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arXiv:quant-ph/0205097 · Journal of Optics B Quantum and Semiclassical Optics

Recovery of continuous wave squeezing at low frequencies

Warwick P. Bowen, Roman Schnabel, Nicolas Treps, Hans‐A. Bachor, Ping Koy Lam

Published Oct 31, 2002• 44 citations
arXivHTMLPDF

Abstract

We propose and demonstrate a system that produces squeezed vacuum using a pair of optical parametric amplifiers (OPAs). This scheme allows the production of phase sidebands on the squeezed vacuum which facilitate phase locking in downstream applications. We observe strong, stably locked, continuous wave vacuum squeezing at frequencies as low as 220 kHz. We propose an alternative resonator configuration to overcome low frequency squeezing degradation caused by the OPAs.

DOI
10.1088/1464-4266/4/6/309

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We propose and demonstrate a system that produces squeezed vacuum using a pair of optical parametric amplifiers (OPAs). This scheme allows the production of phase sidebands on the squeezed vacuum which facilitate phase locking in downstream applications. We observe strong, stably locked, continuous wave vacuum squeezing at frequencies as low as 220 kHz. We propose an alternative resonator configuration to overcome low frequency squeezing degradation caused by the OPAs.
Squeezed vacuum states were generated using a subthreshold optical parametric oscillator (OPO) and a home-made, continuous-wave, single-frequency 532 nm and 1064 nm dual-wavelength laser. The OPO was pumped by the 532 nm output of the laser. The 1064 nm output of the laser was used to generate an auxiliary beam by optical serrodyne sideband modulation and a coherent control field (CCF). When the subthreshold OPO was locked using the auxiliary beam and the squeezing angle controlled using the CCF, 6.1 ± 0.3 dB phase-squeezed vacuum states at audio frequencies from 5 to 100 kHz were stably generated with a compact configuration.
In high-precision optical measurements, squeezed vacuum states are a promising resource for reducing the shot noise. To utilize a squeezed vacuum, it is important to lock the phase of the local oscillator (LO) to the squeezed light. The coherent control sideband (CCSB) scheme has been established for the precise phase locking, while the previous CCSB scheme was designed for the squeezed vacuum generated with an optical parametric oscillator (OPO). Thus the previous CCSB scheme is not applicable to squeezing by a single-pass optical parametric amplifier (OPA), which is attractive for generating broadband squeezed vacuum states. In this study, we propose a variant of CCSB scheme, which is applicable to the squeezing by single-pass OPA. In this scheme, we inject pump light and frequency-shifted signal light into an OPA crystal in the same way as the previous CCSB scheme. The parametric process in the OPA crystal generates a squeezed vacuum, amplifies the signal light, generates an idler light, and causes the pump depletion reflecting the interference of the amplified signal light and the idler light. Through the lock-in detection of the pump depletion, we can phase-lock the injected signal light to the pump light. Then, after the heterodyne detection of the signal and the idler light, we get the error signal of LO and realize the precise phase locking of LO to the squeezed quadrature. We show the feasibility of the proposed scheme by deriving the signal-to-noise ratio (SNR) of the modulated pump signal. We experimentally demonstrate the proposed scheme on pulsed squeezing by a single-pass OPA.
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.
Two-mode orthogonal squeezed vacuum states are an important quantum resource for quantum communication, quantum computing, quantum simulation, quantum precision measurement and sensing. It is essential to obtain stable two-mode orthogonal squeezed vacuum states in a low frequency range and compact configurations for practical applications, especially in quantum precision measurement and sensing. Two-mode orthogonal squeezed vacuum states are commonly produced via a subthreshold nondegenerate optical parametric amplifier (NOPA) in a continuous variable system. However, it is a difficult problem that the subthreshold NOPA cavity is phase sensitive manipulated to obtain stable squeezed vacuum states. Previous signal light injecting scheme relies on an injection of a weak light field into the subthreshold NOPA for phase sensitive manipulation. The injected signal light has the same frequency as the generated squeezed vacuum state. Thereby even the weakest injected signal light can introduce large amounts of excessive noise at low frequencies and the squeezing degree of two-mode squeezed vacuum states will be reduced or squeezing cannot be achieved. In this paper, a single sideband frequency shifted light injecting scheme is proposed for phase sensitive manipulation of NOPA. The comparison between the single sideband frequency shifted light injecting scheme and the signal light injecting scheme for realization of phase sensitive manipulation of NOPA is conducted. The effects of the two schemes on the generation of the low-frequency two-mode orthogonal squeezed vacuum state light field are investigated experimentally . The experimental results show that in the signal light injecting scheme for phase sensitive manipulation, the squeezing degree of the two-mode orthogonal squeezed vacuum state continuously decreases until it disappears as the power of injected signal light increases. In the process of phase sensitive manipulation of NOPA by using the single sideband frequency shifted light injecting scheme, the squeezing degree of the two-mode orthogonal squeezed vacuum state does not change with the power of the injected frequency shifted light increasing. Stable phase sensitive manipulation is realized by injecting single sideband frequency shifted light into NOPA. The NOPA is operated in a phase sensitive amplification state for 30 min. Stable low-frequency two-mode orthogonal squeezed vacuum states are obtained. The (4.1 ± 0.1) dB amplitude orthogonal squeezed vacuum states and (4.0 ± 0.2) dB phase orthogonal squeezed vacuum states at a frequency of 200 kHz are generated stably, in a compact NOPA configuration.
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.
Single-pass optical parametric amplification (OPA) is attractive in the generation of broadband squeezed vacuum. To achieve a higher squeezing level, precise control of the local oscillator (LO) phase is needed. However, the conventional control scheme was designed for squeezing by optical parametric oscillation (OPO). In this study, we propose a new scheme, which is applicable to single-pass OPA. In this scheme, we measure the modulation in the residual pump light after the OPA crystal to realize precise phase-locking of the LO. We anticipate that the proposed scheme will be indispensable for practical applications of squeezing with single-pass OPA.