Mechanical loss and its significance in the test mass mirrors of gravitational wave detectors
The work of this thesis involves the analysis of mechanical losses associated with coated test masses manufactured from fused silica, to determine the existence and level of excess loss associated with the coatings on these substrates. In particular, a major part of this analysis requires the calculation of the ratio of the strain energy stored in the dielectric coating to the strain energy stored in the substrate for a number of the resonant modes of the test mass. This is extremely difficult to calculate analytically for all but the simplest of modes. Finite element analysis had to be used to calculate the modeshapes of a number of resonant modes of the test masses. A piece of analytical software was specifically written to use the output of the finite element analysis package to calculate these energy ratios. The majority of this thesis is concerned with the methodology and usage of this software in the context of a number of analyses of different coated test masses. In addition, a technique was developed to allow experimental determination of modeshapes. This method could then be used to confirm or identify the nature of different modes. An initial investigation suggested that the loss associated with the coating s may prove significant for future generations of detectors such as Advanced LIGO. Further investigations suggested that the principle source of coating loss was due to the materials used in the coatings themselves. These investigations also suggested that for the coatings used, which were manufactured using tantalum pentoxide and silica, the tantalum pentoxide had a higher mechanical loss than the silica. Investigations into different coating materials have been initiated. Finally, preliminary tests on a coated sapphire mirror have been completed which give an upper limit to the loss of a coating on a sapphire mass. These tests required comprehensive changes to be made to the analytical energy ratio software to allow the analysis of anisotropic materials such as sapphire and to allow the output from different finite element packages to be used.