The squeezed light field at the band of alkali metal atoms’ transitions is an important quantum resource in the field of quantum information and precision measurement. The wavelengths of atomic transition lines (760-860 nm) are relative short. Limited by the gray-tracking effect of nonlinear crystals, the squeezing degree of the squeezed light at this band generated by the optical parametric amplifiers is low. For now, the squeezing is about 3~5 dB. Considering the problems in the experimental generation of the squeezed light at the wavelengths of atomic transitions, the variation law of quantum noise of the light field output from the single optical parametric amplifier with its physical parameters is theoretically studied, and the optimal physical parameters are obtained. To further improve the squeezing at the band of alkali metal atoms’ transitions, the cascaded optical parametric amplifiers are considered. Based on the basic theory of the optical parametric amplifiers, the theoretical model of the cascaded optical parametric amplifiers is constructed, in which the optical loss and phase noise of the cascaded optical loops are considered. Based on this, the quantum noise characteristics of the light field output from the cascaded system versus the optical loss and phase noise are analyzed at the frequencies of 2 MHz and 100 kHz, respectively. It is found that for the squeezing at 2 MHz, cascading 2 to 3 optical parametric amplifiers can significantly improve the squeezing under the premise of the low optical path loss and phase noise; for the squeezing at low-frequency band, the enhancement of the squeezing for the cascaded system is quite weak. Under the current experimental parameters, the squeezing at 2 MHz of the squeezed light on rubidium resonance can be improved from -5 dB to -7 dB by cascading another DOPA. For the squeezing at low frequency band, the cascaded is proved to be useless, and the efforts should be made in reducing the technique noise at low frequency band. Furthermore, the quantum limit and spectral characteristics of the squeezed light field output from the cascaded system are further explored. This study can provide reference and guidance for the improvement of the squeezing at the band of atomic transitions.
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