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Acta Optica Sinica

Quantum theory of the pulse train from mode-locked laser

HE Long-qing

Published Jan 1, 1983• 0 citations
OpenAlex

Abstract

A quantum theory for pulse-train from the mode-looked laser has been developed. In the theory the author uses superposition model, i. e. the atomic states are coherent after absorbed photons from different modes of a mode-locked laser. So the final atomic state should be coherently added. The resolvent operator and projector operation are used to derive the operator U. Considering the phase relationship between the modes, the light field are described by multi-mode coherent state. This method can be used to describe the resonant multi-photon processes that the pulse train from the mode locked laser interacting with atoms.

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A quantum theory for pulse-train from the mode-looked laser has been developed. In the theory the author uses superposition model, i. e. the atomic states are coherent after absorbed photons from different modes of a mode-locked laser. So the final atomic state should be coherently added. The resolvent operator and projector operation are used to derive the operator U. Considering the phase relationship between the modes, the light field are described by multi-mode coherent state. This method can be used to describe the resonant multi-photon processes that the pulse train from the mode locked laser interacting with atoms.
A quantum theory of two-photon correlated-spontaneous-emission lasers (CEL's) is developed, starting from the exact atom-field interaction Hamiltonian for cascade three-level atoms interacting with a single-mode radiation field. We consider the situation where the active atoms are prepared initially in a coherent superposition of three atomic levels and derive a master equation for the field-density operator by using a quantum theory for coherently pumped lasers. The master equation is transformed into a Fokker-Planck equation for the antinormal-ordering Q function. The drift coefficients of the Fokker-Planck equation enable us to study the steady-state operation of the two-photon CEL's analytically. We have studied both resonant two-photon CEL for which there is no threshold, and off-resonant two-photon CEL for which there exists a threshold. In both cases the initial atomic coherences provide phase locking, and squeezing in the phase quadrature of the field is found. The off-resonant two-photon CEL can build up from a vacuum when its linear gain is larger than the cavity loss (even without population inversion). Maximum squeezing is found in the no-population-inversion region with the laser intensities far below saturation in both cases, which are more than 90% for the resonant two-photon CEL and nearly 50% for the off-resonant one. Approximate steady-state Q functions are obtained for the resonant two-photon CEL and, in certain circumstances, for the off-resonant one.
A quantum theory of a two-level single-mode laser with injected atomic coherence is developed by generalizing the Scully-Lamb laser theory to a form appropriate for the analysis of a coherently pumped laser. We assume that the active atoms are prepared initially in a coherent superposition of the upper and lower levels, and we derive the master equation for the field density operator by treating the interaction of the laser field with many active atoms simultaneously. It is shown that the photon-number distribution can be exactly Poissonian. The laser operation is analyzed in terms of the Fokker-Planck equation for the laser field. Both the intensity and phase diffusion coefficients are phase sensitive and, for stable laser operation, become much smaller than those of an ordinary laser. Consequently, the injected atomic coherence reduces both the photon-number noise and phase noise simultaneously. The intensity diffusion coefficient can vanish exactly, and at the same time the phase diffusion coefficient can become very small. This leads to spontaneous-emission noise quenching in the photon-number distribution, and the laser field can become very close to a coherent state. A scheme to generate the proper form of the initial atomic coherence necessary for the quantum noise quenching is proposed and analyzed.