2]. It would be very interesting to study a 16- channel transmission system using Er3+ -doped AI2O3 waveguide amplifier cascades using in-line gain equalising filters and find out whether a further increase in the maximum transmission distance is possible.

One of the limitations imposed by using EDFA's as optical amplifiers is the fact that an efficient amplifier requires the input signals to be within the wavelength range -1540- 1565nm to obtain relatively flat gain [3]. This available bandwidth is much narrower than the 200nm low-loss bandwidth of the optical fibre at 1.55|Lim. The -2 5 n m usable bandwidth of the EDFA will be insufficient to accommodate a WDM network for which (1)

Chapter 8: Conclusions

either many wavelengths are required or (2) channel spacings must be large (>lnm ). Therefore, since AI2O3 host provides a much broader optical bandwidth compared to SiO] it would be very interesting to try and take advantage of the whole Er3+ emission bandwidth (1.52|am-1.56|im).