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Physical Review A

Squeezed light from conventionally pumped lasers with nonuniform spatial structure

Craig M. Savage, Timothy C. Ralph

Published Sep 1, 1992• 9 citations
Publisher

Abstract

Spatial variations of the laser mode and pumping rate are incorporated into the theory of conventionally pumped lasers that produce squeezed light. Both a quantum-mechanical theory and a heuristic statistical model are used. While variations in the laser mode are found to have a negligible effect on squeezing, variations in the pumping rate are significant. The maximum attainable squeezing is always reduced compared with the spatially uniform case. However, resonantly enhancing a low-power pump in a Fabry-P\'erot cavity, rather than a ring cavity, may give better squeezing.

DOI
10.1103/physreva.46.2803

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Spatial variations of the laser mode and pumping rate are incorporated into the theory of conventionally pumped lasers that produce squeezed light. Both a quantum-mechanical theory and a heuristic statistical model are used. While variations in the laser mode are found to have a negligible effect on squeezing, variations in the pumping rate are significant. The maximum attainable squeezing is always reduced compared with the spatially uniform case. However, resonantly enhancing a low-power pump in a Fabry-P'erot cavity, rather than a ring cavity, may give better squeezing.
It has recently been discovered that the quantum theory of the laser allows conventional lasers to produce squeezed light.1-3 The physics of this squeezing is uncomplicated and robust. It relies on the fact that a sequence of Poissonian transitions having the same total mean transition time as a single Poissonian process have less time variance than the single process. Hence the sequence of transitions has sub-Poissonian time statistics. This can be transformed into sub-Poissonian laser pumping and hence into sub-Poissonian and quadrature squeezed laser light. We will discuss how this theory can be put into practice. The effects of mode structure and inefficiences will be assessed. Candidate lasers and progress towards their experimental realization will be discussed.
A recent analysis of the influence of the pump statistics on the laser output fluctuations is applied to the experimentally feasible case of a laser being pumped by light coming from a separate pump field cavity that is driven by a broad-band squeezed vacuum plus a coherent driving field. In contrast to the 'squeezed-reservoir laser', where a reservoir of squeezed modes coming from all directions coupled to the dipole moment of the lasing transition and leads to atomic phase decay inhibition and phase locking of the laser field, here the squeezed light merely pumps the gain atoms, that is gives rise to inversion of the laser transition. In analogy to photodetection theory the photon statistics of the laser output is calculated as a function of the photon statistics of the absorbed squeezed pump light. For the case of a squeezed pump field chosen to display sub-Poissonian statistics, it is demonstrated that the Mandel Q-parameter of the laser cavity output can theoretically reach the optimum value of Q=- 1 / 2 , similar to the case of a deterministic pumping mechanism.
In this paper, we explore the squeezing effect generated by two coupled optical cavities. Each cavity contains a second-order nonlinear material and coherently pumped by a laser. Our results show that light intensity is strongly improved due to the presence of the nonlinearities and mainly depends on the detunings between external laser frequencies and cavity modes. More interestingly, the proposed scheme could enhance light squeezing for moderate coupling between cavities : the squeezing generated by one cavity is enhanced by the other one. For resonant interaction, highest squeezing effect is obtained near resonance. When fields are non resonant, squeezing increases near resonance of the considered cavity, but decreases for large detunings relative to the second cavity. Further, when the dissipation rate of the second cavity is smaller than the first, the squeezing could be improved, attaining nearly the perfect squeezing. While the temperature elevation has a negative impact overall on the nonclassical light, squeezing shows an appreciable resistance against thermal baths for appropriate parameter sets.
We investigate the effect of a spatially varying field mode on the squeezing generated by two-level atoms interacting with a single mode of an optical cavity. The mode structure is incorporated into a quantum theory that does not employ adiabatic elimination of field or atomic variables. Thus we are able to consider the regime in which cavity and atomic decay rates are approximately equal as well as the good- and bad-cavity limits. Calculations show that squeezing is reduced by the inclusion of a Gaussian transverse mode profile and a standing-wave structure for a particular value of atomic cooperativity C. The model is consistent with recent measurements of squeezing. The degree of squeezing reduction depends on the ratio of cavity and atomic damping rates, in the regime where they are similar in size.
The statistical and squeezing properties of the cavity light produced by a three-level laser are studied. In the laser, N three-level atoms in an open cavity are coupled with a two-mode vacuum reservoir and are pumped to the top level by means of the electron bombardment. Applying the steady-state solutions of the equations of evolution of the expectation values of the atomic operators and the quantum Langevin equations for the cavity mode operators, we have obtained the global and local photon statistics for single-mode cavity light beams and for two-mode cavity light. It is found that the global mean photon number and the global photon-number variance of the light emitted from the top are greater than those for the light emitted from intermediate level. The cavity lights emitted from the top and intermediate levels can be separately in a chaotic state under certain conditions. However, the two-mode cavity light is in a squeezed state under certain conditions. We have established that the maximum quadrature squeezing of the two-mode cavity light to be about 46% below the coherent-state level. The presence of the vacuum reservoir noise has the effect of increasing the photon-number variance and decreasing the quadrature squeezing of the cavity light but has no effect on the mean photon number. We have shown that the local mean photon number and photon number variance of the cavity light approach the global mean photon number and photon number variance of the cavity light as the frequency interval increases.