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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Squeezed vacuum pulse generation by χ(3) Nonlinearity in a quasi-phase-matched PPLN and a short fibre

Hisayuki Nakagome, Kenichi Hirosawa, Y. Fujiwara, Hidetake Ushio, Y. Ito, Fumihiko Kannari

Published Jun 1, 2009• 0 citations

Abstract

Squeezed vacuum (SV) generation through third-order optical nonlinearity chi(3)is useful for generating a quantum entanglement pair based on continuous variable. However, third-order nonlinearity is usually small. Therefore, the typical interaction length should be large and ultrashort laser pulses should be used. Then, it becomes important to avoid excess phase noise, especially guided-acoustic-wave Brillouin scattering (GAWBS) in optical fibres. In fact, so far, GAWBS has been preventing from generating SV pulses at sufficiently high squeezing levels. In this paper, we examine insusceptible chi(3)squeezing to GAWBS.

DOI: 10.1109/cleoe-eqec.2009.5192132

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Squeezed vacuum (SV) generation through third-order optical nonlinearity chi(3)is useful for generating a quantum entanglement pair based on continuous variable. However, third-order nonlinearity is usually small. Therefore, the typical interaction length should be large and ultrashort laser pulses should be used. Then, it becomes important to avoid excess phase noise, especially guided-acoustic-wave Brillouin scattering (GAWBS) in optical fibres. In fact, so far, GAWBS has been preventing from generating SV pulses at sufficiently high squeezing levels. In this paper, we examine insusceptible chi(3)squeezing to GAWBS.

We experimentally demonstrate the generation of a squeezed vacuum pulse at 810 nm with a fiber polarization interferometer. During femtosecond laser pulse propagation through an optical fiber in the normal dispersion regime, only self-phase modulation within a short length contributes to pulse squeezing since the laser pulse is immediately broadened. Guided acoustic-wave Brillouin scattering (GAWBS) noise that increases in proportional to the fiber length is also lower with shorter fibers. Consequently, a maximum noise reduction of 2.1 dB (4.8 dB when corrected for losses) is obtained using a 40-cm-long single mode optical fiber.

The realization of a compact squeezed-state-generation device at 1.5 µm may have potential applications in optical communication. The squeezed state has been generated in both χ(2) and χ(3) media. In χ(2) media the frequency of the squeezed vacuum is half that of the pump. Therefore frequency doubling is necessary for generating a local oscillator (LO). Such frequency doubling is not needed in χ(3) media. However, squeezing in y(3) media has not been as successful as in X(2) media, for example, in atomic vapor,1 self-focusing seriously limits the amount of squeezing, whereas in optical fibers,2 guided-acoustic-wave Brillouin-scattering (GAWBS) noise causes difficulties that can be circumvented through low- temperature cooling of the fiber or the use of soli ton pulses.

Squeezing with degenerate parametric amplification sacrifices the power needed for the pump.1 Squeezing by using the third- order nonlinearity does not necessarily sacrifice the pump power, because it remains at the same frequency as the squeezed radiation. Because the third-order nonlinearity is weak, long interaction lengths must be used, i.e., optical fibers. Past experiments with quasi excitation in optical fibers were plagued by stimulated Brillouin scattering (SBS) and guided acoustic wave acoustic Brillouin scattering (GAWBS).2 The use of pulses raises the peak power and the SBS threshold, allowing use of shorter fibers. A Mach-Zehnder interferometer configuration separates the pump from the squeezed vacuum.3 This has two advantages: the pump power can be reused as the local oscillator power, and the squeezed radiation can be measured closer to zero frequency (30 kHz) because the pump fluctuations are absent.

Photon-number squeezing and quadrature squeezing were studied using ultrafast fibre nonlinear optics pumped by 1.5 µm femtosecond laser pulses. To obtain quadrature squeezing using an Erbium doped fibre amplifiers (EDFA) light source, we developed temporal homodyne technology to exclude the excess noise of an EDFA light source. We also performed a quadrature squeezing experiment using an optical parametric oscillator light source in stead of an EDFA light source exhibiting substantially high excess noise. Although the photon-number squeezing of -3dB was obtained from an asymmetric Sagnac fibre loop, the quadrature noise could not decrease to less than the Shot noise level without cooling the fibre, which is presumably due to GAWBS. To suppress guided acoustic wave Brillouin scattering, we refrigerated the fibre loop by liquid nitrogen, and squeezing at -0.7 dB was obtained by cooling the fiber at liquid nitrogen temperature.