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Advanced Solid-State Photonics (TOPS)

Generation of a Squeezed Vacuum field with PPKTP at 1064nm for Gravitational Wave Interferometers

Shailendhar Saraf, Keisuke Goda, Eugeniy E. Mikhailov, N. Mavalvala, O. Miyakawa

Published Jan 1, 2007• 0 citations
Publisher

Abstract

Squeezed vacuum generation at 1064 nm in a PPKTP-based optical parametric oscillator is demonstrated for sensitivity improvements in gravitational wave detectors. Broadband squeezing of >2.5dB is reported in the 1-100 KHz frequency range.
DOI
10.1364/assp.2007.tua4

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We report the generation of a stable continuous-wave low-frequency squeezed vacuum field with a squeezing level of 7.4+/-0.1 dB at 1064 nm, the wavelength at which laser-interferometric gravitational wave (GW) detectors operate, using periodically poled KTiOPO4 (PPKTP) in a subthreshold optical parametric oscillator. The squeezing was observed in a broad band of frequencies above 700 Hz where the sensitivity of the currently operational GW detectors is limited by shot noise. PPKTP has the advantages of higher nonlinearity, smaller pump-induced seed absorption, and wider temperature tuning range than alternative nonlinear materials such as MgO-doped or periodically poled LiNbO3, and is, therefore, an excellent material for generation of squeezed vacuum fields for application to laser interferometers for GW detection.
Squeezed state of light is an important resource of optical measuerments below the shot noise limit and has been used to improve measurement sensitivity in many areas such as gravitational wave detection, especially in audio frequency region. Compared with the high-frequency squeezed states, the generation of the low-frequency squeezed states is more difficult, because it is limited by several technical noise sources. In this paper we report the observation of more than 2 dB of vacuum squeezing at 1064 nm in the gravitational-wave detection band down to 3 kHz with a double-resonant optical parametric oscillator (OPO). The OPO has a configuration of linear cavity consisting of an input coupling mirror with a transmission of 11% at 532 nm and an output coupling mirror with the transmission of 12% at 1064 nm. The nonlinear materials in the OPO is type-I periodically poled potassium titanyl phosphate (PPKTP) crystal which is chosen for this experiment due to its higher nonlinearity, broader phase matching temperature, and smaller photo-thermal effect. The OPO is pumped by the light of 532 nm from Nd: YVO4/KTP solid-state laser of maximum optical power 3 W. To avoid various noise coupled from the seed beam, the OPO is seeded by vacuum fluctuations instead of coherent field at the fundamental wavelength (1064 nm). A Pound-Drever-Hall (PDH) locking scheme is used to lock the OPO cavity length with the signal derived from the reflected pump beam, so as to lock the pump field and also lock the fundamental field. To make both the pump and seed beams resonant simultaneously, the temperature of the PPKTP is carefully adjusted. The squeezed state can be detected on a homodyne detection by interfering it with the local oscillator (LO) and detected by a balanced detector with two photodiodes (EXT500 T) but having the same quantum efficiency of 86% at 1064 nm. The subsequent electronic noise is analyzed with a low-frequency spectrum analyzer, which shows that the audio noise sources from lab enviroment, locking quality, escape efficiency, propagation loss, homodyne efficiency and detection efficiency have effect on the squeezing pruced by an OPO.
We report on the generation of squeezed states with optical parametric oscillators. Firstly, we discuss experiments aimed toward producing stronger squeezing. We characterise three different nonlinear materials (PPKTP, PPSLN and PPSLT) in terms of their nonlinearity and optical losses to evaluate their squeezing efficacy. Secondly, we present results showing the development of squeezed states at audio sideband frequencies. Squeezing in this frequency range is required for applications in interferometric gravitational wave (GW) detectors. We discuss the significance of the results in terms of potential sensitivity improvements of current and future GW detectors and also potential upgrades to our experiment.
We demonstrate the generation of broadband continuous-wave optical squeezing from 280 Hz-100 kHz using a below-threshold optical parametric oscillator (OPO). The squeezed state phase was controlled using a noise locking technique. We show that low frequency noise sources, such as seed noise, pump noise, and detuning fluctuations, present in optical parametric amplifiers, have negligible effect on squeezing produced by a below-threshold OPO. This low frequency squeezing is ideal for improving the sensitivity of audio frequency measuring devices such as gravitational-wave detectors.
We propose and demonstrate a coherent control scheme for stable phase locking of squeezed vacuum fields. We focus on sideband fields at frequencies from 10 Hz to 10 kHz, which is a frequency regime of particular interest in gravitational-wave detection and for which conventional control schemes have failed so far. A vacuum field with broadband squeezing covering this entire band was produced using optical parametric oscillation and characterized with balanced homodyne detection. The system was stably controlled over long periods utilizing two coherent but frequency shifted control fields. In order to demonstrate the performance of our setup the squeezed field was used for a nonclassical sensitivity improvement of a Michelson interferometer at audio frequencies.
We present the observation of optical fields carrying squeezed vacuum states at sideband frequencies from 10Hz10\phantom{\rule{0.3em}{0ex}}\mathrm{Hz}10Hz to above 35MHz35\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}35MHz. The field was generated with type-I optical parametric oscillation below threshold at 1064nm1064\phantom{\rule{0.3em}{0ex}}\mathrm{nm}1064nm. A coherent, unbalanced classical modulation field at 40MHz40\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}40MHz enabled the generation of error signals for stable phase control of the squeezed vacuum field with respect to a strong local oscillator. Broadband squeezing of approximately $\ensuremath{-}4\phantom{\rule{0.3em}{0ex}}\mathrm{dB}$ was measured with balanced homodyne detection. The spectrum of the squeezed field allows a quantum noise reduction of ground-based gravitational wave detectors over their full detection band, regardless of whether homodyne readout or radio-frequency heterodyne readout is used.