Upgrades and Improvements of Laser Interferometric Gravitational Wave Detectors

Gravitational wave astronomy has rapidly developed into a powerful means of probing compact objects and understanding the evolution of the Universe. To improve sensitivity and extend the detection band, ground-based laser interferometers such as LIGO, Virgo, and KAGRA have undergone continuous upgrades. This review summarizes their systematic development with an emphasis on noise sources and mitigation strategies. After outlining the principle of gravitational wave detection with laser interferometry, we analyze dominant noise sources including quantum vacuum fluctuations, thermal noise, and seismic disturbances, and introduce techniques such as frequency-dependent squeezed light, advanced seismic isolation, multi-stage suspensions, and cryogenic mirrors. For LIGO, we highlight the transition from the Initial to Advanced configurations, which enabled strain sensitivities of the order of 10-24/√Hz and led directly to the first detection GW150914 and over one hundred subsequent events during O1 to O4. The unique superattenuator system of Virgo and its recent implementation of squeezed light, as well as the underground design of KAGRA and the use of cryogenic sapphire test masses, represent different approaches to suppress low-frequency and thermal noise. In addition, we compare the technical routes adopted by different detectors and summarize the lessons learned from their upgrades, which provide valuable guidance for future detector designs. Finally, we present next-generation projects, including LIGO Voyager, the Cosmic Explorer and the Einstein Telescope, which aim to achieve up to orders of magnitude improvements in sensitivity and provide new research opportunities for gravitational-wave cosmology and fundamental physics. Overall, the evolution of detector technologies has been the key driver of progress in gravitational wave astronomy, and the forthcoming facilities will transform our ability to explore the Universe.