I have now discussed how to design and fabricate GCs, and the parameters that must be considered to do so. Next, I will discuss the testing procedures for these devices. Firstly, we must decide what parameters we wish to be able to extract from the devices, and then lay out an appropriates set of structures. In particular, we would like to know our grating efficiency and centre frequency, the two parameters that can show how closely the real devices compare to the designs.
The measurement setup used to characterise these devices consists of a tunable laser, polarisation rotator, a polarisation maintaining fibre, the sample, output fibre, and a photodetector. Therefore, characterisation is fibre-to-fibre; which gives the simplest and most realistic expression of the coupling behaviour
10. For 1D devices the PMF is not strictly necessary. The basic device consists
of two gratings connected by a waveguide. Mechanical considerations make the minimum waveguide length approximately 2mm. The total efficiency can be expressed as Ioutput = ηinηoute−αLIinput; where ηin = ηout is the efficiency
of each individual grating. Assuming negligible fibre losses, Iinput is known
as a function of wavelength. The device loss, for a waveguide, is modelled as e−αL, which can be determined by making waveguides of several different
lengths, and using the cutback method. This is important because we expect the waveguides to be quite lossy - the fabrication process is optimised for the gratings, not the waveguides.
Ultimately, we would want to use a GC to couple light from a fibre into a photonic wire or photonic crystal, typically in the fundamental mode. To assess the efficiency of coupling into this mode, we must include a photonic wire as a mode filter, plus appropriate tapering structures, and then we must also account for the loss of the wire. If we use four waveguide lengths to determine losses via cutback, we now need twice this many devices to calculate the coupling efficiency of the designed grating.
We also need to determine the centre frequency. Ideally, this would be the same for all the gratings already needed, but to improve accuracy, we also fabricate gratings with a slight lithographic tuning, and another fillfactor as insurance against fabrication tolerances.
For the polarisation diversity couplers, we need all of this in two dimensions,
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hence both waveguide and wire bends, hence more calibration structures. A typical layout to characterise a 2D single-mode polarisation diversity splitter is shown in figure 4.22. The lithography time tends towards days rather than hours. Although this is dominated by the time to write the waveguides rather than the actual GCs, it has largely been found impractical to use optical lithography to reduce the writing time, as photonic wires, and photonic wire bends in particular, are beyond the reach of the available tools.
Figure 4.22: Mask layout showing all of the devices necessary to fully characterise
the efficiency and centre frequency of a single polarisation diversity GC design. The different colours represent different types of structure, such as photonic wires and ridge waveguides, and their bends. The pattern is approximately 2mm wide, and the actual GCs are not visible at this scale.
results on complete 2D polarisation diversity couplers. However, we do have results for 1D gratings, which have one of the highest coupling efficiencies ever recorded [48]. The best devices make use of the gold mirror and wafer bonding techniques developed in this project, with almost 70% coupling efficiency [47]. However, these are for SOI11. Accordingly, I will show the InP results here.
The characterisation has been performed by F. van Laere. An SEM image of the structure is shown in figure 4.23, and the normalised spectra in figure 4.24.
Figure 4.23: SEM image of a FIB cross-section of a measured GC. The lithography
is mine, performed at St Andrews. In particular, this image demonstrates the success of the wafer bonding, as the grating is fully filled with BCB. The grating linewidth and etch depth are also found to be suitable.
The AR coating in these results is a simple step towards increasing the upwards coupling, not otherwise discussed here as the thickness is not reliable and the refractive index poorly characterised. The gold mirror technique is much preferable, and we expect results comparable to those from the SOI structures soon. Figure 4.25 shows the coupling efficiency with a gold mirror
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and use lithography and etching developed as part of the PICCO project in the DUV fab at IMEC, Belgium [28].
Figure 4.24: Coupling efficiencies for the measured InP GCs. The peak coupling is around 30%. These are first generation devices, with 1D gratings, made using the etch-first procedure discussed above, with no bottom reflector. These results agree very well with modeling results.
added (the AR coating is no longer useful), where an attempt has been made to control the thickness of the BCB layer. The quoted efficiency here is 56%, although simulations suggest a 78% efficiency for the best BCB thickness [49]. Further work on these devices is in progress, both to improve the fabrication and hence the coupling efficiencies further, and to integrate the GCs with other devices. In particular, we are evaluating methods to make the lithography more repeatable, such as using other resists; trying to deposit the gold for the grating mirrors solely under the grating as a means of reducing waveguide loss and increasing bonding yield, and optimising the etching for 2D as opposed to 1D structures.
Figure 4.25: Coupling efficiencies for 1D InP devices with a gold mirror added, bonded in vacuum using the simple method - i.e. attempting to control the BCB thickness. The peak efficiency is around 56%.