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Conference on Lasers and Electro-Optics

Numerical and experimental investigations of spatial soliton squeezing

Stephen R. Friberg, M. Werner

Published Nov 27, 2002• 0 citations

Abstract

Summary form only given.We describe numerical and experimental investigations of the use of soliton spectral filtering in the spatial domain. As is well known, the nonlinear Schrodinger equation description of optical soliton propagation in single-mode fibers applies equally well to spatial solitons in nonlinear planar waveguides. Therefore, we can expect that the photon-number squeezing methods that work for temporal solitons in optical fibers apply equally well to spatial solitons. The spatial-soliton equivalent of spectral filtering is to use a spatial slit to remove the edge portions of the spatial soliton after it emerges from the planar waveguide. To optimize the experimental configuration, we have modeled the spatial-soliton propagation using a numerical quantum propagation code. Our results show that optimal squeezing is obtained at propagation distances of several soliton periods for solitons with the order N/spl sim/1.2. To experimentally demonstrate photon number squeezing by spatial filtering of spatial solitons, we have fabricated an AlGaAs planar waveguide that is 1.5 /spl mu/m thick and 1-3 cm long. For our device, the bandgap is in the vicinity of 0.8 /spl mu/m, so by using short pulses from a color-center laser tuned to 1.5- to 1.6-/spl mu/m wavelengths, we can excite reasonably large nonlinearities due to two-photon and three-photon resonances in the bandgap. This allows us to create spatial solitons. Spatial solitons created by color-center laser pulses are spatially windowed by slits at the output of the planar waveguide. The spatially filtered pulses are transmitted to a balanced mixer-detector arrangement that allows measurement of the noise of the windowed pulses relative to the shot-noise level.

DOI: 10.1109/iqec.1998.680401

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Summary form only given.We describe numerical and experimental investigations of the use of soliton spectral filtering in the spatial domain. As is well known, the nonlinear Schrodinger equation description of optical soliton propagation in single-mode fibers applies equally well to spatial solitons in nonlinear planar waveguides. Therefore, we can expect that the photon-number squeezing methods that work for temporal solitons in optical fibers apply equally well to spatial solitons. The spatial-soliton equivalent of spectral filtering is to use a spatial slit to remove the edge portions of the spatial soliton after it emerges from the planar waveguide. To optimize the experimental configuration, we have modeled the spatial-soliton propagation using a numerical quantum propagation code. Our results show that optimal squeezing is obtained at propagation distances of several soliton periods for solitons with the order N/spl sim/1.2. To experimentally demonstrate photon number squeezing by spatial filtering of spatial solitons, we have fabricated an AlGaAs planar waveguide that is 1.5 /spl mu/m thick and 1-3 cm long. For our device, the bandgap is in the vicinity of 0.8 /spl mu/m, so by using short pulses from a color-center laser tuned to 1.5- to 1.6-/spl mu/m wavelengths, we can excite reasonably large nonlinearities due to two-photon and three-photon resonances in the bandgap. This allows us to create spatial solitons. Spatial solitons created by color-center laser pulses are spatially windowed by slits at the output of the planar waveguide. The spatially filtered pulses are transmitted to a balanced mixer-detector arrangement that allows measurement of the noise of the windowed pulses relative to the shot-noise level.

Summary form only given. Several experiments have been performed in optical fibers to generate photon-number squeezed pulses of light. Among these, are experiments employing a spectral filter to exploit the spectral correlations created during pulse propagation along a fiber. We recently reported the generation of photon-number squeezing in an AlGaAs waveguide as a result of multi-photon absorption and we now extend this work to show an increase in the degree of squeezing obtained with appropriate spectral filtering. Unlike much of the fiber work, which used long propagation distances, the results here are produced in a 2-mm-long semiconductor waveguide. The waveguide structure is composed of AlGaAs grown on a GaAs substrate and consisted of a l /spl mu/mm planar high-index waveguiding layer with lateral confinement provided by a surface rib. The estimated effective modal area is 6 /spl mu/m/sup 2/.

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.

Summary form only given.Recently, a novel way of generating an amplitude-squeezed state using the Kerr nonlinearity has been demonstrated. In this scheme, a pulse with an energy greater than that of the fundamental soliton is launched into an optical fiber with a length of several soliton periods. At the fiber output, the pulse is filtered and detected. The resulting photocurrent has a noise level lower then the shot noise, indicating an amplitude-squeezed state. Here, we demonstrate that soliton amplitude squeezing can be obtained without postfiltering.

We study the quantum-noise properties of spectrally filtered solitons in optical fibers. Perturbation theory, including a quantum description of the continuum, is used to derive a complete analytical expression for the second-order correlator of the amplitude quadrature. This correlator is subsequently used to optimize the frequency response of the filter numerically in order to achieve the minimum photon-number noise. For propagation distances up to three soliton periods, the length at which the best noise reduction occurs, a square filter is found to be approximately optimum. For longer distances, more-complicated filter shapes are predicted for the best noise reduction.

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.