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Physical Review Letters

Correlated spontaneous emission in a Zeeman laser

M. P. Winters, J. L. Hall, P. E. Toschek

Published Dec 17, 1990• 82 citations
Publisher

Abstract

We have observed phase-diffusion noise 40% below the Schawlow-Townes limit in the relative phase of a two-mode HeNe Zeeman laser due to the correlated-emission-laser effect.Received 8 June 1990DOI:https://doi.org/10.1103/PhysRevLett.65.3116©1990 American Physical Society

DOI
10.1103/physrevlett.65.3116

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We have observed phase-diffusion noise 40% below the Schawlow-Townes limit in the relative phase of a two-mode HeNe Zeeman laser due to the correlated-emission-laser effect.Received 8 June 1990DOI:https://doi.org/10.1103/PhysRevLett.65.3116©1990 American Physical Society
Measuring the phase dynamics of the beat note in the output of a HeNe Zeeman laser, we (i) unravel the interplay of phase locking and correlated spontaneous emission, (ii) observe the phase-diffusion noise reduced to 114\frac{1}{14}141​ of the Schawlow-Townes quantum level (the largest reduction of quantum noise reported so far), and (iii) show that balanced pumping of the two upper laser levels would achieve correlation of the laser phase noise with no locking at all. The resulting reduction in the phase noise of the beat note allows interferometric measurements of vastly enhanced sensitivity.
As discussed in the last three chapters, the fundamental source of noise in a laser is spontaneous emission. A simple pictorial model for the origin of the laser linewidth envisions it as being due to the random phase diffusion process arising from the addition of spontaneously emitted photons with random phases to the laser field. In this chapter we show that the quantum noise leading to the laser linewidth can be suppressed below the standard, i.e., Schawlow–Townes limit by preparing the atomic systems in a coherent superposition of states as in the Hanle effect and quantum beat experiments discussed in Chapter 7. In such coherently prepared atoms the spontaneous emission is said to be correlated. Lasers operating via such a phase coherent atomic ensemble are known as correlated spontaneous emission lasers (CEL). An interesting aspect of the CEL is that it is possible to eliminate the spontaneous emission quantum noise in the relative linewidths by correlating the two spontaneous emission noise events. A number of schemes exist in which quantum noise quenching below the standard limit can be achieved. In two-mode schemes a correlation between the spontaneous emisson events in two different modes of the radiation field is established via atomic coherence so that the relative phase between them does not diffuse or fluctuate. In a Hanle laser and a quantum beat laser this is achieved by pumping the atoms coherently such that every spontaneously emitting atom contributes equally to the two modes of the radiation, leading to a reduction and even vanishing of the noise in the phase difference.