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9.1 Conclusions

In this research, the fabrication of periodically poled lithium niobate (PPLN) and MgO-doped lithium niobate (PPMgLN) with grating periods ranging from 30µm to 5.0µm has been demonstrated. Different poling methods have been tried and compared, the work also shows that the grating quality of PPLN is determined by several factors, in particular, supplier variation, grating pattern design, thickness of the photo-resist, and the UV exposure time have all been investigated and analysed in detail.

It is widely believed that poling processes are governed by two mechanisms, nucleation of new domains and domain wall spreading under applied field [1,2,3]. The surface quality of the sample thus directly influences poling quality. Two factors were considered in this research, conductivity and surface defects, generally low conductivity is good for poling, because the low conductivity can reduce charge motion beneath the patterned electrodes [4], and surface defects are undesirable for forming new domains in term of uniformity. It was found that by carefully baking the sample after spin-coating the photo-resist can effectively reduce the spontaneous poling dots on the surface of sample, and, therefore, improve the poling quality.

The photo-resist thickness is more critical for fine period gratings than for longer period ones, increasing the thickness of the photo-resist was found to greatly improve the quality of poling. Good quality PPLN samples with fine period gratings were obtained by using 1.8µm thick photo-resist S1813.

The grating pattern design on the mask is crucial for high poling quality of PPLN, a new mask was designed for different period gratings. During the scope of this project, it was found that for large period gratings, better poling quality was achieved by using multiple small openings within each domain of the grating, allowing an increased

Chapter 9: Conclusions and Future Work

density of edges and corners in the photo-resist. For fine period gratings, narrow width gratings or a set of narrow sub-sections in the grating structure result in better poling quality. In addition, the mark-space ratio of the mask is important for poling quality, for large periods of PPLN, a suitable mark-space ratio is 40:60. For small periods of PPLN, a 30:70 mark-space ratio is suggested.

There is little difference in outcome between gel and metal electrodes used for the poling processes, but the gel method does show some advantages in lower electrode remnant on the surface of the PPLN grating after poling. However, Al electrodes are much better than gel electrodes in the backswitch poling method [5].

HexLN samples with a spatial period of 18.05µm and PPLT bulk samples with periods ranging from 7.0µm to 10µm were successfully fabricated. A planar buried waveguide was fabricated by Dr. Gallo in a HexLN sample using proton exchange. Second harmonic generation (red light) and third harmonic generation (green light) for 1536nm wavelength were observed. A PPLT sample was characterized by A. A. Lagatsky et al., where 40% conversion efficiency was obtained.

A novel process for forming zinc indiffused channel waveguides in PPLN was demonstrated. Both TE and TM modes are well confined, and guided in the zinc waveguides thermally indiffused at temperatures of 900°C or above. The SHG conversion efficiency for a 1552.4nm wavelength was measured, individual modes of the second harmonic were observed by tuning the temperature, a peak conversion efficiency of 59%W-1⋅cm-2 was achieved at the room temperature. Using a short pulse source and an appropriately short waveguide, an SHG conversion efficiency of 81% was achieved.

Visible light generation by using the PPLN waveguide devices was investigated. Green light (532nm) was obtained by frequency doubling incident beams from a Nd:YAG laser and a conversion efficiency of 16.7%W-1⋅cm-2 for first order SHG was achieved. Blue light (417.5nm) SHG was achieved through third order QPM by frequency doubling a tuneable titanium sapphire (Ti:Al2O3) laser and a conversion

9.2 The Future Work

Despite the accomplishments of this research, a great deal remains to be done to realize the full potential of electric field periodic poling techniques and zinc indiffused PPLN waveguide devices. Further investigations are needed to optimize the waveguide fabrication conditions for different wavelengths. Efforts to achieve shorter domain periods while maintaining duty cycle uniformity over large areas must continue. With even finer periods to allow first order operation in the blue and better power handling capability, greater conversion efficiency for visible lasers can be realized. The following topics provide important areas for future work.

9.2.1 Segmented Tapered Waveguides

The input coupling efficiency of the waveguides in experiments described in the previous chapters is around 20-35% which is lower than ideal. The main reason is that the NA and mode size of a launched fibre (or the input beam through the objective lens) cannot match with that of the zinc indiffused channel waveguide due to the different refractive index and core size of the waveguide. The mode size of the single mode zinc indiffused waveguide can be modified to match better that of the fibre through decreasing the ∆n (refractive index difference) between the core and substrate, but a small ∆n also results in a waveguide that is more sensitive to the deviation of core size, hence decreasing the SHG efficiency.

Axial tapering of a dielectric waveguide structure is an effective method in transforming the mode properties of the input beam. In a tapered waveguide the mode size can be transformed in different portions of the waveguide. This increases the input and output coupling efficiency [6,7]. Tapered waveguide structure also allow effective coupling between single-mode to multi-mode waveguides, which is important in nonlinear guided-wave mixers, for example SHG device as described above.

The axial tapering of a dielectric waveguide can be effectively achieved by using a segmented taper structure which consists of segments that repeat with a period Λ.Each segment with the length of l would be zinc indiffused producing an index-change ∆n and is separated by an un-diffused region [8], Figure 9.01(a) shows a schematic plot

Chapter 9: Conclusions and Future Work

of a periodically segmented waveguide. Based on theoretical and experimental investigations in previous literature [9,10,11,12], the equivalent refractive index in the segmented waveguide is taken to be: neq =∆nΓ, where the duty cycle Γ=l/Λ.

Figure 9.01. Segmented taper waveguide structure. (a) Schematic picture of a

periodically segmented waveguide; (b) Tapered waveguide using segmented structure. Duty cycle and the width of segments can be modified simultaneously to optimize the mode size transformation.

The effective refractive index of the waveguide can be controlled and modified by changing the physical width and duty cycle of the segmented structure, resulting in mode size transformation in different portions of the waveguide, as illustrated in figure 9.01(b).

9.2.2 Further Improvement of Device Efficiency

Although high conversion efficiency has been demonstrated for a 1550nm wavelength in this research, there is still a need to increase the conversion efficiency especially for visible light generation. Fine period PPLN gratings are crucial for first order QPM blue light generation, improving the uniformity of the PPLN grating for fine periods and the quality of the waveguide could effectively increase the conversion efficiency. Thus the existing poling techniques need to be improved in order to get fine periods of PPLN with high quality.

Recently, several poling methods have been reported to pole fine period PPLN, such as using the corona technique and the atomic force microscopy (AFM) poling method [13,14]. PPLN sample with periods lower than 1µm have been obtained, results show