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Squeezed vacuum pulse generation with third-order optical nonlinearity in short fibers

Yasuyuki Ito, Hisayuki Nakagome, Hidetake Ushio, Kenichi Hirosawa, Fumihiko Kannari

Published Aug 1, 2009• 0 citations
Publisher

Abstract

Short length optical fibers are proved to be effective to reduce Brillouin scattering noise and produce squeezed vacuum even at low repetition pulse rates. Noise reduction of 2.4 dB below the shot noise level is observed.
DOI
10.1109/cleopr.2009.5292499

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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.
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.
Squeezed vacuum generation is demonstrated using an ultrashort pulse fiber laser and a symmetric nonlinear polarization interferometer (NOPI). In this system, the squeezed vacuum can be generated only by passing the ultrashort pulse along the optical fiber. Noise reduction of -1.7 dB below shot noise level is observed. Considering the effect of quantum efficiency in this system, the corresponding magnitude of squeezing is estimated to be -2.7 dB. The system is constructed with all fiber devices and is compact, stable, and useful for practical applications.
Optical solitons are pulses that propagate in optical fibers without temporal or spectral distortion, owing to balancing of the second-order dispersion of the refractive index by nonlinear self-phase modulation induced by the intensity-dependent refractive index. Classically, this stationary distortion-free propagation only requires the proper choice of pulse width and shape for a given pulse energy. However, quantum fluctuations do not undergo stationary propagation but rather evolve due to dispersion and self-phase modulation. As a soliton pulse propagates, zero-point or vacuum noise associated with the pulse evolves into squeezed noise. Vacuum fluctuations are responsible for the fundamental detector noise floor known as the shot noise limit. Reduction of the noise in the detection of squeezed soliton pulses has achieved a noise level 34% below this limit. In optical fibers, thermal fluctuation of the refractive index limit the noise reduction that can be achieved by adding phase fluctuations in excess of the vacuum noise. At low frequencies thermal noise originates from local structural dynamics of fused silica which produces a 1/f power spectrum and from guided acoustic wage Brillouin scattering which modulates the refractive index at frequencies characteristic of the fiber structure. The bandwidth of these fluctuations is ~2 GHz; and for soliton pulses with spectral bandwidth much >2 GHz, we have shown that this noise can be made insignificant. For very short pulses, however, noise from Raman scattering must be considered. Recent calculations incorporating this noise mechanism are compared with experiment.