• Gravitational waves
• LIGO
• Optics
• Virgo

In the first part of this manuscript, I have presented the work done in upgrades and commissioning of the Thermal Compensation system and of Interfer- ometer Sensing and Control. Second part of my thesis was a research activity dedicated to a preparatory work of designing and characterizing a mode cleaner for Double Axicon System in TCS in Advanced Virgo Plus.
For the first part, I discussed the upgrade of the central heating actuator in the Thermal Compensation System, with a dedicated laser and optical line, to correct the cold defects during the locking acquisition. The design and charac- terization were performed in Roma Tor Vergata (Virgo INFN laboratory). The installation of central heating and its integration with DAS system was performed directly at the site in a "standalone" mode (without interacting with ITF). The pre-commissioning and commissioning of Central Heating have been engaged to improve the optical quality of the interferometer while increasing the sidebands’ power and improving the quality of relevant error signals in DRMI.
The locking of the central interferometer with two marginally stable cavities was performed for the first time. During the commissioning phase, the newly implemented locking acquisition strategies have been reported, signifying the in- terplay between TCS and ISC system. One of the most challenging factors during central interferometer locking was finding the correct pre-alignment procedure. The role of Central Heating during DRMI locking and its effect on longitudinal error signals have been studied with modal and Fast Fourier Transform simu- lations. Results obtained experimentally during the commissioning phase have been presented in this thesis work and published in [75].
The next part of the research activity was dedicated to a preparatory phase to upgrade the Thermal Compensation System to guarantee a clean CO2 laser beam
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and an optimal correction for axis-symmetric aberration. The motivation and requirements for a mode cleaner have been intensively investigated. The main challenges faced during this work were the complexity involved in mode cleaner requirements in terms of the content of HOMs and the locking procedure where 5 kW power was circulating in a 50 cm cavity. Since the general PDH technique cannot be used to lock the cavity due to the unavailability of the EOM for the CO2 wavelength, frequency noise measurement of the CO2 laser was performed to verify the feasibility of the dither lock technique. The relevant simulations and experimental activities to finalize the locking procedure have been presented. The thermal effects due to high circulating power and mirror optical absorption were also considered. A temperature control system has been implemented to mitigate the impact of power absorption on the cavity length.
In parallel, the dither locking acquisition was also studied with simulations. Various experimental tests and optical simulations have been reported to obtain the optimal locking acquisition for the mode cleaner while fulfilling the requirements set by the interferometer on the DAS compensation for the O5 run. Accounting for the O5 run requirements, using two-mode cleaners for each DAS ring has been considered to prevent damage and power absorption in the cavity from the mirrors. The next step is to finalize and test the locking procedure of the mode cleaner with the dithering technique.