The resulting thermal lens has a perfect flatness in the central region. The goal is to create a profile such that, combined with the readout beam heat source, it gives that ideal profile. Consider, for instance, an intensity mask of the form
see the profile on Figure 34. This is nothing but the complement to a source of heat corresponding to a mode dissipating W on the coating. The resulting global thermal lens (thus corrected) can be seen in Figure 35.The price to pay is to provide the correcting power. According to Equation (4.15), the integrated power of the mask is
dissipated power | initial losses | compensation power | minimal losses | wavefront curvature |
10 mW | 350 ppm | 1.9 W | 0.7 ppm | |
20 mW | 1,400 ppm | 3.8 W | 3 ppm | |
30 mW | 3,100 ppm | 5.6 W | 6.4 ppm | |
100 mW | 34,300 ppm | 18.8 W | 71 ppm | |
We see in this rather academic case (Table 13) that the residual losses are much less than in the preceding case (by a factor of about 20), but at the price of higher TCS power. The expansion in terms of Zernike polynomials is given in Table 14.
heating ring | Axicon | CO_{2} scan | |
µm/W | µm/W | µm/W | |
0 | 0.759 | 0.018 | 0.774 |
1 | 0.016 | –0.008 | –0.058 |
2 | –0.044 | 0.003 | –0.016 |
3 | –0.012 | 0.001 | 0.001 |
4 | 0.002 | –0.001 | –0.002 |
5 | 0.004 | 0.001 | 0.002 |
6 | 0.002 | 0 | –0.001 |
7 | 0 | 0 | 0.001 |
8 | –0.001 | 0 | –0.001 |
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