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Soviet Journal of Quantum Electronics

Does a laser generate coherent radiation?

S V Amel'kin, N B Arkhangel'skiĭ, А. Н. Ораевский

Published Jan 31, 1984• 1 citation
Publisher

Abstract

An analysis is made of the role of the polarization properties of the pump radiation in the appearance of a quasiclassical laser field. It is shown that a coherent quasiclassical field in a laser is of fluctuation origin and its appearance does not require the assumption that the active atoms are polarized by the pump source. The process of growth of a coherent laser field is simulated on a computer.

DOI
10.1070/qe1984v014n01abeh004625

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An analysis is made of the role of the polarization properties of the pump radiation in the appearance of a quasiclassical laser field. It is shown that a coherent quasiclassical field in a laser is of fluctuation origin and its appearance does not require the assumption that the active atoms are polarized by the pump source. The process of growth of a coherent laser field is simulated on a computer.
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The influence of light polarization on the dynamics of an optically pumped single-mode laser with a homogeneously broadened four-level medium is theoretically investigated in detail. Pump and laser fields with either parallel or crossed linear polarizations are considered, as are typical in far-infrared-laser experiments. Numerical simulations reveal dramatically different dynamic behaviors for these two polarization configurations. The analysis of the model equations allows us to find the physical origin of both behaviors. In particular, the crossed-polarization configuration is shown to be effective in decoupling the pump and laser fields, thus allowing for the appearance of Lorenz-type dynamics.
In this Letter, we give a new, to the best of our knowledge, perspective on the origin of light coherence in lasers. We demonstrate that a coherence appears below the lasing threshold and manifests itself as long-range correlations between polarizations of active medium atoms. These correlations contribute to the formation of a collective state of atomic polarizations and electromagnetic field modes, which interacts more effectively with the active medium and lases when pumping exceeds the lasing threshold. We demonstrate that inhibiting these atomic correlations leads to the destruction of the collective state and suppression of lasing. The obtained results open up new ways to control coherence.
The fundamental source of noise in laser and optical amplifier is spontaneous emission. However, it is possible to reduce this noise by preparing the gain medium (atoms) in coherent superposition of states. There are essentially two schemes for generating atomic coherence in optical amplifiers. \n \nFirst scheme involves the preparation of atoms in a coherent superposition of states before injecting them inside the cavity. In the other scheme atoms (initially pumped incoherently) are continuously driven by an external pump field inside the cavity. Almost all the studies concerning the quenching of spontaneous emission noise in optical amplifiers, based on the notion of atomic coherent superposition, ignore the finite bandwidth of the field that is responsible for generating coherence. In this dissertation, we have examined the effect of the finite bandwidth associated with the phase fluctuations of the driving field on a number of noise quenching schemes. \n \nIt is known that coherent preparation of atoms can, under certain conditions, lead to noise-free amplification and almost 50 percent phase squeezing in a two-photon correlated emission laser. We study the effect of phase fluctuations in the injected atoms on the phase noise of the field in these systems. We discuss the conditions under which squeezing is preserved in the two-photon linear amplifiers. We also show that, even for very large phase fluctuations in the injected atoms, almost 50 percent phase squeezing can be achieved. The corresponding gain is however significantly reduced. \n \nWe also study a system of phase-sensitive amplifier where atoms are prepared in a coherent superposition of states by a strong classical field. We study the role of pump phase fluctuations associated with the finite linewidth on the quantum statistical properties of the radiation field. A study of output field characteristics reveals that the presence of a weak pump field gives rise to equal variances in both the field quadratures which are independent of linewidth. On the other hand, the presence of strong pump field alters the output field characteristics with the generation of squeezing which is later on destroyed due to the presence of phase fluctuations
The Scully-Lamb quantum-mechanical laser theory is generalized to the case of active atoms situated in superposed upper and lower active states. In this case, a laser emits a field of nonzero average intensity. If there is no interference between the active levels of the active atoms, a state of the field may be found for which the average quantum-mechanical intensity is zero. An investigation is made of pumping in a molecular oscillator and in gas lasers. It is shown in which cases pumping causes interference between the active levels of the active atoms. An interference experiment which can distinguish between fields having zero and nonzero average intensities is discussed.