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Figure 2
(a) Two measured focus profiles for a partially coherent setting (SSA size 20 µm, blue dashed curve) and a quasi-coherent setting (SSA size 5 µm, red line), shown on linear (left) and logarithmic (right) scale. (b) Intensity profiles of the three guided WG modes (left), superposition of the three modes (right). (c) Simulated and experimental focus tails, for different coherence settings. The blue line shows the simulated intensity of the HFM in a fully coherent setting; the red dashed line is convolved with the WG mode structure; the green line is calculated for a finite SSA size (horizontal) of 15 µm (and convolved with the WG guiding channel). The black circles represent experimental data for an SSA size of 15 µm, with a sinusoidal fit shown as a black line. Experimental data have been shifted by a factor of 1:3.5 vertically for clarity. (d) Fringe contrast, quantifying the degree of coherence, as a function of the SSA size. The thin blue line is simulated for ideal sampling, whilst the thick red line accounts for the convolution with the WG modes. The orange circles show contrast fits for experimental data; orange lines correspond to 2σ error bars of the fit.

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