Departmental Seminar

Technologies for future silicon-based interferometric gravitational wave detectors

Ms Disha Kapasi
Centre for Gravitational Astrophysics

Future terrestrial gravitational wave detectors (GWDs) are designed to be limited by fundamental noise sources. One of these fundamental noise sources is the thermal noise arising in the test masses and suspensions in the frequency band where ground-based detectors are sensitive. To mitigate this noise, future detectors are envisioned to be using silicon suspensions and test masses at cryogenic temperatures since the coefficient of thermal expansion for silicon goes to zero at 123 K. When using silicon test masses for optics, the operating wavelength of future GWDs will change to 1.55μm - 2μm regime. This requirement is to accommodate the optical absorption properties for the silicon material.

Silicon ribbons resemble a cantilever topology, and therefore studying the thermal noise in the flexing of a gram-scale silicon cantilever is analogous to the suspension thermal noise encountered in these ribbon suspensions. I measure the off-resonant thermal noise displacement using a cavity-enhanced interferometric readout. A multi-stage pendulum system is used to create a quiet low vibration environment in which a short optical cavity is operational. In this study, the cavity is cooled to cryogenic temperatures, and operated at around 123 K where the thermal expansion coefficient of silicon crosses zero, thereby allowing us to study the contribution of thermo-elastic fluctuations to the displacement noise of the flexure. I will present initial results of our cool-down tests, thermal noise measurements at room temperature and future plans.

The development of a stable, low-noise and narrow linewidth 2μm-band laser is a requirement for operating silicon-based detectors. It is crucial for research into squeezing, high quantum efficiency photodiodes, and optical coatings at 2μm all of which are active research areas in the GW community.

Here, I present the development of the 2μm laser and highlight the performance of our low-noise laser. Additionally, this laser has the potential to be used as the seed laser for high powered laser amplifiers needed to operate GWDs at 2μm.

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