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Communications in Theoretical Physics

Effect of Pump Bandwidth: Enhancement of Output Intensity Noise Squeezing in Raman Lasers

Xiangming Hu, Peng JinSheng

Published Jun 15, 2000• 0 citations
Publisher

Abstract

The effect of the pump bandwidth on the quantum statistical properties of Raman lasers is investigated. It is shown that the pump bandwidth may lead to the enhancement of output intensity noise squeezing even though it reduces the Mandel parameter.
DOI
10.1088/0253-6102/33/4/649

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We investigate the influence of pump bandwidth and pump intensity noise on the quantum-noise properties of lasers based on a Raman atomic-gain medium, which recently have been predicted to emit narrow-bandwidth, sub-shot-noise light. We show that, using finite-bandwidth pump light, the predicted limit to the laser bandwidth can still be well below the value given by the Shawlow-Townes formula and much smaller than the input bandwidth. The laser intensity noise is shown to be relatively insensitive to input phase noise as long as the Rabi frequency on the lasing transition is large compared to the bandwidth. On the other hand, even small amounts of pump amplitude noise tend to increase strongly the laser spectral bandwidth, especially in the high-intensity limit. Including dynamic pump-noise reduction through multilevel atomic cycling in our model leads to a Mandel Q parameter value even below Q=-1/2, together with a significant enhancement of the output intensity-noise squeezing.
We investigate the effect of collisional broadening, of population trapping and repumping, and of detuning of the atomic levels on the quantum-statistical properties of Raman laser radiation. We find that under suitably chosen operating conditions the effect of such additional incoherent relaxation mechanisms for the atomic polarization and populations can be made very small. One obtains highly nonclassical values for the Mandel Q parameter and the output-intensity fluctuation spectrum S(\ensuremath{\omega}) for experimentally reasonable parameters. Including detuning of the laser and the pump light from the atomic levels in certain cases can lead to further narrowing of the intracavity photon number distribution, as compared to the resonant case, down to a Mandel Q parameter close to the optimum possible value of Q=-1. A subsequent preliminary analysis prevails that inhomogeneous Doppler broadening of the atomic lines, both for collinear and for counterpropagating pump and laser light, is not detrimental to the nonclassical intracavity state.
We study in detail the influence of pumping statistics on the laser dynamics. We apply the technique of quantum Langevin operators and generalize the corresponding noise operators to incorporate the statistical properties of the pump mechanism. These equations are then used to derive expressions for the phase and intensity fluctuations of lasers with various pump statistics. We find that a reduction of pump noise can lead to a significant squeezing of the photon number noise below the shot-noise limit. This is in complete agreement with our previous analysis, which used a density-operator approach.
We demonstrate that the intensity fluctuations of a laser with a Raman gain medium, described by a large number of independent 3-level atoms, can drop well below the shot noise level. In the limit of a good cavity, where one can adiabatically eliminate the atoms, we find an optimum Mandel Q parameter of Q ≈ −1/2 over a large range of parameters, when the laser is operated well above threshold. This leads to a strong suppression of the low-frequency quantum fluctuations in the laser output. In addition we find that due to suppression of spontaneous emission into the lasing mode, the spectral linewidth of the emitted light can be reduced below the Schawlow-Townes limit by more than one order of magnitude.
Finite input laser bandwidths alter the output quantum statistics in an intracativity parametric oscillator. We show that in an above-threshold experiment, the obtainable squeezing can be greatly reduced by these laser fluctuations. The effect is not eliminated just by reducing the laser bandwidth to less than the cavity bandwidth. Instead, the cavity/laser bandwidth ratio must be improved to the order of the intracavity photon number. Thus, highly stabilized lasers are essential in above-threshold squeezing experiments.