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Journal of Lightwave Technology

Quantum-statistical properties of pulse amplification in optical fibers with gain saturation

Gokalp Kahraman, B.E.A. Saleb

Published Jun 1, 1995• 2 citations
Publisher

Abstract

We studied the quantum-statistical properties of pulse amplification in erbium-doped fiber amplifiers (OFAs), including saturation of the atomic population inversion and pump depletion. We use a fully quantum theory to describe the atom-field interaction as wed as the light propagation. The generating function of the output photon number distribution (PND) is determined as a function of time during the course of the pulse, with an arbitrary input PND assumed. For input light with Poisson PND, the output PND is shown to be the Laguerre distribution with parameter 1 at all times smaller than the coherence time /spl tau//sub c/, even in the presence of nonlinear effects. An expression for the photon count moment generating function is found for counting times T/spl Gt//spl tau//sub c/. The mean pulse shape is shown to be altered by the nonlinear amplification. The variance is similarly altered, and the excess amplifier noise is greater at the leading side of the pulse.>

DOI
10.1109/50.390230

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We studied the quantum-statistical properties of pulse amplification in erbium-doped fiber amplifiers (OFAs), including saturation of the atomic population inversion and pump depletion. We use a fully quantum theory to describe the atom-field interaction as wed as the light propagation. The generating function of the output photon number distribution (PND) is determined as a function of time during the course of the pulse, with an arbitrary input PND assumed. For input light with Poisson PND, the output PND is shown to be the Laguerre distribution with parameter 1 at all times smaller than the coherence time /spl tau//sub c/, even in the presence of nonlinear effects. An expression for the photon count moment generating function is found for counting times T/spl Gt//spl tau//sub c/. The mean pulse shape is shown to be altered by the nonlinear amplification. The variance is similarly altered, and the excess amplifier noise is greater at the leading side of the pulse.>
Erbium-doped fiber amplifiers (EDFA) are gaining increasing importance in the lightwave communications arena due mainly to their low insertion loss and easy manufacturability. Consequently there is significant motivation in obtaining the photon number distribution at the output of an EDFA taking finite bandwith and line shape of the signal lasers into account. In this paper the photon number distribution at the output of an EDFA resulting from an input wave packet having an arbitrary photon number distribution is evaluated. Previously the output photon number distribution was derived [1] for input fields having Poisson photon number distributions, i.e. for coherent sate input fields. Our treatment is more general in the sense that the input field can be in an arbitrary quantum sate. This generality will enable researchers to evaluate EDFA photon number distributions resulting from non-monochromatic incident fields [for example laser sources with phase and/or intensity noise] simply by using the appropriate input photon number distributions in the equations that will be derived below [see for example [2] for several photon number distributions corresponding to various laser line shapes].
The intensity fluctuations in the output of a saturable optical amplifier are studied as the amplifier length is increased. Characteristics are determined by calculation of the diagonal elements of the density matrix of the optical field. Saturation affecting both upper-state and lower-state atomic populations is considered. Spontaneous emission distributed along the amplifier causes the "photon-number" distribution to approach a Bose-Einstein distribution during linear amplification if the input field is not Bose-Einstein. The onset of saturation reduces the relative fluctuations of the optical field both by introducing nonlinear gain and by reducing the spontaneous emission. Limiting solutions show that strong saturation leads to a major reduction in the fluctuations causing the photon-number distribution to approach a Poisson distribution.
We present a theoretical analysis and an experimental study of the statistical properties of the noise accompanying an optical pulse propagating in a nonlinear semiconductor optical amplifier. Several degrees of gain saturation corresponding to different levels of optical signal-to-noise ratios (OSNRs) are examined. We employ the Heun numerical procedure to ensure proper convergence to the Stratonovich solution of the multiplicative propagation equation. This algorithm takes also into account the effect of gain saturation due to the amplified spontaneous emission noise. Moreover, the multicanonical Monte Carlo algorithm is used to efficiently calculate the probability density functions (pdfs), including the tails, of the peak of a pulse which emerges at the output of a saturated semiconductor optical amplifier. The results are compared to the corresponding pdfs obtained in a linear amplification system (where the optical noise is additive and Gaussian) having the same gain and under identical OSNR levels. We demonstrate that the pdf of the saturated amplifier is shifted toward lower power levels and is narrower, or equivalently, the mean and variance for the saturated amplifier case are smaller. Also, the difference between the two configurations increases with the degree of gain saturation. The theoretical predictions are confirmed by a series of experiments in which the pdfs at the output of the linear amplification scheme and saturated semiconductor optical amplifier are measured for an optical pulse of /spl sim/ 70-ps duration.
The properties of a linear optical amplifier or attenuator in which the input light is coupled to a collection of non-saturable atoms are considered. The photon-number factorial moments and probability distribution of the amplifier output are derived for arbitrary input statistics. Relations are obtained between the output and input variances in photon number, phase-angle cosine, and electric-field magnitude. The effects of amplification and attenuation on signal-to-noise ratio are obtained for both direct and homodyne detection. Particular attention is paid to the extents to which the non-classical properties of photon antibunching and squeezing are preserved by amplification and attenuation. It is found that both properties can at best survive only twofold intensity amplification.