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Physical Review A

Quantum fluctuations of optical solitons in fibers

Demin Yao

Published Dec 1, 1995• 14 citations
Publisher

Abstract

Quantum fluctuations of optical solitons in fibers are discussed on the basis of the exact quantum states in the frame of the quantum nonlinear Schr\"odinger equation. Position and momentum fluctuations as well as quadrature phase fluctuations are calculated, and explicit results are expressed in terms of the intrinsic quantum parameters. Soliton wave-packet spreading and phase-diffusion phenomena are clearly demonstrated, and the corresponding squeezing effects are analyzed. In particular, soliton amplitude squeezing is investigated in an experimental setup of a Mach-Zehnder interferometer containing an optical fiber. Coherent states for quantum solitons are discussed with attention given to their dynamical preservability. Compared with previous linearized approaches, the present study may provide a more fundamental treatment to quantum fluctuations of solitons.

DOI
10.1103/physreva.52.4871

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Quantum fluctuations of optical solitons in fibers are discussed on the basis of the exact quantum states in the frame of the quantum nonlinear Schr"odinger equation. Position and momentum fluctuations as well as quadrature phase fluctuations are calculated, and explicit results are expressed in terms of the intrinsic quantum parameters. Soliton wave-packet spreading and phase-diffusion phenomena are clearly demonstrated, and the corresponding squeezing effects are analyzed. In particular, soliton amplitude squeezing is investigated in an experimental setup of a Mach-Zehnder interferometer containing an optical fiber. Coherent states for quantum solitons are discussed with attention given to their dynamical preservability. Compared with previous linearized approaches, the present study may provide a more fundamental treatment to quantum fluctuations of solitons.
The usual view of a soliton in an optical fiber is that it obeys the classical Maxwell equations, and propagates unchanged down a lossless fiber. Of course, it is well known that an optical amplifier introduces noise in position. It is less well known that even a perfectly lossless fiber, without an amplifier, is unable to support the classical invariant solution. Instead, all solitons experience quantum noise in position and phase, and this noise increases with propagation distance. Experimenters at IBM laboratories have recently observed the first quantum effects in solitons. These experiments focussed on quadrature squeezing, and were able to detect the large increase in phase-quadrature noise that is predicted quantum-mechanically. In this paper we treat the effects of pulse-position noise, and indicate how this noise source can limit information transmission using solitons. While these effects are small in currently proposed communication systems, they place a fundamental limit on future enhancements of soliton communications using pulse-position modulation. Of most scientific interest is the macroscopic nature of the quantum fluctuations, which can alter the location of a huge number of cooperatively positioned photons, and possible implications for QND momentum measurements.
The quantum dynamics of coherent optical pulses are studied using a nondiagonal coherent-state generalized PPP representation including photon-phonon interactions. Photon-number squeezing of coherent quantum solitons using spectral filtering is theoretically predicted. It is shown that Raman noise does not significantly reduce photon-number squeezing produced by spectral filtering of 1-ps fundamental coherent quantum solitons in optical fibers. Coherent NsechNsechNsech pulses with N>1N>1N>1 can show a larger reduction in photon-number fluctuations even at room temperature. The reduction in quantum noise for N>1N>1N>1 is not restricted to photon number and an improvement of more than 3 dB is also found for the quadrature-phase squeezed soliton experiments using a fiber Sagnac interferometer at 77 K.
An investigation is made of the evolution of quantum fluctuations of a fundamental soliton in the course of its propagation in a nonlinear fiber waveguide characterized by losses and compensated by amplification. Simple relationships are obtained for the amplitude and phase noise, quantum uncertainty of the position and momentum, and also fluctuations of the quadrature components of the radiation field. Numerical estimates are obtained. It is shown that loss-compensating amplification is unnecessary for efficient formation of squeezed states of a soliton.
Quantum fluctuations of one-dimensional dark dissipative solitons sustained by an external radiation in an interferometer with a Kerr nonlinearity are analyzed theoretically. The stability region of classical solitons in this interferometer is studied. The boundaries of this region are determined, and types of excited solitons are classified. Quantum fluctuations of solitons are analyzed in an approximation linear in fluctuations. This problem was solved by linearizing the quantum Langevin equation in a neighborhood of a classical solution for the main type of a soliton from the obtained stability region. The main attention has been paid to studying quantum fluctuations of collective variables of dissipative solitons, namely, the coordinate of the center and momentum of the soliton. Based on the expansion of solutions of the linearized equation in eigenfunctions of the discrete spectrum of this equation, a solution describing quantum fluctuations of these variables is constructed. Using this expansion scheme made it possible to give a rigorous definition of the dissipative soliton position fluctuation operator. The study performed based on this scheme has made it also possible to construct a solution for a one-dimensional dark relaxing dissipative soliton. This soliton generalizes the stationary soliton with allowance for the shift of its center and deformation of its profile followed by the recovery of its initial shape. Average squares of quantum fluctuations of collective variables are calculated. A domain of parameters in which there exist quantum states of solitons with an initially high degree of squeezing with respect to the momentum is found. It is shown that such states are in correspondence with significantly higher velocities of soliton center drift. An experiment that could detect the relative squeezing with respect to the momentum due to the soliton center drift is discussed.
In the preceding paper [paper I of a two-part study; Lai and Haus, Phys. Rev. A 40, 844 (1989)] we have used the time-dependent Hartree approximation to solve the quantum nonlinear Schr\"odinger equation. In the present paper, the eigenstates of the Hamiltonian are constructed exactly by Bethe's ansatz method and are superimposed to construct exact soliton states. Both fundamental and higher-order soliton states are constructed and their mean fields are calculated. The quantum effects of soliton propagation and soliton collisions are studied in the framework of this construction. It is shown that a soliton experiences dispersion as well as phase spreading. The magnitude of this dispersion is estimated and is shown to be very small when the average photon number of the soliton is much larger than unity. The phase and position shifts due to a collision and the uncertainty of these shifts are also calculated.
We develop a quantum theory of propagation in dispersive nonlinear media via a quantized field theory and a coherent state expansion. The resulting stochastic nonlinear Schrodinger equation includes both quantum fluctuations and thermal noise due to refractive-index fluctuations in the medium. The theory can describe either cw propagation with dispersion or soliton propagation. In the cw case, enhanced squeezing occurs with anomalous dispersion. This is reduced by thermal noise, just as it is in the normal dispersion case. The noise-induced reduction in squeezing is more severe with normal dispersion owing to the intrinsic lack of phase matching in the normal dispersion regime. With anomalous dispersion, the cancellation of nonlinear by linear dispersion allows squeezing to occur over a wide bandwidth even in the presence of thermal fluctuations. Our theory is also able to treat soliton propagation in the anomalous dispersion case. A substantial degree of pulsed squeezing is obtained, and we point out that an optimal detection scheme requires a pulsed local oscillator. Results for the effects of thermal noise on soliton propagation are presented, in which case improvement is obtained by reducing the soliton time duration.