Quantum noise reduction in a laser by cascade coherent excitation

Abstract

We study the statistical properties of a three-level laser where cascade coherent excitation as a two-step process is used as a coherent pump and a laser field is generated on the upper transition. It is shown that the laser intensity fluctuations are suppressed up to 50% below the shot noise limit and that the laser linewidth is reduced below the Schawlow-Townes formula by more than one order of magnitude of the laser intensity. By transforming to the picture dressed by the coherent field on the upper transition, we identify a transfer of the lower coherent excitation to be responsible for the quantum noise reduction. Cascade coherent excitation turns out to be a new way in which one can create a pathway for recycling of active electrons and regularization of laser excitation process. The conditions for the physical realization of the Raman scheme (Ritsch et al 1992 Europhys. Lett. 19 7) as a limiting case is discussed.

Effect of biased noise fluctuations on the output radiation of coherent beat laser

“…When there is no injected atomic coherence ( = 1) and N = 0, squeezing close to 50% is obtained for a very large amplitude of the external radiation ( ¯ ). This result agrees with the recent claim that three-level laser in which the atoms are initially prepared in the bottom level and externally pumped by strong radiation resembles parametric oscillator [2,14]. Moreover, comparison of the results shown in Figs.…”

“…The atomic coherence can be induced in a three-level cascade scheme by coupling the upper energy level |a and lower energy level |c, between which a direct transition is dipole forbidden, with external radiation [1][2][3][4][5][6] or by preparing the atom initially in coherent superposition of these two levels [7][8][9][10][11][12] or using these mechanisms at the same time [13]. In addition to these options, Xu and Hu [14] have considered the two-step cascade coherent excitation. For the sake of convenience, the amplification of light when spontaneously emitted photons in the cascade transitions are correlated by the atomic coherence resulting from the initial preparation of the superposition and external driving mechanism can be taken as coherent beat laser (CBL).…”

REMOVED: Driven degenerate three-level cascade laser

“…Interaction of three-level atoms with a radiation has attracted a great deal of interest in recent years [1,2,3,4,5,6,7,8,9,10,11,12]. It is believed that an atomic coherence is found to be responsible for various important quantum features of the emitted light.…”

Anomalous linewidth of driven Mollow lasers

“…Finally, it should be pointed out that the above results hold when the lower lasing level |2 instead of the upper lasing level |1 is coupled to the auxiliary level |3 by a coherent pump 2 , and the level |3 decays to the ground state |2 instead of the excited state |1 . In addition, for the present cascade system (figure 1(a)), if the cavity field is applied to the upper transition 3-1 instead of the lower transition 1-2, the linewidth may have a fast 1/I 3 decay [20], which is in sharp contrast to the present anomalous linewidth. The difference is because the equivalent system is the V system when the cavity field is applied to the lower transition, while the equivalent system is the system when the cavity field is applied to the upper transition.…”