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Journal of Modern Optics

Fibre-optic photon-number squeezing in the normal group-velocity dispersion regime

Friedrich König, Stefan J. Spalter, I. L. Shumay, Andreas Sizmann, Thomas Fauster, Gerd Leuchs

Published Dec 1, 1998• 10 citations
Publisher

Abstract

The nonlinear optical Kerr effect, acting on optical pulses in fibres, creates spectral sidebands and noise correlations between these sidebands. The reduction of photon-number fluctuations of these pulses below the shot-noise limit by spectral filtering is well established in the anomalous dispersion regime which allows for soliton formation. Here it is demonstrated that a significant quantum-noise reduction with spectral filtering can also be reached for pulses in the normal dispersion regime. The filter function was optimized and the power dependence of the noise reduction was investigated. The best squeezing result is (1.2 ± 0.2) dB (corresponding to (2.6 ± 0.7) dB inferred for 100% detection efficiency).

DOI
10.1080/09500349808230495

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The nonlinear optical Kerr effect, acting on optical pulses in fibres, creates spectral sidebands and noise correlations between these sidebands. The reduction of photon-number fluctuations of these pulses below the shot-noise limit by spectral filtering is well established in the anomalous dispersion regime which allows for soliton formation. Here it is demonstrated that a significant quantum-noise reduction with spectral filtering can also be reached for pulses in the normal dispersion regime. The filter function was optimized and the power dependence of the noise reduction was investigated. The best squeezing result is (1.2 ± 0.2) dB (corresponding to (2.6 ± 0.7) dB inferred for 100% detection efficiency).
Photon number squeezing of spectrally filtered optical solitons has been observed in the sub-picosecond regime, where the Raman effect changes the internal quantum structure of each pulse. 160 fs pulses were propagated down a 10 m optical fibre of anomalous group velocity dispersion. At energies below the soliton energy subsequent spectral filtering with an optical band-pass squeezed the directly detected photocurrent fluctuations to 3.2 ± 0.1 dB (52%) below the shot-noise limit. This implies 4.5 dB (65%) of soliton photon number squeezing, if detection losses are taken into account. Using a spectral edge filter the spectral components of the pulses are shown to contribute asymmetrically to the squeezing due to stimulated Raman scattering.
Noise reduction of optical pulses using variable spectral filters and optical fibers in the anomalous dispersion region is investigated numerically and experimentally. In the numerical analysis, quantum noise evolution of the optical pulse is calculated using a quantized nonlinear Schrödinger equation and linearization approximation. It is shown that when the optical spectrum is in the broadening process, photon-number squeezing can be obtained using a band-pass filter. On the other hand, when the optical spectrum is in the narrowing process, a correlated pulse pair is generated using a band-rejection filter and photon number squeezing can be also observed. In the experiment, the characteristics of noise reduction of optical pulses are investigated using a passively mode-locked fiber laser at 1.55 µm and a variable spectral filter. Noise distribution of optical pulses changes periodically as the fiber input power is changed. Intensity noise of the fiber laser is reduced by as much as 23 dB using the band-pass filter and is slightly reduced below the shot noise level. Using the band-rejection filter, the noise reduction by the correlated pulse pair is confirmed. The characteristics of the experimental results are almost in agreement with those of the numerical ones.
Summary form only given.Spectral filtering of optical solitons has been demonstrated to squeeze their photon-number uncertainty up to a measured value of 3.8 dB. The squeezing has been explained in terms of an uneven spectral distribution of photon-number noise caused by the Kerr nonlinearity of the fiber. Some indication for correlations among different frequency components has been found in a numerical analysis. We present the first experimental evidence for such spectral correlations. The experimental setup is shown. Pulses of 130 fs centered at 1.5 /spl mu/m are launched into a 2.9-m polarization-presenting single-mode optical fiber with energies close to the fundamental soliton energy.
We present new results on photon number squeezing of spectrally filtered solitons in fibers. The impact of frequency low-, high-, and bandpass filtering on noise reduction has been measured as a function of fiber length for 130-fs pulses close to the soliton energy. For short fibers our results agree qualitatively with theoretical predictions. For longer fibers, however, the measured squeezing increases to an unexpectedly large value. Filtering out the long-wavelength components of strongly Raman-shifted, higher energy pulses squeezed the directly detected photocurrent fluctuations down to 3.8+/-0,2 dB (59%) below the shot noise level. The measured noise reductions are broadband from 5 to 90 MHz.
We report the photon-number squeezing of optical solitons. 2.7 ps pulses were launched as solitons down a 1.5 km optical fiber. For energies slightly above that of fundamental solitons, they broadened spectrally due to self-phase modulation caused by ${\ensuremath{\chi}}^{(3)}$ nonlinearities. Filtering away outlying components of the broadened spectra squeezed the soliton's photon-number fluctuations to 2.3 dB (41%) below the shot-noise limit. Accounting for losses, this corresponds to 3.7 dB (57%) photon-number squeezing. A quantum field-theoretic model shows that the outlying spectral components have large energy fluctuations, so that their removal causes squeezing.