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Description
The low-frequency interferometer of the proposed Einstein Telescope (ET-LF) will employ cryogenically cooled silicon test masses to reduce thermal noise. Silicon, however, strongly absorbs the currently well-established laser wavelength of 1064 nm, motivating the investigation of alternative wavelengths such as 1550 nm.
This change introduces challenges for interferometer control and lock acquisition. Current gravitational-wave detectors rely on the second harmonic generation (SHG) of 1064 nm, but the SHG of 1550 nm (775 nm) is strongly absorbed in silicon and therefore unsuitable for ET-LF.
We investigate a two-colour laser scheme in which phase-coherent 2090 nm light is generated from the 1550 nm main laser via nonlinear difference-frequency generation (DFG). The resulting 2090 nm field, generated using an 890 nm pump, can serve as a secondary reference for the lock acquisition of ET-LF silicon cavities. We present the creation and characterisation of a DFG-based 2090 nm light source and demonstrate proof-of-principle operation with a small silicon test cavity.