Abstract
Our laboratory has participated in the research project of the Laser Interferometer Gravitational Wave Observatory(LIGO), which focused on the study of optical coating on the high reflective mirror in large-scale Michelson interferometers. As the gravity wave signal is extremely weak, how to reduce the noise has become an important issue. According to LIGO's theoretical noise spectrum, the source of the noise comes mostly from the Coating Brownian noise, which is the thermal noise generated by the optical coating on the high reflective mirror. In the light of fluctuation-dissipation theorem, it can be seen that the thermal noise is proportional to the mechanical loss of the film. Knowing the mechanical loss of the film is equivalent to understanding the level of thermal noise. The mechanical loss of the SiNxHy produced by PECVD in our laboratory has reached 10-5 order at room temperature. It is obviously much lower than the loss of common optical films Ta2O5 (2x10-4). To meet the LIGO specifications of the high reflector material, apart from a low mechanical loss, a extreme low optical absorption is also necessary. The absorption will make great influence on the quality of high reflector. The latest low-temperature LIGO large-scale Michelson interferometer uses silicon substrate. In response to decrease the optical absorption of silicon substrates, CW laser light sources in 1550nm will be used. Therefore, the study of optical absorption of silicon nitride film in 1550nm is very important. At first, our laboratory used an ellipsometer to study the optical absorption of silicon nitride film in the 1550nm. However, because the absorption value was too low, the ellipsometer can not resolve the accurate absorption value. As a result, our laboratory deployed a Photothermal Common-path Interferometry(PCI) that can be used to estimate extreme low optical absorption. This thesis will expound the principle of measurement of PCI system, discuss the flow and measurement steps, and analyze the measurement results of silicon nitride film. After the data of optical absorption of the silicon nitride film was obtained, the number of layers of SiN0.40H0.79/SiO2 was calculated by Macleod optical simulation software according to the specifications of LIGO white paper. Next, the number of layers was used to calculate the thermal noise of SiN0.4/SiO2 layers. Finally, the data of the noise will be compared with the LIGO thermal noise theoretical curve.