Nonclassical Interferometry towards Gravitational-Wave Detectors at a Laser Wavelength of 2.1um
Final Report Abstract
Gravitational wave observatories use ultra-stable, quasi-monochromatic laser light with its signal sideband spectrum in squeezed vacuum states to measure the tiny oscillations of space-time at the Earth’s location, but which are generated in distant galaxies by merging black holes and neutron stars. Future observatories of this type should have much higher sensitivity, allowing them to observe so many gravitational waves that their recorded signals overlap in time. Increasing their range by a factor of ten increases the average event rate by a factor of a thousand. This is the ambitious goal for future GW observatories - at a minimum. It requires reducing the thermal noise of the observatories as well as the quantum noise of the laser light. Prior to the start of this project, a promising path was identified. New mirror materials and new materials for the mirror coatings have been identified that allow cryogenic operation of the mirrors and significant reduction of all types of thermal noise. However, the new materials require a new laser wavelength in the 2 µm region. This project provided evidence of a concept for generating ultra-stable laser light as well as squeezed light in the required wavelength range. The project was based on the innovative idea of starting from existing and widely developed laser technology at 1064 nm and doubling the wavelength in a highly efficient degenerate optical parametric oscillator (OPO). The project was able to build such an OPO stably controlled for degeneracy, i.e., stable generation of a quasi-monochromatic field at wavelength 2128 nm confined in the beam of a nearly perfect transverse electromagnetic mode. About 52 mW of 1064 nm light was converted to about 0.45 mW, corresponding to an exceptionally high external conversion efficiency of 87%. The maximum converted power was limited by the relatively high specular reflectance of the OPO resonator. A new OPO design is also expected to allow conversion of hundreds of watts. The same OPO technology was used to achieve quantum noise suppression of more than 7 dB, a sensitivity benefit equivalent to a factor of 5 increase in light output. The second parametric resonator was operated slightly below the oscillation threshold required to produce squeezed vacuum states. The squeeze factor was limited by optical losses. In particular, optical loss due to the imperfect quantum efficiency of the photodiodes used was limiting. Better photodiodes with quantum efficiency greater than 99% at wavelengths in the 2µm range and with low dark noise are not currently commercially available, but are in principle possible based on extended InGaAs and at low temperature operation. The results of this project are an excellent starting point for further targeted research and development work. The conversion of existing laser radiation of more than 100 W at 1064 nm needs to be tested, low noise/quantum efficient photodiodes need to be developed and squeeze factors of well above 10 dB need to be demonstrated, especially at sub-audio frequencies. This range of signal frequencies is the main target of future GW observatories, which will open the GW window to the universe much wider.
Publications
- Highly efficient generation of coherent light at 2128 nm via degenerate optical-parametric oscillation, Opt. Lett. 45, 6194 (2020)
C. Darsow-Fromm, M. Schröder, J. Gurs, R. Schnabel, S. Steinlechner
(See online at https://doi.org/10.1364/ol.405396) - NQontrol: An open-source platform for digital control-loops in quantum-optical experiments, Review of Scientific Instruments 91, 035114 (2020)
C. Darsow-Fromm, L. Dekant, S. Grebien, M. Schröder, R. Schnabel, S. Steinlechner
(See online at https://doi.org/10.1063/1.5135873) - Squeezed light at 2128 nm for future gravitational-wave observatories, Opt. Lett. 46, 5850 (2021)
C. Darsow-Fromm, J. Gurs, R. Schnabel, S. Steinlechner
(See online at https://doi.org/10.1364/ol.433878)
