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Progress on Silicon Test Masses and 1550nm Laser Light for Future Gravitational-Wave Detectors

The sensitivity of next generation's interferometric gravitational wave detectors will be limited by thermal noise and quantum noise. Cryogenic cooling of the interferometer test masses and the use of nonclassical, squeezed light are promising techniques for further enhancement of the detection sensitivity. While the mechanical properties of fused silica degrade towards low temperatures, silicon is a promising candidate for test masses in future cryogenic detectors. It shows very high mechanical quality factors, and values exceeding 10^-9 have been demonstrated [1]. Due to the non-transparency of silicon at 1064nm, a change of the laser wavelength to 1550nm will be necessary, where the absorption of silicon is expected to be at or even below 10^-8cm^-1 [2]. Stable continuous-wave lasers operating at 1550nm are widely available for telecommunication purposes and development of reliable high power laser sources for gravitational wave detection has been started. Strongly squeezed light at detection-band frequencies and with long-term stability has been demonstrated in a table-top experiment [3]. On a prototype scale, compatibility of squeezed light input with today's interferometer topologies was shown [4]. Up to now, the non-classical regime around 1.5µm has been primarily explored with laser pulses and the feasibility of squeezed vacuum at this wavelength had yet to be investigated. We give an overview of recent results regarding the mechanical and optical characterization of silicon as a test-mass material. In addition, we report on the generation of continuous-wave squeezed vacuum states at a wavelength of 1550nm that where produced by type I optical parametric amplification in PPKTP [5]. In an improved setup, a non-classical noise reduction of 6.4dB was achieved.

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