The construction of a large-scale cryogenic gravitational wave telescope, KAGRA, has been completed, and four sapphire test masses have been installed in cryostat vacuum chambers. Recently, it was ...observed that a sapphire substrate used for one of the input test masses showed a characteristic structure in its transmission map due to nonuniformity of the crystal. We performed an interferometer simulation to observe the influence of the nonuniformity using measured transmission/reflection maps. Shot noise increased by 7% at low frequencies and by 16% at high frequencies. Laser frequency noise coupling increased by 1 order of magnitude. We also found deformation of an alignment control signal and higher tilt coupling in the output optics.
An optical cavity consisting of optically trapped mirrors makes a resonant bar that can be stiffer than diamond. A limitation of the stiffness arises in the length of the optical bar as a consequence ...of the finite light speed. High laser power and light mass mirrors are essential for realization of a long and stiff optical bar that can be useful for example in the gravitational-wave detector aiming at the observation of a signal from neutron-star collisions, supernovae, etc. In this letter, we introduce a parametric signal amplification scheme that realizes the long and stiff optical bar with a non-linear crystal inside the signal-recycling cavity.
•A stiff optical spring bar is a way to detect gravitational waves at high frequencies.•A parametric amplification technique realizes the stiff optical bar without increasing the circulating laser power.•This letter introduces our theoretical work on a new interferometric configuration with the parametric amplifier.
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