Auxiliary Optics R&D Activities

Thermal Compensation

The initial prototyping of the two schemes for thermal compensation concluded this year and resulted in the PhD thesis of a LIGO student. The lens formed in the substrates due to the absorption of the laser light in the substrate make the interferometer sensitive to the power level. Including a thermal compensation system allows the interferometer to be used with a wide range of input powers, allowing e.g., better low frequency sensitivity with a reduction in the power. It also allows a trade to be made with the material properties of the substrate; this is useful for sapphire, and necessary in the fallback case of fused silica.

The basic approach for compensation is to add a complementary additional heat source, so that the sum of the laser and compensation heating leads to a uniform optical path. In one technique, a circular heater adds heat to the edge of the optic. In this way, the scattering effect of lensing can be reduced (in experiments and models that show excellent agreement) by more than a factor of 50; see Figure 11, upper plots. This is a very effective approach for the case of uniform absorption, which is expected to dominate.

Figure 1 Thermal compensation demonstration results.
Top left: The contour map for a uniform absorption of a Gaussian beam.
Top right: The residual deformation after compensation with a ring heater.
Bottom left: the distortion due to a "point absorber" (mimicked by a small probe laser beam);
Bottom right: the map after compensation with a scanned compensation beam

In the second approach, a scanning laser beam is played on the substrate and the dwell time and/or intensity can be modulated to deposit heat in a pattern optimized to compensate for a specific defect, for example a volume of higher thermal absorption. As shown in the lower plots in Figure 1, an additional suppression of a factor of 8 can be achieved for this example of point defect.