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Science in China Series A-Mathematics Physics Astronomy & Technological Science

Generation of Sub-Poisson Light in a Negative Feedback and Cascade Three-Level System

Jin-Hua Chai, Guang-Can Guo

Published Jan 1, 1993• 0 citations
Publisher

Abstract

A scheme for generating sub-Poisson light in a negative feedback and cascade three-level system is proposed in this paper. By using Langevin quantum theory, it is shown that there is a partial correlation between the two output light beams, one of which is fed back to control pump source so that the pump noise may be suppressed. When the pump noise is perfectly suppressed, the photon noise level of the internal field is 50% below the standard quantum limit (SQL), but the reduction of the output photon noise is related with the atomic number N. When N is very large, the output noise level is SQL. When N is very small, the sub-Poisson light can be produced within the cavity bandwidth in the external field, and the fluctuation of output photon flow approaches zero when N approaches 1. The conclusion in this paper is of generality.

DOI
10.1360/ya1993-36-2-209

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A scheme for generating sub-Poisson light in a negative feedback and cascade three-level system is proposed in this paper. By using Langevin quantum theory, it is shown that there is a partial correlation between the two output light beams, one of which is fed back to control pump source so that the pump noise may be suppressed. When the pump noise is perfectly suppressed, the photon noise level of the internal field is 50% below the standard quantum limit (SQL), but the reduction of the output photon noise is related with the atomic number N. When N is very large, the output noise level is SQL. When N is very small, the sub-Poisson light can be produced within the cavity bandwidth in the external field, and the fluctuation of output photon flow approaches zero when N approaches 1. The conclusion in this paper is of generality.
The aim of this paper is to try to find out the possibility of reducing the photon number noise in an optically pumped three-level atomic system. Consider a three-level atomic system. The atomic transition between level 1 and level 3 is forbidden. Each atom is incoherently excited to upper level 3 from level 1, transits to level 1 through intermediate level 2, and emits photons at frequency (omega) 1 and (omega) 2. We place the atoms with the above feature into an oscillator and may obtain two coherent light beams, whose frequency are (omega) 1 and (omega) 2, respectively. There may be some correlation between these two light beams. We make one beam to control the pump source by a feedback loop and expect to reduce the noise of photon number of the other beam.
We show that the injection of an external field in a laser helps to improve amplitude squeezing and sub-Poissonian light generation close to threshold, but spoils quantum noise reduction for very high pumping rates. A general nonlinear theory is developed for the resonant case (no detunings), without any assumption on the relative magnitudes of field and atomic decay constants, thus encompassing a large class of lasers. Several possible pumping schemes are accommodated, ranging from regular to Poissonian. We carry out a detailed stability analysis of the general equations of motion, relating it to the noise-reduction problem. Both the photon-number variance inside the cavity and the homodyne spectra of the output are discussed.
The author reviews methods for generating sub-Poissonian light and related concepts. This light has energy fluctuations reduced below the level which corresponds to a classical Poissonian process (shot-noise level). After an introduction to the concept of nonclassical light, an overview is given of the main methods of quantum-noise reduction. Sub-Poissonian processes are exemplified in different areas of optics, ranging from single-atom resonance fluorescence to nonlinear optics, laser physics, and cavity quantum electrodynamics. Emphasis is placed on the conceptual foundations, and on developments in laser theory that lead to the possibility, already demonstrated experimentally, of linewidth narrowing and sub-Poissonian light generation in lasers and masers. The sources of quantum noise in these devices are analyzed, and four noise-suppression methods are discussed in detail: regularization of the pumping, suppression of spontaneous-emission noise, nonadiabatic evolution of the atomic variables, and twin-beam generation.
We present new results for the quantum noise reduction owing to the mth harmonic generation (m = ½, 2, 3) in a laser. Study of such a source of light with noise beyond the quantum limit is now in progress.1,2 We consider this problem for both the laser with conventional random pumping and the laser with reduced pump noise, i.e., the sub-Poissonian laser. In the case of the laser with a conventional pump, the noise in the mth harmonic can be suppressed by m times as compared to the shot noise limit. For the sub-Poissonian laser, perfect noise reduction can be achieved in any harmonic mode. Cross correlations between the fundamental mode and the harmonic mode lead to the quantum noise interference effect, which manifests itself in the noise of the total photon flux from the laser. The system with two nonlinear crystals inside the laser cavity can serve as a source of two perfectly squeezed modes with divisible frequencies. The first crystal is driven by the external field of frequency 2ω. This field is downconverted in the crystal to the frequency ω, which is the frequency of lasing. In the second crystal the lasing field is again downconverted to the frequency ω/2. The lasing mode m and the subharmonic mode ω/2 from such a laser can be perfectly squeezed; that means complete quantumnoise reduction of these two modes if mixing with local oscillators of phase shifts π/2 is used.
Summary form only given. Among a variety of schemes for nonclassical light generation, the generation of sub-Poissonian photon-states with semiconductor lasers or light-emitting-diodes (LEDs) has been attracting much attention, primarily from engineering point of view. In this talk, we shall present our experimental results, manifesting substantial noise suppression of output photon fluxes below standard quantum limit (SQL) level from a single LED surprisingly without recourse to high-impedance circuit at room temperature. The observed squeezing is interpreted semiquantitatively in terms of our new model including effects of microscopic backward pump process.
Using an extension of the Scully—Lamb laser theory we show that by insertion of nonlinear elements in the cavity, laser light with nonclassical sub—Poissonian photon statistics can be generated. For an N—photon absorber in the limit of small saturation the variance of the photon number distribution can be reduced by a factor of (+1/N) compared to Poissonian limit of the usual laser. When we generalize the type of such nonlinear elements to the case of a nonlinear feedback or Raman type nonlinear transition, the noise in the laser intensity can be reduced even further and approach the ideal limit, characterized by a Mandel Q parameter of Q = —1. Furthermore we show how this noise reduction via nonlinearities can be combined with a regular pump mechanism, which recently has been sugessted as a way to generate sub—Poissonian light. It turns out that in certain configurations the effect of both methods can be combined. Finally we show how this nonclassical behaviour is also reflected in a reduction of the output intensity power spectra below the shot noise level.