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Squeezed light at 1550 nm with a quantum noise reduction of 12.3 dB

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Abstract

Continuous-wave squeezed states of light at the wavelength of 1550 nm have recently been demonstrated, but so far the obtained factors of noise suppression still lag behind today’s best squeezing values demonstrated at 1064 nm. Here we report on the realization of a half-monolithic nonlinear resonator based on periodically-poled potassium titanyl phosphate which enabled the direct detection of up to 12.3 dB of squeezing at 5 MHz. Squeezing was observed down to a frequency of 2 kHz which is well within the detection band of gravitational wave interferometers. Our results suggest that a long-term stable 1550 nm squeezed light source can be realized with strong squeezing covering the entire detection band of a 3rd generation gravitational-wave detector such as the Einstein Telescope.

© 2011 Optical Society of America

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Figures (5)

Fig. 1
Fig. 1 Schematic of the experiment. Laser: 1550 nm fiber laser. FCs, filter cavities for spatio-temporal mode cleaning at both wavelengths. PZT, piezoelectric transducer; SHG, external second-harmonic generator to produce the 775 nm pump field; BS, beam splitter; DBS, dichroic beam splitter; PDs, photo diodes, SA, spectrum analyzer that converts the differential current of the two photo diodes into variances; BHD, balanced homodyne detector for the characterization of the squeezed field.
Fig. 2
Fig. 2 Balanced homodyne measurements of the quadrature noise variances. The vacuum reference was recorded with a LO power of 11.8 mW and a blocked signal port. The OPA was driven with a pump power of approximately 171 mW. With an open signal port, the squeezed variance and the anti-squeezed variance were measured by choosing the corresponding relative phase between signal field and LO. The depicted measurements are averages of 2 traces that were recorded at a Fourier frequency of 5 MHz, with a resolution bandwidth of 200 kHz and a video bandwidth of 200 Hz. No data postprocessing was applied, i.e., the data still include electronic dark noise, and thus represent direct observations. A linear fit to the linearized variances was used to determine the variance mean values and the associated standard deviation. The anti-squeezing was at +19.3±0.2 dB relative to the vacuum noise, whereas the squeezing was at −12.3±0.2 dB.
Fig. 3
Fig. 3 Pump power dependence of anti-squeezed and squeezed quadrature variances. All values were obtained from zero-span measurements at 5 MHz. In order to fit the numerical model, all the data were dark-noise corrected and subsequently normalized to the vacuum reference.
Fig. 4
Fig. 4 Pump power dependence of the squeezing spectra, experiment and theory. All depicted traces are averages of 10 individual traces each recorded with a resolution bandwidth RBW = 500 kHz and video bandwidth VBW = 2 kHz. All traces were corrected for electronic detector dark noise and were normalized to the vacuum level. The theoretical predictions (solid lines) were obtained from Eq. (2) using η = 0.965, Pthr = 221 mW, and θfluc = 0.66° with the respective pump power values of P1 = 6 mW, P2 = 56 mW, P3 = 106 mW, and P4 = 180 mW. The remaining cavity parameters were used as given in the text. The bottom curve shows the squeezing spectrum under the assumption of zero phase noise.
Fig. 5
Fig. 5 Squeezing at frequencies between 1.5 kHz and 80 kHz. All traces were measured with RBW of 250 Hz and were normalized to the vacuum noise level corresponding to a LO power of 4.8 mW. Each measurement point is the averaged root mean square value of 20 [40 for trace(c)] measurements. Trace (a) shows the direct observation of more than 10 dB of squeezing down to a frequency of 18 kHz with a maximum noise suppression of −11.4± 0.5 dB between 60 kHz and 80 kHz. Below 18 kHz the squeezing level degraded, reaching approximately −5 dB at 2 kHz. The peak at 8 kHz was caused by a residual, non-stationary amplitude modulation from FC1. The anti-squeezing [trace (b)] was 16.8±0.5 dB above the vacuum noise level as measured by the homodyne detector [trace (c)]. Electronic dark noise was not subtracted from the measurement data.

Equations (2)

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V 1 , 2 = 1 ± η det η esc 4 P / P thr ( 1 P / P thr ) 2 + 4 ( 2 π f κ 1 ) 2 ,
V 1 , 2 = V 1 , 2 cos 2 θ fluc + V 2 , 1 sin 2 θ fluc .
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