Input Optics and Laser Frequency Noise Suppression for Einstein Telescope and Third-Generation Gravitational-Wave Detectors

Future gravitational-wave observatories such as Einstein Telescope and Cosmic Explorer will require a substantial evolution of the laser frequency stabilization strategies currently employed in second-generation interferometric detectors. In Advanced Virgo and Advanced LIGO, residual laser frequency noise is strongly suppressed by feedback from the interferometer common-arm degree of freedom, whose long baseline provides an excellent frequency reference over most of the observation band. In third-generation detectors, however, the much longer arm cavities significantly reduce both the free spectral range and the coupled-cavity pole frequency, limiting the achievable bandwidth of the common-arm control loop and degrading the sensing-noise performance at high frequencies. As a consequence, the interferometer itself may no longer provide the broadband frequency stabilization presently achieved in second-generation instruments. This shifts a much larger fraction of the stabilization burden toward the input-optics system. In this review the implications of such a transition are discussed, with particular emphasis on long suspended input mode cleaners, multi-stage stabilization architectures, higher-order optical mode coupling, sensing-noise limitations, and optical layout considerations relevant for future detectors.